Somatosensory System and Receptors

• The somatosensory system helps perceive touch, texture, body position, pain, temperature, and other sensations
• This is achieved by integrating information from various receptors
• Most receptors have long axons
• Touch and position receptors use large-diameter, heavily myelinated axons
• Whereas pain and temperature receptors use small-diameter, thinly myelinated or unmyelinated axons

Receptor Types

• Large-diameter fibers are mechanoreceptors, often with connective tissue capsules or accessory structures to filter mechanical stimuli
• Small-diameter fibers are typically free nerve endings, some responding to painful stimuli, others to temperature changes or light touch

Cutaneous and Subcutaneous Receptors

• Responsible for discriminative touch (shape, texture, movement across the skin)
• They have large-diameter axons and encapsulated endings or accessory structures
• Examples include Meissner corpuscles, Pacinian corpuscles, and Merkel endings
• The density of these receptors, especially Meissner corpuscles and Merkel endings, determines tactile acuity
• Tactile acuity is high on fingertips and lips

Major Somatosensory Receptor Types

• Free nerve endings: ubiquitous, respond to pain, temperature, and light touch using Ad and C axon types
• Meissner corpuscles: in glabrous skin, respond to changing touch using Aβ axon type
• Pacinian corpuscles: in skin, joints, and deep connective tissue respond to vibration using Aβ axon type
• Muscle spindles: in skeletal muscle, respond to muscle stretch using la and Aβ axon types
• Golgi tendon organs: at the muscle-tendon junction, respond to muscle tension using lb axon type
• Merkel endings: in glabrous and hairy skin, respond to touch using Aβ axon type
• Endings around hairs: in hairy skin, respond to touch using various axon types

Meissner Corpuscles

• Located in the dermal papillae of glabrous (hairless) skin
• Their capsules contain flattened disks of Schwann cell cytoplasm
• Rapidly adapting receptors, important for detecting details of things moving across the skin
• They are involved when fingertips move across something, or when something in grasp begins to slip

Pacinian Corpuscles

• Widespread in subcutaneous connective tissue
• Have a multilayered capsule shaped like an onion
• Rapidly adapting receptors that respond briefly at the beginning and end of a mechanical stimulus
• Good at detecting rapidly changing stimuli, such as vibrations

Merkel Endings

• Abundant in both glabrous and hairy skin
• They are sensitive, slowly adapting receptors that are important for detecting the shape and texture of stationary objects touching the skin
• They are the flattened sensory terminals of large-diameter axons applied to Merkel cells in the basal layer of the epidermis
• Merkel cells might be the actual receptor cells because they make synaptic contacts on Merkel endings

Other Receptors

• Sensory endings wrapped around hair bases detect tactile stimuli
• Unmyelinated (C) fibers with free nerve endings in hairy skin respond best to slow, gentle brushing
• The C-fiber touch receptors are probably more important for pleasurable feelings associated with this kind of touch

Receptors for Coolness and Warmth

• The receptors are all free nerve endings
• Receptors for cooling have thinly myelinated (Ad) axons
• Receptors for warming have unmyelinated (C) axons
• There are different receptors from those that detect painful heat and cold

Nociceptors

• They detect events that damage or threaten to damage tissue
• Some have thinly myelinated (Ad) fibers, and others have C fibers
• These two groups correspond to the two-part sensation of pain where sharp, well-localized fast pain is followed by a dull, poorly localized, aching sensation of slow pain
• Fast pain is initiated by firing of Ad nociceptors
• The delayed onset of slow pain is directly related to the slower conduction velocity of the C fibers that mediate it
• Ad nociceptors respond specifically to intense mechanical stimulation, to painful heat or cold, or to both
• C-fiber nociceptors respond to intense mechanical stimulation, to painful heat or cold, or to both, as well as to a variety of substances released in damaged tissue, and are referred to as polymodal nociceptors

Muscle Receptors

• Skeletal muscles contain free nerve endings that detect muscle pain and others that probably keep track of the metabolic status of the muscle.
• They also contain muscle spindles and Golgi tendon organs, both encapsulated receptors that monitor the length and tension of a muscle, respectively

Muscle Spindles

• They are small, cigar-shaped organs consisting of a few slender, intrafusal ("inside the spindle") muscle fibers which are enclosed in a capsular continuation of the perineurium
• They are found in nearly all skeletal muscles
• The number of spindles in a muscle relates to the degree of fine control over its contraction
• Each intrafusal fiber has a central, nucleated, noncontractile zone with sensory endings wrapped around it, flanked by contractile zones
• Both ends of the spindle are anchored in the muscle among the extrafusal ("outside the spindle") muscle fibers,so stretching the muscle also stretches the central zones of the intrafusal fibers and distorts the mechanosensory endings
• The contractile portions of the intrafusal fibers are innervated by gamma motor neurons, so named because their axons are in the Ay category, but are too small to generate a significant amount of force
• This provides an elegant mechanism for maintaining constant sensitivity of a spindle during contraction and relaxation of a muscle
• Monitor changes in the length of muscles and provides information important for the coordination of movement and awareness of changes in position

Golgi Tendon Organs

• They are encapsulated regions of myotendinous junctions in which large-diameter sensory axons enter the perineurial capsule and divide into branches that are interdigitated with the collagen bundles there
• By squeezing the endings between the collagen bundles, Golgi tendon organs monitor the tension in a muscle, something that muscle spindles cannot provide much information about

Position Sense

• Is given through the movements at a joint stimulates receptors in the joint itself, changes the length of muscles that insert around the joint, and distorts the skin and subcutaneous tissues surrounding the joint
• The CNS uses all of this information both to coordinate movements and for conscious awareness of the position of parts in space (proprioception)
• Muscle spindles play a major role and joint receptors, surprisingly, are the least important.

Distribution of Somatosensory Information in the Central Nervous System

• Each vertebrate sensory system has one collection of receptors that are used for multiple functions—at the very least, feeding into pathways to the cerebral cortex (conscious awareness), into reflex circuitry, and in most or all cases into pathways to the cerebellum as well
• The primary afferents branch repeatedly and make synapses on multiple sets of interneurons on the ipsilateral side of the CNS
• Somatosensory pathways to the cerebral cortex cross the midline
• Pathways to the cerebellum do not include a stop in the thalamus
• The cerebellum affects the ipsilateral side of the body, so most spinocerebellar neurons are uncrossed

Reflexes

• Reflexes are automatic responses to sensory inputs, such as pupillary constriction in response to bright light and salivation in response to something appetizing
• They include the stretch reflex and the flexor reflex, one that withdraws a body part from a painful stimulus
• The stretch reflex is as simple as a reflex can be, involving a large-diameter primary afferent from a muscle spindle and a motor neuron that projects back to the same muscle
• All other reflexes involve one or more interneurons, generally because they affect multiple muscles

Touch and Position: Posterior Column-Medial Lemniscus Pathway

• Sensory axons of all sizes are intermingled in spinal nerves, but as dorsal rootlets approach the spinal cord the large- and small-diameter fibers separate from each other
• The large-diameter axons move medially, enter the ipsilateral posterior funiculus (also called the posterior column), and give off numerous branches
• Their axons in turn cross the midline in the medulla, forming the medial lemniscus
• The medial lemniscus ascends through the brainstem and terminates in the ventral posterior nucleus of the thalamus (in a lateral part called the ventral posterolateral nucleus, or VPL) projects through the posterior limb of the internal capsule to somatosensory cortex in the postcentral gyrus
• Comparable information from the face arrives in the trigeminal nerve and terminates at a midpontine level in the main sensory nucleus of the trigeminal nerve
• Axons arising in the main sensory nucleus cross the midline, join the medial lemniscus, and terminate in the medial part of the ventral posterior nucleus (the ventral posteromedial nucleus, or VPM)
• The face and body are represented systematically in the postcentral gyrus in a distorted somatotopic map
• The area devoted to any part is related to the packing density of receptors there, so the somatosensory homunculus (a figurative "little person") has disproportionately large fingers and lips

Pain and Temperature: Anterolateral Pathway

• It has not only a discriminative aspect but also a powerful emotional and motivational component
• The discriminative aspect (judging the location, nature, and intensity of a painful stimulus) is similar in all and consistently
• Distributed more widely than touch and position information in the CNS
• Pain and temperature pathways begin with small-diameter primary afferent fibers that separate from large-diameter fibers as the dorsal rootlets join the spinal cord

Pain-Temperature Pathway

• The primary afferents enter the cord lateral to the larger fibers, join Lissauer's tract, and divide into ascending and descending branches that distribute synapses to neurons in the posterior horn over several segments
• Some of these are interneurons in flexor reflex arcs, others are tract cells that project to the cerebellum, many are tract cells with axons that cross the midline in the spinal cord to form the anterolateral pathway
• The anterolateral pathway is actually a composite of several tracts conveying pain and temperature information
• VPL then relays some of this information to somatosensory cortex, subserving the discriminative aspects of pain and temperature sensation
• Other thalamic nuclei project to limbic areas of cortex such as the cingulate gyrus and insula, subserving the emotional and motivational aspects

Trigeminal Pain Pathway

• Contributes to this pathway, bringing information about pain and temperature in the face and head
• The primary afferents take an unexpected course with four trigeminal nuclei in the brainstem-one motor and three sensory
• Finally, the spinal trigeminal nucleus extends from the level of the main sensory nucleus all the way to the cervical spinal cord
• Small-diameter trigeminal afferents enter the brainstem at a midpontine level with the rest of the trigeminal nerve, descend through the spinal trigeminal tract and synapse on the trigeminal's equivalent of spinothalamic tract cells in the part of the spinal nucleus in the caudal medulla

Clinical conditions

• Trigeminal neuralgia is a rare chronic pain condition that affects the trigeminal nerve
• It causes extreme, sporadic burning or shock-like pain in the face

Control of Pain Transmission

• The CNS controls the sensitivity of all of its sensory systems
• The is done with a descending pain-control pathway that originates in the periaqueductal gray
• Opioid peptides have inhibitory postsynaptic effects and are used by interneurons in the substantia gelatinosa to inhibit transmission from nociceptors to spinothalamic tract cells