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The Role of the Nervous System

The Role of the Nervous System. Applied Kinesiology 420:151. Agenda. Introduction to the nervous system Structural considerations Motor efferents and gradations of force Sensory afferents Reflex movement. Introduction to the NS. Functions: Sensory input  afferent neurons Integration

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The Role of the Nervous System

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  1. The Role of the Nervous System Applied Kinesiology 420:151

  2. Agenda • Introduction to the nervous system • Structural considerations • Motor efferents and gradations of force • Sensory afferents • Reflex movement

  3. Introduction to the NS • Functions: • Sensory input  afferent neurons • Integration • Motor output  efferent neurons • Properties: • Irritability • Conductivity

  4. Introduction to NS • Levels of Control: • Cerebral cortex • Consciousness • Basal ganglia • Homeostasis  posture and equilibrium • Cerebellum • Timing and intensity  smooth and precise motion • Brain stem • Arousal and cardiorespiratory function • Spinal cord • Link b/w CNS and PNS  interneurons and synapses

  5. Cerebral cortex Basal ganglia Cerebellum Brain stem Spinal cord Overide? Figure 4.14, Hamilton

  6. Introduction to the NS • Basic divisions of the nervous system:

  7. Figure 14.1, Marieb & Mallett (2003)

  8. Agenda • Introduction to the nervous system • Structural considerations • Motor efferents and gradations of force • Sensory afferents • Reflex movement

  9. Structural Considerations • The neuron • The nerve • The synapse • The motor unit

  10. The Neuron • Functional unit of nervous tissue • Three main types of neurons • Sensory/afferent neurons • Motor/efferent neurons • Interneurons • Common structures

  11. Figure 12.11, Marieb & Mallett (2003) Dendrites Cell Body Axon Differences: Peripheral body, location of dendrites/synaptic knobs, direction of transmission

  12. The Neuron • Other considerations • Cell body • Nucleus • Almost all cell bodies are in spinal cord (ganglia?) • Dendrites • Afferents  cell body via peripheral body • Efferents  cell body via axon • Axon • Myelin sheath • Axon collaterals • Extensive terminal branching (10,000) • Synaptic knobs

  13. Figure 12.4, Marieb & Mallett (2003)

  14. Structural Considerations • The neuron • The nerve • The synapse • The motor unit

  15. The Nerve • Nerve = bundle of neurons • Not unlike skeletal muscle architecture Figure 12.17, Marieb & Mallett (2003)

  16. The Nerve • Nerves can contain both afferent and efferent neurons. • Spinal/peripheral nerves connect to the spinal cord via: • Anterior root (motor efferent neurons) • Posterior root (sensory afferent neurons)

  17. Posterior root Anterior root

  18. The Nerve • Thirty one pairs of spinal/peripheral nerves: • Cervical  8 • Thoracic  12 • Lumbar  5 • Sacral  5 • Coccygeal 1 Figure 13.29, Marieb & Mallett (2003)

  19. Structural Considerations • The neuron • The nerve • The synapse • The motor unit

  20. Figure 12.7, Marieb & Mallett (2003) The Synapse • Synapse: Area between the synaptic knob of one neuron and the membrane of another neuron

  21. Neurons have thousands of synaptic knobs Some neurons are excitatory, some inhibitory Competition between excitation and inhibition occurs  Threshold stimulus reached? NMJ or motor end plate Neurotransmitter Figure 12.8, Marieb & Mallett (2003)

  22. Excitatory and Inhibitory Postsynaptic Potentials: EPSP, IPSP EPSP - IPSP = Stimulus Stimulus > Threshold = Excitation of impulse Stimulus < Threshold = Inhibition of impulse Impulse itself can be excitatory or inhibitory in nature

  23. Structural Considerations • The neuron • The nerve • The synapse • The motor unit

  24. The Motor Unit • Functional unit of neuromuscular system • Consists of: • Neuron + all muscle fibers • Eye muscles vs. gastrocnemius (10-2000) • Fewer fibers/neuron = precision • More fibers/neuron = force

  25. Figure 14.6, Marieb & Mallett (2003)

  26. Agenda • Introduction to the nervous system • Structural considerations • Motor efferents and gradations of force • Sensory afferents • Reflex movement

  27. Efferents: Gradations of Force • Motor efferent: Sends signal away from the CNS (skeletal muscle) • Dendrites in spinal cord • Synaptic knobs  muscle • Excitatory or inhibitory • Gradation of force: • Concept: Muscles are able to activate with varying degrees of force

  28. Efferents: Gradations of Force • Two factors influence the gradation of force: • Number coding: The number of motor units participating • Rate coding: The frequency of stimulation

  29. Number Coding • All-or-none principle of single motor units  threshold • Gradation of force • Small force = fewer motor units or motor units with less fibers • Large force = more motor units or motor units with more fibers • Orderly sequence  Size principle

  30. Resting muscle tonus achieved via alternating activation of some muscle fibers Figure 19.13, Plowman & Smith (2003)

  31. Rate Coding • Effects of different stimulus frequencies on motor units: • Single stimulus  twitch • Second stimulus added prior to full relaxation  temporal summation • Multiple stimuli added so that any relaxation is prohibited  irregular and fused tetanus

  32. Maximum number coding + maximum rate coding = maximum force As frequency increases, force/tension increases

  33. Agenda • Introduction to the nervous system • Structural considerations • Motor efferents and gradations of force • Sensory afferents • Reflex movement

  34. Sensory Afferents • Sensory afferents: Sends signal towards the CNS • Dendrites are all over body (not in CNS) • Synaptic knobs are in spinal cord • Classifications of afferents: • Exteroceptors • Interoceptors (visceroceptors) • Proprioceptors

  35. Figure 14.1, Marieb & Mallett (2003) Proprioceptors are main concern

  36. Proprioceptors • Location: Tendons, skeletal muscle, ligaments, joint capsules and inner ear • Functions: • Transmit movement information  CNS • CNS integrates and initiates appropriate response (consciously/subconsciously) • Provide sense of body awareness • Provide stimulus for reflexes

  37. Proprioceptor Classification • Muscle proprioceptors: • Muscle spindles • Golgi tendon organs • Joint and skin proprioceptors • Ruffini endings • Pacinian corpuscles • Labyrinthine and neck proprioceptors • Labyrinthine proprioceptors • Neck proprioceptors

  38. Muscle Proprioceptors: Muscle Spindles • Location: Lay between and parallel to muscle fibers • Structure: • Tiny capsules (1 mm) • Filled with fluid and intrafusal muscle fibers • Nucleated and supplied with afferent neuron • Function: • Sensitive to stretch and tension of skeletal muscle tissue • Transmit to CNS • Excitatory impulse  agonist and synergists • Inhibitory impulse  antagonists (reciprocal inhibition)

  39. Figure 14.5, Knutzen & Hamill (2004) Stretch Excitatory activation of agonists Interneurons Activation of synergists Reciprocal inhibition of antagonists

  40. Muscle Proprioceptors: GTOs • Location: Musculotendon junction of skeletal muscle • Structure: • Mass of terminal endings in connective tissue capsule • Connections both with tendon and fibers • Function: • Sensitive to tension in tendon due to both stretch and shortening of muscle • Transmit to CNS: • Inhibitory impulse  agonists and synergists • Excitatory impulse  antagonists

  41. 1. High muscle tension 3. GTO activation 4. Inhibition of agonist 2. High tendon tension

  42. Joint and Skin Proprioceptors: Ruffini Endings • Location: Beneath skin, joint capsules • Structure: Spray of dendrites in flattened connective tissue capsule • Functions: Sensitive to  • Rapid changes in joint angle • Constant pressure resulting in deformation of capsule

  43. Skin and Joint Proprioceptors:Pacinian Corpuscles • Location: Beneath skin, joint capsules, ligaments and tendons • Structure: • Relatively large (naked eye) • Tip of single dendrite in connective tissue capsule • Function: Sensitive to  • Rapid changes in joint angle • Rapid, short-term changes in pressure resulting in deformation of capsule

  44. Pacinian corpuscle Ruffini endings Free nerve endings

  45. Labyrinthine Proprioceptors • Location: Inner ear • Structure: Several structures within the ear • Function: • Detect orientation and movements of the head

  46. Neck Proprioceptors • Location: Ligaments of cervical vertebrae • Function: • Head/neck movement  transmit opposite signals • Prevents sense of imbalance

  47. Agenda • Introduction to the nervous system • Structural considerations • Motor efferents and gradations of force • Sensory afferents • Reflex movement

  48. Reflexes • Reflex: Specific pattern response that occurs without volition • The reflex arc consists of: • Receptor organ • Afferent neuron • Interneuron (sometimes) • Efferent neuron

  49. Figure 12.18, Marieb & Mallett (2003)

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