Human Physiology   7/e   Vander/Sherman/Luciano
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Chapter 12: Control of Body Movement


Study Outline

Chapter 12: Control of Body Movement

  1. MOTOR NEURON POOLS

    1. Skeletal muscles are controlled by their motor neurons. All the motor neurons that control a given muscle form a motor neuron pool.

  2. MOTOR CONTROL HIERARCHY

    1. The neural systems that control body movements can be conceptualized as being arranged in a motor control hierarchy.

      1. The highest level determines the general intention of an action.
      2. The middle level specifies the postures and movements needed to carry out the intended action and, taking account of sensory information that indicates the body's position, establishes a motor program.
      3. The lowest level determines which motor neurons will be activated.
      4. As the movement progresses, information about what the muscles are doing is fed back to the motor control centers, which make any needed program corrections.
      5. Actions are voluntary when we are aware of what we are doing and why, or when we are paying attention to the action or its purpose.
      6. Almost all actions have conscious and unconscious components.

  3. LOCAL CONTROL OF MOTOR NEURONS

    1. Most input to motor neurons is from local interneurons, which themselves receive input from peripheral receptors, descending pathways, and other interneurons.
    2. Muscle length and the velocity of changes in length are monitored by muscle-spindle stretch receptors.

      1. Activation of these receptors initiates the stretch reflex, in which motor neurons of ipsilateral antagonists are inhibited and those of synergists are activated.
      2. Tension on the stretch receptors is maintained during muscle contraction by gamma efferent activation of the spindle muscle fibers.
      3. Alpha and gamma motor neurons are often coactivated.

    3. Muscle tension is monitored by Golgi tendon organs, which, via interneurons, activate inhibitory synapses on motor neurons of the contracting muscle and excitatory synapses on motor neurons of ipsilateral antagonists.
    4. The flexion reflex excites the ipsilateral flexor muscles and inhibits the ipsilateral extensors. The crossed-extensor reflex excites the contralateral extensor muscles during excitation of the ipsilateral flexors.

  4. THE BRAIN MOTOR CENTERS AND THE DESCENDING PATHWAYS THEY CONTROL

    1. The location of the neurons in the motor cortex varies in general with the part of the body the neurons serve.
    2. Different areas of sensorimotor cortex have different functions, but there is much overlap in activity.
    3. The basal ganglia form a link in a circuit that originates in and returns to sensorimotor cortex. These subcortical nuclei facilitate some motor behaviors and inhibit others.
    4. The cerebellum coordinates posture and movement and plays a role in motor learning.
    5. The corticospinal pathways pass directly from the sensorimotor cortex to motor neurons in the spinal cord (or the brainstem, in the case of the corticobulbar pathways) or to interneurons near the motor neurons.

      1. In general, neurons on one side of the brain control muscles on the other side of the body.
      2. Corticospinal pathways serve predominately fine, precise movements.
      3. Some corticospinal fibers affect the transmission of information in afferent pathways.

    6. Other (brainstem) pathways arise in the brainstem and are involved mainly in the coordination of large groups of muscles used in posture and locomotion.
    7. There is some duplication of function between the two descending pathways.

  5. MUSCLE TONE

    1. Hypertonia, as seen in spasticity, rigidity, spasms, and cramps, usually occurs with disorders of the descending pathways.
    2. Hypotonia can be seen with cerebellar disease or, more commonly, with disease of the alpha motor neurons or muscle.

  6. MAINTENANCE OF UPRIGHT POSTURE AND BALANCE

    1. To maintain balance, the body's center of gravity must be maintained over the body's base.
    2. The crossed-extensor reflex is a postural reflex.

  7. WALKING

    1. The activity of networks of interneurons in the spinal cord brings about the cyclical, alternating movements of locomotion.
    2. These pattern generators are affected by feedback and motor programs.

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