Chapter 8Movement

The Control of Movement

• Animal movement depends on muscle contractions.

1. Smooth muscles - control the digestive system and other organs

2. Skeletal muscles/striated muscles - control movement of the body in relation to the environment.

3. Cardiac muscles - heart muscles that have properties of skeletal and smooth muscles

The Control of Movement

• Muscles are composed of many individual fibers.– Each muscle fiber receives information

form only one axon but an axon may innervate many muscle fibers.

• A neuromuscular junction is a synapse between a motor neuron axon and a muscle fiber.

• Release of chemicals causes the muscle to contract.

The Control of Movement

• 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.

The Control of Movement

• Myasthenia gravis is an autoimmune disease in which the immune system forms antibodies that attack the acetylcholine receptors at neuromuscular junctions.– Slight decline in acetylcholine is

problematic– Causes the progressive weakness and

rapid fatigue of the skeletal muscles.

The Control of Movement

• Skeletal muscle types range from:– Fast-twitch- fibers produce fast

contractions but fatigue rapidly.– Slow-twitch- fibers produce less vigorous

contraction without fatigue. • People have varying percentage of fast-twitch

and slow-twitch muscles.

The Control of Movement

• Slow-twitch fibers are aerobic and require oxygen during movement and therefore do not fatigue.– Nonstrenuous activities utilize slow-twitch

and intermediate fibers.• Fast-twitch fibers are anaerobic and use

reactions that do not require oxygen, resulting in fatigue.– Behaviors requiring quick movements

utilize fast-twitch fibers.

The Control of Movement

• The human anatomy is specialized for endurance in running.– Reflected in the shape of our toes, leg

bones, muscles and tendons and the high percentage of slow-twitch muscles in our legs.

– Extensive sweat glands and reduced body hair improve temperature regulation.

The Control of Movement

• Fluidity of movement depends on:• Proprioceptors - receptors that detect the

position or movement of a part of the body • Muscle spindles are proprioceptors parallel to

the muscle that responds to a stretch.– causes a contraction of the muscle.

• 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 Control of Movement

• 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.

The Control of Movement

• Reflexes are involuntary, consistent, and automatic responses to stimuli.

• Infants have several reflexes not seen in adults:– Grasp reflex - grasps objects placed in the

hand.– Babinski reflex - extends big toe and fans

others when the sole of the foot is stroked.– Rooting reflex - turns head and sucks when

cheek is stimulated.

The Control of Movement

• Few behaviors are purely reflexive or non-reflexive and movements vary in their sensitivity to feedback.

• Ballistic movements are movement that once initiated can not be altered or corrected.– Example: stretch reflex, dilation of the

pupil.

The Control of Movement

• Many behaviors consist of rapid sequences of individual movements.

• Central pattern generators are neural mechanisms in the spinal cord or elsewhere that generate rhythmic patterns of motor output.– Example: wing flapping in birds.

The Control of Movement

• A motor program refers to a fixed sequence of movements that is either learned or built into the nervous system.– once begun, the sequence is fixed from

beginning to end.– Automatic in the sense that thinking or

talking about it interferes with the action.– Example: Mouse grooming itself, skilled

musicians playing a piece, or a gymnast’s routine.

Brain Mechanisms of Movement

• 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

Brain Mechanisms of 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 exists

• The primary motor cortex is active when people intend a movement

Brain Mechanisms of Movement

• Other areas near the primary motor cortex also contribute to movement:

• Posterior parietal cortex- keeps track of the position of the body relative to the world– Damage to this area causes difficulty

coordinating visual stimuli with movement.– Important for planning movement

• Primary somatosensory cortex integrates touch information and movement.

Brain Mechanisms of Movement

• Cells in the following areas are involved in the preparation and instigation of movement:

• Prefrontal cortex:– Responds to lights, noises and other

sensory signals that lead to movement.– Calculates predictable outcomes of actions

and plans movement according to those outcomes.

Brain Mechanisms of Movement

• Premotor cortex:– active during preparation for movement– receives information about a target– integrates information about position and

posture of the body and organizes the direction of the movement in space.

• Supplementary motor cortex:– Important for organizing a rapid sequence

of movements in a particular order.– Active seconds before the movement

occurs

Brain Mechanisms of Movement

• Mirror neurons are neurons that are active during both preparation of a movement and while watching someone else perform a similar movement.

• May be important for understanding, identifying and imitating other people.

• May be involved in social behaviors.• Unknown whether they cause or result from

social behavior.

Brain Mechanisms of 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 itself– Implies that we become conscious of the

decision to move after the process has already begun.

The Control of Movement

• 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:

1. Lateral corticospinal tract

2. Medial corticospinal tract

Brain Mechanisms of Movement

• Lateral corticospinal tract - a set of axons from the primary motor cortex, surrounding areas, and the red nucleus to the spinal cord– Controls movement in peripheral areas

(hands and feet)– (Red nucleus - a midbrain area with output

mainly to the arm muscles)• Axons extend from one side of the brain to

the opposite side of the spinal cord and control opposite side of the body.

Brain Mechanisms of Movement

• medial corticospinal tract - set of axons from many parts of the cortex, the reticular formation, midbrain tectum and vestibular nucleus.

• Vestibular nucleus is a brain area that receives information from the vestibular system.

• Axons go to both sides of the spinal cord• Allow control of muscles of the neck, shoulders and

trunk.• Enables movements such as walking, turning,

bending, standing up, and sitting down.

Brain Mechanisms of Movement

• The ventromedial tract also includes axons from the midbrain tectum, reticular formation, and the vestibular nucleus.– Vestibular nucleus - brain area that

receives input from the vestibular system.

Brain Mechanisms of Movement

• The cerebellum is a structure in the brain often associated with balance and coordination.

• Damage to the cerebellum causes trouble with rapid movement requiring aim and timing.– Examples: clapping hands, speaking,

writing, etc.

Brain Mechanisms of Movement

• Studies suggest that the cerebellum is important for the establishment of new motor programs that allow the execution of a sequence of actions as a whole.– Important for task that require timing

• The cerebellum also seems critical for certain aspects of attention such as the ability to shift attention and attend to visual stimuli.

Brain Mechanisms of Movement

• The cerebellum receives input from the spinal cord, from each of the sensory systems, and from the cerebral cortex and sends it to the cerebellar cortex.

• The cerebellar cortex is the surface of the cerebellum.

Brain Mechanisms of Movement

• Cerebellar cortex neurons are arranged in precise geometrical patterns:

• Purkinje cells- flat parallel cells in sequential planes• Parallel fibers -axons parallel to one another and

perpendicular to the planes of Purkinje cells.• Regular pattern of arrangement allows outputs of

well-controlled duration.• The greater the number of excited Purkinje cells, the

greater their collective duration of response.

Brain Mechanisms of Movement

• Parallel fibers excite Pukinje cells• Pukinje cells transmit inhibitory messages to

the cells in the nuclei of the cerebellum (clusters of cell bodies in the interior of the cerebellum) and the vestibular nuclei in the brain stem

• Messages then sent to the midbrain and the thalamus

The Control of Movement

• The basal ganglia is a group of large subcortical structures in the forebrain responsible for initiating an action not guided by a stimulus

• Comprised of the following structures:– Caudate nucleus– Putamen– Globus pallidus

The Control of Movement

• Caudate nucleus and putamen receive input from the cerebral cortex and send output to the globus pallidus.

• Globus pallidus connects to the thalamus which relays information to the motor areas and the prefrontal cortex.– Inhibits the thalamus

• Basal ganglia select a movement to make by ceasing to inhibit it.

The Control of Movement

• The learning of new skills requires multiple brain areas involved in the control of movement.– Basal ganglia is critical for learning motor

skills, organizing sequences of movement, “automatic” behaviors, and new habits .• Example: driving a car

– Relevant neurons in the motor cortex also increase their firing rate and the pattern of activity becomes more consistent as the skill is learned.

Movement Disorders

• Parkinson’s disease is 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.

• Symptoms also include depression and memory and reasoning deficits and other cognitive deficits.

Movement Disorders

• Caused by gradual and progressive death of neurons, especially in the substantia nigra.

• Substantia nigra 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.

Movement Disorders

• Studies suggest early-onset Parkinson’s has a genetic link.

• Genetic factors are only a small factor of late on-set Parkinson’s disease (after 50).

Movement Disorders

• Exposure to toxins are one environmental influence.– MPTP found in some illegal drugs and

pesticides. – MPTP is converted to MPP+ which

accumulates and destroys neurons that release dopamine.

Movement Disorders

• Cigarette smoking and coffee drinking are related to a decreased chance of developing Parkinson’s disease.

• Research suggests marijuana use increases the risk of Parkinson’s disease.

• Damaged mitochondria of cells seems to be common to most factors that increase the risk of Parkinson’s disease.

Movement Disorders

• The drug L-dopa is the primary treatment for Parkinson’s and is a precursor to dopamine that easily crosses the blood-brain barrier.– Often ineffective and especially for those in

the late stages of the disease.• Does not prevent the continued loss of

neurons.• Enters other brain cells producing unpleasant

side effects.

Movement Disorders

• Other possible treatments for Parkinson’s include:– Antioxidant drugs– Drugs that stimulate dopamine receptors or

block glutamate or adenosine receptors– Neurotrophins to promote neuron survival – Drugs that decrease apoptosis.– High frequency electrical stimulation of the

globus pallidus.– Stimulation of cannabinoid receptors

Movement Disorders

• Implantation of neurons from aborted fetuses remains controversial and only partially effective.

• Only moderate benefits seen.• Stem cells are immature cells grown in tissue

culture that are capable of differentiating and are an attractive alternative.

Movement Disorders

• Huntington’s disease is a neurological disorder characterized by various motors symptoms.– affects 1 in 10,000 in the United States– usually appears between the ages of 30

and 50.• Associated with gradual and extensive brain

damage especially in the caudate nucleus, putamen, globus pallidus, and the cerebral cortex.

Movement Disorders

• Initial motor symptoms include arm jerks and facial twitches.

• Motors 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 and delusions, poor judgment, alcoholism, drug abuse, and sexual disorders.

Movement Disorders

• Presymptomatic tests can identify with high accuracy who will develop the disease.– Controlled by an autosomal dominant gene

on chromosome #4.– The higher the number of consecutive

repeats of the combination C-A-G, the more certain and earlier the person is to develop the disease.

• No treatment is effective in controlling the symptoms or slowing the course of the disease.

Movement Disorders

• A variety of neurological diseases are related to C-A-G repeats in genes.

• For a variety of disorders, the earlier the onset, the greater the probability of a strong genetic influence.

• Identification of the gene for Huntingtin’s disease led to the discovery of the protein that codes it. (huntingtin)

• Mutant form impairs neurons in the brain and future drug therapy may address huntintin.