SomatosensorySystems andReceptors Department of Neurology Loyola University Medical Center 2012.
The primacy of touch And I found that of all the senses the eye was the most superficial, the ear the most haughty, smell the most voluptuous, taste the most superstitious and inconstant, touch the most profound and philosophical.Diderot, ‘Letter on the Blind,’ 1749
Somatosensory SystemsInformation Input from the somatosensory systems informs the organism about events impinging on it. Sensation can be divided into four types: superficial, deep, visceral, and special.
Types of sensation Superficial sensation touch, pain, temperature, and two-point discrimination. Deep sensation muscle and joint position sense (proprioception), deep muscle pain, and vibration sense. Visceral sensations relayed by autonomic afferent fibers and include hunger, nausea, and visceral pain Special senses—smell, vision, hearing, taste, and equilibrium
Categories of sensation Conscious sensation perceived at the level of cerebral cortex. Non conscious sensation not perceived, they have reference to the cerebellum
Conscious sensations Exteroceptive: external word. Impinge either somatic receptors : body surface (touch, pressure, heat, cold, pain) Or, telereceptors: vision and hearing. Propioceptive: arise within the body. locomotor system, or vestibular labyrinth. Position sense (stationary) and kinesthetic sense (movement)
Non conscious sensations Non conscious proprioception:Smooth motor coordination, spinocerebellar pathways (afferent information) Interoception:Unconscious afferent signals involved in visceral reflexes
Receptors Receptors are specialized cells for detecting particular changes in the environment. Exteroceptors include receptors affected mainly by the external environment Meissners corpuscles, Merkels corpuscles, and hair cells for touch; Krauses end-bulbs for cold; Ruffinis corpuscles for warmth; and free nerve endings for pain. Receptors are not absolutely specific for a given sensation; strong stimuli can cause various sensations, even pain, even though the inciting stimuli are not necessarily painful. Proprioceptors receive impulses mainly from pacinian corpuscles, joint receptors, muscle spindles, and Golgi tendon organs. Painful stimuli are detected at the free endings of nerve fibers.
Sensory units Stem fiber and the “family of endings” constitute a sensory unit The territory from which a sensory unit can be excited is its receptive field. Inverse relationship between sized of receptive fields and sensory acuity. Example, 2cm2 upper arm, 1cm2 wrist, 5mm2 finger pads.
Adaptation Each efferent fiber from a receptor relays stimuli that originate in a receptive field and gives rise to a component of an afferent sensory system. Each individual receptor fires either completely or not at all when stimulated. The greater the intensity of a stimulus, the more end-organs that are stimulated, the higher the rate of discharge is, and the longer the duration of effect is. Adaptation denotes the diminution in rate of discharge of some receptors on repeated or continuous stimulation of constant intensity; the sensation of sitting in a chair or walking on even ground is suppressed
Nerve endingsFree nerve endings.Run toward skin surface.Schwann cells sheaths open to permit naked axons to terminate between collagen bundles (dermal nerve endings) or within epidermis (epidermal nerve endings)
Free nerve endings Thermoreceptors: supply either “warm spots” or “cold spots” Nociceptors (pain-transducing): 1. mechanical deformation, finely myelinated, Aδ fibers 2. polymodal nociceptors, C-fibers: transduce mechanical deformation, intense heat or cold, irritant chemicals, and are responsible for the axon reflex.
Follicular nerve endings Myelinated fibers apply a “palisade” of naked terminals along the outer root sheath epithelium of the hair follicles. Circumferential set of terminals. Territorial overlap = each unit supplies many follicles. Rapidly adapting.
Merkel cell neurite complexes Expanded nerve terminals in the basal epithelium of epidermal pegs and ridges. Slowly adapting Discharge continuously in response of sustained pressure (wear glasses, holding a pen)
Posterior (dorsal) Column-medial lemniscalpathway 1st order: largest somas in the posterior root ganglia. Process largest sensory receptors. touch, joint sensation, two-point discrimination, and vibratory sense from receptors to the cortex Lower limb and lower trunk give branches (fasciculus gracilis) to gray matter to reach gracile nucleus in medulla oblongata Upper limb and upper trunk cuneate fasciculus to reach cuneatus nucleus.
Posterior (dorsal) Column-medial lemniscalpathway 2nd order afferents, start nucleus gracilis and nucleus cuneatus. Ventrally in medulla oblongata for sensory decusation and the fibers turn rostrally in medial lemniscus Ending lateral part ventral posterior nucleus of the thalamus, also ending medial part same nucleus, trigeminal lemniscus. 3rd order afferents from the thalamus to the somatic sensory cortex.
Spinothalamic pathway Second order neurons, projecting from laminae I-II, IV-V posterior gray horn, to the contralateral thalamus. Cells receive excitatory and inhibitory synapses from neurons of the substantia gelatinosa (modulatory effects) Cross midline anterior comissure at all segmental levels. Anterior spinothalamic tract, and lateral spinothalamic tract. In the brainstem is spinal lemniscus Spinal lemniscus join trigeminal afferents to the VP nucleus thalamus
And I found that of all the senses the eye was the mostsuperficial, the ear the most haughty, smell the mostvoluptuous, taste the most superstitious and inconstant,touch the most profound and philosophical.Diderot, ‘Letter on the Blind,’ 1749 Thank you.