SKELETAL MUSCLE RECEPTORS
Student Preparation
Textbook of Medical Physiology, 10 ed. Guyton and Hall, Chapter 54
Neuroscience, 2nd ed. 2001, Bear et al., Editors, pp. 449-464
Nerve Fiber Classification
• General classification scheme (Erlanger-Gasser):– A fibers: Myelinated
• Subtypes: some overlap in ranges• Fastest conducting and largest diameter – m/sec,
• “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 )
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
Erlanger-Gasser
Lloyd-Hunt
Nerve FiberClassification
Motor Unit and Neuron Pool• Skeletal muscles are
innervated by -motor neurons from the spinal cord ventral horn & brain stem
• Motor unit = muscle fibers innervated by one -motor neuron
• Neuron pool = all -motor neurons that innervate one muscle
Muscle Spindle & Golgi Tendon Organ - Proprioceptors
• Muscle spindle - fusiform shaped– 3-12 or more
intrafusal fibers– Parallel attachment
to sheaths of extrafusal skeletal muscle fibers
– Detect stretch of skeletal muscle
• Golgi tendon organ - capsular structure located in muscle tendons– Connected in series
to extrafusal skeletal muscle fibers
– Detect changes in muscle tension
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 – motor neurons
Intrafusal fibers – 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 motor neurons – to increase the sensitivity of muscle spindles
Motor innervation () of muscle spindle
• Dynamic gamma fiber (-d) - excites mainly nuclear bag intrafusal fibers
• Static gamma fiber (-s) - excites mainly nuclear chain fibers
Function of muscle spindle cont.
• Stimulation of -d fibers enhances the dynamic response of the primary afferent
• Stimulation of -s fibers enhances the static response of the secondary afferent
• Fusimotor fibers () 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 stimulation
Primary = dynamicmuscle spindle
Secondary responses to passive stretch without and with
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.