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| The two main divisions of the nervous system are |
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| The _________ nervous system is divided into the sympathetic and parasympathetic systems. |
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| Which branch of the nervous system is most active when you are in a fight? |
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| The fluid filled spaces in the brain are called |
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| The limbic system function is |
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| The thalamus relays information to the brain cortex about |
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| 4 out of 5 sensory modalities |
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| Information from terminal button of one neuron is communicated to a second neuron through the __________ of the second neuron |
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| dendrites (receiving antenna) |
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| The chemical released by a presynaptic neuron into the synapse is called a |
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| Most of the cells in the central nervous system are |
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| glial cells of one sort or another |
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| the electrically charged chemical particles that responsible for electric currents in and around the neuron are called . |
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| The levels of ions that generate an electric charge across a neuronal membrane |
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include Na+, K+ & Cl-
are constantly maintained by "pumps" & "channels" in the neuron's membrane |
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| Calcium entering a nerve terminal is critical for |
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| the release of neurotransmitters. |
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| Which of the following neurotransmitters is derived from the amino acid, tyrosine? |
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| Which of the following neurotransmitters has ONLY G-protein linked receptors? |
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| The major source of the neurotransmitter norepinephrine (NE) in the brain is |
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| Acetylcholine (ACh) is degraded in the synapse by |
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| G-protein linked receptors |
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take longer to respond than ligand-gated receptors
can signal distant parts of a neuron, including the nucleus |
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| In classical conditioning, the stimulus that is originally neutral in regard to the response to be learned is the: |
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| When advertisers pair their products with attractive people or enjoyable surroundings, in the hope that the pairings will cause their products to evoke good feelings, they are using principles derived from: |
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| According to someone like B. F. Skinner, a stimulus is a reinforcer if it: |
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| increases the probability of the response that produced it |
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| A child discovers that her classmates stop taunting her if she just ignores them. This is an example of: |
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| A rat is reinforced for the first lever-pressing response that occurs after each 60 seconds. Which sort of schedule is the rat on? |
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| Which of the following is most concerned with ushering memories from short-term to long-term memory? |
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| Which of the following is most important in emotional aspects of learning (i.e., about emotionally significant information)? |
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| The basal ganglia, and in particular the dorsal striatum, are important for |
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| The order of the basic memory processes in which information enters our memory system and is used later is: |
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| encoding -> storage -> retrieval |
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| The type of memory where information is stored for the shortest period of time is: |
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| The serial-position effect refers to the enhanced recall of information: |
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| at the beginning and end of a list, relative to information in the middle of a list |
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| __________ occurs when new information impairs the retention of previously learned information. |
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| Dave is reminiscing about the first car he owned in high school and how he felt the first time he drove it through town. This is an example of __________ memory. |
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| peripheral nervous system |
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Definition
It consists of the nerves which directly connect to the skin, muscles, blood vessels and organs of the body.
As a general simplification, if nerve tissue not encased in bone (skull, spinal column), it is part of the PNS. |
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| somatic nervous system (voluntary) |
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Definition
innervates the muscles, and connective tissues attached to the skeleton and our skin.
It is responsible for our voluntary movements and the physical sensations (heat, cold, pressure, vibration, pain) we experience
made up of both afferent and efferent nerve fibers. |
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the direction of the impulses it transmits go toward the nervous system from the body's muscles and skin.
conduct sensory information towards the nervous system. |
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sends impulses away from the nervous system in the direction of the body's muscles.
generate movements of the skeleton and hence are motor nerve fibers |
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responsible for sensory and motor functions outside of our voluntary control, such as internal organs and glands.
subdivisions, the sympathetic and parasympathetic |
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| Sympathetic (Fight or Flight) |
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Definition
| involved in preparing us for the expenditure of energy. |
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| Parasympathetic (Rest & Relaxation) |
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Definition
involved in acquiring and storing energy and restoration of the body.
most active during digestion and resting |
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| those that are encased in bone for protection, namely the brain and spinal cord |
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a tough plastic like covering
part of the meninges |
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acts like cushioning and has a spider web-like appearance
part of the meninges |
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a thin, relatively fragile covering
part of the meninges |
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| help to isolate the CNS from the rest of the body and protect it from possible infections. |
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| The main function of the spinal cord is to conduct nerve impulses from the afferent (sensory) nerves to the brain and efferent (motor) impulses to the peripheral nervous system |
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control reflex responses which are enacted without a command from the brain.
An example of this is the knee-jerk reflex sometimes performed during a doctor's examination. |
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The brain is the organ responsible for guiding and controlling behavior.
The brain is the organ responsible for guiding and controlling behavior. |
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| Across animal species, the more complex and highly developed the brain, the more complex the forms of sensory processing, the greater memory capacity and the wider range of motor responses than can be made. |
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Definition
The ventricles are a system of interconnected fluid-filled chambers in the brain that are contiguous with the central canal of the spinal cord.
reservoir for and the producer of the cerebrospinal fluid
EX.disorders like schizophrenia or Alzheimer’s disease, one sign of the disorders can be enlarged ventricles
When the brain loses large numbers of cells across a wide area of the brain
the shrinkage occurs inside-out, hence the enlargement of the ventricles in those cases. |
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| allows the two hemispheres to communicate. |
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| predominantly involved in analytical tasks, breaking down problems, and in language production and comprehension |
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| specializes in emotional processing, math, music, and synthetic processing, |
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| Structural Organization of the Brain |
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| The brain's organization proceeds from its lowest levels which control simple, basic functions necessary for life and awareness to progressively more complex structures which are eventually associated with thought, creativity and reasoning. |
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| structures of the hindbrain are responsible for functions not under voluntary control which maintain general physiological functions of the body and both voluntary and involuntary movements |
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| region of the brain where centers which control autonomic functions such heart rate, blood pressure, respiration, arousal, startle and sleep/wake are located. |
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Pons
part of the hindbrain |
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Definition
| serve as an input and output fiber pathway connecting the brain and the cerebellum |
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Definition
primary job of the cerebellum is to fine-tune the motor signals generated by higher brain motor centers.
Damage to the cerebellum can disrupt the smoothness and grace of movements as well as impair their timing, force, coordination and balance |
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control functions that are more complex than the basic ones controlled by the midbrain
linked to our awareness of the world around us and coping and attending to new stimuli and as well as the roots of motivation |
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superior colliculus
part of midbrain |
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| controls involuntary eye movements and the targeting of the eyes |
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inferior colliculus
part of midbrain |
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Definition
| auditory information, bringing the source of sounds in our environment to our attention and locating them relative to our position. |
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periqueductal gray
part of the midbrain |
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Definition
| process information regarding pain |
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analgesia
part of midbrain |
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Definition
| generate active (fighting) and passive (freezing all movement) behaviors to cope with painful or threatening stimuli. |
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substantia nigra
part of midbrain |
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Definition
| producing essential neurotransmitters for forebrain circuits which are involved in voluntary movements |
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ventral tegmental area
part of midbrain |
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| producing essential neurotransmitters for forebrain circuits which are involved in pleasure, reward and attention |
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reticular activating system
part of midbrain |
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system alertnes
ex.we may come to ignore the steady hum of an air conditioner, but when it suddenly stops the sudden change may make us take notice. |
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Definition
| most highly developed part of the mammalian brain, increasing in complexity |
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Hypothalamus
part of forebrain |
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Definition
located below the thalamus and can be thought of as the seat of most our behaviors which would be called emotional
controls the release of hormones
involved in other basic functions such as hunger, thirst, body temperature regulation, fear, aggression, mating and our response to stress among other things |
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Thalamus
part of forebrain |
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gating of incoming sensory information (taste, touch, hearing, vision, but NOT smell)
functions like an old-time telephone switchboard, routing incoming signals to the appropriate destination. |
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Term
Limbic System
part of forebrain |
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Definition
a group of various specialized structures, each with different functions spanning from memory, planning, emotion, reinforcement and attention.
Together, their actions to provide us with the ability to compare our internal physiological and psychological states with the conditions in the external environment and select an appropriate response based on our expected or desired outcomes. |
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Cerebrum
part of forebrain |
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Definition
the bulk of the forebrain
consists of the cortex
can be divided into four regions, called lobes |
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cortex
part of cerebum and forebrain |
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Definition
outermost part of the cerebrum
highly folded with ridges called gyri and grooves called sulci |
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| what are the four lobes and their functions |
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Definition
occipital lobe-concerned with processing visual information.
parietal lobe-processes somatosensory (for example, touch, cold, heat pain
temporal lobe-(lateral)auditory processing and spoken language.
(medial)involved in memory functions.
frontal lobes- involved in planning, foresight, understanding the consequences of actions and the selection and initiation of motor movements. |
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| Functions of Sensory Systems |
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Definition
sensory stimulation of the sense organs is transmitted into the CNS where in some cases very basic initial sensory processing is done in lower brain regions
main target is the thalamus where the projections are organized and sent to the appropriate cerebral cortical target for intensive processing
exception is for the sense of smell (olfaction) which bypasses the thalamus. smell goes directly to nose |
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| Functions of Motor System |
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has its beginnings in the frontal lobes
The premotor cortex and supplemental motor area are involved in the planning of voluntary motor movements. They project to the primary motor cortex which is responsible for the nerve impulses initiating the movements.
primary motor cortex projects to a group of structures called the basal ganglia
The basal ganglia projects to midbrain motor areas which then project to the motor neurons of the spinal cord. |
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| Functions of Limbic System |
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Definition
is a constellation of structures that involve the temporal lobes, the hypothalamus and the cerebral cortex (most notably the frontal lobes).
The hippocampus is involved in transferring information from our short-term memory to our long-term memory.
The amygdala adds emotional impact and significance to the facts and events being transferred into our long-term memories. |
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Definition
neurons, are not the only cells of the nervous system
The majority of cells in the nervous system are cells called glia |
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Definition
| Glial cells are support cells for the neurons. Neurons are highly specialized and are far less robust and self-sufficient than other types of cells, like muscle cells or liver cells. They rely heavily on glia for a variety of functions which allow the neurons to function effectively. There are even three major types of glial cells to meet the variety of roles that neurons need to be filled. |
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Definition
| Schwann cells are the myelin producing cells of the PNS, the only place where they are found. Each Schwann cell provides one segment of insulation for one projection of a single neuron |
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Definition
| myelin producing cells found only in the CNS. Each oligodendrocyte is star-shaped and provides a segment of myelin insulation for neurons. Each star-like "branch" of an oligodendrocyte, however, provides myelin segment for the projection of a different neuron. |
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Definition
| Astrocytes are the general housekeepers, environmental engineers and nurses of the CNS. They maintain the stability and buffer the chemical content of the extracellular fluid surrounding neurons |
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Definition
| Microglia are the resident immune system cells of the CNS. They are embryologically derived from the immune system cells of the rest of the body. Because of the blood-brain barrier's effectiveness at preventing access to the CNS, microglia do the job of the immune system within the CNS. However, immune cells can eventually enter to supplement the activity of the microglia. |
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Definition
| Nerve cells are called neurons. Neurons are highly specialized for the maintenance of a slight electrical charge across the entire surface of their membranes, intercellular communication, processing electrochemical signals and storing information. While cells of other organs are remarkably similar and uniform with respect to each other, neurons display a wide variety of sizes, shapes and chemical messengers between cells. However, they do share some things in common. |
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Definition
| Given the role of neurons as a communicators and signal processors, all neurons have dendrites, which can be thought of as the "input" side of a neuron. Neurons receive signals from other neurons though their dendrites. Neurons have numerous dendrites forming what is called a dendritic arbor or dendritic field, a highly branched and sub-branched structure which can resemble a tree without leaves. Inputs from thousands of other neurons, three to ten thousand, can make contact with a single neuron's dendritic field. |
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Definition
| The soma is the neuronal cell body. Like the cell bodies found in other tissues the neuronal soma is the site of the neuron's nucleus, organelles and protein manufacturing and metabolic machinery. At one end of the soma is the dendritic arbor at the other side of the soma is the axon. |
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Definition
| If the dendrites are the "input" side of the neuron, then the axon is the "output" side of the neuron. The neurons signals other neurons via electrical activation of its axon. While numerous dendrites attach to the soma, only one axon exits the soma. However, once the axon proceeds some distance from the soma it can subdivide and branch up to several thousand times. Just as a neuron can receive inputs from three to ten thousand neurons, the same neuron can make just as many contacts with other neurons. |
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Definition
| The terminal button is the tip of the axon which makes contact with the dendrite of the next neuron and enables electrochemical communication between the two neurons. The junction where the terminal button of one neuron meets the dendrite of the next and intercellular signal processing and communication occurs is called the synapse. |
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Term
| The (Resting) Membrane Potential |
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Definition
The baseline difference in electrical charge between the inside of the neuron and the exterior is typically -70 millivolts and is called the resting membrane potential, sometimes just called the resting potential. This value can vary slightly in different neurons and in different animal species, but most mammalian species have a resting potential of -70 millivolts. The inside of the neuron is just slightly more negative than the outside of the cell; -70 millivolts is -0.07 volts, less than one-tenth of a volt. The establishment of the resting potential is generated by moving electrically charged ions against their chemical concentration gradients or against their electrical gradients or sometimes both gradients.
The concentration gradient is determined by simple chemical diffusion, i.e., that atoms or molecules tend to move from regions of higher concentration to lower concentration. The higher the concentration of a chemical, the stronger the diffusion force is pushing all the atoms or molecules of that chemical away from each other. Hence, the higher the concentration, the greater the energy needed to resist or overcome the diffusion force.
The electrical gradient is present because the atoms or molecules being moved across the neuronal membranes carry either positive or negative electric charges. Like charges repel each other. When particles of similar (same sign) charge are moved physically closer together, energy is required to overcome the electrical repulsion between those similarly charged particles. |
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Definition
Ions are electrically charged atoms or molecules. Positively charged ions are called cations. Negatively charged ions are called anions. The most important ions for neuronal function are potassium (K+), sodium, (Na+) chloride (Cl-), calcium (Ca++) and large structural proteins with negatively charged groups attached to them called large protein anions (A-). The protein anions are embedded near the neuron’s inner cell surface as part of the cellular structural framework and can never leave the cell. They basically establish the initial conditions that will make the resting potential possible. At rest, K+ is the only ion that can freely move across the cell membrane. It comes into the neuron and builds up to high concentration in the vicinity of the protein anions until the electrical attraction between A- and K+ is balanced by the diffusion force trying to push K+ outward. This “break-even point” between electrical attraction and chemical repulsion is what sets the resting membrane potential at -70 millivolts; hence K+ is basically responsible for the potential.
Under these baseline conditions, K+ is in high concentration inside the cell and low concentration outside. In contrast, Na+, Cl- and Ca++ are in extremely low concentration inside the neuron and relatively high concentration outside. In the case of Na+ and Ca++, diffusion and electrical forces are trying to force their entry into the cell at the -70 millivolt resting potential. In the case of Cl- diffusion force tries to force the anion in and electrical forces are opposing its entry. When a cell fires in the act of signaling another neuron or when it receives a signal from another neuron these other ions can enter as part of that process and basically temporarily and very briefly (for a few milliseconds) “short out the battery.” But then the neuron then goes through processes to reset the membrane potential. |
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Definition
The management and regulation of the ion imbalance is due in part to a series of ion-specific channels made of proteins that span the membrane. The basic shape of these ion channels once assembled and implanted in the neuronal membrane is that of a hollow tube. Some are constantly open (only K+ has some channels that are always open and the rest are normally “pinched” closed and only open very, very briefly under some specific conditions of changes in membrane voltage. The specificity of the channels (K+, Na+, Cl- and Ca++) of the hole or pore in the tubular channel. For instance Na+ is smaller than Ca++, and therefore the pore of a Na+ is smaller than the pore of a Ca++ specific channel. Also, since the channels are proteins and like the protein anions, charged particles can be incorporated into their structure, positive charges are incorporated along the inside of the channel pore that is selective for Cl- and vice-versa for channels selective for the positively charged ions.
When these channels open and close and cause changes in the chemical concentration of ions, there are protein-based pumps (which require energy to operate) that correct the imbalances and work to maintain the relative ion imbalances that sustain the resting potential. The most prominent pump is the Na+/K+ ATPase, also called the Na+/K+ pump, which pumps K+ into the cell and Na+ out of the cell at the cost of one molecule of adenosine triphosphate (ATP), the energy currency of the cell, for each cycle of ion transfer that it completes. This pump runs continuously as long as the cell is alive and is a major component of the neuron’s total energy requirement. |
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Definition
| The action potential is a very rapid rise away from the resting membrane potential and then the equally rapid reversal back to the resting potential. When we refer to a neuron "firing" we're referring to the production of an action potential. This phenomenon only appears on the axon and travels down the axon to each and every terminal button of the axon. The action potential, once initiated, never decreases in peak strength all the way to the terminal button, no matter how long the axon. When an action potential occurs voltage-dependent Na+ channels open briefly and Na+ rushes in and the membrane potential rises until the electrical and diffusion forces on Na+ cancel each other out (at about +55 millivolts) and the electrical force pushing Na+ out matches the magnitude of the diffusion force pushing it in. At this point the Na+ channels close. However, some there are also some voltage-dependent K+ channels that open while the membrane potential is rising. Once the Na+ channels close, K+ leaves the neuron (taking its positive charges with it) through these additional open voltage-dependent K+ channels, as well as the other constantly open K+ channels. This exit of K+ and its charge is what returns the neuron to the resting potential. These two types of ion-selective voltage-dependent channels exist from the base of the axon where it attaches to the soma, called the axon hillock, or the spike initiation zone (SIZ), all the way to the terminal buttons. Once an action potential starts at the SIZ, the action potential starts a chain reaction, of sorts, which triggers the activation of all the neighboring voltage-dependent Na+ and K+ channels all along the axons length and allows the action potential to propagate down the entire length of the axon without diminishing in its peak voltage along the way. This chain reaction opening of voltage-dependent channels is called active conduction. |
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Definition
You might ask, "How does this action potential stuff start in the first place," and that would be a good question. Here's how: When a neuron receives a signal from another neuron at its dendrite, it usually triggers an electrical response which also involves Na+ entering the receiving neuron and a rise in the membrane potential to +55 millivolts. However, this entry of Na+ only occurs at that synapse. There are no voltage-dependent Na+ channels on the dendrites and soma so the potential generated at the synapse does not regenerate but only decreases as K+ leaves through the constantly open K+ channels. This is called an electrotonic potential (also called a graded potential) and since no voltage-dependent channels open along the way it travels by passive conduction. This form of conduction is very, very fast; faster than an action potential. But it doesn’t regenerate itself so it decays rapidly, also unlike the action potential. But if the electrotonic/graded potential is strong enough when it reaches the axon hillock/SIZ (sufficiently high above resting potential), then it will meet or exceed the threshold potential of activation, sometimes just called the threshold potential, which is the voltage level required to open the voltage-dependent Na+ channels. So, if membrane at the SIZ is sufficiently depolarized (more positive/less negative than the resting potential) to reach the threshold potential, then the graded potential triggers the opening of voltage-dependent Na+ channels and an action potential is born.
Once the action potential is generated, there are two possibilities. One is the active conduction of the action potential we've talked about already. That occurs in only a relatively few nerve fibers in most animals. This is because overwhelming majority of nerve fibers in most vertebrate animals are wrapped in myelin for electrical insulation and that presents a complication for simple active conduction. Myelin may sound like a problem then but it's not. Here is why. Invertebrates like insects and snails slugs and cephalopods don't have myelin. Their axons, like vertebrate axons posses a characteristic very similar to the electrical resistance found in electronic components. Many invertebrates like flying insects and squid need very fast action potentials to control the muscles which quickly propel them through air or water. Because of the electrical resistance and the lack of myelin they are forced to have very large diameter axons to achieve the sort of conduction velocity they need, because like in electrical wiring, electrical resistance goes down as the diameter of the conducting element (a wire or an axon) increases. For example, the squid has a single axon which leads to it's main propulsion muscle in its mantle that is so large it is visible to the naked eye, about 1 millimeter in diameter. This adds to the energy requirements for the cell because the large axon is alive and Na+ ions cross the membrane over the entire length of the large axon and that increase the workload of the Na+/K+ pump.
In myelinated vertebrate axons, however, the myelin covers most of the axon and the action potential travels along the axon in a combination of fast (yet decaying) passive conduction and slower (yet non-decaying) active conduction. This hybrid from of conduction is called saltatory conduction. In this form of action potential, the axon hillock fires and starts at full strength but it encounters a segment of myelin. Under the myelin sheath, the signal now speeds up and propagates via passive conduction. Before the fast moving potential decays in strength below the threshold potential of voltage-dependent Na+ channels, the potential encounters a bare spot on the axon called a node of Ranvier where the action potential is now able to fully regenerate in strength before traveling again at high speed under the next myelinated segment. Thus myelination allows for very fast conduction of nerve signals with smaller axons and Na+ ions only enter the axon at the nodes of Ranvier. Together the smaller size of the axon and the lower total Na+ levels serve to minimize the neuron's energy requirements. |
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Definition
| Another question might be how fast can a neuron fire? The answer is 500-100 times per second, maximum; though in practice the limit is about 500 times a second. What determines this is the refractory period, the minimum time required for the neuron to prepare for firing another action potential. The refractory period has two components, the absolute refractory period and the relative refractory period. The absolute refractory period is caused by the voltage-dependent Na+ channels which have to physically and mechanically "reset" themselves after they close but before they can reopen. When they open voltage-dependent Na+ channels stay open for about 1 millisecond and the "reset" process takes about 1-2 milliseconds. Hence, the absolute refractory period is about 1-2 milliseconds. During the absolute refractory period it is physically impossible for the neuron to fire another action potential; cannot be done. The duration of the relative refractory period varies but typically it lasts several milliseconds. The basis of the relative refractory period lies in the movement of K+ through the constantly open and voltage-dependent K+ channels in response to the depolarization phase of the action potential. As K+ leaves the cell, repolarizing the neuron back towards the resting potential, more K+ than is necessary to repolarize leaves the neuron due to all the additional open channels. This excess loss of K+ results in hyperpolarization, the membrane potential becoming more negative than resting potential, but the membrane quickly returns to resting. Unlike the absolute refractory period, the neuron can fire during the relative refractory period. However, since the neuron is hyperpolarized, slightly more negative than usual, the task of the incoming graded potential is more difficult. The electrotonic potential must still reach the threshold potential despite the more negative membrane potential. So, only unusually strong signals can trigger a second action potential during the relative refractory period. |
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Definition
| You might ask how easy is it to make a neuron fire an action potential. Well, it's not that easy. Because of all the K+ channels and how well they can counter the effects of any depolarization, one lone signal from one synapse has no chance in practical terms to trigger an action potential at the axon hillock. Multiple electrotonic potentials are required. These potentials are either excitatory potentials (depolarizing, where a positively charged ion enters at the synapse) or inhibitory (hyperpolarizing, where a negatively charged ion enters at the synapse) potentials. The excitatory signals and inhibitory signals compete and if the total effect of the excitatory signals is strong enough an action potential is triggered. This totaling of all the excitatory and inhibitory signals is called summation. Summation is a practical requirement for an action potential to be generated. However, there are two types of summation. |
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Term
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Definition
| One type of summation is temporal summation, in which one terminal button at one synapse is firing repeatedly, over and over. Each excitatory postsynaptic potential (EPSP) in the receiving neuron has not completely died away before the following EPSP is generated. Therefore, each EPSP "piggybacks" on the tail-end of the other, all of them merging together. This merging can yield a strong enough EPSP traveling electrotonically to trigger an action potential at the SIZ. The term temporal summation refers to the repeated rapid-fire signals from a single synapse "summing up" in time. |
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Term
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Definition
| The second type of summation is called spatial summation, where individual signals from separate synapses located at different spatial locations across the neuron, occur close enough in time that they converge and "add up" as they travel across the neuron. If the sum of the EPSPs and any inhibitory postsynaptic potentials (IPSPs) from the various activated synapses is strong enough, the spatially summated graded potential will trigger an action potential. |
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Term
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Definition
| The synapse is a structure where neuronal signaling and communication occurs that includes parts of both the sending (presynaptic) and receiving (postsynaptic) neurons. It is also thought that some long-lasting psychological changes such as learning, memory, habits, recovery from a brain injury and even addiction may reflect long-term changes in the function of the synapses involved in those functions. The structural integrity of the synapse is partially maintained by proteins called neuronal cell adhesion molecules (NCAMs), which act like velcro and serve to anchor the terminal button to the dendrite. |
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Term
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Definition
| The terminal button is the tip of the axon, but it does have some unique properties that make it specialized for communication. For instance, while it does have voltage-dependent Na+ channels like the rest of the axon, the button also has voltage-dependent Ca++ channels which are not found elsewhere on the axon. It is the entry of Ca++ into the terminal button that is absolutely critical for the release of chemical messengers, called neurotransmitters, from the terminal button. If Ca++ doesn't enter the terminal button, it doesnÕt matter what else happens, no neurotransmitter will be released. The terminal button is also the site of manufacture and storage of the neurotransmitters. All the machinery to synthesize them, store them and release them as well as generate the energy to power all these processes is found in the terminal button. |
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Definition
| The gap between the terminal button and the dendrite is incredibly small, only 20 to 50 nanometers. That is smaller than the shortest visible wavelength of light which is about 400 nanometers. The neurotransmitter released from the terminal button only takes 1-2 milliseconds to passively diffuse across the gap/cleft where the transmitter molecules make contact with receptors for them. |
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The postsynaptic density is the dendritic part of the synapse where the receptors for the neurotransmitters, as well as other structural and biochemical machinery involved in generating a response to the transmitter, are located and anchored.
Each neuron only makes one type of neurotransmitter and releases only that transmitter from all of its terminal buttons. When an action potential reaches the terminal button voltage-dependent Ca++ channels open and Ca++ enters the button which triggers a series of cellular events which cause small membrane packets (vesicles) filled with a chemical transmitter to move forward within the terminal button. The vesicles closest to the cell membrane move forward and fuse with the membrane and dump their contents into the synaptic cleft. The transmitter molecules drift across the gap and make contact with receptors specialized to respond to only those specific molecules. Once the transmitter binds to the receptors, the receptors are activated and a postsynaptic response occurs, the chemical message being received by the postsynaptic neuron. Because the signaling involves both electrical and chemical elements, neurons are said to engage in electrochemical signaling.
However, such signaling between neurons would be essentially meaningless if the signal cannot be also turned off as well as on. There are two major ways in which transmitter signals are terminated. The overwhelmingly most prevalent manner is through the process of reuptake. In reuptake after the transmitter disengages from the receptor, it is transported back inside the terminal button of the releasing neuron where the molecules are broken down for remanufacture. The reuptake is accomplished via a transporter protein that functions much like the Na+/K+ pump. All transmitters except for one rely on reuptake to terminate the chemical signaling. The only transmitter which does not rely on reuptake is a molecule called acetylcholine. That transmitter system utilizes enzymatic degradation in the synaptic cleft. An enzyme, acetylcholinesterase, is attached to the postsynaptic density. The acetylcholine released by the terminal button which doesn’t make contact with the receptors or disengages from them is broken apart by the enzyme. One of the two break-down fragments of acetylcholine, choline, is transported back inside the terminal button for reprocessing. The other break-down fragment, acetate, is allowed to drift away into the extracellular fluid. The acetylcholine transmitter system is the only transmitter system that does not terminate the neurotransmitter signal by reuptake. |
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There have been over one hundred compounds that have been identified or proposed as neurotransmitters. Transmitters must be synthesized, packaged and stored, released, activate a receptor and metabolized. The terminal button is the site of all these functions. The receptors have specific binding sites on them that allow the transmitters to activate them. The action of the transmitters at their receptors depends primarily on the physical shape of the transmitter molecule and its chemical nature. With over a hundred transmitters, we will concern ourselves with only a few of the most studied and most important ones.
It is important to note that every drug that has a psychological effect, whether as a medicine or a drug of abuse, has that effect by somehow influencing the activity of a neurotransmitter system that is already present in the brain. These drugs may influence the transmitter system at any one of the following levels: synthesis, storage, vesicle release, receptor activation, enzymatic breakdown or reuptake. If the drug increases or facilitates the activity or effect of the transmitter it is called an agonist. If the drug decreases, interferes with or blocks the activity of the transmitter, it is called an antagonist. It is also important to note that if the drug successfully impersonates the transmitter at the receptor and activates it, it is referred to as a direct agonist. In fact the transmitter itself may be referred to as a direct agonist for its receptor. If the drug unsuccessfully impersonates the transmitter at the receptor, attaching itself to it, but not activating it, it is then referred to as an indirect agonist. |
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| Acetylcholine (ACh) is our lone transmitter that relies on enzymatic degradation in the cleft to terminate its signal. ACh is synthesized from choline, which is a typically used as a component of the cell membrane, and the enzyme Acetyl-CoA. By an enzyme called choline acetyltransferase (ChAT). It is broken down by acetylcholinesterase (AChase). Most of the limbic system's and cerebral cortex's source of ACh comes from a group of structures called the cholinergic basal forebrain (BF). The two most prominent structures of the basal forebrain are the septum and the nucleus basalis. ACh is involved in learning, memory, sleeping and waking and sensory processing. |
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| The monoamines are grouped together because they share a lone amino group at one end of their chemical structure. The monoamines are in turn classified further into two groups, catecholamines and indolamines based on their chemical structures at the other end of the molecules. |
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| Tyrosine is an amino acid found in the diet that is the basic building block for the catecholamines. The term catecholamine denotes that the compounds share a catechol as part of their structure (a benzene ring with two hydroxide groups attached to it). |
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| For the synthesis of dopamine, tyrosine is first converted to L-DOPA by a relatively slow-acting enzyme called tyrosine hydroxylase. Then the L-DOPA is converted to dopamine by an enzyme called DOPA decarboxylase. There are two major sources for the brain's supply of dopamine. One is the substantia nigra (SN; literally, the black substance in Latin) which provides its dopamine exclusively to systems in charge of voluntary movements. The other is the ventral tegmental area (VTA) which provides its dopamine to brain systems involved in learning, memory, reward and cognition. |
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| The brain's supply of norepinephrine, is produced by the locus ceruleus (LC; Latin for the blue spot). Norepinephrine cells first go through all the steps of manufacturing DA and then dopamine-beta-hydroxylase converts dopamine into norepinephrine. The LC, which is associated with the reticular activating system, provides NE for the entire forebrain and this transmitter seems to be especially important in memory, attention, emotional arousal and response to novelty. |
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| There are very, very few epinephrine producing cells in the brain. The few cells that do are in the lower parts of the brain and produce epinephrine from norepinephrine by way of the enzyme phenylethanolamine N-methyltransferase. |
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| Tryptophan is another amino acid found in the diet that is the basic building block for an indolamine neurotransmitter. The term indolamine denotes that the compound has an indole ring as part of its structure (a benzene ring with another five member nitrogen containing ring four attached to it). |
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| The majority of the brain's serotonin is produced and distributed throughout the entire forebrain by the cells of the dorsal raphe nucleus, which is also associated with the reticular activating system. Serotonin is among the most widely distributed transmitters and has roles in learning, memory, attention, mood, aggression, appetite, sleeping and waking, sensory processing, and arousal. Serotonin is often referred to as 5HT, for 5-hydroxytryptamine, serotonin's official chemical name. |
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| Along with serotonin, the most widely used and important transmitters are the amino acid neurotransmitters. |
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| Glutamate is the most common neurotransmitter in the brain and is the primary excitatory transmitter in the brain. Glutamate is found in the diet but it can also be synthesized as a byproduct from the Krebs citric acid cycle in the mitochondria. It is especially important in sensory processing and learning and memory. |
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| GABA, gamma-aminobutyric acid, is far less common than glutamate but it is the primary inhibitory neurotransmitter in the brain. While the diet is not a primary source of GABA, it can be synthesized from glutamate. By virtue of its role in quieting the activity of nerve cells, it plays a role learning, memory, sensory processing, sleep and waking and relief from anxiety. |
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| As neurotransmitters, peptides are an exception to the rule that transmitters are manufactured at the terminal button. All peptides, which are short proteins, are made in the cell body and shipped in vesicles out the axon and to the terminal buttons. There are several peptides used as neurotransmitters. Many of them, outside of the brain, act as hormones. But within the brain because the blood-brain barrier prevents them from exiting, the same peptide molecules serve a second use as neurotransmitters. One example is cholecystokinin, which in the digestive system is a hormone which stimulates the pancreas and the liver, but in the brain is a neurotransmitter with roles in pain, memory, anxiety and coincidentally, hunger, as well. |
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| Neurotransmitter Receptors |
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| Every neurotransmitter receptor is a complex protein that spans the neuron’s membrane and has an external and an internal facing side. All receptors have TWO separate binding sites for the transmitter. These binding sites are depressions or pits in the receptor that can be though of as keyholes in a door lock and their shape is a match for the shape of the transmitter molecules. When two molecules of the transmitter, or any other direct agonist, attach to the receptor binding sites simultaneously, the receptor is activated. There are however, two general classes of receptor for almost each transmitter. |
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| Ligand-Gated Ion Channels (Ionotropic) |
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| The ligand-gated ion channels are distant relatives if the voltage-dependent ion channels found on the axon. The most obvious difference is that the voltage-dependent channels had a mechanism for sensing changes in electrical charge, while the ligand-gated ion channels have receptor binding sites what act as a mechanism for detecting the presence of the transmitter. Other than that they are tubes that are normally pinched closed until the binding sites have been activated and then they briefly open and allow charged ions to enter for a short time. It is the entrance of these ions that cause the EPSPs and IPSPs we talked about earlier. These receptors are also called ionotropic since they allow ions to pass into the neurons. |
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| B. G-Protein Linked (Metabotropic) |
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Definition
The second general class of receptor is the G-protein linked, also called metabotropic since they do not directly cause an electrical response but cause biochemical changes within the neuron which can cause widespread changes in the neuron's metabolism. The G-protein linked does not have a pore as part of the receptor. It is bound to the membrane. Within the two layers of the membrane are entities, separate from the receptor, called G-proteins. When the receptor is activated, the G-protein that comes in contact with the receptor is, in turn, also activated. The activated G-protein then disengages from the receptor and the active subunit of the G-protein travels along the inner part of the membrane coming into contact with distant ion channels or membrane-bound enzymes and changes their activity. Often the result of this G-protein caused activation is the release of other signaling compounds into the cell’s interior, called second messengers since the neurotransmitter is considered the first messenger. These second messengers can go from the dendrite to as far as the cell nucleus, activating other enzymes or triggering the new manufacture of cellular proteins along the way.
These two classes of receptors work together, often within the same synapse. The ionotropic cause EPSPs or IPSPs, the electrical signals which directly determine if an action potential will occur. But the metabotropic, by changing the neuron's metabolic state, can also make the cell more or less sensitive to the EPSPs or IPSPs making it indirectly easier or harder to trigger an action potential. The ionotropic receptors are in a sense like a telegraph key sending a message, but the metabotropic receptors can be thought of as volume controls, making the message louder or fainter.
Within each class of receptor, there can be further subtypes of receptor for each transmitter. In all this makes each transmitter capable of generating many different types of responses in the range of neurons that are responsive to that transmitter. For instance, for serotonin in humans there are 13 subtypes of receptor: one ligand-gated (the 5HT3) and 12 G-protein linked subtypes. In another example, dopamine has 5 subtypes, all G-protein linked (called D1, D2, D3, D4, and D5) with no ligand-gated subtypes at all, while glutamate has 2 types of ligand-gated receptors (called AMPA and NMDA) and one metabotropic (called ACPD). The variety of receptor subtypes, while sometimes confusing, does offer some hope for treating disorders. If a disorder is found to be more closely linked to a particular receptor subtype, it may be possible to target pharmaceutical treatments specifically to that receptor and hopefully treat that disorder with a minimum of side-effects. |
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| a change in behavior or knowledge due to experience |
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| past expericen is drawn on to guide or direct behavior or thoughts in the present |
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predictive learning 1 stimuli predixts 2 stimuli ex. bell----salviging |
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| Innitial learning of the relationship between c.s. and c.r |
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presenting cs without the cr ex. bell----no food |
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applies both the cs and cr the aquisition is never forgotten ex.---after extinction bell rings----dog does cr but not as much as before |
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stuff with similar qualities of cs may elict cr ex----a dog heres a keys and does cr because he thinks its the bell |
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| ex---after a while the dog can tell the difference between the keys jingling and the actual sound of the bell |
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| Higher Order Conditioning |
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cs learned so long it becomes the u.c.s ex---a beach ball is thrown after the bell is rung then the dog gets the food the bell is now the ucs the beach ball is now the cs and the salvaging is still the cr |
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| learning is based by consequences |
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increase likelihood posotive---give kid money for cleaning room negative--whoop kid so that he will clean room |
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removing the posotive stimuli (time out) ex--no desert taking car keys |
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| REINFORCEMENT AFTER A CERTAIN EXACT # |
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| REINFORCEMENT OF 1 RESPONSE AFTER A FIXED AMOUNT OF TIME HAS PASSED BY |
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| VARIABLE RATION AND VARIABLE INTERVAL |
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| NUMBER ON AVERAGE....UNCERTAIN # |
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WHEN A FR OR A FI STOPS THE BEHAVIOR THAT IS LEARNED IS STOP FOR VR OR VI BEHAVIOR LEARNED DOES NOT STOP BECAUSE THEY ARE USE TO UNCERTAINTY EX--SLOT MACHINE |
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| LEARNING THAT OCCURS WHEN OBSERVING AND DUPLICATION THE BEHAVIOR THAT WAS OBSERVED |
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| KIDS SAW ADULT ACTING VIOLENTLY TO A DOLL SO THEY WENT IN ROOM AND ACTED VIOLENTLY |
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| PROCESSING INPUT OF INFORMATION |
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| foccuses on one element brings to short term memory |
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| processing the physical structure only |
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deeper yet basic form of processing ex. this word rhymes or sounds like this word |
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| having a chomprehension and understanding of the word |
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| retaining memory storing memeory |
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old interfering with the new |
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| new interferes with the old |
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| constantly updated and refreshed....short lived |
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| short term memory/ working memory |
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| decays after about 30 sec max is 3-5 min |
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| can hold 7 plus or minus 2 elements or chunks or information |
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all the words in the middle are lost because proactive is fighting reto the strugle |
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| access or bring up the memory |
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| strength of synaptic changeble but stable |
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responsible from moving short term to long term episodic |
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attaching emotion to it procedural |
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| ltp long term potentiation |
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| biochemical mechanism of synaptic plasticity |
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| requires activation for ltp |
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in stressful simulation or events memory is stored to remind us of that incident for future reference ex---picture of that monster in class |
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| fact based memory-----words |
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general knowledge of the world ex---eggs are breakfast food stop at a traffic light or stop sign |
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personal biography----facts relating to your life ex----first kiss first times |
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does or cannor be verbally expressed ex----riding a bike |
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