Integrative Physiology: Control of Body Movement (Chapter 13 Study Notes)

Integrative Physiology: Control of Body Movement

Overview

This chapter explores the mechanisms underlying the control of body movement, focusing on neural reflexes, autonomic reflexes, skeletal muscle reflexes, and the integrated control of movement in both skeletal and visceral muscles. Understanding these processes is essential for comprehending how the nervous system coordinates voluntary and involuntary actions.

Neural Reflexes

Classification of Neural Reflexes

Neural reflex pathways are rapid, automatic responses to stimuli, and can be classified in several ways:

• By the efferent division controlling the response:

  • Somatic reflexes: Involve somatic motor neurons controlling skeletal muscles.

  • Autonomic reflexes: Involve autonomic neurons controlling smooth muscle, cardiac muscle, glands, and adipose tissue.

• By the CNS location of integration:

  • Spinal reflexes: Integrated within the spinal cord.

  • Cranial reflexes: Integrated within the brain.

• By whether the reflex is innate or learned:

  • Innate reflexes: Genetically determined and present at birth.

  • Learned (conditioned) reflexes: Acquired through experience.

• By the number of neurons in the pathway:

  • Monosynaptic reflexes: Only two neurons (one afferent and one efferent), with a single synapse.

  • Polysynaptic reflexes: Multiple neurons and synapses, including interneurons.

Classification Criteria	Types	Description
Efferent Division	Somatic / Autonomic	Somatic controls skeletal muscle; autonomic controls smooth/cardiac muscle, glands, adipose tissue.
CNS Integration Site	Spinal / Cranial	Spinal reflexes do not require brain input; cranial reflexes are integrated in the brain.
Developmental Timing	Innate / Learned	Innate are inborn; learned are acquired through experience.
Number of Neurons	Monosynaptic / Polysynaptic	Monosynaptic: one afferent, one efferent; polysynaptic: multiple neurons and synapses.

Monosynaptic vs. Polysynaptic Reflexes

• Monosynaptic reflex: Direct synapse between sensory and motor neuron (e.g., stretch reflex).

• Polysynaptic reflex: Involves one or more interneurons between sensory and motor neurons (e.g., withdrawal reflex).

• Example: The patellar tendon (knee-jerk) reflex is monosynaptic.

Autonomic Reflexes

Characteristics of Autonomic (Visceral) Reflexes

Autonomic reflexes, also known as visceral reflexes, regulate the function of internal organs and are essential for homeostasis.

• Integration: Some are spinal reflexes modulated by the brain; others are integrated entirely within the brain.

• Limbic system: Acts as the "visceral brain," linking emotional stimuli to visceral responses.

• Polysynaptic: Typically involve multiple neurons and synapses.

• Involuntary activity: Control smooth muscle, cardiac muscle, glands, and adipose tissue.

• Example: Regulation of heart rate and digestion.

Skeletal Muscle Reflexes

Proprioceptors and Reflex Pathways

Skeletal muscle reflexes are mediated by sensory receptors called proprioceptors, which provide information about body position and movement.

• Location: Found in skeletal muscle, joint capsules, and ligaments.

• Input: Sensory neurons transmit signals to the CNS.

• Integration: CNS processes input via excitatory and inhibitory interneurons.

• Output: Somatic motor neurons (alpha motor neurons) carry signals to extrafusal muscle fibers (contractile fibers).

• Clinical relevance: Reflex testing assesses muscle tone and nervous system function.

Golgi Tendon Organs (GTO)

• Location: Junction of tendons and muscle fibers.

• Structure: Free nerve endings interwoven among collagen fibers within a connective tissue capsule.

• Function: Respond to muscle tension, especially during isometric contraction.

• Role: Send sensory information to the CNS to prevent muscle damage from excessive force.

Muscle Spindles

• Type: Stretch receptors that detect changes in muscle length.

• Structure: Capsule containing intrafusal fibers, innervated by gamma motor neurons.

• Function: Initiate stretch reflexes to maintain muscle length and tone.

• Alpha-gamma coactivation: Simultaneous activation of alpha and gamma motor neurons ensures spindle sensitivity during muscle contraction.

• Example: When a muscle shortens, gamma motor neurons contract intrafusal fibers, keeping the spindle active.

Stretch Reflexes and Reciprocal Inhibition

• Monosynaptic stretch reflex: Involves a direct connection between sensory and motor neuron (e.g., patellar reflex).

• Reciprocal inhibition: Antagonistic muscles relax as the primary mover contracts, allowing smooth movement.

• Polysynaptic reflexes: Withdrawal (flexion) reflexes pull limbs away from painful stimuli.

• Crossed extensor reflex: Coordinates movement of opposite limb for balance during withdrawal.

The Integrated Control of Body Movement

Types of Movement

Body movements are classified based on complexity and neural integration:

• Reflex movements: Least complex, integrated at spinal cord or brain stem (e.g., postural reflexes).

• Rhythmic movements: Intermediate complexity, involve central pattern generators (CPGs) in the spinal cord with input from higher centers (e.g., walking).

• Voluntary movements: Most complex, integrated in the cerebral cortex (e.g., writing, speaking).

Type of Movement	Integration Site	Examples
Reflex	Spinal cord, brain stem	Postural reflexes, withdrawal reflex
Rhythmic	Spinal cord (CPGs), higher centers	Walking, running
Voluntary	Cerebral cortex	Writing, speaking

Neural Control of Movement

• Spinal cord: Integrates spinal reflexes and contains central pattern generators.

• Brain stem and cerebellum: Control postural reflexes and coordinate eye/hand movements.

• Cerebral cortex and basal ganglia: Responsible for voluntary movements.

• Corticospinal tract: Major pathway for voluntary motor control.

• Parkinson's disease: Results from loss of basal ganglia neurons that release dopamine, causing abnormal movements, speech difficulties, and cognitive changes.

Feedforward and Feedback Mechanisms

• Feedforward reflexes: Anticipate and adjust posture for expected disturbances.

• Feedback reflexes: Respond to unanticipated disturbances to maintain posture and balance.

Control of Movement in Visceral Muscles

Mechanisms of Visceral Muscle Contraction

Movement in smooth and cardiac muscles is regulated differently than in skeletal muscle:

• Pacemaker fibers: Specialized cells that spontaneously depolarize, initiating contractions.

• Hormonal and autonomic control: Contraction is modulated by hormones and the autonomic nervous system.

• Example: Regulation of heart rate and peristalsis in the digestive tract.

Summary of Key Topics:

• Neural Reflexes: Classification and mechanisms

• Autonomic Reflexes: Visceral control and integration

• Skeletal Muscle Reflexes: Proprioceptors, muscle spindles, Golgi tendon organs

• Integrated Control of Body Movement: Types, neural pathways, and feedback mechanisms

• Control of Movement in Visceral Muscles: Pacemaker activity and autonomic regulation