Chapter 4: The Neuromuscular Basis of Human Motion

1. Chapter 4:The Neuromuscular Basis of Human Motion KINESIOLOGY Scientific Basis of Human Motion, 11th edition Hamilton, Weimar & Luttgens Presentation Created by TK Koesterer, Ph.D., ATC Humboldt State University Revised by Hamilton & Weimar

2. Objectives 1. Name and describe the function of the basic structure of the nervous system. 2. Explain how gradations in strength of muscle contraction and precision of movements occur. 3. Name and define the receptors important in musculoskeletal movement. 4. Explain how the various receptors function, and describe the effect each has on musculoskeletal movement.

3. Objectives 5. Describe reflex action, and enumerate and differentiate among the reflexes that affect musculoskeletal action. 6. Demonstrate a basic understanding of volitional movement by describing the nature of the participation of the anatomical structures and mechanisms. 7. Perform an analysis of the neuromuscular factors influencing the performance of a variety of motor skills.

4. THE NERVOUS SYSTEM AND BASIC NERVE STRUCTURES • Central nervous system (CNS) A. Brain B. Spinal cord II. Peripheral nervous system (PNS) A. Cranial nerves (12 pairs) B. Spinal nerves (31 pairs) III. Autonomic nervous system A. Sympathetic B. Parasympthetic

5. Neurons • A single nerve cell consists of a cell body and one or more projections. • Axons: Carry impulses away from cell body. • Dendrites: Carry impulses toward cell body. Fig 4.1

6. Motor Neurons • Situated in anterior horns of spinal cord • Dendrite that synapses with sensory neurons or connector neurons. • Axon emerges from the spinal cord, travels by way of a peripheral nerve to muscle. • Each terminal branch ends at the motor end plate of a single muscle fiber

7. Sensory Neurons • Situated in a dorsal root ganglion just outside the spinal cord. • Neuron may terminate in spinal cord or brain. • A long peripheral fiber comes from a receptor. Fig 4.1b

8. Connector Neurons • Exist completely within the CNS. • Serve as connecting links. • May be a single neuron, connecting sensory to motor neurons. OR • An intricate system of neurons, whereby a sensory impulse may be relayed to many motor neurons.

9. Nerves • A bundle of fibers, enclosed within a connective tissue sheath, for transmission of impulses. • A typical spinal nerve consists of: • Motor, outgoing (efferent) fibers • Sensory, incoming (afferent) fibers

10. Nerves • Each spinal nerve is attached to spinal cord by an anterior (motor) root and a posterior (sensory) root • Posterior root bears a ganglion – a collection of cell bodies Fig 4.2

11. Spinal Nerves • 31 pairs – exit both sides of the vertebral column • 8 Cervical • 12 Thoracic • 5 Lumbar • 5 Sacral • 1 Coccyx • Table 4.1 outlines spinal innervation patterns

12. The Synapse • Connection between neurons. • May be thousands between any two neurons. • Is a proximity of the membrane of an axon to the membrane of a dendrite or cell body. • The more often a synapse is used the faster a signal will pass through it. • The greater the number of synapses from receptor to effector, the longer the time from stimulus to response.

13. The Synapse • Transmission across the synapse depends on a chemical transmitter. • Substance diffuses the synapse and produces an action potential in the postsynaptic neuron. Fig 4.3

14. Action Potentials • Threshold level is the minimum level of stimulus (chemical transmitter) necessary to initiate or propagate a signal. • Facilitation – an excitatory stimulus • Inhibition – an inhibitory stimulus • Stimulus may be from more than one neuron • The sum total of excitatory and inhibitory determine impulses determine if the postsynaptic neuron will produce an action potential.

15. THE MOTOR UNIT • Consists of a single motor neuron and all the muscle fibers its axon supplies • All muscle fibers in a motor unit are of the same muscle fiber type Fig 4.4

16. Size of Motor Units • Vary widely in the number of muscle fibers. • Gastrocnemius: 2,000 or more muscle fibers per motor unit. • Eye muscles: may have fewer than 10 fibers per motor unit. • A small ratio of muscle fibers to motor neurons is capable of more precise movements. • Size of the motor unit has a direct bearing on the precision of movement.

17. Gradations in the Strength of Muscular Contractions • Experience tells us that the same muscles contract with various gradations of strength. How do they adjust to such extremes? • Number of motor units that are activated. • Frequency of stimulation.

18. Gradations in the Strength of Muscular Contractions • All-or-None Principle: If the stimulus is of threshold value, all muscle fibers of the motor unit will contract. • Motor unit recruitment: has an orderly sequence: • Smaller slow twitch fibers are recruited first. • They have lower thresholds • Larger fast twitch fibers are recruited later. • They have higher thresholds

19. Gradations in the Strength of Muscular Contractions • At low frequency stimuli, muscle fibers relax between impulses. • At high frequency stimuli, fibers do not have time to relax and result in summation or maximal contraction. • A combination of maximum number of fibers stimulated and high frequency of stimuli results in a maximal strength of contraction.

20. SENSORY RECEPTORS • Respond to different stimuli • Exteroceptors: near body surface stimuli come from outside the body. • Interoceptors: sense heat, cold, pain and pressure. Fig 4.5

21. Proprioceptors • Stimulated by body movements. • Transmit information to CNS. • Two primary categories: • Muscle receptors • Joint & skin receptors Fig 4.6

22. Muscle ProprioceptorsMuscle Spindles • Located in muscle belly, parallel with fibers. • When stretched, sensory nerve sends impulses to CNS, which activates the motor neurons facilitating contraction of the same muscle. • More spindles are located in muscles controlling precise movements.

23. Muscle ProprioceptorsMuscle Spindles • Extrafusal fibers “regular” muscle fibers. • Intrafusal fibers muscle fibers inside spindles. • Non-contractile central portion Fig 4.7

24. Muscle ProprioceptorsMuscle Spindles • Spindles contains two type of nerve endings. • Primary or annulospiral endings: coiled around noncontractile midsection. • Sensitive to velocity of change (phasic). • Sharp decline in impulses with static change. • Flower-spray endings: at end of noncontractile midsection. • Respond to static muscle length. • Impulses directly proportional to change in length.

25. Muscle ProprioceptorsMuscle Spindles • Gamma motor neurons: stimulate the intrafusal fibers to contract, shortening the muscle spindle.

26. Muscle ProprioceptorsGolgi Tendon Organ (GTO) • Embedded “in series” in the tendon. • As tension in tendon increases GTO is activated. • Signals CNS to relax muscle. • Protective mechanism. Fig 4.8

27. Joint and Skin ProprioceptorsPacinian Corpuscles • In regions around joint capsules, ligament, and tendons sheaths. • End-organ has concentric layers of capsule. • Activated by joint angle changes & pressure. • Transmits impulses for only a very brief time.

28. Joint and Skin ProprioceptorsRuffini Endings • In deep layers of skin and joint capsule. • Activated by mechanical deformation. • Stimulated strongly by sudden joint movement. • Sense joint position and changes in joint angle. • The CNS knowing which receptors is stimulated can tell the joint angle.

29. Joint and Skin ProprioceptorsCutaneous Receptors • Meissner corpuscles: touch • Pacinian corpuscles: pressure • Free nerve endings: pain • Serve as proprioceptors when they show sensitivity to texture, hardness, softness and shape, and participate in reflexes.

30. Labyrinthine and Neck Proprioceptors • Labyrinthine system: Concerned with sense of balance. • Consists of utricle, saccule, and semicircular canals. • Filled with endolymph whose motion triggers hair cells. • Contain otoliths that shift with gravity. • Utricle: sensitive to linear acceleration and head position with respect to gravity. • Semicircular canals: sensitive to angular acceleration. • Three canals oriented in different planes at right angles to each other.

31. Labyrinthine and Neck Proprioceptors Joint receptors of the neck: sensitive to angle between the body and the head. • Prevent labyrinthine proprioceptors from producing feeling of imbalance.

32. REFLEX MOVEMENT • A specific pattern of response without volition from the cerebrum. • Stimulus - receptor organ - sensory neuron - motor neuron - muscle (response) • Connector neurons often used. Fig 4.10

33. Exteroceptive ReflexesExtensor Thrust Reflex • Stimulus - pressure. • Receptor -Pacinian corpuscles. • Response - contraction of the extensor muscles of that limb. Fig 4.11

34. Exteroceptive Reflexes • Flexor Reflex: • Stimulus - pain, noxious stimuli. • Receptor - free nerve endings. • Response - Quick withdrawal from source of pain (flexion). • Crossed Extensor Reflex: • Stimulus - flexor reflex or unweighting • Response - opposite extensors contract for support.

35. Proprioceptive ReflexesStretch Reflex • A reflex contraction of stretched muscle and synergists and relaxation of antagonists. • Phasic Response: • Stimulus - high velocity stretch. • Receptor - annulospiral endings of muscle spindle. • Response - facilitates proportional contraction of stretched muscle.

36. Proprioceptive ReflexesStretch Reflex • Knee jerk response • Sudden addition of weight to hand, elbow at 90°- • Biceps stretches, then contracts. Weight dropped Muscle contracts

37. Proprioceptive ReflexesStretch Reflex • Tonic Response: • Stimulus - Slow, sustained stretch(>.02 s). • Receptor - flower spray endings of muscle spindle. • Response - gamma efferent system resets spindle tension using intrafusal fiber contraction or relaxation.

38. Proprioceptive ReflexesStretch Reflex • Phasic Response Application: • Phasic preparatory phase can take advantage of the stretch reflex. • Results in a stronger contraction. • Postural control feedback. Fig 4.13a

39. Proprioceptive ReflexesStretch Reflex • Tonic Response Application: • Slow preparatory phase should be used when the desired outcome is accuracy. • Result in a low level, sustained contraction. Fig 4.13b

40. Tendon Reflex • Stimulus - high level of stretch, due to muscle stretch, or muscle contraction. • Receptor - Golgi tendon organ. • Response - relaxation of stretched muscle and facilitation of antagonist. • Feedback mechanism to control tension. • May effect skills of beginners until GTO threshold develops.

41. Labyrinthine and Neck Reflexes • Righting Reflex: • Stimulus - head not upright with respect to gravity. • Receptor - utricle and semicircular canals. • Response - bring the head to the upright position. • This reflex is integrated with movements of the arms and legs as described in the tonic neck and labyrinthine reflexes.

42. Labyrinthine and Neck Reflexes • Tonic Neck Reflex (TNR): • Symmetrical response: • Stimulus - head/neck(H/N) flexion/hyperextension. • Receptor - neck receptors. • Response - H/N flexion facilitates upper extremity flexion; H/N hyperextension facilitates upper extremity extension.

43. Labyrinthine and Neck Reflexes • Tonic Neck Reflex (TNR): • Asymmetrical response: • Stimulus - head/neck rotation. • Receptor - neck receptors. • Response - H/N rotation facilitates upper extremity chin side abduction & extension; back of head side adduction and flexion. • In both TNR responses, action in the lower extremities is in opposition to the upper extremities. • The TNR is a primitive reflex and not easily elicited in adults. May still be observed to be facilitory (throwing motions) or inhibitory (golf drive).

44. Labyrinthine and Neck Reflexes • Tonic labyrinthine reflexes: • Response is in opposition to the TNR. • Most apparent in the lower extremity.

45. VOLITIONAL MOVEMENTCNS: Levels of Control 1. Cerebral cortex: where consciousness occurs, initiation of voluntary movement. 2. Basal ganglia: responsible for homeostasis, coordination & some learned acts of posture. 3. Cerebellum “little brain”: key role in sensory integration, regulates timing & intensity of muscle contraction.

46. VOLITIONAL MOVEMENTCNS: Levels of Control 4. Brain stem: arousal and monitoring of physiological parameters, key facilitory and inhibitory centers. • Spinal cord: contains cell bodies of lower motor neurons, common pathway between CNS & PNS, final point for integration and control. Functions of the 5 levels overlap depending on classification scheme used

47. VOLITIONAL MOVEMENTPyramidal & Extrapyramidal Tracts • The two tracts originate in cerebral cortex and end at the spinal cord. • Pyramidal Tracts: predominately axons of motor neurons, controls muscles for precision. • Extrapyramidal Tracts: synapses with all levels of CNS, functions in stabilization and control of posture.

48. Kinesthesis • The conscious awareness of position of body parts and the amount and rate or joint movement. • Without rapid transmission & processing, accurately controlled movements could not proceed. • Kinesthetic perception and memory are the basis for voluntary movement and motor learning.

49. Reciprocal Inhibition • When motor neurons are transmitting impulses to an agonist, antagonistic are simultaneously & reciprocally inhibited. • Antagonists remain relaxed & agonists contract without opposition. • Automatic in reflexes & familiar movements. • In more complicated movements this depends on the degree of skill developed by performer.

50. Coactivation or Reciprocal Activation • Most frequently appears in movement when there is uncertainty about movement task. • Practice increases familiarity, and coactivation decreases in favor of reciprocal inhibition. • Efficiency of movement increases. • Coactivation also occurs to maintain joint stiffness.