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Lecture 5
Nervous System: Sensory (Basic, Touch, Pain)
18
Physiology
Undergraduate 3
09/25/2012

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Term
Nervous System Distinctions
Definition
  • CNS=brain and spinal cord.
  • PNS=afferent neurons or efferent neurons
    • afferent neurons: input, from the somatic, special, or visceral senses. Usually pseudo-unipolar neurons that send signals to the CNS via interneurons, which are multipolar). 
    • efferent neurons: send input to the somatic and autonomic muscle of both sympathetic and parasympathetic nervous system.
Term
Structural Classes of Neurons
Definition
  • Bipolar: have an axon and a dendrite coming off the cell body.
  • Pseudo-Unipolar: subclass of bipolar neurons; axon and dendrite appear as a single extension.
  • Multipolar: multiple projections from the cell body; one axon, everything else is dendrites.
Term
Sensory Pathway
Definition

Stimulus→Receptors on Afferent Neuron (1st Order Neuron)→Brain→Interneuron (2nd Order Neuron)→Thalamus→Interneuron (3rd Order Neuron)→Cortex

 

Dorsal Root Ganglion=where afferent neurons reach the CNS.

Term
General Principles of Sensory Physiology
Definition
  • Deals with afferent nervous system signals (info from periphery to CNS).
  • Sensory Information (external): comes from somatic sensations (somesthetic/skin touch or proprioception (limb position) or special senses (vision, hearing, equilibrium, taste, smell)
Term
Sensory Receptors
Definition
  • Have specialized nerve endings that detect a sensory stimulus.
  • Must convert stimulus energy into electrical energy.
  • Types: photoreceptors, chemoreceptors, thermoreceptors, mechanoreceptors.
  • Stimulus causes a change in the membrane potential of the sensory receptor.
    • Graded potential (caused by opening/closing of an ion channel); can lead to action potential if threshold is reached.
    • Decrease amplitude of receptor potential over time with constant stimulus.
    • This decreases our perception of a stimulus.
Term
Sensory Units and Sensory Pathways
Definition
  • Labeled Line Theory: says that there is a specific neural pathway for each sense.
  • Sensory Unit: a single afferent neuron + associated receptors.
  • Sensory Pathway: when all receptors/receptor fields are the same.
    • Receptor Field: all axons of a particular nerve.
    • The location of a stimulus is gauged by receptor fields; if multiple receptive fields are involved, the location is determined by which neuron induces the greatest number of action potenials.
  • The intensity of a stimulus is gauged by the frequency of the action potential.
  • A stronger stimulus activates more receptors (recruitment), either from the same afferent neuron or not.
  • Two-Point Discrimination: ability to perceive 2 points on skin. 
Term
Skin Mechanoreceptors
Definition
  • Rapidly Adapting:
    • Pacinian Corpuscle
    • Meissner's Corpuscle
    • Hair Follicle
  • Slowly Adapting:
    • Free Nerve Endings
    • Merkel's Disk
    • Ruffini's Ending
Term
Skin Thermoreceptors
Definition
  • Warm Receptors
    • Free Nerve Endings that sense between 30-43 degrees.
    • Increase frequency with increase in temperature
  • Cold Receptors
    • Free Nerve Endings that sense between 35 and 20 degrees.
Term
Pathways for Somatosensory System
Definition
  • Receptors: Mechanoreceptors, Thermoreceptors, Nocireceptors (for pain)
  • Dorsal Column-Medial Lemniscal Pathway
    • Somesthetic Sensations: associated with skin
    • Proprioception: awareness of body's position in space.
  • Spinothalamic Tract
    • Pain
    • Temperature
  • Pain Perception
    • Pain Response
    • Visceral Pain (Referred Pain)
    • Modulation of Main
Term
Dorsal Column-Medial Lemniscal Pathway
Definition
  • Transfers info from mechanoreceptors and prorioceptors to the CNS; crosses to the other side of the CNS in the medulla oblongata.
  • 1st Order Neurons initiate in the periphery and enter the dorsal horn of the spinal cord.
    • The collateral from the main axon may end up in the spinal cord
    • This causes communication with interneurons, resulting reflexes.
  • Main 1st Order Axon has ipsilateral ascension (ascends from the spinal cord to the brainstem, on the same side as the stimulus, in dorsal columns (tracts of white matter).
  • 1st Order Neurons terminate in dorsal column nuclei, located in the medulla, where they form synapses with second order neurons.
  • 2nd order neurons cross over to the other side of the medulla, via a tract called the medial lemniscus, and then ascend to the thalamus.
  • In the thalamus, 2nd order neurons form synapses with 3rd order neurons, which transmit info to the somatosensory cortex.
Term
Spinothalamic Tract
Definition
  • Transmits info from thermoreceptors and nocireceptors to the thalamus; crosses to the other side of the CNS within the spinal cord. 
  • 1st Order Neurons initiate in periphery and enter the dorsal horn of the spinal cord.
  • 1st order may ascend or descend a short distance (a few spinal segments) along Lissauer's tract--> Eventually form synapses with second order neurons in the dorsal horn.
  • Second order cross over to the other side of the spinal cord (contralateral), ascends in the anterolateral quadrant of the spinal cord to the brainstem.
  • 2nd order terminates in the thalamus, forming a synapse with 3rd order neurons that ascend to the somatosensory system.
Term
Pain Response
Definition
sensation produced by tissue damaging stimulus or stimulus that potentiall causes tissue damage; pain elicits sensation, autonomic responses, emotional responses, pain perception depends on past experience.
Term
Chemicals Activating Nociceptors
Definition
potassium, histamine, prostaglandins, bradykinin, serotonin. 
Term
Fast vs. Slow Pain
Definition

Fast Pain: A Delta fibers; sharp pricking sensation; well localized.

 

Slow Pain: C Fibers; dull aching; poorly localized.

Term
Pathways for Pain
Definition
  • A Delta or C; both terminate in spinal cord dorsal horn.
  • The Neurotransmitter of C Fibers (and maybe A Delta) is Substance P.
  • Specific Pathway: spinothalamic tract
  • Non-Specific Pathway: to reticular formation, hypothalamus, limbic system.
Term
Visceral Pain
Definition

pain originating in internal organs; sensation "referred" to body surface

 

ex. angina due to heart damage.

 

Mechanism: Nocireceptors in organs detect damage, 2nd order neurons from the visceral 1st order afferent also receive input from skin's 1st order afferents--> the brain interprets visceral input as input from somatic afferents, causing pain to be perceived in the skin. 

Term
Gate-Control Theory of Pain Modulation
Definition
  • The perception of pain varies depending on circumstances and past experiences.
  • Gate-Control Theory: somatic signals of nonpainful sources can inhibit signals of pain at the spinal level.
  • In unmodulated pain, Collaterals of nocireceptor afferents (C Fibers) inhibit the inhibitory interneurons, allowing pain signal transmission.
  • In modulated pain, Collaterals of large diameter afferents (A Beta fibers) that branch from touch and pressure receptors excite the inhibitory neuron, decreasing pain signal transmission).
Term
Endogenous Analgesia Systems
Definition
  • Analgesia=blocking of pain
  • Brain can block pain through this system.
  • Stress activates signals in the periaqueductal gray matter (in the midbrain).
  • The periaqueductal gray matter communicates with the nucleus raphe magnus (in the medulla) and the lateral reticular formation (extends the length of the brainstem).
  • Neurons from these two areas go down to the dorsal horn and block communication between nocireceptor primary afferent neurons and second order neurons.
  • They do this by releasing the neurotransmitter enkephalin, which binds to receptors on the second order neurons and the receptors of the nociceptive afferent neuron.
  • This causes presynaptic inhibition (so no release of the pain neurotransmitter Substance P) and production of IPSPs on second order neuron.
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