Chapter 1. Somatosensory System: How We Feel

1.0.1 Somatosensory System: How We Feel

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Somatosensory System: How We Feel
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Neuroscience in Action
somatosensory neuron (receptor)
Brain cell that brings sensory information from the body into the spinal cord.
rapidly adapting receptor
Body sensory receptor that responds briefly to the onset of a stimulus on the body; these neurons signal both the beginning and end of the sensation.
slowly adapting receptors
Body sensory receptor that responds as long as a sensory stimulus is on the body.
cell body (soma)
Core region of the cell containing the nucleus and other organelles for making proteins.
haptic
Perceptual ability to discriminate objects on the basis of touch.
proprioceptive
Perception of the position and movement of the body, limbs, and head.
posterior spinothalamic tract
Pathway that carries fine-touch and pressure fibers toward the brain.
nociceptive
Perception of pain, temperature, and itch.
anterior spinothalamic tract
Pathway from the spinal cord to the thalamus that carries information about pain and temperature toward the brain.
pain gate
Hypothetical neural circuit in which activity in fine-touch and pressure pathways diminishes the activity in pain and temperature pathways.
spinal cord
Part of the central nervous system encased within the vertebrae; provides most of the connections between the brain and the rest of the body.
haptic
Perceptual ability to discriminate objects on the basis of touch.
proprioceptive
Perception of the position and movement of the body, limbs, and head.
nociceptive
Perception of pain, temperature, and itch.
excitatory
Increase in the activity of a neuron or brain area.
interneuron
Association cell interposed between a sensory neuron and a motor neuron; in mammals, interneurons constitute most of the brain’s neurons.
inhibitory
Decrease in the activity of a neuron or brain area.
action potential
Large, brief reversal in the polarity of an axon membrane.
primary somatosensory cortex
region in the parietal lobe that receives sensory information from the body and begins to construct perceptions from that information.

The Somatosensory System and the Somatosensory Cortex

By: Dr. Aileen M. Bailey, St. Mary's College of Maryland

1.1 Introduction

Introduction
Somatosensory System: How We Feel

Our somatosensory system provides sensations such as touch, pressure, pain, and proprioception (body awareness) through various specialized somatosensory neurons. The somatosensory neurons send information via the spinal cord and spinothalamic tracts to the cortex for perception of an event. In this activity, you will explore the somatosensory system and how it tells us about the physical contact we make with the world.

After completing this activity, you should be able to:

  • Describe the relationship between the physiological properties and functions of somatosensory receptors.
  • Describe the connections between somatosensory information and the ascending spinothalamic tracts.
  • Explain the pain gate theory of pain perception.

This activity relates to the following principles of nervous system function:

  • Principle 4: The CNS Function on Multiple Levels
  • Principle 6: Brain Systems Are Organized Hierarchically and in Parallel
  • Principle 7: Sensory and Motor Divisions Permeate the Nervous System

1.2 Somatosensory Receptors

Somatosensory Receptors

The receptive ends of somatosensory neurons (receptors)—which are located in the skin, muscles, and internal organs—send sensory information to the spinal cord and brain. Somatosensory receptors are specialized by their physiological properties to assist with distinct perceptual experiences.

Somatosensory receptors with myelinated axons are specialized to send messages quickly; those axons with less myelination send messages more slowly, those with more myeliation send messages more quickly. Rapidly adapting receptors respond quickly at the onset of an event. Slowly adapting receptors help to signal that an event is still occurring by their continued response (action potential) to the stimulus.

First, let's see the response from a myelinated somatosensory receptor in the finger and follow the resulting impulse.

[insert animation 11.1a Somatosensory]

Somatosensory neurons (receptors) with extensive myelination are equipped to send quick messages (action potentials) about changes in the environment. These quick messages allow us to make fast behavioral adaptations, such as moving your finger away from a sharp object.

Now let's follow the path of an unmyelinated somatosensory neuron.

[insert animation 11.1b Somatosensory]

The messages (action potentials) sent from unmyelinated somatosensory receptors are slower to reach the spinal cord and cortex. These receptors tend to alert us to ongoing events that do not necessarily require immediate action, such as our skin being rubbed or massaged.

1.3 Somatosensory Receptors and Perception

Somatosensory Receptors and Perception

For each of the sensory experiences below, select the level of myelination in the somatosensory receptor (A. myelinated axon or B. unmyelinated axon) that is most likely related to the perception of the event.

Question 1.1

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1.4 The Spinothalamic Tracts

The Spinothalamic Tracts

Now that we have considered some differences in somatosensory receptor types, let's examine how somatosensory information travels into the central nervous system (CNS).

The cell bodies (also known as the soma) of somatosensory receptors are located in the posterior root ganglion, and their axons enter the CNS via the spinal cord. Axons carrying haptic (touch) or proprioceptive (body awareness) information ascend the spinal cord on the same side of the body (ipsilaterally), forming the posterior spinothalamic tract. Axons carrying nociceptive (pain, temperature, itch) information synapse with spinal neurons whose axons cross immediately to the contralateral side and form the anterior spinothalamic tract.

Select various points of the spinothalamic tract to learn more details.

The spinothalamic tracts cross at the spinal cord or the brainstem and terminate in the somatosensory cortex on the opposite side of the body (contralateral side). The right hemisphere somatosensory cortex processes sensation from the left side of the body and vice versa.
This is the posterior spinothalamic tract, which carries haptic (touch) and proprioceptive (body awareness) information along the spinal cord from the ipsilateral (same) side of the body. This tract crosses in the brainstem and terminates in the somatosensory cortex on the contralateral (opposite) side.
This is the anterior spinothalamic tract, which carries nociceptive (pain, temperature, itch) information. This tract crosses in the spinal cord and travels up the contralateral (opposite) side of the body. Therefore, nociceptive information is processed in the somatosensory cortex on the contralateral side.
This is the posterior root ganglion, which contains the soma for the somatosensory neurons (receptors).

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1.5 The Spinothalamic Tracts and Perception

The Spinothalamic Tracts and Perception

Now that you have considered the basics of the somatosensory pathway from the one side of the body to the opposite (contralateral) somatosensory cortex, let’s consider the connection between perceived sensations (or lack of them) and the spinothalamic tract.

The figure below shows the somatosensory pathway originating from the right side of the body and terminating in the left somatosensory cortex. Of the four regions indicated here, select the region(s) of the somatosensory pathway most likely damaged based on the symptoms of each of the following patients.

Question

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Correct! Patient A.B. has lost all sensory information (haptic, proprioceptive, nociceptive) from the right side body. Such extensive loss of somatosensation from the right side body will only result from damage to the left hemisphere somatosensory cortex or damage to the right posterior root ganglia. The spinothalamic tracts carry only a portion of sensory information, so damage to either of these tracts would produce a different pattern of sensory loss.
Incorrect. Patient A.B. has lost all sensory information (haptic, proprioceptive, nociceptive) from the right side body. Such extensive loss of somatosensation from the right side body will only result from damage to the left hemisphere somatosensory cortex or damage to the right posterior root ganglia. The spinothalamic tracts carry only a portion of sensory information, so damage to either of these tracts would produce a different pattern of sensory loss.

Question 1.2

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Correct! The posterior spinothalamic tract carries haptic and proprioceptive information from the right side of the body, which is lost in this patient. However, the anterior spinothalamic tract carries nociceptive information, which remains intact in this patient. This suggests that the damage in this patient is isolated to the posterior spinothalamic tract.
Incorrect. The posterior spinothalamic tract carries haptic and proprioceptive information from the right side of the body, which is lost in this patient. However, the anterior spinothalamic tract carries nociceptive information, which remains intact in this patient. This suggests that the damage in this patient is isolated to the posterior spinothalamic tract.

Question

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Correct! The anterior spinothalamic tract carries nociceptive information from the right side of the body, which is lost in this patient. However, the posterior spinothalamic tract carries haptic and proprioceptive information from the right side, which remains intact in this patient. Therefore, this suggests that the damage in this patient is isolated to the anterior spinothalamic tract.
Incorrect. The anterior spinothalamic tract carries nociceptive information from the right side of the body, which is lost in this patient. However, the posterior spinothalamic tract carries haptic and proprioceptive information from the right side, which remains intact in this patient. Therefore, this suggests that the damage in this patient is isolated to the anterior spinothalamic tract.
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{"general":"Scheme of the spinothalamic tract","ids":["zero"],"query_hs": [["Left somatosensory cortex","Posterior spinothalamic tract","Anterior spinothalamic tract","Posterior root ganglion"]]}

1.6 The Pain Gate Theory

The Pain Gate Theory

Neuronal circuits in the spinal cord allow haptic-proprioceptive and nociceptive pathways to interact. These interactions may be responsible for our variable responses to pain. The pain gate theory describes a possible mechanism for these pathways to interact and influence our perception of pain.

This figure depicts the pain gate of sensory inputs. Imagine that you have an injury that continually sends a pain message through the body’s nociceptive somatosensory receptors. If you massage that general area, it will cause the haptic sensory receptors to respond by sending action potentials. Use the slider to increase or decrease the response rate from the haptic somatosensory receptors and watch the change in action potential signal of the nociceptive pathway.

1.7 Treating Pain

Treating Pain

Question

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Correct! The receptor that an opioid pain reliever binds to is located on the postsynaptic side of the spinal nociceptive neuron. Activation of this receptor with an opioid will create an inhibitory postsynaptic potential and make it harder for the spinal nociceptive neuron to reach threshold and fire an action potential. Decreasing the number of action potentials to the brain reduces a person’s perception of pain.
Incorrect! The receptor that an opioid pain reliever binds to is located on the postsynaptic side of the spinal nociceptive neuron. Activation of this receptor with an opioid will create an inhibitory postsynaptic potential and make it harder for the spinal nociceptive neuron to reach threshold and fire an action potential. Decreasing the number of action potentials to the brain reduces a person’s perception of pain.
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{"query_hs": [["Spinal nociceptive neuron","Large fibers","Small fibers","Interneuron"]]}

1.8 Summary

Summary
Take the quiz

Congratulations! You have successfully completed the activity. You reviewed the connection between the physiological properties and the function of the somatosensory receptors. You have considered how somatosensory information travels from peripheral areas to the somatosensory cortex on the contralateral side of the body. Finally, you examined the response rates of various neurons related to the pain gate theory of pain perception.

Your instructor may now have you take a short quiz about this activity. Good luck!