The Nervous System: Structure and Control of Movement

The Nervous System: Structure and Control of Movement

Presentation Information

  • Title: Exercise Physiology: Theory and Application to Fitness and Performance, 11th Edition

  • Authors: Scott K. Powers, Ph.D., Ed.D., Edward T. Howley, Ph.D., John Quindry, Ph.D.

  • Copyright: © 2021 McGraw-Hill Education. All rights reserved.

Lecture Outline

  • General Nervous System Functions

  • Organization of the Nervous System

  • Sensory Information and Reflexes

  • Muscle Chemoreceptors

  • Somatic Motor Function and Motor Neurons

  • Motor Control Functions of the Brain

  • Motor Functions of the Spinal Cord

  • Control of Motor Functions

  • Autonomic Nervous System

Functions of the Nervous System - Big Picture

  • Control of the Internal Environment: Works in conjunction with the endocrine system to maintain homeostasis.

  • Voluntary Control of Movement: Facilitates voluntary muscle movements.

  • Spinal Cord Reflexes: Enables reflex actions without direct involvement of the brain.

  • Memory and Learning: Integrates experiences to facilitate memory and learning.

Organization of the Nervous System

  • Central Nervous System (CNS)

    • Composed of the brain and spinal cord.

  • Peripheral Nervous System (PNS)

    • Consists of neurons outside the CNS.

    • Sensory Division: Afferent fibers transmit impulses from receptors to CNS.

    • Motor Division: Efferent fibers transmit impulses from CNS to effectors (muscles and glands).

Structure of a Neuron

  • Cell Body: Contains the nucleus.

  • Dendrites: Conduct impulses toward the cell body.

  • Axon: Carries electrical impulse away from the cell body; may be covered by Schwann cells forming a myelin sheath.

  • Synapse: Contact points between axons of one neuron and dendrites of another neuron.

Resting Membrane Potential

  • Negative charge inside cells at rest: ranges from -5 to -100 mV (typically -40 to -75 mV in neurons).

  • Charge determined by:

    • Permeability of the plasma membrane to ions.

    • Differences in ion concentrations across the membrane (in particular, Na extsuperscript{+}, K extsuperscript{+}, and Cl extsuperscript{−}).

  • Maintained by the sodium-potassium pump:

    • Potassium ions (K extsuperscript{+}) tend to diffuse out of the cell.

    • The Na extsuperscript{+/}K extsuperscript{+} pump moves 2 K extsuperscript{+} into the cell and 3 Na extsuperscript{+} out.

Action Potential

  • Neurons transmit messages via action potentials:

    • Occur when a stimulus of sufficient strength depolarizes the neuron.

    • Na extsuperscript{+} channels open and Na extsuperscript{+} diffuses into the cell, making the inside more positive.

    • Repolarization: Return to resting membrane potential where K extsuperscript{+} leaves the cell rapidly and Na extsuperscript{+} channels close.

    • All-or-None Law: Once a nerve impulse is initiated, it will travel the length of the neuron.

Synapses and Neurotransmitters

  • Synapse: Small gap between presynaptic neuron and postsynaptic neuron.

  • Neurotransmitter: Chemical messenger released from presynaptic membrane; binds to receptor on the postsynaptic membrane, causing depolarization of the postsynaptic membrane.

Graded Potentials

  • Excitatory: Promote depolarization.

  • Inhibitory: Promote hyperpolarization.

    • Temporal Summation: Summing potentials from one presynaptic neuron.

    • Spatial Summation: Summing potentials from several different presynaptic neurons.

Muscle Proprioceptors

  • Muscle Proprioceptors (Mechanoreceptors): Provide CNS with information about body position.

  • Muscle Spindle: Provides information about muscle length and rate of shortening.

  • Golgi Tendon Organ (GTO): Monitors muscle force production and can prevent injury by causing reflex relaxation of muscles during excessive force.

Somatic Motor Function

  • Motor neurons located within the spinal cord responsible for conveying neural messages to skeletal muscles.

  • Motor Unit: The motor neuron and all muscle fibers it innervates.

  • Innervation Ratio: Number of muscle fibers per motor neuron; low in muscles with fine motor control (e.g., 23/1 in extraocular muscles) and higher in larger muscles (e.g., 1,000/1 or greater in large limb muscles).

Motor Unit Recruitment and the Size Principle

  • Motor Unit Recruitment: Activation of additional muscle fibers by activating more motor units.

  • Size Principle: The smallest motor units are recruited first during exercise.

  • Types of motor units based on fiber type:

    • Type S (Slow, Type I fibers)

    • Type FR (Fast, Fatigue Resistant, Type IIa fibers)

    • Type FF (Fast, Fatigable, Type IIx fibers)

Central Governor Theory

  • Central Governor Theory: A central control center regulates exercise performance and may reduce motor output to protect homeostasis during exercise.

  • Fatigue may be a result of both central factors (depletion of neurotransmitters) and peripheral factors.

Control of Voluntary Movement

  • Involves cooperation among various brain areas (e.g., motor cortex, basal nuclei, cerebellum, thalamus) and spinal mechanisms (e.g., spinal tuning).

  • Feedback from muscle proprioceptors leads to further refinements in movement control.

Lecture Summary

  • The key points discussed include general nervous system functions, organization of the nervous system, sensory information and reflex mechanisms, muscle chemoreceptors, control of motor function, and connections between the brain and spinal cord in motor control.

Skeletal Muscle: Structure and Function

Lecture Outline

  • Structure of Skeletal Muscle

  • Neuromuscular Junction

  • Muscular Contraction

  • Exercise and Muscle Fatigue

  • Exercise-Associated Muscle Cramps

  • Fiber Types

  • Muscle Actions

  • Speed of Muscle Action and Relaxation

  • Force Regulation in Muscle

  • Force-Velocity Relationships

Structure of Skeletal Muscle

  • Composed of over 600 skeletal muscles, accounting for 40-50% of total body weight.

  • Functions include:

    • Force production for locomotion and breathing.

    • Force production for postural support.

    • Heat production during cold stress.

Satellite Cells

  • Play a significant role in muscle growth and repair.

  • During muscle growth, satellite cells increase the number of nuclei in mature muscle fibers.

  • Myonuclear Domain: The volume of cytoplasm surrounding each nucleus, with more nuclei allowing for greater protein synthesis, essential for adaptation to strength training.

Muscle Fiber Structure

  • Myofibrils: Contain contractile proteins (actin and myosin).

  • Sarcoplasmic Reticulum: Calcium storage sites that release calcium during contraction.

  • Transverse Tubules: Extend from sarcolemma to sarcoplasmic reticulum and propagate action potential.

Neuromuscular Junction

  • Motor Unit: Includes the motor neuron and all fibers it innervates.

  • Motor End Plate: The region of the muscle fiber that interacts with the motor neuron.

  • Neuromuscular Cleft: The gap between the neuron and muscle fiber where neurotransmitter acetylcholine is released, initiating depolarization.

Sliding Filament Model of Muscle Contraction

  • Muscle shortening occurs due to actin filaments moving over myosin filaments.

  • Cross-bridges form between actin and myosin during contraction leading to a power stroke.

ATP Requirements for Contraction

  • Energy released from ATP breakdown is essential for the power stroke during contraction:

    • ATP is broken down to ADP and Pi by the enzyme Myosin ATPase.

    • Sources of ATP include Phosphocreatine (PC), glycolysis, and oxidative phosphorylation.

Excitation-Contraction Coupling

  • Depolarization at the motor end plate leads to an action potential that is coupled to calcium release, initiating muscle contraction through the binding of calcium to troponin, altering tropomyosin's position.

Muscle Fatigue

  • Fatigue: Decline in muscle power output; dependent on exercise intensity.

  • Mechanisms of fatigue may include:

    • In heavy exercise: Reduced calcium release from the sarcoplasmic reticulum and metabolite accumulation.

    • In prolonged endurance exercises (>60 min): Increased radical production, glycogen depletion, and accumulation of metabolites.

Muscle Cramps

  • Defined as spasmodic, involuntary contractions, often linked to prolonged intense exercise.

  • Likely correlated with hyperactivity of motor neurons rather than dehydration or electrolyte imbalances.

Muscle Fiber Types

  • Three types exist:

    • Type I (Slow): Endurance-oriented, oxidative.

    • Type IIa (Fast, Fatigue Resistant): Intermediate properties.

    • Type IIx (Fast, Fatigable): Quick, powerful movements but fatigue quickly.

  • Performance characteristics differ between these fiber types.

Force Regulation in Muscle

  • Tension Generating Characteristics:

    • All-or-None principle: Muscles contract fully when stimulated.

    • Gradation of force can be adjusted through firing rate and number of motor units recruited.

Force-Velocity Relationship

  • At any force exerted by the muscle, the speed of movement is greater in muscles with a higher proportion of fast-twitch fibers.

  • Typically, maximum velocity occurs at the lowest force levels.

Aging and Muscle Loss

  • Sarcopenia: Represents age-related muscle loss, with a significant reduction in muscle mass and functionality.

    • Muscle mass loss is approximately 10% between ages 25-50, followed by an additional 40% loss from 50-80.

  • Resistance training may help mitigate age-associated muscle loss.

Lecture Summary

  • Focused on the structure and function of skeletal muscle, neuromuscular junction mechanics, muscle contraction dynamics, fatigue mechanisms, muscle fiber types, and the physiological principles governing muscle actions and force production.