Chapter 13: Integrative Physiology I - Control of Body Movement

Neural Reflexes

  • Neural reflex pathways are classified by:
    • Efferent division of the nervous system:
      • Somatic reflexes: Control skeletal muscles.
      • Autonomic reflexes (visceral reflexes): Regulate internal organs.
    • CNS location of integration:
      • Spinal reflexes: Integrated in the spinal cord.
      • Cranial reflexes: Integrated in the brain.
    • Whether the reflex is innate or learned:
      • Innate reflexes: Genetically determined.
      • Learned reflexes (conditioned reflexes): Acquired through experience.
    • Number of neurons in the pathway:
      • Monosynaptic reflexes: Involve only two neurons (sensory and motor).
      • Polysynaptic reflexes: Involve one or more interneurons.

Autonomic Reflexes

  • Also known as visceral reflexes.
  • Some are spinal reflexes modulated by the brain, while others are integrated in the brain.
  • Limbic system: "Visceral brain" linking emotional stimuli to visceral responses.
  • Typically polysynaptic.
  • Exhibit tonic activity (continuous, low-level activity).

Skeletal Muscle Reflexes

  • Involve sensory receptors (proprioceptors) in skeletal muscles, joint capsules, and ligaments.
    • Proprioceptors: Monitor position, movement, and effort; include joint receptors, Golgi tendon organs, and muscle spindles.
  • Input signals travel to the CNS via sensory neurons.
  • The CNS integrates signals using excitatory and inhibitory interneurons.
  • Output signals are carried by somatic motor neurons (alpha motor neurons).
  • Effectors: Contractile skeletal muscle fibers (extrafusal muscle fibers).
  • Clinical assessment:
    • Reflexes are tested to assess neural pathways and synaptic transmission.
    • Muscle tone (resistance to stretch) is evaluated.
  • Normal Reflexes:
    *Normal conduction through all neurons in the pathway.
    *Normal synaptic transmission at NMJ.
    *Normal muscle contraction
  • Muscle tone:
    *Resistance to stretch in relaxed/resting muscle
    *Due to continuous alpha motor neuron activation of extrafusal fibers
  • Pathology:
    *Absent reflex
    *Abnormally slow reflex (eg. Hypothyroidism)
    *Hyperactive reflex
    *Absence of tone or resistance to stretch

Golgi Tendon Organs (GTOs)

  • Location: Junction of tendons and muscle fibers.
  • Structure: Free nerve endings intertwined with collagen fibers inside a connective tissue capsule.
  • Function: Respond to muscle tension during contraction; relatively insensitive to stretch.
  • Mechanism: Muscle contraction → tendons act as series elastic elements → GTOs send sensory information to the CNS.
  • Role: Integrate sensory input for optimal motor control of posture and movement.

Muscle Spindles

  • Function: Stretch receptors that signal the CNS about changes in muscle length.
  • Distribution: Present in almost every skeletal muscle (except one in the jaw).
  • Structure: Capsule enclosing intrafusal fibers (small muscle fibers) innervated by gamma motor neurons.
  • Resting state: Muscles stretch enough to activate sensory fibers, resulting in tonic activity.

Muscle Spindles at Rest

  • Sensory neurons synapse with alpha motor neurons, causing them to fire and leading to resting muscle tone.

Muscle Spindles Respond to Muscle Stretch

  • Resting muscle tone is maintained by the firing of alpha motor neurons due to sensory neuron input.
  • Stretch reflex:
    • Simultaneous activation of alpha and gamma motor neurons.
    • Movement activates sensory neurons.
    • Gamma motor neurons fire, causing intrafusal fibers to contract and prevent overstretching.

Alpha-Gamma Coactivation

  • Alpha motor neuron fires: Muscle contracts (shortens), releasing tension on the muscle spindle capsule.
  • Gamma motor neurons fire: Intrafusal fibers contract, pulling on the central region of the spindle and maintaining stretch on sensory nerves.
  • Significance: Ensures the spindle remains active even during muscle contraction.

Stretch Reflexes and Reciprocal Inhibition

  • Myotatic unit: Collection of pathways controlling a single joint.
    • Monosynaptic stretch reflex: Involves only two neurons (sensory neuron from spindle → somatic motor neuron to muscle); example: patellar tendon reflex.
  • Reciprocal inhibition: Antagonistic muscles relax as prime mover muscles contract.
  • Flexion reflexes: Polysynaptic pathways that pull limbs away from painful stimuli.
  • Crossed extensor reflex: Compensatory reflex that occurs in conjunction with the flexion reflex.

Integrated Control of Body Movement

  • Movement types: Reflex, voluntary, and rhythmic.
  • Integrated responses require input from multiple brain regions.
  • Reflex movements: Least complex, integrated at the spinal cord or brain stem.
  • Postural reflexes: Maintain body position; integrated in the brain stem/cerebellum.
  • Voluntary movements: Most complex, integrated in the cerebral cortex.
  • Rhythmic movements: Intermediate complexity, integrated in the spinal cord with higher center input; involve central pattern generators (CPGs).
  • Feedforward reflexes: Anticipatory adjustments.

CNS Integration of Movement

  • Three levels of nervous system control:
    • Spinal cord: Integrates spinal reflexes and contains central pattern generators.
    • Brain stem and cerebellum: Control postural reflexes and eye/hand movements.
    • Cerebral cortex and basal ganglia: Control voluntary movements.
  • Corticospinal tract: Descending pathway for voluntary motor control.
  • Parkinson’s disease: Reflects basal ganglia dysfunction due to loss of dopamine-releasing neurons; characterized by abnormal movements, speech difficulties, and cognitive changes.

Control of Movement in Visceral Muscles

  • Contraction in smooth and cardiac muscles:
    • Spontaneously depolarizing fibers (pacemakers).
    • Regulation by hormones or the autonomic nervous system.