SKELETAL MUSCLE RECEPTORS Nerve Fiber Classification

• General classification scheme (Erlanger-Gasser): – A fibers: Myelinated • Subtypes: α, β, γ, δ, some overlap in ranges • Fastest conducting and largest diameter – α (120 m/sec, 20 μ) • “A” often dropped: alpha motor neuron – B fibers: Slower myelinated (seldom used) – C fibers: Unmyelinated • Slower conducting than As and smallest diameter (0.5 m/sec, 0.5 μ) Nerve Fiber Classification

• Sensory nerve classification (Lloyd- Hunt): – I, II, III fibers: Myelinated • Subtypes: Ia, Ib • Fastest conducting and largest diameter – Ia – IV fibers: Unmyelinated • Slower conducting than IIIs and smallest diameter Nerve Fiber Classification Erlanger-Gasser

Lloyd-Hunt Motor Unit and Neuron Pool • Skeletal muscles are innervated by α-motor neurons from the spinal cord ventral horn & brain stem • Motor unit = muscle fibers innervated by one α-motor neuron • Neuron pool = all α- motor neurons that innervate one muscle Muscle Spindle & Golgi Tendon Organ - Proprioceptors • Muscle spindle - • Golgi tendon organ - fusiform shaped capsular structure – 3-12 or more located in muscle intrafusal fibers tendons – Parallel attachment – Connected in series to sheaths of to extrafusal skeletal extrafusal skeletal muscle fibers muscle fibers – Detect changes in – Detect stretch of muscle tension skeletal muscle Muscle spindles are in parallel with extrafusal muscle fibers Muscle spindle Muscle Spindle Intrafusal Fibers

• Central region devoid of contractile elements • Contain several nuclei • Nuclear bag fibers -nuclei clumped in central region (dynamic responders) • Nuclear chain fibers - nuclei arranged in rows (static responders) Efferent innervation of skeletal muscles:

Extrafusal fibers – α motor neurons

Intrafusal fibers – γ motor neurons Sensory Innervation of Intrafusal Fibers • Primary spindle afferent – group Ia afferent, annulospiral ending; spirals around and innervates the central region of both nuclear bag and nuclear chain fibers • Secondary spindle afferent – group II afferent, flower spray ending; mostly innervate distal ends of nuclear chain fibers Sensory innervation of the muscle spindle Muscle spindle function

• Primary spindle afferents (Ia) - respond primarily to dynamic changes in muscle length (rate of change in length) • Secondary spindle afferents (II) - respond primarily to static changes in muscle length (steady state changes) • Contraction of extrafusal fibers causes collapse of intrafusal fibers with cessation of discharge Role of γ motor neurons – to increase the sensitivity of muscle spindles Motor innervation (γ) of muscle spindle • Dynamic gamma fiber (γ-d) - excites mainly nuclear bag intrafusal fibers • Static gamma fiber (γ-s) - excites mainly nuclear chain fibers Function of muscle spindle cont. • Stimulation of γ-d fibers enhances the dynamic response of the primary afferent • Stimulation of γ-s fibers enhances the static response of the secondary afferent • Fusimotor fibers (γ) contract intrafusal fibers (usually during active extrafusal contraction) and resensitizes them for accurate detection of muscle length over a wide range Recording arrangement Ia response to tendon tap Spindle responses to passive muscle stretch

dynamic

static Primary spindle responses to passive stretch without and with γ stimulation

Primary = dynamic muscle spindle Secondary responses to passive stretch without and with γ stimulation

Secondary = static muscle spindle Connections of Ia afferent fibers within the spinal cord

• Cell body in dorsal root ganglion • Alpha motor neuron in ventral gray • Single synapse • Motor effector same (homonymous) muscle • Purpose - oppose stretch and maintain tone or posture Connections of group II fibers in spinal cord

• Cell body in dorsal root ganglion • Alpha motor neuron in ventral gray • Single synapse • Motor effector same (homonymous) muscle • Purpose - oppose stretch and maintain tone or posture; locomotion Spinal reflex connections of Ia afferent fibers

• Excitatory to motor neurons of the homonymous muscle • Excitatory to neurons of synergistic muscles (facilitation) • Inhibitory to neurons of antagonistic muscle

Summary of spindle physiology

• Muscle spindle afferents and gamma motor efferents allow the CNS to adjust the contraction of skeletal muscle for changes in muscle length. The spindle is active in the stretch reflex. Golgi tendon organ

• Golgi tendon organ - capsular structure located in muscle tendons • Connected in series to extrafusal skeletal muscle fibers • Detects changes in muscle tension

Golgi tendon organ

Ib afferent Excess muscle stretch (tension) fires Golgi tendon organ

muscle spindle

Golgi tendon organ Contrast Golgi tendon organ and primary spindle during contraction

• Golgi tendon organ (Ib) responds to increased tension during extrafusal muscle contraction • Muscle spindle (Ia) is silent during muscle contraction Spinal connections of Ib afferent fibers • Inhibitory to neurons of homonymous m. • Inhibitory to neurons of synergistic muscles • Excitatory to neurons of antagonistic muscle • Purpose - oppose development of excessive muscle tension Summary of Golgi tendon organ physiology

• Contraction or excessive lengthening of extrafusal fibers cause discharge of impulses in afferents of Golgi tendon organ. • Afferents of group Ib fibers are inhibitory to alpha motor neurons of homonymous muscle, synergist muscle and excitatory to neurons of antagonistic muscle.