Somatosensory system
Merkel cell nerve endings sit in the basal epidermis and hair follicles. They react to low vibrations between 5 and 15 Hz. These receptors handle deep static touch like shapes and edges. Their small receptive field provides extremely detailed information for fingertips. Tactile corpuscles respond to moderate vibration from 10 to 50 Hz. They live in the dermal papillae of the skin. These cells are primarily located in fingertips and lips. They respond with quick action potentials unlike Merkel nerve endings. Pacinian corpuscles determine gross touch and distinguish rough or soft substances. They react especially to vibrations around 250 Hz even up to centimeters away. Bulbous corpuscles react slowly and respond to sustained skin stretch. All these receptors activate upon pressures that distort their shape causing an action potential.
The first-order neuron is a type of pseudounipolar neuron. Its cell body always sits in the dorsal root ganglion of the spinal nerve. A peripheral axon innervates touch mechanoreceptors while a central axon synapses on the second-order neuron. The second-order neuron has its cell body either in the spinal cord or in the brainstem. This neuron's ascending axons will cross to the opposite side either in the spinal cord or in the brainstem. In the case of touch and certain types of pain the third-order neuron has its cell body in the ventral posterior nucleus of the thalamus. It ends in the postcentral gyrus of the parietal lobe in the primary somatosensory cortex. All afferent touch information ascends the spinal cord via the dorsal column-medial lemniscus pathway. Cuneatus sends signals indirectly via spinal grey matter for sound determination. All fibers cross in the medulla.
Wilder Penfield devised the cortical homunculus as a map of somatosensory areas. The postcentral gyrus lies in the parietal lobe. Its cortex is the primary somatosensory cortex known as Brodmann areas 3, 2 and 1 collectively referred to as S1. BA3 receives the densest projections from the thalamus. Region S2 divides into Area S2 and parietal ventral area. Area S2 is involved with specific touch perception and is thus integrally linked with the amygdala and hippocampus. Parietal ventral area serves as the somatosensory relay to the premotor cortex. BA5 acts as the topographically organized somato memory field. BA1 processes texture info while BA2 processes size and shape information. The insular cortex plays a role in bodily-ownership and self-awareness. It conveys info about sensual touch pain temperature itch and local oxygen status.
The feeling of pleasantness associated with affective touch activates the anterior cingulate cortex more than the primary somatosensory cortex. Functional magnetic resonance imaging data shows increased blood-oxygen-level contrast signal in the anterior cingulate cortex highly correlated with pleasantness scores. Inhibitory transcranial magnetic stimulation of the primary somatosensory cortex inhibits the perception of affective touch intensity but not affective touch pleasantness. Tactile contact between two animals occurs through stroking licking or grooming. These behaviors induce the release of oxytocin in the hypothalamus. Oxytocin decreases stress and anxiety and increases social bonding between animals. The consistency of oxytocin neuron activation has been observed in rats stroked by humans especially in the caudal paraventricular nucleus. Longer stroking induced a greater release of the hormone over time.
Many studies have shown a decline in passive tactile spatial acuity with age. Index finger passive tactile spatial acuity is better among adults with smaller index fingertips. This effect of finger size underlies the better passive tactile spatial acuity of women on average compared to men. The density of tactile corpuscles is greater in smaller fingers. Among children of the same age those with smaller fingers also tend to have better tactile acuity. Passive tactile spatial acuity is enhanced among blind individuals compared to sighted individuals of the same age. Individuals who have been blind since birth reportedly consolidate tactile information more rapidly than sighted people. Cross modal plasticity in the cerebral cortex of blind individuals may explain these differences.
A somatosensory deficiency may be caused by a peripheral neuropathy involving peripheral nerves of the somatosensory system. This may present as numbness or paresthesia. Crude touch allows the subject to sense that something has touched them without being able to localize where they were touched. Its fibers are carried in the spinothalamic tract unlike fine touch which is carried in the dorsal column. A person will be able to localize touch until fibers carrying fine touch have been disrupted. Then the subject will feel the touch but be unable to identify where they were touched. Phantom limb sensations remain a key symptom of such deficiencies.
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Common questions
What are the specific functions of Merkel cell nerve endings in the somatosensory system?
Merkel cell nerve endings sit in the basal epidermis and hair follicles to handle deep static touch like shapes and edges. These receptors react to low vibrations between 5 and 15 Hz and provide extremely detailed information for fingertips due to their small receptive field.
How does the first-order neuron function within the somatosensory pathway?
The first-order neuron is a type of pseudounipolar neuron with its cell body always sitting in the dorsal root ganglion of the spinal nerve. Its peripheral axon innervates touch mechanoreceptors while its central axon synapses on the second-order neuron.
Which brain regions process texture versus size and shape information in the primary somatosensory cortex?
Area BA1 processes texture info while Area BA2 processes size and shape information within the primary somatosensory cortex known as Brodmann areas 3, 2 and 1 collectively referred to as S1. Region S2 divides into Area S2 and parietal ventral area where Area S2 is involved with specific touch perception.
What physiological effects occur when tactile contact induces oxytocin release in animals?
Tactile contact between two animals occurs through stroking licking or grooming which induces the release of oxytocin in the hypothalamus. Oxytocin decreases stress and anxiety and increases social bonding between animals.
Why do blind individuals exhibit better passive tactile spatial acuity compared to sighted people?
Passive tactile spatial acuity is enhanced among blind individuals compared to sighted individuals of the same age because they reportedly consolidate tactile information more rapidly than sighted people. Cross modal plasticity in the cerebral cortex of blind individuals may explain these differences.