1. PHYSIOLOGY 1 LECTURE 20 SKELETAL MUSCLE MOTOR CONTROL

2. SKELETAL MUSCLE MOTOR CONTROL Objectives: The student should learn: Objectives: The student should learn: 1. The muscle control system (afferents and efferents) 1. The muscle control system (afferents and efferents) 2. The feedback information required for limb movement 2. The feedback information required for limb movement 3. Muscle receptors (muscle spindles, Golgi tendon bodies, free nerve endings) 3. Muscle receptors (muscle spindles, Golgi tendon bodies, free nerve endings) 4. Afferent / efferent nerve axon types, receptors, and sensitivity 4. Afferent / efferent nerve axon types, receptors, and sensitivity

3. SKELETAL MUSCLE MOTOR CONTROL 5. Types of muscle spindle fusiform fibers and function 5. Types of muscle spindle fusiform fibers and function 6. Golgi tendon bodies sensitivity 6. Golgi tendon bodies sensitivity 7. Overall muscle control system 7. Overall muscle control system

4. SKELETAL MUSCLE MOTOR CONTROL Introduction: Introduction: –Skeletal muscle control is not a simple matter of command move but rather a complex series of continual feedback loops which supply the brain with continual information on

5. SKELETAL MUSCLE MOTOR CONTROL Joint position Joint position State of muscle contraction State of muscle contraction –Agonists muscle – concentric contraction –Antagonists muscle – eccentric contraction Degree of tension on tendons Degree of tension on tendons Strain and tension on joints Strain and tension on joints

6. SKELETAL MUSCLE MOTOR CONTROL –The feedback information supplies the brain with exact information on Limb position (all the above) Limb position (all the above) Ground conditions (Visual) Ground conditions (Visual) Surroundings (Visual, hearing, touch) Surroundings (Visual, hearing, touch) Continual estimates of weights to be lifted or moved Continual estimates of weights to be lifted or moved

7. SKELETAL MUSCLE MOTOR CONTROL  Speed of limb movement Momentum Momentum Pain Pain Muscle temperature Muscle temperature Chemical environment of the muscle fibers Chemical environment of the muscle fibers Degree of muscle recruitment Degree of muscle recruitment

8. SKELETAL MUSCLE MOTOR CONTROL Allows the motor cortex, cerebellum and spinal column to continually adjust required estimates on force, speed, position, and limb angles needed to accomplish the task required or in the case of repetitive movements to initialize the reverberatory spinal reflexes to accomplish the movement. Allows the motor cortex, cerebellum and spinal column to continually adjust required estimates on force, speed, position, and limb angles needed to accomplish the task required or in the case of repetitive movements to initialize the reverberatory spinal reflexes to accomplish the movement.

9. SKELETAL MUSCLE MOTOR CONTROL Receptors in muscle control Receptors in muscle control –Sensory Receptors – (Afferents) The classic studies of Ruffini, Golgi, and Sherrington showed that skeletal muscle was richly endowed with a large variety of sensory receptors.

10. Receptors in muscle control Muscle spindles Muscle spindles Golgi tendon organ Golgi tendon organ Secondary spindle endings Secondary spindle endings Nonspindle endings Nonspindle endings Free nerve endings Free nerve endings

11. Receptors in muscle control –Control Innervations – (Efferent) These nerve fibers provide skeletal muscle with the necessary commands to maintain continual information flow from the brain, midbrain, and spinal column (Efferent) These nerve fibers provide skeletal muscle with the necessary commands to maintain continual information flow from the brain, midbrain, and spinal column –Alpha motor neuron –Beta fibers (skeletofusimotor) –Gamma fibers (fusimotor)

12. Receptors in muscle control Afferent axon type: Receptor: Sensitive to: Group 1a Primary spindle endings Muscle length and rate of change of length Group 1b Golgi tendon organ Muscle tension

13. Receptors in muscle control Group II Secondary spindle endings Muscle length (little rate sensitivity) Group II Nonspindle endings Deep pressure Groups III & IV Free nerve endings Pain, chemical stimuli, and temperature

14. Receptors in muscle control Efferent axon type: InnervationFunction Alpha motor neuron (Skeletomuscle) Extrafusal muscle fibers Control tension of muscles, strength and rate of pull on skeleton

15. Receptors in muscle control Beta fibers (Skeletofusimotor ) Intrafusal muscle fibers (collateral from alpha motor neuron) Control sensitivity of muscle spindles – no independent control of muscle spindles Gamma motor neurons (fusimotor) Intrafusal fibers Control sensitivity of spindles independent of alpha motor neurons - unloading does not impair information

16. Receptors in muscle control Muscle Spindles – Encapsulated sensory receptor whose complexity is as great as the eye – each individual muscle contains several muscle spindles – a general rule is that muscles requiring fine delicate control have a greater density of muscle spindles (intravertebral muscles) than do gross muscles (gluteus maximums). They are randomly distributed deep within the belly of the muscle Muscle Spindles – Encapsulated sensory receptor whose complexity is as great as the eye – each individual muscle contains several muscle spindles – a general rule is that muscles requiring fine delicate control have a greater density of muscle spindles (intravertebral muscles) than do gross muscles (gluteus maximums). They are randomly distributed deep within the belly of the muscle

19. Receptors in muscle control Capsule – Muscle spindles are encapsulated receptors with fusiform (spindle) shape and range from 2 to 10 mm in length and 0.5 mm to 1 mm in diameter containing a gel perhaps to lubricate the intrafusal fibers. Capsule is connected to the extrafusal skeletal muscle fibers and to intrafusal fibers. Capsule – Muscle spindles are encapsulated receptors with fusiform (spindle) shape and range from 2 to 10 mm in length and 0.5 mm to 1 mm in diameter containing a gel perhaps to lubricate the intrafusal fibers. Capsule is connected to the extrafusal skeletal muscle fibers and to intrafusal fibers.

21. Receptors in muscle control Intrafusal fibers – specialized muscle fibers contained within the muscle spindle capsule and embedded at random within the extrafusal skeletal muscle fibers Intrafusal fibers – specialized muscle fibers contained within the muscle spindle capsule and embedded at random within the extrafusal skeletal muscle fibers –Range from 2 to 12 per capsule –Intrafusal fibers are arranged in parallel with themselves and with the extrafusal fibers

22. Receptors in muscle control –Intrafusal fiber types 1] Nuclear bag fibers – contain numerous nuclei concentrated near the center of the fiber and lying parallel with each other – 2 types – one of each in each muscle spindle 1] Nuclear bag fibers – contain numerous nuclei concentrated near the center of the fiber and lying parallel with each other – 2 types – one of each in each muscle spindle a] Dynamic fiber – motion sensor a] Dynamic fiber – motion sensor b] Static fiber – position sensor b] Static fiber – position sensor 2] Nuclear chain fibers- contain nuclei lined up in series – varying numbers usually 5 in mammals – degree of stretch and strain 2] Nuclear chain fibers- contain nuclei lined up in series – varying numbers usually 5 in mammals – degree of stretch and strain

23. Receptors in muscle control d. Central regions of each fiber are non contractile and innervated by one or more neurons wrapped around the intrafusal fiber (annulospiral endings – primary spindle endings and secondary endings) d. Central regions of each fiber are non contractile and innervated by one or more neurons wrapped around the intrafusal fiber (annulospiral endings – primary spindle endings and secondary endings) e. Polar regions of the intrafusal fibers are contractile – pull on central region from both ends e. Polar regions of the intrafusal fibers are contractile – pull on central region from both ends

25. Receptors in muscle control Primary spindle endings (afferents) – Group 1a fiber axon – dendritic endings wrap around all Intrafusal fibers (dynamic and static nuclear bag fibers, and all nuclear chain fibers) - may be one or more but usually just one - and provide feedback information on muscle length and rate of change in muscle length (stretch) – nerve fires continually increase firing rate when stretched and decrease firing rate when relaxed - information on developed muscle tension (force), speed of contraction, and momentum Primary spindle endings (afferents) – Group 1a fiber axon – dendritic endings wrap around all Intrafusal fibers (dynamic and static nuclear bag fibers, and all nuclear chain fibers) - may be one or more but usually just one - and provide feedback information on muscle length and rate of change in muscle length (stretch) – nerve fires continually increase firing rate when stretched and decrease firing rate when relaxed - information on developed muscle tension (force), speed of contraction, and momentum

27. Receptors in muscle control Secondary spindle endings (afferents) - Group II fiber axon – dendritic endings wrap around nuclear chain fiber and static nuclear bag fiber intrafusal fibers – may be as many as 5 dendrites on each fiber and provide feedback on muscle length – nerve fires continually stretch increases firing rate and relaxation decreases firing rate but differs from primary firing rate– gives information on joint angles and muscle length even when muscle is relaxed Secondary spindle endings (afferents) - Group II fiber axon – dendritic endings wrap around nuclear chain fiber and static nuclear bag fiber intrafusal fibers – may be as many as 5 dendrites on each fiber and provide feedback on muscle length – nerve fires continually stretch increases firing rate and relaxation decreases firing rate but differs from primary firing rate– gives information on joint angles and muscle length even when muscle is relaxed

29. Receptors in muscle control Beta fiber axon endings (efferents) – Synapse with intrafusal muscle fibers – these are collateral axons from the alpha motor neuron – adjust tension in the intrafusal fibers to maintain sensitivity as the fiber contracts Beta fiber axon endings (efferents) – Synapse with intrafusal muscle fibers – these are collateral axons from the alpha motor neuron – adjust tension in the intrafusal fibers to maintain sensitivity as the fiber contracts

31. Receptors in muscle control Gamma motor neurons (efferents) - Synapse with intrafusal muscle fibers near polar regions – Control spindle sensitivity in unloaded state (non contractile) Gamma motor neurons (efferents) - Synapse with intrafusal muscle fibers near polar regions – Control spindle sensitivity in unloaded state (non contractile)

34. Receptors in muscle control D. Golgi Tendon Organs - Encapsulated sensory receptor – much simpler than the muscle spindles – D. Golgi Tendon Organs - Encapsulated sensory receptor – much simpler than the muscle spindles – Capsule – Golgi tendon organs are thin encapsulated receptors with an elongated tube shape approximately 1 mm in length and 0.1 mm in diameter Capsule – Golgi tendon organs are thin encapsulated receptors with an elongated tube shape approximately 1 mm in length and 0.1 mm in diameter

35. Receptors in muscle control Type 1b nerve fiber enters the capsule and splits into hundreds of dendritic endings which are associated with collagen fibers at the muscle tendon junction – nerve fires continually – increased firing rate with increased tension and decreased rate with relaxation Type 1b nerve fiber enters the capsule and splits into hundreds of dendritic endings which are associated with collagen fibers at the muscle tendon junction – nerve fires continually – increased firing rate with increased tension and decreased rate with relaxation Collagen fibers twist together at this point so that the Golgi tendon body may associate with as many as 15 different muscle motor units Collagen fibers twist together at this point so that the Golgi tendon body may associate with as many as 15 different muscle motor units

36. Receptors in muscle control Sensitive to minute changes in muscle tension - Stretch of the tendons or passive changes in tension Sensitive to minute changes in muscle tension - Stretch of the tendons or passive changes in tension

37. Receptors in muscle control Free Nerve Endings – Skeletal muscle is richly endowed with numerous free nerve endings (type II, III, and IV fiber types). However very little is known about these other than type and conjecture as to function. Free Nerve Endings – Skeletal muscle is richly endowed with numerous free nerve endings (type II, III, and IV fiber types). However very little is known about these other than type and conjecture as to function.

38. Receptors in muscle control Type II fibers – respond mostly to pressure – may be giving information on blood flow and tissue pressures Type II fibers – respond mostly to pressure – may be giving information on blood flow and tissue pressures Type III fibers – respond to pain, temperature, and chemical stimuli Type III fibers – respond to pain, temperature, and chemical stimuli Type IV fibers - temperature and chemical stimuli Type IV fibers - temperature and chemical stimuli There is some evidence that these fibers may also act on the vasomotor and pulmonary centers in the medulla, to signal for increased blood flow and increased breathing rate during exercise. There is some evidence that these fibers may also act on the vasomotor and pulmonary centers in the medulla, to signal for increased blood flow and increased breathing rate during exercise.

39. Receptors in muscle control

42. REFLEX CONTROL Reflex control – When a skeletal muscle is passively stretched it responds with a momentary contraction (Knee jerk reaction to a hammer strike just below the patella) Reflex control – When a skeletal muscle is passively stretched it responds with a momentary contraction (Knee jerk reaction to a hammer strike just below the patella) –Classic experiments of Sherrington and Lindel (1924 – 30) on decerebrate cats showed that the stretch reflex has two components and showed reciprocal and synergist innervation

43. REFLEX CONTROL Brisk phasic component – initiatial stretch of the muscle results in a brief but fairly powerful contraction of the skeletal muscle Weaker but longer lasting component triggered by the new static stretch of the lengthen skeletal muscle fibers Reciprocal innervation – stretch of the agonist muscle fibers produced relaxation in the antagonists muscle

44. REFLEX CONTROL Synergist innervation – stretch of the agonists muscle fibers also produced contractions in all synergistic muscles

45. REFLEX CONTROL –Lloyd and Eccles (1946 – 48) showed that group 1a axons from the muscle spindles pass into the dorsal root of the spinal cord and split into thousands of collaterals Group 1a collaterals form an excitatory synapse with every alpha motor neuron acting on the original muscle Group 1a collaterals form an excitatory synapse with every alpha motor neuron acting on the original muscle

46. REFLEX CONTROL Group 1a collaterals form an excitatory synapse with all synergistic muscle alpha motor neurons (Synergist innervation) Group 1a collaterals form an excitatory synapse with all synergistic muscle alpha motor neurons (Synergist innervation) Group 1a collaterals form an excitatory synapse with inhibitory interneurons which synapse with all antagonist muscle alpha motor neurons (Reciprocal innervation) Group 1a collaterals form an excitatory synapse with inhibitory interneurons which synapse with all antagonist muscle alpha motor neurons (Reciprocal innervation)

47. REFLEX CONTROL –Stretch reflex tends to resists passive lengthing of the muscles from their original resting position –Chiropractically this reflex can be used to test for vertebral and nerve damage – compression or extension of the dorsal root would tend to slow or block the reflex

48. REFLEX CONTROL –Flexion reflex afferent pathways – Group II fibers from the secondary muscle spindle nerves and some free nerve endings (group III and IV fibers) seem to stimulate homogenous alpha motor neurons and inhibit antagonist muscle alpha motor neurons

49. REFLEX CONTROL Motor control mechanism Motor control mechanism –Central motor command – Alpha motor neuron Single action potential produces a sub max twitch Single action potential produces a sub max twitch Collaterals synapse on agonists muscle alpha motor neurons and on antagonists alpha motor neurons – agonists muscle begins contraction and antagonist muscle begins contraction at same time Collaterals synapse on agonists muscle alpha motor neurons and on antagonists alpha motor neurons – agonists muscle begins contraction and antagonist muscle begins contraction at same time

50. REFLEX CONTROL

51. Feedback from muscle spindles, Golgi tendon organ, and free nerve endings constantly modify central motor command to properly position limb in space and time Feedback from muscle spindles, Golgi tendon organ, and free nerve endings constantly modify central motor command to properly position limb in space and time Feedback from muscle spindles (Group 1a fibers) excites synergistic muscles to contract in concert Feedback from muscle spindles (Group 1a fibers) excites synergistic muscles to contract in concert

52. REFLEX CONTROL Feedback from muscle spindles (Group 1a fibers) Feedback from muscle spindles (Group 1a fibers) cause inhibition of antagonists muscle alpha motor neurons – begins to relax antagonists muscle fibers to allow eccentric contraction cause inhibition of antagonists muscle alpha motor neurons – begins to relax antagonists muscle fibers to allow eccentric contraction Secondary command continues movement while making necessary adjustments based on feedback information to properly align speed and force of the movement Secondary command continues movement while making necessary adjustments based on feedback information to properly align speed and force of the movement

54. SKELETAL MUSCLE MOTOR CONTROL Summary Summary 1. What is the difference between intrafusal and extrafusal fibers? 1. What is the difference between intrafusal and extrafusal fibers? 2. How does fiber type tie into function? (chart) 2. How does fiber type tie into function? (chart) 3. What is a muscle spindle body and how does it operate? 3. What is a muscle spindle body and how does it operate? 4. What is a Golgi tendon body and how does it operate? 4. What is a Golgi tendon body and how does it operate?

55. SKELETAL MUSCLE MOTOR CONTROL Summary cont. Summary cont. 5. What is reflex control, how does it operate? 5. What is reflex control, how does it operate? 6. How does skeletal muscle control operate, affect on synergistic muscle, agonist muscle and antagonists muscle? 6. How does skeletal muscle control operate, affect on synergistic muscle, agonist muscle and antagonists muscle?