Neurology for the PTA - Week 6

Flashcards about the Somatosensory System for PTA students.

Peripheral Nerves

Bundles of axons (fascicles) that travel to/from the extremities, each with a cutaneous and muscular branch containing afferent and efferent axons.

Nerve Plexus

Site where axons from multiple spinal segments merge; includes cervical, brachial, lumbar, and sacral plexuses.

Spinal Nerves

Contain neurons from one spinal level; sensory and motor information travel together but divide to enter through dorsal or ventral roots.

Tracts

Groups of myelinated axons connecting the spinal cord with the brain, running vertically within the spinal cord with a common origin and destination.

Brainstem

Relay center for information between the spinal cord, cerebellum, and cerebrum; contains nuclei for synapse and pathways for information travel.

Thalamus

Part of the diencephalon, superior to the brainstem, containing nuclei for sensory and motor tracts; acts as a relay station.

First-order neuron

Relays sensory information from skin, muscles, and joints; afferent.

Somatosensory System Function

Provides information about the external world, body position in space, contributes to movement control and balance, and protects against injury.

Types of Somatosensation

Touch, pressure, vibration, temperature, nociception, and proprioception.

First-order neuron pathway

Sensory receptors to spinal cord (PNS).

Second-order neuron pathway

Spinal cord/brainstem to thalamus (CNS).

Third-order neuron pathway

Thalamus to cortex (CNS).

Sensory Receptors Location

Embedded in the dermal layer of the skin and within muscle fibers, tendons, ligaments, and joint capsules.

Mechanoreceptors

Respond to deformation including touch, pressure, stretch, vibration; encapsulated receptors and free nerve endings.

Chemoreceptors

Respond to chemicals in the internal or external environment; free nerve endings.

Thermoreceptors

Respond to temperature; free nerve endings.

Nociceptors

Detect tissue damage or harmful stimuli; free nerve endings.

Cutaneous Receptors: Receptive Field

Area of the skin innervated by a single neuron; small field = more localization, large field = less localization.

Adaptation

Decline in action potential generation despite ongoing exposure to a stimulus.

Tonic receptors

Slowly adapting; provide constant input and sustained response.

Phasic receptors

Rapidly adapting; alert the body to change and transient response.

Unencapsulated nerve endings

Dendrites are directly embedded in the tissue; less selective, most prolific; free nerve endings, Merkel discs, hair follicle receptors.

Encapsulated nerve endings

Dendrites are wrapped in layers of connective tissue; improves selectivity for a specific stimulus; majority of mechanoreceptors.

Meissner's corpuscle

Slip, motion on skin.

Merkel cell

Light pressure, object shape.

Ruffini’s corpuscle

Stretch.

Pacinian corpuscle

Vibration, texture.

Muscle spindle

Muscle stretch, changes in muscle length.

Golgi tendon organ

Tension in tendons/muscles.

Ligament receptor

Tension on ligaments.

Joint receptors

Deformation of the capsule and ligaments.

Muscle Spindle: Function

Embedded within skeletal muscle, providing information about static muscle length and detecting amplitude and velocity of muscle length changes; muscles for fine motor control have more.

Extrafusal fibers

Muscle fibers outside of the muscle spindle; primary muscle cells.

Intrafusal fibers

Muscle fibers within the muscle spindle that are surrounded by a connective tissue capsule; nuclear chain fibers for static positioning, nuclear bag fibers for dynamic changes.

Gamma motor neurons

Ensure that intrafusal fibers will contract simultaneously with extrafusal fibers, maintaining spindle sensitivity.

Muscle Spindles: Protective Function

Prevents potential damage if a muscle is stretched rapidly beyond its limit.

Golgi Tendon Organs: Function

Woven within the collagen strands of tendons; monitor the tension produced by muscle contraction.

Golgi Tendon Organ: Protective Function

Protects the tendon/muscle from injury that could occur from excessive tensile forces.

1st Order Neurons

Relay information from peripheral sensory receptors to the spinal cord via peripheral nerves.

Aϐ

Discriminative touch, proprioception, vibration, pressure.

Aδ

Temperature, nociception (fast).

C

Nociception (slow).

Clinical Application

Each type of somatosensation has a unique pathway and destination; understanding helps with hypothesis, differential diagnosis, treatment plan, and prognosis.

Fine touch

Ability to localize and detect a specific stimulus.

Crude touch

Ability to sense that touch has occurred with limited specificity or localization.

2-Point Discrimination

Ability to resolve small differences in the location of a mechanical stimulus; related to receptor density and receptive field size.

Proprioception

Ability to sense muscle tension, limb or body position, limb or body movement; muscular mechanoreceptors with cutaneous stretch receptors.

Nociception

Detects tissue damage, ischemia, noxious stimuli; typically perceived as pain; meaning applied centrally; receptor determines modality.

Warm temperature receptors

Respond to temperatures above body temperature.

Cold temperature receptors

Respond to temperatures below body temperature; nociceptive thermoreceptors activated below 50°F or above 115-120 °F.

Dermatome

Area of the skin innervated by a specific spinal nerve (one level).

Peripheral nerve innervation

Area of the skin innervated by the cutaneous branch of a specific peripheral nerve (multiple spinal levels).

Peripheral Somatosensory System - Summary

Peripheral somatosensory receptors in skin (cutaneous) and muscles/joints (muscular); multiple receptor types for specific stimuli; axons are 1st order neurons classified by diameter and myelination; enter via dorsal root and synapse with 2nd order neurons in dorsal horn.

Ascending Somatosensory Tracts

Collection of myelinated axons traveling between a common origin and destination; named for origin and termination; includes DCML, spinothalamic, spinocerebellar, spinomesencephalic, spinoreticular, spinolimbic.

Primary Somatosensory Pathways

Dorsal Column/Medial Lemniscus (DCML) and Spinothalamic (anterolateral system).

Somatotopic organization

Specific areas within a neural structure are mapped for specific body regions; improves localization.

Fasciculus cuneatus

Carries information from upper trunk and arms; named for site of synapse in the medulla (nucleus cuneatus).

Fasciculus gracilis

Carries information from lower trunk and legs; named for site of synapse in the medulla (nucleus cuneatus).

Dorsal Column/Medial Lemniscus: Function

Carries afferent information about light (fine) touch, two-point discrimination, vibration, conscious proprioception.

Lateral Spinothalamic Pathway: Function

The lateral spinothalamic tract carries afferent information about temperature and nociception.

Anterior Spinothalamic Pathway: Function

The anterior spinothalamic tract carries afferent information about crude touch and pressure.

Somatosensory Cortex

Site where information from ascending tracts is brought into conscious awareness, located in the parietal lobe.

Homunculus

Describes the map of the brain regions where each body area is processed; related to receptor density.

Secondary Somatosensory Pathways

Slow nociceptive pathways with poorly localized or unconscious afferent information.

Slow Nociceptive Pathways

Experienced as a dull, achy, unlocalized pain; often trigger an efferent response.

Spinocerebellar Tracts: Function

Carries afferent information from touch and pressure receptors, muscle spindles, and Golgi tendon organs to the cerebellum for movement coordination and body positioning; unconscious processing.