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Lecture 5
Nervous System: Sensory (Basic, Touch, Pain)
Undergraduate 3

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Nervous System Distinctions
  • 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.
Structural Classes of Neurons
  • 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.
Sensory Pathway

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.

General Principles of Sensory Physiology
  • 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)
Sensory Receptors
  • 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.
Sensory Units and Sensory Pathways
  • 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. 
Skin Mechanoreceptors
  • Rapidly Adapting:
    • Pacinian Corpuscle
    • Meissner's Corpuscle
    • Hair Follicle
  • Slowly Adapting:
    • Free Nerve Endings
    • Merkel's Disk
    • Ruffini's Ending
Skin Thermoreceptors
  • 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.
Pathways for Somatosensory System
  • 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
Dorsal Column-Medial Lemniscal Pathway
  • 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.
Spinothalamic Tract
  • 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.
Pain Response
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.
Chemicals Activating Nociceptors
potassium, histamine, prostaglandins, bradykinin, serotonin. 
Fast vs. Slow Pain

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


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

Pathways for Pain
  • 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.
Visceral Pain

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. 

Gate-Control Theory of Pain Modulation
  • 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).
Endogenous Analgesia Systems
  • 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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