Neurological Control of Movement

10. Neurological Control of Movement

NEUROLOGICAL CONTROL OF MOVEMENT

A. The structure & Function of the Nervous System

1. The Neuron

Individual nerve fibers or nerve cells are called neurons Neuron is composed of three regions:-

1. The cell body or soma 2. The dendrites 3. The axon

Most neurons contain many dendrites (neuron’s receivers) – receive impulses then carry toward the cell body.

Most neurons have only one axon (neuron’s transmitter) – conducts impulses away from the cell body.

Axon splits near its end into branches called axon terminals (terminal fibrils).

The tips of the axon terminals are called the synaptic knobs containing vesicle (sacs) filled with chemicals, known as neurotransmitter – used for communication between neuron and another cell.

2. The Nerve Impulse

Nerve impulse is an electrical charge – is the signal that passes from one neuron to the next and finally to an end organ.

Resting Membrane Potential (RMP) The cell membrane of a neuron at rest has a negative electrical potential

of about -70 mV. (mV=milivolt) That means, the electrical charges found inside and outside the cell were

differ by 70 mV, and the inside was negative relative to the outside. This potential difference (-70 mV) is called the resting membrane

potential or RMP. It is caused by a separation of charges across the membrane. When the charges across the membrane differ, the membrane is said to be

polarized. The neuron (axon) has a high concentration of potassium ions (K+) on the

inside and a high concentration of sodium ions (Na+) on the outside because the sodium-potassium pump actively moves sodium out of the cell and potassium into the cell.

The “Na-K Pump” moves three (3) Na+ ions out of the cell for each two (2) K+ ions it brings into the cell. The cell membrane is much more permeable to K+ ions, so some of the K+ ions may also move to the outside. The Na+ cannot move in this manner. Therefore, the inside of the cell is more negative than outside, creating the potential difference across the membrane.

Maintenance of a constant RMP of -70mV is primarily a function of the “Na-K pump”.

Depolarization & Hyperpolarization If the inside of the cell becomes less negative relative to the outside, the

potential difference across the membrane will decrease. The membrane will

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be less polarized. When this happen, the membrane is said to be depolarized.

Thus, depolarization happens any time when the charge difference is less than the RMP of -70 mV, moving closer to zero. This is result from a change in the membrane’s Na+ permeability.

The opposite can also occur. If the charge difference across the membrane increases, moving from the RMP to an even more negative number, then the membrane becomes more polarized. This is known as hyperpolarization.

Changes in the membrane potential are signals used to receive, transmit and integrate information within & between cells.

These signals are of two (2) types – graded potentials & action potentials. Both are electrical currents created by the movement of ions

Graded Potentials These are localized changes in the membrane potential – can be either

depolarizations or hyperpolarizations. These are triggered by local changes in the neuron’s local environment.

Action Potentials An action potential is a rapid and substantial depolarization of the neuron’s

membrane. Typically, membrane potential changes from the RMP -70 mV to a value of

+30 mV, and then rapidly returns to its resting value. All action potentials begin as graded potentials. Action potentials are

generated when enough stimulation occurs to cause a depolarization (at least 15 – 20 mV).

That means if the membrane depolarizes from the RMP of -70 mV to a value of -50 mV to -55 mV, the cell will experience an action potential.

The minimum depolarization required to produce an action potential is called the “threshold”.

Any depolarization less than the threshold value of 15 – 20 mV will not result in an action potential. This is the “All-or-None Principle”.

Sequence of Events in an Action Potential1. Increased sodium (Na+) permeability through opening of sodium gates,2. Decreased sodium (Na+) permeability as sodium gates close, &3. Opening of potassium gates and repolarization.

Propagation of the Action Potential The myelin sheath – a fatty sheath that insulates the cell membrane of axons. The gaps between sheaths which are not insulated are called nodes of Ranvier. The action potential appears to jump from one node to the next as it

traverses a myelinated fibers. This is referred to as “saltatory conduction”, a much faster rate (5-50 times faster) of conduction than in unmyelinated fibers of the same size.

Diameter of the neuron – the velocity of nerve impulses transmission is also determined by the neuron’s size.

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The larger diameter neurons conduct nerve impulses faster because larger neurons present less resistance to local current flow.

3. The Synapse

A synapse is the site of impulse transmission from one neuron to another. A synapse between two neurons includes:

1. Axon terminals of the presynaptic neuron (neuron carrying impulse),2. Postsynaptic receptors on the dendrite or cell body of the next neuron, &3. Space (synaptic cleft) between the two neurons.

Impulses are transmitted in 1 direction only. The presynaptic terminals of the axon contain a large number of sac-like

structure, called synaptic vesicles. Synaptic vesicles contain neurotransmitters chemicals. When the impulse reaches the presynaptic terminals, the synaptic vesicles

respond by dumping their chemicals into the synaptic cleft. These neurotransmitters then diffuse across the synaptic cleft to the

postsynaptic neuron’s receptors. Once the postsynaptic receptors bind with the neurotransmitters, the impulse

has been transmitted successfully to the next neuron and can be transmitted onward.

4. The Neuromuscular Junction

The neuromuscular junction is where motor neuron communicates with the muscle fiber.

It involves:-1. Presynaptic axon terminals (motor endplates),2. The synaptic cleft, &3. Receptors on the sarcolemma of the muscle fiber.

The neuromuscular junction functions much like a neural synapse.

5. Neurotransmitters

There are more than 40 identified neurotransmitters. Acetylcholine and Norepinephrine are the two major neurotransmitters

involved in regulating our physiology responses to exercise. A nerve impulse causes the release of neurotransmitters from presynaptic

axon terminals into the synaptic cleft which then diffuse across the cleft and bound to postsynaptic receptors.

Once the neurotransmitter binds to the postsynaptic receptors, the nerve impulse has been successfully transmitted.

Neurotransmitters are then either destroyed by enzymes or return into presynaptic neuron for reuse when the next impulse arrives.

Neurotransmitter binding at the postsynaptic receptors can cause either depolarization ( excitation) or hyperpolarization (inhibition).

6. The Postsynaptic Response

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The incoming impulse may be excitatory or inhibitory. An excitatory impulse causes depolarization, known as an Excitatory

Postsynaptic Potential (ESSP). An inhibitory impulse causes hyperpolarization, known as an Inhibitory

Postsynaptic Potential (IPSP). A single presynaptic terminal is not sufficient to generate enough

depolarization to fire an action potential. Multiple signals are needed which may come from numerous axon terminals

that release neurotransmitters repeatedly and rapidly. Summation is the process of accumulation of the incoming signals. The

summation must reach the threshold for an action potential to be released.

SUMMARY

1. Nerve impulses typically pass from the dendrites to the cell body and from the cell body along the length of the axon to its terminal fibrils.

2. A neuron’s RMP of -70 mV results from the separation of sodium & potassium ions maintained primarily by the sodium-potassium pump, coupled with low sodium permeability & high potassium permeability of the neuron membrane

3. Any change making the membrane potential more positive is a depolarization. Any change making this potential more negative is a hyperpolarization. These changes occur when ion gates in the membrane open, permitting ions to move from one side to the other.

4. If the membrane is depolarized by 15 -10 mV, threshold is reached & an action potential results. Action potentials are not generated if threshold is not met.

5. The chain of events for action potential are: increased sodium permeability through opening of sodium gates, decreased sodium permeability as sodium gates close, and opening of potassium gates and repolarization.

6. In myelinated neurons, the impulse travels through the axon by jumping between nodes of Ranvier (gaps between the cells that form the myelin sheath). This process, salutatory conduction, is 5 to 50 times faster than in unmylinated fibers of the same size.

7. Impulses also travel faster in neurons of larger diameters.

8. Neurons communicate with each other across synapses.

9. A synapse involves: the axon terminals of the presynaptic neuron, the postsynaptic receptors on the dendrite or cell body of the next neuron, & the space (synaptic cleft) between the two neurons.

10. A nerve impulse causes chemicals called neurotransmitters to be released from the presynaptic axon terminals into the synaptic cleft.

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Peripheral Nervous System (PNS)

Cranial Nerves Spinal Nerves

Sensory Division (Afferent)

Motor Division (Efferent)

Autonomic NervousSystem

(Involuntary)

Somatic NervousSystem

(Voluntary)


11. Neurotransmitters diffuse across the cleft and are bound to the postsynapticreceptors.

12. Once neurotransmitters are bound, the impulse has been successfully transmitted and the neurotransmitter is then either destroyed by enzymes or actively returned to the presynaptic neuron for future use.

13. Neurotransmitter binding at the postsynaptic receptors opens the ion gates in that membrane and can cause depolarization (excitation) or hyperpolarization (inhibition), depending on the specific neurotransmitter and the receptors to which it binds.

14. Neurons communicate with muscle cells at the neuromuscular junctions.

15. The neurotransmitters most important to regulation of exercise are acetycholine and norepinephrine.

B. The Central Nervous System (CNS)

The functional organization of the nervous system

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Central Nervous System (CNS)

Brain Spinal Cord


CNS CNS is composed of the brain and the spinal cord. CNS houses more than 100 billion neurons.

1. The Brain

Subdivide into 4 regions:- cerebrum, diencephalon, cerebellum & brain stem.

Cerebrum Composed of the right & left cerebral hemispheres. These are connected to each other by fiber bundles (tracts) referred to as the

corpus callosum, allowing the 2 hemispheres to communicate with each other.

The cerebral cortex (gray matter) forms the outer portion has been referred to as the site of the mind & intellect.

Cerebral cortex is the conscious brain. It allows us to think, to be aware of sensory stimuli, & to voluntary control of movements.

Cerebrum consists of 5 lobes: - 4 outer lobes & the central insula (not discuss)1. Frontal lobe – general intellect & motor control,2. Temporal lobe – auditory input & its interpretation,3. Parietal lobe – general sensory & its interpretation,4. Occipital lobe - visual input & its interpretation.

Diencephalon Composed of the thalamus & the hypothalamus. Thalamus is an important sensory integration center. All sensory input (except smell) enters the thalamus & is relayed to the

appropriate area of the cortex. Thalamus regulates what sensory input reaches the conscious brain, & thus is

very important for motor control.

Hypothalamus, directly below the thalamus, is responsible for maintaining homeostasis by regulating almost all processes that affect the body’s internal environment.

Hypothalamus regulate:-1. the autonomic nervous system (BP, HR, respiration, digestion, etc.),2. body temperature,3. fluid balance,4. neuroendocrine control,5. emotions,6. thirst,7. food intake, &8. sleep-wake cycle.

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Cerebellum Located behind the brain stem. Connected to numerous parts of the brain & has a crucial role in controlling

movement.

Brain stem Composed of the midbrain, the pons, & the medulla oblongata. Is the stalk of the brain, connecting the brain & the spinal cord. All sensory & motor nerves pass through the brain stem as they relay

information between the brain & the spinal cord. Also contains the major autonomic regulatory centers that exert control over

the respiratory & cardiovascular systems.

A specialized collection of neurons running the entire length of the brain stem, known as the reticular formation, are influenced by & have an influence on nearly all areas of the CNS. These neurons help:-1. coordinate skeletal muscle function,2. maintain muscle tone,3. control cardiovascular & respiratory functions, &4. determine our state of consciousness (both arousal & sleep).

The brain has a pain control system, called an analgesia system. The enkephalins & ß-enorphin are important opiate substances that act on the opiate receptors in

the analgesia system to help reduce pain. Exercise of long duration has been postulated to increase the natural levels of these opiate substances.

2. The Spinal Cord

Composed of tracts of nerves fibers that allow two-way conduction of nerves impulses.

The sensory (afferent) fibers carry neural signals from sensory receptors, such those in the muscle & joints, to the upper levels of the CNS.

Motor (efferent) fibers from the brain & upper spinal cord travel down to end organs (muscle, glands).

C. The Peripheral Nervous System (PNS)

PNS contains 43 pairs of nerves: 12 pairs of cranial nerves that connect with the brain, &

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31 pairs of spinal nerves that connect with the spinal cord.

Functionally, the PNS has 2 major divisions: sensory division & motor division.

1. The Sensory Division

The sensory division of PNS carries sensory information from sensory receptors toward the CNS.

Sensory (afferent) neurons originate in such areas: blood & lymph vessels, internal organs, special sense organs (taste, touch, smell, hearing, vision), the skin, and muscles & tendons.

Sensory neurons in the PNS end either in the spinal cord or in the brain.

The sensory division receives information from 5 primary types of receptors:-o Mechanoreceptors that respond to mechanical forces such as pressure,

touch, vibration, or stretch.o Thermoreceprtors that respond to changes in temperature.o Nociceptors that respond to painful stimuli.o Photoreceptors that respond to electromagnetic radiation (light) to allow

vision.o Chemoreceptors that respond to chemical stimuli, such as from foods,

odors, or changes in blood concentrations of substances such as O2, CO2, glucose, electrolytes, & so on.

The nerve endings of mechanoreceptors, thermoreceprtors & nociceptors are important for the prevention of injury during athletic performance.

Special muscle & joint nerve endings are of many types & functions, and each type is sensitive to a specific stimulus:o Joint kinesthetic receptors located in the joint capsules are sensitive to

joint angles & rates of change in these angles. Thus, they sense the position & any movement of the joints.

o Muscles spindles sense how much a muscle is stretched.o Golgi tendon organs detect the tension applied by a muscle to its

tendon, providing information about the strength of muscle contraction.

2. The Motor Division

The motor division of PNS carries motor impulses out from the CNS to various part of the body (target areas – muscles) through the motor (efferent) neurons.

The motor division is divided into 2 components: the autonomic nervous system (involuntary) & the somatic nervous system (voluntary).

3. The Autonomic Nervous System (ANS)

The ANS controls the body’s involuntary internal functions. Some of these functions that are important to sport & activity include heart rate, blood pressure, blood distribution, & respiration.

ANS has 2 major divisions: the sympathetic NS & the parasympathetic NS.

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The effects of the two systems are often antagonistic, but both systems are always functioning together.

The Sympathetic Nervous System

Sympathetic NS is the “fight-or-flight system” : It prepares the body to face a crisis (acute stress or physical activity).

When we are excited, our sympathetic NS produces a massive discharge throughout the body, preparing us for action.

The effects of sympathetic stimulation are important to the athlete: Heart rate & strength of cardiac contraction increase (Heart muscle). Coronary vessels dilate, increasing the blood supply to the heart

muscle to meet its increased demands (Coronary blood vessels). Peripheral vasodilation allows more blood to enter the active skeletal

muscles (Blood vessels). Vasoconstriction in most other tissues diverts blood away from them &

to the active muscles (Blood vessels). Blood pressure increases, allow better perfusion of the muscles &

improving the return of venous blood to the heart (Blood vessels). Bronchodilation improves gas exchange (Lungs). Metabolic rate increases, reflecting the body’s effort to meet the

increased demands of physical activity (Cellular metabolism). Mental activity increases, allowing better perception of sensory stimuli

& more concentration on performance (Brain). Glucose is released from the liver into the blood as an energy source

(Liver). Stimulates lipolysis (Adipose tissue). Increases sweating (Sweat glands). Stimulates secretion of epinephrine & norepinephrine (Adrenal glands). Functions not directly needed are slowed (e.g., renal function,

digestion), conserving energy so that it can be used for action. Causes vasoconstriction; decreases urine formation (Kidney). Decreases activity of glands & muscles; constricts sphincters

(Digestive System).The Parasympathetic Nervous System

Parasympathetic NS is the body’s “housekeeping system” : It has a major role in carrying out such processes as digestion, urination, glandular secretion, & conservation of energy.

This system is more active when we are calm & at rest. Its effects tend to oppose those of the sympathetic system.

The parasympathetic division causes: Decreased HR & the force of the heart muscle contraction (Heart muscle), constriction of coronary vessels (Coronary blood vessels), bronchoconstriction (Lungs), & Increases peristalsis & glandular secretion; relaxes sphincters (Digestive

System).

D. The Sensory-Motor Integration 9


Sensory-motor integration is the process by which the periphery NS relays sensory input to the CNS & the CNS interprets this information then sends out the appropriate motor signal to elicit the desired motor response.

1. Sensory Input Sensory input can terminate in sensory areas of the brain stem, the

cerebellum, the thalamus, or the cerebral cortex. An area in which the sensory impulses terminate is referred to as an

integration center. This is where the sensory input is interpreted & linked to the motor system.

2. Motor Control Skeletal muscles are controlled by impulses conducted by motor (efferent)

neurons that originate from any of 3 levels: the spinal cord, the lower regions of the brain, & the motor area of the cerebral cortex.

The degree of movement complexity increases from simple reflex control to complicated movements requiring thought processes.

Motor responses for more complex movement patterns typically originate in the motor cortex of the brain.

3. Reflex Activity A motor reflex is a preprogrammed response; any time the sensory nerves

transmit certain impulses, the body responds instantly & identically Reflexes are the simplest form of motor control. They are preprogrammed

responses, therefore not the conscious response. All neural activity occurs extremely rapidly, but the reflex is the fastest mode

of response because the body does not need time to make a conscious decision.

Two reflexes that help control muscle function: 1. Muscle spindles trigger reflexive muscle action when the muscle

spindle is stretched.2. Golgi tendon organs trigger a reflex that inhibits contraction muscles if

the tendon fibers are overstretched.

Muscle Spindles MS are sensory receptors located in the muscle that senses how much the

muscle is stretched. A muscle spindle comprises specialized muscle fibers called intrafusal fibers

(inside the spindle) & these fibers are controlled by specialized motor neurons, called gamma motor neurons.

Golgi Tendon Organs GTO are encapsulated sensory receptors located in muscle tendon fibers that

monitor tension. GTO are sensitive to tension in the muscle tendon & operate like a strain

gauge, a device that senses changes in tension. GTO are inhibitory in nature, performing a protective function by reducing the

potential for injury.

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4. The Higher Brain Centers

Most movements used in sport activities involve control & coordination through the higher brain centers specially: The primary motoe cortex, The basal ganglia, & The cerebellum.

The Primary Motor Cortex PMC which located in the frontal lobe. Neurons here, known as pyramidal

cells, allow us consciously control movement of the skeletal muscles. PMC is responsible for the control of fine discrete muscle movements.

The Basal Ganglia Basal ganglia (nuclei) located in the cerebral white matter, deep to the

cortex. BG are known to be important in the initiation of movements of a sustained &

repetitive nature (such as arm swinging while walking), & thus they control complex semivoluntary movements such as walking & running.

BG also involved in maintaining posture & muscle tone.

The Cerebellum Cerebellum is crucial to control of all rapid & complex muscular activities. It helps coordinate the timing of motor activities & the rapid progression from

one movement to the next by monitoring & making corrective adjustments in the motor activities that are elicited by other parts of the brain.

It assists the functions of both the primary motor cortex & the basal ganglia. It facilitates movement patterns by smoothing out the movement, which

would otherwise be jerky & uncontrolled.

5. Engrams

Specific learned motor patterns appear to be stored in the brain, to be replayed on request. These memorized motor patterns are referred to as motor programs, or engrams.

E. The Motor Response

1. The Motor Unit

The motor nerve (neuron) and the group of muscle fibers it innervates form a single motor unit.

Each muscle fiber is innervated by only one motor neuron, but each motor neuron can innervates up to several thousand muscle fibers.

All muscle fibers within a single specific motor unit are homogeneous with respect to fiber type. Thus we do not find a motor unit has both FT & ST fibers.

2. The Orderly Recruitment of Muscle Fibers & the Size Principle

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Motor units are generally activated on the basis of a fixed order of recruitment. This is known as the principle of orderly recruitment.

Motor unit are recruited in an orderly manner, therefore specific ones are called on each time a specific activity is performed.

The size principle explained that the order of recruitment of motor units is directly related to their motor neuron size.

Motor units with smaller neurons (ST fibers) will be recruited first before larger neurons (FT fibers).

SUMMARY

1. The central nervous system is composed of the brain and the spinal cord.

2. The 4 major divisions of the brain are the cerebrum, the diencephalon, the cerebellum & the brain stem.

3. The cerebral cortex is the conscious brain.

4. The diencephalon includes the thalamus, which reveices all sensory input entering the brain & the hypothalamus, which is a major control center for homeostasis.

5. The cerebellum, which is connected to numerous parts of the brain, is critical for coordinating movement.

6. The brain stem is composed of the midbrain, the pons, & the medulla oblongata.

7. The spinal cord carries both sensory & motor fibers between the brain and the periphery.

8. The PNS contains 43 pairs of nerves: 12 pairs of cranial nerves & 31 pairs of spinal nerves.

9. The PNS subdivided into sensory division & motor division. The motor division also includes the autonomic nervous system.

10. The sensory division carries information from sensory receptors to the CNS so that the CNS is constantly aware of the current status & environment.

11. The motor division carries motor impulses out from the CNS to the muscles.

12. The autonomic nervous system includes the sympathetic NS, which is the fight-or-flight system, & the parasympathetic NS, which is the housekeeping system. Though these systems often oppose each other, they always function together.

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13. Sensory-motor integration is the process by which the PNS relays sensory input to the CNS and the CNS interprets this information then sends out the appropriate motor signal to elicit the desired motor response.

14. Sensory input can terminate at various levels of the CNS. Not all information reaches the brain.

15. Reflexes are the simplest form of motor control. They are not the conscious response. For a given sensory stimulus, the motor response is always identical and instantaneous.

16. The level of nervous system control varies in response to sensory input according to the complexity of movement necessary. Simple reflexes are handled by the spinal cord, whereas complex reactions require involvement of the brain.

17. Muscle spindles trigger reflexive muscle action when the muscle spindle is stretched.

18. Golgi tendon organs trigger a reflex that inhibits contraction if the tendon fibers are overstretched.

19. The primary motor cortex, located in the frontal lobe, is the center of conscious motor control.

20. The basal ganglia, in the cerebral white matter, help initiate some movements (sustained & repetitive ones) & help control posture & muscle tone.

21. The cerebellum is involved in all rapid & complex movement processes & assists the primary motor cortex & the basal ganglia in coordinating the response. It is an integration center that decides how to best execute the desired movement, given the body’s current position & the muscle’s current status.

22. Though not well understood, engrams are memorized motor patterns, stored in both the sensory & motor areas of the brain, that are called upon as needed.

23. Each muscle fiber is innervated by only one motor neuron, but each neuron can innervates up to several thousand muscle fibers.

24. All muscle fibers within a single motor unit are of the same fiber type.

25. Motor units are recruited in an orderly manner, so that specific ones are called on each time a specific activity is performed.

26. Motor units with smaller neurons (ST fibers) are called on before those with larger neurons (FT fibers).

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Neurological Control of Movement