Chapter 1. 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

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.

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.

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.

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.

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.

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!