Somatosensory system

Peripheral Somatosensory neurons Sensory receptors Peripheral nerve axon Dorsal root ganglion Spinal cord Brain Peripheral Somatosensory neurons

Introduction Somatosensation: Skin : Musculoskeletal : the sensory information from the skin and musculoskeletal systems Skin : Superficial sensory or cutaneous Superficial sensory information : touch, pain, temperature Touch Superficial pressure and vibration Musculoskeletal : proprioception and pain

Proprioception stretch of muscles, tension on tendons, position of joints, and deep vibration both static joint position sense and kinesthetic sense, sensory information about movement information in the somatosensory system proceeds from the receptor through a series of neuron to the brain Sensory information: nerve impulses generated from the original stimuli Sensation: awarness of stimuli from the senses Perception: the integration of sensation into meaningful forms (an active process of interaction between the brain and the environment)

Pathway convey Somatosensory information share similar anatomic arrangements Receptors in the periphery encode the mechanical, chemical or thermal stimulation received into receptor potentials Action potential  peripheral axon soma in dorsal root ganglion  proximal axon in spinal cord via axons white matter brain The diameter of the axons, the degree of axonal myelination, and the number of synapse in the pathway determine how quickly the information is processed

Peripheral somatosensory neurons Sensory receptors Somatosensory peripheral neurons Cutaneous innervation Musculoskeletal innervation

Sensory receptors : located at the distal ends of peripheral nerves mechanoreceptors, responding to mechanical deformation of the receptor by touch, pressure, stretch, or vibration chemoreceptors, responding to substances released by cells, including damaged cells following injury or infection thermoreceptors, responding to heating or cooling Nociceptors Tonic receptors Phasic receptors

Somatosensory peripheral neurons cell body of most peripheral sensory neurons : outside the spinal cord in the dorsal root ganglion or outside the brain in cranial nerve ganglia Two axons : distal axons conduct message from receptors to the cell body proximal axons project from the cell body into the spinal cord or brain stem

Classification of afferent axons Factors that influence nerve conduction velocity -Diameter of the axons(the larger is the faster) -The degree of axonal myelination

Cutaneous innervation - receptive field : the area of skin innervated by each neuron

Receptive field: tend to be smaller distally and larger proximally Skin sensations: Touch Pain Temperature Fine touch: superficial fine touch receptors : Meissner’s corpuscles, Merkel’s disks, hair follicle receptors subcutaneous fine touch : pacinian corpuscles, Ruffini’s corpuscles

Cutaneous receptors Coarse touch: Nociceptors Thermal receptors -free endings throughout the skin (localized touch or pressure and sensations of tickle and itch) Nociceptors -Free nerve endings, responsive to stimuli that damage or threaten tissue. Thermal receptors -Also free nerve endings, respond to either warmth or cold within the temperature Cutaneous receptors -respond to touch, pressure, vibration, stretch, noxious stimuli, and temperature Dermatome -the area of skin innervated by axons from cell bodies in a single dorsal root

Musculoskeletal innervation Muscle spindle Are embedded in skeletal muscle. the sensory organ in muscle – muscle fibers, sensory endings, and motor endings Sensory endings stretch that is changes in muscle length and the velocity of length change Quick and tonic stretch of the spindle type Ia afferents Tonic muscle stretch type II afferents

Intrafusal and extrafusal fibers Intrafusal fibers contractile only at their ends; the central region cannot contract nuclear bag fibers Have a clump of nuclei in the central region nuclear chain fibers Have nuclei arranged single fiber Two different sensory endings primary endings : phasic and tonic (annulospiral ending) secondary endings : tonic(flower-spry ending)

Muscle length is signaled by type Ia and II afferents reflecting stretch of the central region of both types of intrafusal fibers. Spindle sensitivity to changes in length is adjusted by gamma static efferents. Velocity of change in muscle length is signalized only by type Ia afferents, with information mainly from nuclear bag fibers whose sensitivity is adjusted by gamma dynamic efferents.

Golgi tendon organs - sensitive to very slight changes in the tension on a tendon and respond to tension exerted both by active contraction and by passive stretch of muscle - Ib afferents Joint receptors - respond to mechanical deformation of capsule and ligaments -ruffini’s endings(II) : extremes of joint range and respond more to passive than to active movement - paciniform corpuscles(II) : respond to movement - free nerve endings(Aδ and C) : stimulated by inflammation - ligament receptors(Ib) : similar to Golgi tendon organs and signal tension

Fully normal proprioception Requires muscle spindles, joint receptors, and cutaneous mechanoreceptors.

Golgi tendon organs and muscle spindles: how do they compare? GTO Muscle spindle Location Within tendon near the muscle-tendon junction in series with muscle fibers Interspersed among muscle fibers in parallel with the fibers Stimulus Increase in muscle tension Increase in muscle length responses 1)Inhibits tension development in streteched muscle 2)Initiate tension development in antagonist muscles 1)Initiate rapid contraction of stretched muscle 2)Inhibits tension development in antagonist muscles

Summary of the function of different-diameter axons large-diameter afferents: muscle, tendons, and joints medium-sized afferents: joint capsule, muscle spindles, and cutaneous touch, stretch, and pressure receptors smallest-diameter afferents: crude touch, nociceptive, and temperature information

Pathway to the brain Conscious relay pathways Divergent pathways Unconscious relay pathways

Conscious relay pathways to cerebral cortex - touch - proprioception - pain - temperature Three projection neurons Two routes - discriminative touch and conscious proprioception (dorsal columns, medial leminiscus system) discriminative pain and temperature (anterolateral tracts=neospinothalamic and trigeminospinothalamic) : fast pain To be aware of sensory information The information must reach the thalamus

Discriminative touch and conscious proprioception pathway localization of touch and vibration and ability to discriminate between two closely spaced points touching the skin Conscious proprioception: awareness of the movements and relative position of body parts Stereognosis: the ability to use touch and proprioceptive information to identify an object

Three-neuron relay the primary, or first-order, neuron conveys information from the receptors to the medulla the secondary, or second-order, neuron conveys information from the medulla to the thalamus the third-order, neuron conveys information from the thalamus to the cerebral cortex

Dorsal Column/Medial Lemniscus System Fasciculus gracilis Axons from the lower limb occupy the more medial section of the dorsal column Nucleus gracilis Fasciculus cuneatus Axons from the upper limb occupy the more the lateral section of the dorsal column Nucleus cuneatus

Tactile information from the face Sensory information essential for identifying objects by palpation, distinguishing between closely spaced stimuli, and controlling fine movement and smoothness of movements travels in the dorsal columns  medial leminiscus VPLS1 cortex Tactile information from the face travels in the trigeminal nerve thalamus(VPM)  sensory cortex

Discriminative pain and temperature, coarse touch Anterolateral columns Pain Temperature Coarse touch Conveys less information than the dorsal column/MLS, and thus coarse touch cannot be tested independently when discriminative touch is intact. Is involved with pleasant touch and skin-to-skin contacts. Spinothalamic tract

Temperature sensation warmth and cold free nerve ending of small myelinated and unmyelinated neurons Aδ : cooling, C : heat proximal axon (segment of the spinal cord) the second axon (cross the midline and then ascend contralaterally to VPL nucleus of the thalamus) sensory cortex

Neospinothalamic pain: Spinolimbic pain: Both pain: complex phenomenon; persistent pain affects emotional, autonomic, and social functioning Pain is composed Protective and the emotional response Nociceptive Describes receptors or neurons that receive or transmit information about stimuli that damage or threaten to damage tissue. Several different pathways Neospinothalamic pain: fast pain (conscious relay pathway) Spinolimbic pain: slow pain (divergent pathway) Both pain: anterolateral section of the spinal cord

Fast, localized pain the first-order neuron brings information into the dorsal horn of the spinal cord the axon of the second-order neuron crosses the midline and projects from the spinal cord to the thalamus the third-order neuron projects from the thalamus to the cerebral cortex

Divergent pathways Medial pain system Projection neurons synapse in medial locations in the central nervous system Several pathways with variable numbers of projection neurons, not a three neuron pathway like fast pain. The information from the medial pain systems is not somatotopically organized, so slow pain cannot be precisely localized.

Divergent pathways Solw, aching pain: First neuron: depends on a divergent network, not a three-neuron pathway like fast pain First neuron: small, unmyelinated C fibers, sensitive to noxious heat, chemical, or mechanical stimulation

Ascending projection neurons: wide-dynamic-range neuron The ascending axons reach the midbrain, reticular formation, and thalamus via three tracts Spinomesencephalic Spinoreticular Spinolimbic

Spinomesencephalic tract nociceptive information to the superior colliculus and to an area surrounding the cerebral aqueduct, the periaqueductal gray involved in turning the eyes and head toward the source of noxious input and in activating descending tracts that control pain periaqueductal gray is part of the descending pain control system

Spinoreticular ascending neurons: Reticular formation is a neural network in the brainstem that includes the reticular nuclei and their connections. Arousal, attention, and sleep/waking cycles are modulated by the reticular formation. reticular formation  midline and intralaminar nuclei of the thalamus Spinolimbic track midline and intralaminar nuclei of the thalamuscerebral cotrex involved with emotions, sensory integration, personality, and movement

The slow pain pathways: produces automatic movements and autonomic and emotional responses to noxious stimuli Slow pain information from the face : trigeminoreticulothalamic pathway: C fibers in the trigeminal N. reticular formation intralaminar nuclei

Tempeature Reticular formation, nonspecific nuclei of the thalamus, to subcortical nuclei, and to the hypothalamus This temperature information that does not reach conscious awareness contributes to arousal, provides gross localization, and contributes autonomic regulation

Unconscious relay to the cerebellum Information from proprioceptors and information about activity in spinal interneurons are transmitted to the cerebellum via Spinocerebellar tracts Adjusting movements

High-fidelity pathways Two pathways -arranged information to the cerebellar cortex posterior spinocerebellar pathway -Transmits information from the legs and the lower half of the body -1st order; nucleus dorsalis -2nd order; posterior spinocerebellar tract. -the tract remains ipsilateral and projects to the cerebellar cortex via the inferior cerebellar peduncle. cuneocerebellar pathway -begin with primary afferents from the arm and upper half of the body

High fidelity pathway Posterior spinocerebellar pathway : Transmit information from the leg and the lower half of the body Dorsal column to the thoracic or upper lumbar spinal cord(T1-L2)  nucleus dorsalis  posterior spinocerebellar tract Ipsilateral and projects to the cerebellar cortex

Cuneocerebellar pathway : Afferents from the arm and upper half of the body Lateral cuneate nucleus (first – and second-order neurons) in the medulla  cuneocerebellar tract  ipsilateral inferior cerebellar penduncle cerebellar cortex

Internal feedback tracts the two internal feedback tracts monitor the activity of spinal interneurons and of descending motor signals from the cerebral cortex and brain stem anterior spinocerebellar tract Transmits information from the thoracolumbar spinal cord rostrospinocerebellar tract

Internal feedback tracts Anterior spinocerebellar tract : Transmit information from the thoracolumbar spinal cord Lateral and ventral horns -> cross the opposite side and ascend in the contralateral anterior spinocerebellar tract to the midbrain -> cerebellum via superior cerebellar penduncle Gets information from both sides of the lower body Bilateral projection : automatic coordination of lower limb activities

Rostrospinocerebellar tract : Transmits information from the cervical spinal cord to the ipsilateral cerebellum and enters the cerebellum via both the inferior and superior cerebellar peduncles The anterior and rostrospinocerebellar tracts : - apprise the cerebellum of the descending commands delivered to the neurons that control muscle activity via interneurons located between descending motor tracts and motor neurons that innervate muscles - the activity of spinal reflex circuits

Functions of spinocerebellar tracts - not consciously perceived - unconscious adjustments to movements and posture internal feedback tract : *convey descending motor information to the cerebellum prior to the information reaching the motor neurons high-fidelity pathway *convey information spindles, tendon organs, and cutaneous mechanoreceptors; cerebellum obtains information about movements commands and about the movements or postural adjustments -cerebellum can compare the intended motor output with the actual movement output

Information in the spinocerebellar tracts is from proprioceptors, spinal interneurons, and descending motor pathways. This information, which does not reach conscious awarness, contributes to automatic movements and postural adjustments