Chapter 4: The Neuromuscular Basis of Human Motion

1. Chapter 4:The Neuromuscular Basis of Human Motion Lecture Notes

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 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 I. 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 Is a single nerve cell consisting of a cell body and one or more projections

6. Motor Neurons Situated in anterior horns of spinal cord Dendrite that synapse with sensory neurons Axon emerges from 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

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

9. Nerves A bundle of fibers, enclosed within a connective tissue sheath, for transmission of impulses A typical spinal nerve consist of Motor outgoing fibers Sensory incoming 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

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

12. The Synapse Connection between neurons May be thousands between any two neurons Proximity of the membrane of one axon to another dendrites The more often a synapse is used the faster a signal will pass through it The greater the number of synapses for receptor to effector, the longer the time form stimulus to response

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

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 if the postsynaptic neuron will produce an action potential

15. THE MOTOR UNIT (MU) Consist of a single MU and all the muscle fibers its axon supplies All muscle fibers in a MU are of the same muscle fiber type

16. Size of Motor Units Vary widely in the number of muscle fibers Gastrocnemius: 2,000 or more muscle fibers Eye muscles: may have fewer than 10 fibers Small ratio of muscle fibers to MU is capable of more precise movements Size of MU has 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? 1. Number of motor units that are activated 2. Frequency of stimulation

18. All-or-None PrincipleRecruitment of Motor Units All-or-None Principle: If the stimulus is of threshold value, all muscles of MU will contract Applies to muscle fibers not whole muscle MU recruitment: has an orderly sequence to Smaller slow twitch fibers are recruited first They have lower thresholds Larger fast twitch fibers are recruited later They have higher thresholds

19. Frequency of Stimulation At low frequency, muscle fibers relax between impulses At high frequency, 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 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

21. Proprioceptors Respond to degree, direction, & rate of change of body movements Transmit information to CNS Muscle receptors Joint & skin receptors

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

23. Muscle ProprioceptorsMuscle Spindles Extrafusal fibers “regular” muscle fibers Intrafusal fibers muscle fibers inside spindles Noncontractile central portion

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 to static changes Flower-spray endings: at end of noncontractile midsection Respond to static muscle length Impulses directly proportional to 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

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 Predict where body part will be at any time Appropriate adjustment in position can be anticipated and effected

28. Joint and Skin ProprioceptorsRuffini Endings In deep layers of skin and joint capsule Activated by mechanical deformation Signal continuous states of pressure Adapt slowly, then transmit a steady signal 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 Cochlea: is concerned with hearing Semicircular canals: sense balance Labyrinth filled with fluid and is lined with hair cells, senses motion of fluid as head moves Joint receptors of the neck: sensitive to angle between the body and the head Prevent labyrinthine proprioceptors from producing feeling of imbalance

31. REFLEX MOVEMENT A specific pattern of response without volition form the cerebrum Receptor organ, Sensory neuron, Motor neuron, Muscle Connector neurons

32. Exteroceptive ReflexesExtensor Thrust Reflex Pressure on sole of stimulates reflex contraction of extensor muscles