Lecture 6: Touch

Somatosensory system

The subset of the sensory nervous system responsible for feeling of touch and pain.

Mechanoreceptors

The sensory receptors that respond to mechanical pressure or distortion. They have different-sized receptive fields and adaptation rates.

Meissner’s Corpuscle

The type of mechanoreceptors that have small receptive fields and rapid adaptation rates. 

Pacinian Corpuscle

The type of mechanoreceptors that have large receptive fields and rapid adaptation rates due to the connective tissue surrounding the axon tip. Werner Loewenstein's experiment (1960s) found that removing the surrounding tissue made the adaptation rate much slower. 

Merkel’s disk

The type of mechanoreceptors that have small receptive fields and slow adaptation rates. 

Ruffini’s ending

The type of mechanoreceptors that have large receptive fields and slow adaptation rates.

Stretch-sensitive Mechanosensitive Ion Channels

The types of mechanoreceptors on the lipid bilayer that stretch out in response to a force. The stretching opens up the channel and allows for the influx of Na⁺ and Ca²⁺ into the cell, depolarizing it. 

Extracellular Mechanosensitve Ion Channels

The types of mechanoreceptors on the lipid bilayer that are attached to a protein on the outside of the cell. In response to a force, the protein stretches open the channel and allows for the influx of Na⁺ and Ca²⁺ into the cell, depolarizing it. 

Cytoskeletal Mechanosensitive Ion Channels

The types of mechanoreceptors on the lipid bilayer that are attached to a protein on the inside of the cell. In response to a force, the protein stretches open the channel and allows for the influx of Na⁺ and Ca²⁺ into the cell, depolarizing it. 

Lateral Inhibition

The neural mechanism that enhances the perception of touch by allowing excited neurons to inhibit the activity of their neighboring neurons, thereby sharpening sensory input and improving contrast. Lateral inhibition is more pronounced in certain areas than others (i.e., fingers can better discriminate due to the higher density of mechanoreceptors than the back). 

Dorsal Column-Medial Lemniscal Pathway

The sensory pathway of the central nervous system that conveys touch, vibration, and proprioceptive information on the same side from the dorsal root axon to the dorsal column nuclei, where it then crosses over in the medial lemniscus up into the thalamus and into the cerebral cortex. 

Spinothalamic Pathway

The sensory pathway of the central nervous system that conveys pain, temperature, and some touch on the opposite side of the dorsal root axon up the lateral spinothalamic tract, where it ascends contralaterally and reaches the thalamus and cerebral cortex. 

Somatosensory Cortex (S1)

The region in the parietal lobe responsible for processing sensory information from the body, such as touch, pain, temperature, and proprioception. Lesions impair somatic sensations, while electrical stimulation evokes sensory experiences. It projects to areas 1 and 2, but also to the posterior parietal cortex. 

Similar organizational structure as IVC of area V1 (separates information from slowly adapting and rapidly adapting neurons into columns). 

Somatotopy (homunculus)

The point-for-point correspondence of an area of the body to a specific point on the primary somatosensory cortex. Neighboring cortical areas receive information from neighboring skin areas, and the more cortical area is dedicated to areas with a denser number of mechanoreceptors. 

Experiment by Michael Merzenich and colleagues

The experiment examined the primary somatosensory cortex of monkeys from activity in their hands.

At rest, they could map out cortical areas dedicated to specific fingers that were activated in the primary somatosensory cortex due to the activity.

After surgically removing one, they found that neighboring cortical areas expand to compensate for the lost activity and now receive the input from the removed finger.

After having the monkey engage in a task that only uses two of its fingers, they found that these specific areas began to expand due to the continued activation of these two areas. 

Posterior Parietal Cortex

The portion of the parietal cortex that is posterior to the primary somatosensory cortex that is essential for the perception and interpretation of spatial relationships, accurate body image, and coordination of the body through space. 

Agnosia

The inability to recognize objects.

Astereoagnosia

The inability to recognize objects by touch.

Pain

The perception that requires brain activity and is an emotional experience that arises from nociceptive signals. Can feel ____ without any nociceptor receptors active, but can also have free nerve ends active and not experience ____. 

Nociception

The sensory process that detects harmful stimuli and sends signals to the brain, which can trigger a pain response. Can have pain without any activity in ____ receptors, but can also have ____ receptors active and not experience pain. 

Mechanical Nociceptors

The type of nociceptors that respond to excess pressure or disruption of tissue.

Thermal Nociceptors

The type of nociceptors that respond to noxious heat or cold at temperatures above 43 ^oC. They are activated by capsaicin. 

Chemical Nociceptors

The type of nociceptors that respond to endogenous ligands that arise from changes in the internal tissue. Respond through activation of H⁺, Bradykinin (kininogen), and Histamine (mast cells). 

Polymodal Nociceptors

The type of nociceptors that respond to more than one type of noxious stimulus, making them a mix of the other types. 

Hyperalgesia

The symptom of feeling excessive pain in response to normal or mild stimuli

Aδ Fiber

The myelinated nerve fibers that sense the initial pain from a noxious stimulus.

C fiber

The nerve fibers that sense the “second” pain from a noxious stimulus.

Ascending Regulation of Pain

The regulation of pain that two pathways of somatic sensation antagonize one another, and that the activation of non-nociceptive mechanoreceptors can activate interneurons and decrease the input of mechanoreceptors of the other pathway (i.e., activation of Aα mechanoreceptors can inhibit C fiber nociceptive signals to the spinothalamic tract). 

Descending Regulation of Pain

The regulation of pain that involves activation of the periaqueductal gray (PAG) area of the midbrain from nociceptive signals via the spinothalamic tract. It then relays this information to the rostral ventral medulla (RVM), which sends a signal down the spinal cord and activates endogenous opioids that suppress pain. 

The Mechanism of Placebo Analgesia

Naloxone acts as an opioid antagonist that reverses the analgesic effects induced by placebo analgesia, indicating that naloxone disrupts the mechanisms that allow the brain to interpret pain signals differently when expecting a placebo effect. 

Temperature

This perception has a similar pathway to pain, and sensitivity is not spread uniformly across the skin. Some free nerve endings are sensitive to hot (i.e., capsacin endings: Trpv1, Trpv2) temperatures and some are sensitive to cold (menthol: Anktm1, Trpm8) temperatures. 

QOD: Imagine that a person is injured such that the right side of the spinal cord is severed at T9. What would be the effect of this injury on the right leg? The left leg?

If this injury occurred on the right side of the spinal cord, then the right leg would lose the sense of touch and proprioception due to this information being disrupted in the dorsal column-medial lemniscal pathway, which carries this information on the same side of the spinal cord. However, the feeling of pain and temperature would be preserved, as this information travels contralaterally up the spinal cord in the spinothalamic tract, unaffected by the injury in the right side of the spinal cord. 

For the left leg, the opposite would occur. Pain and temperature would be lost, as this information crosses over contralaterally to the right side of the spinal cord. However, touch and proprioception would be preserved as they remain on the same side of the spinal cord, and in this instance, the left side.