Physio 3200 Motor systems1 Filled

Course Overview

  • Course Title: Physio 3200 Human Physiology Motor Systems

  • Instructor: Harpreet Singh, Ph.D. FAHA FCVS

  • Department: Physiology and Cell Biology

Objectives

  • Understand the basic organization of the motor systems.

  • Learn the functions of:

    • Afferent neurons

    • Efferent neurons

    • Interneurons

    • Motor neurons

    • Muscles

  • Understand the roles of muscle spindles and Golgi tendon organs.

  • Define voluntary and involuntary movements, reflexes, and motor programs.

  • Identify brain regions that are important for motor control.

  • Recognize the senses critical for maintaining posture and balance.

Motor Control Hierarchy

  • Levels of Control:

    • 10.1: Voluntary and involuntary actions.

    • 10.2: Local control of motor neurons through interneurons and local afferent input.

    • 10.3: Brain motor centers and the descending pathways they control.

    • 10.4: Muscle tone and abnormalities in muscle tone.

    • 10.5: Maintenance of upright posture and balance.

    • 10.6: Mechanics of walking and its associated highlights.

Control of Body Movement

  • Involves multiple levels of motor control.

Motor Units and Motor Neuron Pool

  • Motor Neurons: Activate muscles causing contractions.

  • Motor Units: Consist of one motor neuron and all skeletal muscle fibers it innervates.

  • Motor Neuron Pool: All motor neurons that supply a given muscle, acting as the final common pathway out of the CNS.

  • The cell bodies of motor neurons for any muscle are located near one another in the spinal cord or brainstem.

Neural Control of Muscle Activity

  • Many neurons converge on motor neurons to regulate their activities.

  • Action potentials (AP) in motor neurons lead to muscle contraction; the frequency determines the strength of contraction.

  • The overall contraction dynamics (force, speed, timing, and extent) depend on a balance of excitatory and inhibitory inputs to motor neurons, which govern AP frequencies for coordinated contractions.

Types of Motor Movements

  • Reflexes:

    • Simplest, mostly involuntary, and rapid responses (e.g., sneezing, knee-jerk).

  • Voluntary Movements:

    • Goal-directed, purposeful, initiated by external stimuli, learned and improved through practice (e.g., playing piano).

  • Rhythmic Motor Patterns:

    • Initial voluntary control with sequences of stereotypical movements performed reflexively (e.g., walking).

Voluntary vs Involuntary Components

  • Most motor behaviors are on a spectrum of voluntary and involuntary components.

  • Reflexes can be consciously overridden; learned behaviors can become almost automatic with practice (e.g., playing tennis).

Tasks of the Motor System

  • Main task: Ensure muscle contraction and relaxation.

  • Additional tasks include:

    • Conveying timed commands to various muscle groups for precise joint movements.

    • Postural adjustments in preparation for movement.

    • Continuous modification of commands to adapt to body position changes.

Motor Control Hierarchy Levels

  • Highest Level:

    • Forms complex plans and issues commands to initiate, modify, or stop movements.

  • Middle Level:

    • Integrates commands and sensory input to create a motor program for the desired action.

  • Local Level:

    • Consists of brainstem and spinal cord interneurons and motor neurons that control muscle activation and timing based on motor programs received.

Proprioception and Motor Programs

  • Proprioception:

    • Afferent information regarding body position in space.

  • Motor programs adjust movements based on updated proprioceptive information, and repeated movements lead to skill improvement through learning.

Local Control and Afferent Input

  • Local Afferent Inputs:

    • Originate from receptors in muscles, tendons, and joints, continuously providing feedback for adjustment.

  • Local control systems are critical for responding to unexpected stimuli and injuries.

  • Interneurons:

    • Majority of spinal cord neurons (90%) that can modulate motor neuron activity by being either excitatory or inhibitory.

Senses Related to Posture and Movement

  • The senses of posture and movement rely on:

    • Vision and Vestibular Organs: key for balance, including muscle-spindle stretch receptors and Golgi tendon organs.

    • Mechanoreceptors: In joints, tendons, ligaments, and skin also contribute.

Muscle Spindles and Golgi Tendon Organs

  • Muscle Spindles:

    • Monitor muscle length and the rate of change.

    • Intrafusal fibers provide feedback through afferent nerve fibers.

  • Golgi Tendon Organs:

    • Monitor muscle tension by detecting the distortion of collagen bundles in tendons.

Reflexes: Stretch and Withdrawal Types

  • Stretch Reflex:

    • Involves monosynaptic pathways (direct synapse) or polysynaptic pathways (with interneurons).

    • Reciprocal innervation occurs when one muscle activates while its antagonist inhibits.

  • Withdrawal Reflex:

    • Activated by painful stimuli leading to flexor contraction and extensor inhibition in the affected leg, while the opposite occurs in the other leg for support.

Summary of Motor Control

  • Skeletal muscles are regulated by motor neurons in pools.

  • Hierarchical control involves:

    • Highest Level: Determining action intention.

    • Middle Level: Developing the motor program informed by sensory data.

    • Local Level: Deciding which motor neurons to activate or inhibit during actions.

  • Feedback and correction mechanisms are crucial for proper motor function, integrating various inputs from different levels and receptors.