Chapter 7: Movement

Muscles and Movement

• Without the ability to respond to the environment, cognition and perception would be biologically irrelevant.
• Muscle Types:
  • Smooth Muscle: Controls involuntary actions (e.g., digestion, blood vessel regulation).
  • Skeletal Muscle: Drives voluntary movements in relation to the external environment.
  • Cardiac Muscle: Specialized heart muscle with properties of both smooth and skeletal muscle.
• Muscles are composed of many individual fibers.
• Each muscle fiber receives information from only one axon, but a single axon may innervate many muscle fibers.
• A neuromuscular junction is a synapse between a motor neuron axon and a muscle fiber.
• Release of acetylcholine causes the muscle to contract.
• Movement requires the alternating contraction of opposing sets of muscles called antagonistic muscles.
• Acetylcholine always excites skeletal muscles to contract.
• A flexor muscle is one that flexes or raises an appendage.
• An extensor muscle is one that extends an appendage or straightens it.

Fast and Slow Muscles

• Skeletal muscle types range from:
  • Fast-twitch: fibers produce fast contractions but fatigue rapidly because these fibers are anaerobic and do not require oxygen for its reactions.
  • Slow-twitch: fibers produce less vigorous contraction without fatigue because these fibers are aerobic and require oxygen during movement.
• People have varying percentages of fast-twitch and slow-twitch muscles.

Muscle Control by Proprioceptors

• Proprioceptors: receptors that detect the position or movement of a part of the body
• Muscle spindles are proprioceptors parallel to the muscle that respond to a stretch: cause a contraction of the muscle
• A stretch reflex occurs when muscle proprioceptors detect the stretch and tension of a muscle and send messages to the spinal cord to contract it.
• The Golgi tendon organ is another type of proprioceptor that responds to increases in muscle tension.
  • Located in the tendons at the opposite ends of the muscle
  • Acts as a “brake” against excessively vigorous contraction by sending an impulse to the spinal cord where motor neurons are inhibited

Voluntary and Involuntary Movements

• Movements range from walking to throwing under pressure.
• Nervous system uses different controls for different actions.
• Reflexes: automatic, involuntary responses.
• Most movements mix voluntary and involuntary control.
• Some are ballistic (unchangeable once started).
• Others are feedback-guided.

Sequences of Behaviors

• Many behaviors are rapid sequences of movements.
• Central patterns generators: neural circuits (e.g., spinal cord) that produce rhythmic motor output.
  • Ex: bird wing flapping, dog shaking.
• Motor program: a fixed sequence of movements, learned or innate
  • Runs start to finish once initiated.
  • Disrupted by conscious thought or speech.
  • Ex: yawning, mouse grooming.

Knowledge Check 7-1

• Why can we move the eye muscles with greater precision than the biceps muscles?
  • Each axon to eye muscles contracts only about three fibers, as opposed to a hundred or more for axons to the biceps.

The Cerebral Cortex

• The primary motor cortex is located in the precentral gyrus located in the frontal lobe.
• Axons from the precentral gyrus connect to the brainstem and the spinal cord, which generate impulses that control the muscles.
• Cerebral cortex is additionally involved in complex movements.

Principal Motor Areas of the Human Cortex

• Supplementary motor cortex
• Premotor cortex
• Primary motor cortex
• Primary somatosensory cortex
• Posterior parietal cortex
• Basal ganglia

Planning a Movement

• Specific areas of the primary motor cortex are responsible for control of specific areas of the opposite side of the body.
  • Some overlap does exist.
• The primary motor cortex is active when people intend a movement.
• The primary motor cortex “orders” an outcome.

Other Areas for Planning a Movement

• Supplementary motor cortex
  • Organizes rapid sequence of movements in a specific order; inhibitory if necessary
  • Active seconds before the movement
  • Active following an error in movement so you can inhibit the incorrect movement the next time
• Premotor cortex
  • Active during preparation for movement
  • Receives information about a target
  • Integrates information about position and posture of the body; organizes the direction of the movement in space
• Prefrontal cortex
  • Active during a delay before movement
  • Stores sensory information relative to a movement
  • Necessary for you to consider the probable outcomes of a movement

Inhibiting of Movements

• Antisaccade task: inhibits a saccade, a voluntary eye movement from one target to another
  • Performing this task well requires sustained activity in parts of the prefrontal cortex and basal ganglia before seeing the moving stimulus.
  • Ability to perform this task matures through adolescence.
  • Performance declines in old age and as a result of schizophrenia, attention-deficit disorder, or alcohol intoxication.

From the Brain to the Spinal Cord

• Messages from the brain must reach the medulla and spinal cord to control the muscles.
• Corticospinal tracts are paths from the cerebral cortex to the spinal cord.
  • Two such tracts:
    • Lateral corticospinal tract
      • Extends from the primary motor cortex, nearby areas of the cortex, and the red nucleus
    • Medial corticospinal tract
      • Extends from many areas of the cerebral cortex, as well as from several areas of the midbrain and medulla

The Corticospinal Tracts

• Lateral corticospinal tract (from contralateral cortex)
• Medial corticospinal tract

The Touch Path and the Lateral Corticospinal Tract

• Discriminative touch (recognition of shape, size, texture)

Disorders of the Spinal Cord

• Paralysis: Loss of voluntary movement in part of the body. Cause: Damage to motor neurons or their axons in the spinal cord
• Paraplegia: Loss of sensation and voluntary muscle control in the legs (Genital stimulation can produce orgasm, despite the lack of conscious sensation.). Cause: A cut through the spinal cord in the thoracic region or lower
• Quadriplegia (or tetraplegia): Loss of sensation and voluntary muscle control in both arms and legs. Cause: Cut through the spinal cord in the cervical (neck) region
• Hemiplegia: Loss of sensation and voluntary muscle control in the arm and leg of either the right or left side. Cause: Cut halfway through the spinal cord
• Tabes dorsalis: Impaired sensations and muscle control in the legs and pelvic region, including bowel and bladder control. Cause: Damage to the dorsal roots of the spinal cord in the late stage of syphilis
• Poliomyelitis: Paralysis. Cause: A virus that damages motor neurons in the spinal cord
• Amyotrophic lateral sclerosis: Gradual weakness and paralysis, starting with the arms and spreading to the legs. Cause: Gradual loss of motor neurons

The Cerebellum

• A structure in the brain often associated with balance and coordination
• More neurons in the cerebellum than in all other brain areas combined
• Damage to the cerebellum causes trouble with rapid movements requiring aim/timing.
  • Examples: clapping hands, speaking, writing, and so on

Cellular Organization of the Cerebellum

• Cerebellum integrates inputs from the spinal cord, sensory systems, and cortex.
• Sends output via the cerebellar cortex (its outer layer).
• Neurons are precisely arranged for timed, controlled responses
  • Purkinje cells: flat, layered output cells.
  • Parallel fibers: run perpendicular, activate Purkinje cells
  • More active Purkinje cells à longer-lasting inhibition/output.

The Basal Ganglia

• Caudate nucleus & putamen get input from cortex → send to globus pallidus
• Globus pallidus → thalamus → motor & prefrontal cortex
• Basal ganglia select movements by removing inhibition
• Key for spontaneous, self-initiated actions
• Crucial for habit learning

Two Pathways Through the Basal Ganglia

• The direct pathway
• The indirect pathway

Conscious Decisions and Movement

• The conscious decision to move, and the movement itself, occur at two different times.
• A readiness potential is a particular type of activity in the motor cortex that occurs before any type of voluntary movement.
  • Begins at least 500 ms before the movement
  • Implies that we become conscious of the decision to move after the process has already begun

Results from Study of Conscious Decision and Movement

• Readiness potential: Brain's readiness potential begins to rise in preparation for the movement.
• Person reports that the conscious decision occurred here.
• The movement itself starts here.

Parkinson’s Disease

• A movement disorder characterized by muscle tremors, rigidity, slow movements, and difficulty initiating physical and mental activity
• Associated with an impairment in initiating spontaneous movement in the absence of stimuli to guide the action.
• Caused by gradual and progressive death of neurons, especially in the substantia nigra
  • Substantia nigra usually sends dopamine-releasing axons to the caudate nucleus and putamen.
  • Loss of dopamine leads to less stimulation of the motor cortex and slower onset of movements.

Connections from the Substantia Nigra:

• Normal
• In Parkinson's Disease
• Decreased excitation from substantia nigra to putamen
• Decreased excitation from thalamus to cortex
• Increased inhibition from globus pallidus to thalmus
• Decreased inhibition from putamen to globus pallidus

Parkinson’s Causes

• Studies suggest early-onset Parkinson’s has a genetic link.
• Genetic factors are only a small factor of late onset Parkinson’s disease (after 50).
• Environmental influences such as exposure to toxins
  • Insecticides, herbicides, certain drugs, and fungicides
• Traumatic head injury
• Cigarette smoking and coffee drinking are related to a decreased chance of developing Parkinson’s disease.
• Damaged mitochondria of cells seems to be common to most factors that increase the risk of Parkinson’s disease.

L-Dopa Treatment

• The drug L-dopa is the primary treatment for Parkinson’s and is a precursor to dopamine that easily crosses the blood–brain barrier.
• They are more effective in some people than in others, probably because of variation in the intestinal bacteria that metabolize L-dopa before it can enter the blood.
• Does not prevent the continued loss of neurons
• Enters other brain cells, producing unpleasant side effects

Non-pharmaceutical Therapies

• Drugs that directly stimulate dopamine receptors
• Implanting electrodes to stimulate areas deep in the brain
• Experimental strategies such as:
  • Transplanting brain tissue of aborted fetuses
  • Implantation of stem cells that are programmed to produce large quantities of L-dopa

Huntington’s Disease

• A neurological disorder characterized by various motors symptoms
• Affects 17 in 100,000 in the United States
• Usually onset occurs between the ages of 30 and 50.
• Associated with gradual and extensive brain damage especially in the basal ganglia but also in the cerebral cortex

Huntington’s Disease

• Initial motor symptoms include arm jerks and facial twitches.
• Motor symptoms progress to tremors and writhing that affect the persons walking, speech, and other voluntary movements.
• Also associated with various psychological disorders:
  • Depression, memory impairment, anxiety, hallucinations/delusions, poor judgment, alcoholism, drug abuse, sexual disorders

Study Questions

• What are the functional differences between fast-twitch and slow-twitch muscles?
• What do proprioceptors do, and why are they important for movement?
• What is a motor program, and how does it work?
• How do different cortical areas contribute to movement control?
• What role does the prefrontal cortex play in inhibiting actions?
• What are mirror neurons, and what is their proposed function?
• How do the lateral and medial corticospinal tracts differ in anatomy and function?
• What roles do the cerebellum and basal ganglia play in movement?
• What does research say about the role of consciousness in movement planning?
• What causes Parkinson’s disease, and how is it treated?
• What are the genetic factors involved in Huntington’s disease?