1. SKELETAL MUSCLE RECEPTORS

2. Student Preparation Textbook of Medical Physiology, 10 ed. Guyton and Hall, Chapter 54 Neuroscience, 2nd ed. 2001, Bear et al., Editors, pp. 449-464

3. 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  )

4. 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

5. Erlanger-Gasser Lloyd-Hunt Nerve Fiber Classification

6. 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

7. 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

8. Muscle spindles are in parallel with extrafusal muscle fibers

9. Muscle spindle

10. 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)

11. Efferent innervation of skeletal muscles: Extrafusal fibers –  motor neurons Intrafusal fibers –  motor neurons

12. 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

13. Sensory innervation of the muscle spindle

14. 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

15. Role of  motor neurons – to increase the sensitivity of muscle spindles

16. Motor innervation (  ) of muscle spindle Dynamic gamma fiber (  -d) - excites mainly nuclear bag intrafusal fibers Static gamma fiber (  -s) - excites mainly nuclear chain fibers

17. 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

18. Recording arrangement

19. Ia response to tendon tap

20. Spindle responses to passive muscle stretch dynamic static

21. Primary spindle responses to passive stretch without and with  stimulation Primary = dynamic muscle spindle

22. Secondary responses to passive stretch without and with  stimulation Secondary = static muscle spindle

23. 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

24. 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

25. 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

27. 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.

28. 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

31. Golgi tendon organ Ib afferent

32. Excess muscle stretch (tension) fires Golgi tendon organ muscle spindle Golgi tendon organ

33. 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

34. 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

35. 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.