• Cells that support and protect neurons
• Do not fire action potentials
• 90% of cells in nervous system are these type of cells

• Most numerous type of glial cell
• stimulate formation of the blood brain barrier
• protects the brain from many kinds of harmful molecules that might be found in the blood
• Helps maintain normal ion concentrations in the ECF in the brain

• Sends out cytoplasmic processes that form an insulating wrap of myelin sheaths around several axons in the neurons in the CNS

• Sheathes only a single axon with myelin sheaths in the peripheral nervous system

CNS

Central nervous system

• consists of the brain and spinal cord

• learning
• memory
• emotions
• thoughts
• language
• and other complex functions

• consists of neurons that provide communication between the CNS and organs throughout the body
• Can be subdivided into two divisions:
  • afferent
  • efferent

• functional unit
• smalles unit of a tissue that can carrry out a function of that tissue
• excitable cells
• communicate by transmitting electrical impulses

• cells capable of producing large, rapid electrical signals called action potentials

Cell Body

(soma)

• contains the cell nucleus
• contains most of the cell's organelles
• carries out most of the functions
  • protein synthesis
  • cellular metabolism

• branch from the cell body
• receive input from other neurons at specalized junctionls called synapses

• releases a chemical messenger called a neurotransmitter that communicates with the dedrite or cell body of a postsynaptic neuron or other cell

• Sends information
• functions in the rapid transmission of information over relatively long distances in the form of electrical signals called action potentials

Specialized junctions in the neuron that receive input

• brief, large changes in membrane potential during which the inside of the cell becomes positively charged relative to the outside

• the site where the axon originates from the cell body
• specialized for the initiation of action potentials

• specialized to release neurotransmitter on arrival of an action potential
• the released neurotransmitter molecules carry a signal to a postsynaptic cell, usually to a dendrite or the cell body of another neuron or to the cells of an effector organ

• nongated channels
• found in the plasma membrane
• always open
• responsible for the resting membrane potential

• open or close in response to the binding of a chemical messenger
• specific receptor
• in the plasma membrane
• in neurons located in the dendrites and cell body

• open or close in response to chanes in membrane potential
• located throughout the neuron but more densely in the axon and in greatest density in the axon hillock
• initiation and progagation of action potential

• classified structurally according to the number of processes that project from the cell body

• located in the CNS
• cell bodies are grouped in the nucleus
• axons travel together in bundles called pathways, tracts or commissures

• located in the PNS
• cell bodies are clustered together in the ganglion
• axons travel together in bundles called nerves

Glial Cells

(glia "glue")

• second class of cell found in the nervous system
• account for 90% of all cells in the nervous system
• do not function directly in signal transmission
• provide structural integrity to the nervous system
• necessary for neurons to carry out their function

• astrocytes
• ependymal
• microglia
• oligodendrocytes

• schwann cell

• myelin, consists of concentric layers of plasma membrane
• forms a layer around an axon

• sends out cytoplasmic processes that form myelin sheaths around several axons
• nodes of ranvier gaps in the myelin sheaths

• a given schwann cell sheathes only a single axon

• Difference in voltage across the plasma membrane
• always given in terms of voltage inside the cell releative to voltage outside the cell

• Difference in voltage across the plasma membrane when a cell is at rest

• a relatively small change in membrane potential produced by some type of stimulus that triggers the opening or closing of ion channels
• strength of graded potential is relative to strength of stimulus

• graded potentials produced in the post-synaptic cell in response to neurotransmitters binding to receptors

• graded potentials produced in response to a stimulus acting on a sensory receptor

• a large, rapid change in membrane potential produced by depolarization of an excitable cell's plasma membrane to threshold
• Location - axon
• Strength - 100 mV (All or none)
• Voltage gated
• Na⁺, K⁺

• the membrane potential that counters the chemical forces acting to move an ion across the membrane, thereby putting the ion at equilibrium

• Sodium ions are at a higher concentration outside the cell
• chloride ions help balance the electrical charge outside the cell
• potassium ions are at a higher concentration inside the cell
• organic anions help balance the electrical charge inside the cell

• occur in neurons via changes in membrane potential when ion channels, called gated channels open or close in a response to a certain stimuli

• open or close in response to a mechanical force on the membrane
• associated with sensory or visceral receptors located at the end of afferent neurons

• a change to a more negative value which causes the membrane to become more polarized

• a change to a less negative or to a postive potential membrane becomes less polarized

• occurs when the membrane potential returns to the resting membrane potential following a depolarization

• graded potentials
• action potentials

• are small electrical signals that act over short ranges
• diminish in size with distance
• can be produced as a result of neurotransmitter molecules binding to receptors on a dendrite or cell body of a neuron
• can result from a sensory stimulus - touch or light - acting on a sensory receptor at the peripheral ending of an afferent neuron

• large signals capable of traveling long distances without decreasng in size
• Occur in the membranes of excitable tissue in response to graded potentials that reach threshold

• they determine whether a cell will generate an action potential

• graded potentials that are depolarized
• bring the membrane potential closer to the threshold to generate an action potential

• graded potentials that are hyperpolarized
• the the membrane potential away from the threshold to elicit an action potential

• change in membrane potential decreases in size as it moves along the membrane away from the site of stimulation

• When a change in potential occurs across a cell membrane at a particular site, this change generates differences in potential within the intracellular and extracelluar fluids
• creates a charge seperation
• causes voltage changes by passive charge movement

• are responsible for the opening of sodium channels during the depolarization phase of an action potential

• are responsible fo the closing of sodium channels during the repolarization phase of an action potential

• Activation and inactivation gates
• both types of gates open and close in response to changes in the membrane potential

• at rest the inactivation gate is open but the activation gate is closed
• the gate can be open by a depolarizing stiumulus that causes the activation gate to open
• both gates open allowing sodium to move through
• inactivation gate closes by the same depolarization that caused it to open
• activation gate opens
• membrane potential returns to resting value which cause repolarization
• inactivation gate opens and activation gate closes returning the channel to resting state

• negative feedback during action potential
  • as the cell repolarizes, the depolarizing stiumulus weakens, and potassium channels slowly close

• sodium

• Potassium

• During and immediately after an action potential, the membrane is less excitable than it is at rest

• During the rapid depolarization phase of an action potential, the regenerative opening of sodium channels that have been set into motion will proceed to its conclusion and will not be affected by a second stimulus
• During the beginning of the repolarization phase, most of the sodium inactivation gates are closed and cannot be opened by a second stimulus

• occurs immediately after the absolute refractory period
• primarily due to the increased permeability to potassium

• more voltage-gated Na⁺ channels open, in response to depolarization of the more proximal region of the axon
• Inflow of Na⁺ ions propagates the depolarization phase forward, so the actin potential travels distally
• on the trailing edge of teh action potential, voltage-gated K⁺ channels open to repolarize the membrane

• multiplication or increase
• spreading to a larger area or greater number down the length of the axon from the trigger zone to the axon termianl without decrement

• Glial cells processes wrap many axons, at internals
  • oligodendrocytes myelinate some axons in the CNS
  • schwann cells myelinate most axons in the PNS
• causes the action potential to travel in jumps (called saltatory conduction) which is much faster
• speeds of action potential progagation are typically 120 meters/sec

• -70mV
• Established and maintained by the Na+/K+ leaking channels
• Na+/K+ pump is involved
• ATP is required to establish and maintain Vrest
• At Vrest, there is a very large inward electrochemical force on Na+

• Starts at the synapse
• Causes changes in the membrane potential, causing it to go away from the resting potential
• Travels over the plasma membrane
• Reaches the axon hillock

• At rest, the sodium inactivation gate is open and the activation gate is closed
• Threshold is depolarizied
• both gates open letting sodium move into the cell
• inactivation gate closes repolarizing the cell to resting state

• opening of sodium channels
• delayed closing of sodium channels
• delayed opening of potassium channels

• can be elicited by a suprathreshold stimulus

• hyperpolarized

• less excitable

• activation gates are closed
• inactivation gates are open

• threshold reaches -55mV by excitatory graded potential changes
• positive feed back loop is generated
• unleases the inward electrochemical force on Na+

• The voltage-gated Na+ channel has an inactivation gate, which closes the channel
  • prevents further flow of Na+ into the cell
• Voltage-gate K+ channels open
  • allows K+ to flow out
  • Vm comes back toward the negative resting potential
  • Repolarizing phase of an action potential