Somatic Senses AND Motor Control PDF

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SOMATIC SENSES AND MOTOR CONTROL

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Classification of senses General Senses Somatic GSA) - Tactition - Nociception - Thermoception - Proprioception Visceral (GVA) Chemoception (blood glucose; blood pH; osmolarity) Baroreception (BP)

Special Senses   -

Somatic (SSA) Balance Hearing Sight Visceral (SVA) Taste Small

Nerve Fibre Classification

Non-encapsulated Neural Receptors  Free nerve endings - Nociceptors - Thermoreceptors - Hair root plexuses - Merkel nerve endings

Encapsulated Neural Receptors  Meissner (tactile) corpuscle.  Ruffini

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General senses Nociception Unpleasant sensation arising from noxious stimuli. Nociceptor types Mechanical Thermo-mechanical Polymodal (chemical, mechanical, and thermal). Variable conduction velocity ‘Fast’ pain  Type III (Aδ-fibres): acute, sharp pricking pain. ‘Slow’ pain  Type IV (C-fibres): burning

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Thermoception Sense of presence or absence of heat. 2 Thermoreceptor types

Cold receptor types  Range: 10-40°C  Location: epidermis (stratum basale).  Type III (Aδ-fibres) Warm Receptors  Range: 32-48°C  Location: dermis  Type IV (C-fibres)

Muscle Spindles

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Tactition Sense of touch, pressure, vibration, tickle, itch.

Superficial mechanoreceptors:  Meissner’s (tactile) corpuscles - Fine touch; vibration 50Hz  Ruffini (bulbous corpuscles) - Stretch due to movement  Hair root plexuses - Movement of hairs.

(bulbous) corpuscle. Pacinian (lamellar) corpuscle.

Proprioception Sense of movement (kinaesthesia), joint position sense, and sense of an objects weight.  Proprioceptors: - Muscle spindles - Golgi tendon organs - Joint kinaesthetic receptors. 

Golgi Tendon Organs

Morphology  Intrafusal muscle fibres embedded within skeletal (extrafusal) muscle fibres. Adequate stimulus  Muscle stretch (lengthening) Fibre group  Type Ia and II (Aα & Aβ) Distribution  Skeletal muscles

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Morphology  Consists of a thin capsule of connective tissue that encloses a few tendon fascicles/ collagen fibres with surrounding sensory nerve endings. Adequate stimulus  Tension Fibre group  Type Ib (Aα) Location  Musculotendinous junction of skeletal muscle

General organisation of somatosensory pathways 1st order Sensory Neurons 2nd Order Sensory Neurons From sensory receptor to spinal cord  From spinal cord to thalamus. or brainstem.  Travel in spinal cord tracts.  Decussate in spinal cord or brainstem. Travel in peripheral nerves. Cell body located in the dorsal root ganglion.

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3rd Order Sensory Neurons From thalamus to primary somatosensory cortex. Travel in internal capsule.

Dorsal column – Medial lemniscus pathway Description Neuron pathway 1st Order Neurons Conveys  Fine touch  Proprioceptors and mechanoreceptors  dorsal gray horn ascend I/L in dorsal  Pressure columns  synapse in gracile and cuneate nuclei of the medulla. 2nd Order Neurons  Vibration  Conscious proprioception  Decussate in medulla (sensory decussation)  ascend in medical lemniscus  synapse in VPL nucleus of thalamus. 3rd Order Neurons  Ascend in internal capsule  somatosensory cortex.

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Anterolateral System Description Spinothalamic tracts. Spinoreticular tract Spinomesencephalic (spinotectal) tract.

Conveys  Pain  Temperature  Itch  Tickle

Tracts (Spinothalamic Tracts) 1st Order Neurons  Nociceptors  Lissauer’s tract  laminae I and V (Aδ-fibres) and II (C fibres) in the dorsal gray horn  synapse. nd 2 Order Neurons  Decussate in anterior white commissure  ascend in lateral or anterior spinothalamic tracts (STT). - Lateral SST – fast pain (Aδ-fibres) and temperature. - Anterior STT – slow pain (C fibres), tickle, itch, crude touch, pressure. - Ascend through medulla  spinal lemniscus (superior to the inferior olivary nuclei) VPL nucleus of thalamus  synapse. 3rd Order Neurons  VPL nucleus of thalamus  somatosensory cortex.

Spinocerebellar Pathways Posterior Spinocerebellar tract  Unconscious proprioception from the trunk and lower limb.  Project to I/L cerebellum. Anterior spinocerebellar tract  Unconscious proprioception from trunk & lower limb.  Project to I/L cerebellum.

Cuneocerebellar tract  Unconscious proprioception from upper limb.  Projects to I/L cerebellum.

Motor Control

Basal Ganglia Functions Indirectly influence the control of movements: - Initiation and termination of movements  Receives input from several cortical areas: - Primary motor cortex, premotor area, and somatosensory cortex.  Projects (via the thalamus) back to cerebral cortex Primary motor cortex, premotor area, and supplementary motor area. 

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Motor Pathways Direct motor (pyramidal) pathway Corticospinal tracts (lateral and anterior) Corticobulbar tract Indirect motor (extrapyramidal) pathways Lateral pathways  Rubrospinal tract Medial pathways  Colliculospinal (tectospinal) tract  Reticulospinal tract  Vestibulospinal tract

Cerebellum Vestibulocerebellum: Balance and spatial orientation Receives input primarily from vestibular nuclei. Spinocerebellum: Fine-tune body and limb movements. Receives primarily proprioceptive input from spinocerebellar tracts. Cerebrocerebellum: Planning movements (also, cognitive functions) Receives input exclusively from the cerebral cortex.

Motor Neurons Upper motor neurons (UMNS’s)  Cerebral cortex or brainstem  ventral gray horn of spinal cord (or brainstem nuclei). Lower motor neurons (LMN’s)  Ventral gray horn of spinal cord (or brainstem nuclei).  skeletal muscle - α motor neurons to extrafusal muscle fibres. - γ motor neurons to intrafusal muscle fibres.  Also known as the ‘final common pathway’

Direct Motor pathway: Corticospinal Tracts  Primary motor cortex  internal capsule  cerebral peduncles (midbrain)  pyramids (medulla) Lateral Corticospinal Tract Anterior Corticospinal Tracts 90% of axons decussate  descend in C/L lateral  10% of axons do no decussate  descend in I/L anterior corticospinal tract synapse onto LMNs in ventral gray horn. corticospinal tract  decussate in anterior white Precise, agile and skilled motor function of the hands and commissure (spinal cord)  synapse onto LMNs in ventral feet. gray horn.  Responsible for trunk and proximal limb movements.

Indirect motor pathways Rubrospinal Tract

Colliculospinal Tracts

Vestibulospinal Tracts

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Input from cerebral cortex and cerebellum. Red nucleus  ventral tegmental decussation (midbrain)  descends in C/L Rubrospinal tract  synapse with LMNs in lateral part of ventral gray horn. Rudimentary in humans.

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Input from optic tracts and visual cortex. Superior colliculus  pass ventromedially around the PAG dorsal tegmental decussation  descend in C/L colliculospinal tractsynapse onto LMNS in ventral gray horn (mostly in cervical segments). Function:  Reflex (defensive) movements of head, neck, and eyes position in response to visual input.

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Input from vestibular apparatus and cerebellum.  Vestibular nuclei (medulla)  descends in I/L Vestibulospinal tracts (anterior funiculus)  synapses with LMNs (via interneurons). Function:  Excitation of trunk and proximal limb muscles = maintain balance in response to I/L head movement.  Maintenance of upright posture, facilitation of anti-gravity muscles.  Controls head and neck position and gaze....