Peripheral nervous system

The PNS contains 43 pairs of nerves: 12 pairs of cranial nerves that connect with the brain and 31 pairs of spinal nerves that connect with the spinal cord. Cranial and spinal nerves directly supply the skeletal muscles. Functionally, the PNS has two major divisions: the sensory division and motor division.

Sensory division

The sensory division of the PNS carries sensory information toward the CNS. Sensory (afferent) neurons originate in such areas as:

• Blood and lymph vessels;
• Internal organs;
• Special sense organs(taste, touch, smell, hearing, vision);
• The skin;
• Muscles and tendons.

Sensory neurons in the PNS end either in the spinal cord or in the brain, and they continuously convey information to the CNS concerning the body’s constantly changing status. By relaying this information, these neurons allow the brain to sense what is going on in all parts of the body and in the immediate environment. Sensory neurons within the CNS carry the sensory input to appropriate areas, where the information can be processed and integrated with other incoming information.

The sensory division receives information from five primary types of receptors:

1)      Mechanoreceptors that respond to mechanical forces such as pressure, touch, vibrations, or stretch.

2)      Thermoreceptors that respond to changes in temperature.

3)      Nocireceptors that respond to painful stimuli.

4)      Photoreceptors that respond to electromagnetic radiation(light) to allow vision.

5)      Chemoreceptors that respond to chemical stimuli, such as from foods, odors, or changes in blood or tissue concentrations of substances such as oxygen, carbon dioxide, glucose and electrolytes

Several of these receptors are important in exercise and sport. Let’s consider just a few. Free nerve endings detect crude touch, pressure, pain, heat, and cold. Thus, they function as mechanoreceptors, nocireceptors, and thermoreceptors. These nerve endings are important for preventing injury during athletic performance. Special muscle and joint nerve endings are of many types and functions, and each type is sensitive to a specific stimulus. Here are some important examples:

·         Joint kinesthetic receptors located in the joint capsules are sensitive to joint angles and rates of change in these angles. Thus, they sense the position and any movement of the joints.

·         Muscle spindles sense muscle length and rate of change in length.

·         Golgi tendon organs detect the tension applied by a muscle to its tendon, providing information about the strength of muscle contraction.

Motor division

The CNS transmits information to various parts of the body through the motor, or efferent, division of the PNS. Once the CNS has processed the information it receives from the sensory division, it decides how the body should respond to that input. From the brain and spinal cord, intricate networks of neurons go out to all parts of the body, providing detailed instructions to the target areas – for our purposes, muscles.

Autonomic nervous system

The autonomic nervous system, often considered part of the motor division of the PNS, controls the body’s involuntarly internal functions. Some of these functions that are important to sport and activity include heart rate, blood pressure, blood distribution, and lung function.

The autonomic nervous system has two major divisions: the sympathetic nervous system and the parasympathetic nervous system. These originate from different sections of the spinal cord and from the base of the brain. The effects of the two systems are often antagonistic, but the systems always function together.

Sympathetic nervous system

The sympathetic nervous system is sometimes called the fight-or-flight system: It prepares the body to face a crisis and sustains its function during that crisis. When  excited, the sympathetic nervous system produces a massive discharge throughout the body, preparing it for action. A sudden loud noise, a life-threatening situation, or those last few seconds before the start of an athletic competition are examples of circumstances in which this massive sympathetic discharge occurs. The effects of sympathetic stimulation are important to the athlete:

• Heart rate and strength of cardiac contraction increase.
• Coronary vessels dilate, increasing the blood supply to the heart muscle to meet its increased demands.
• Peripheral vasodilatation allows more blood to enter the active skeletal muscles.
• Vasoconstriction in most other tissues diverts blood away froom them and to the active muscles.
• Blood pressure increases, allowing better perfusion of the muscles and improving the return of venous blood to the heart.
• Bronchodilatation improves gas exchange.
• Metabolic rate increases, reflecting the body’s effort to meet the increased demands of physical activity.
• Mental activity increases, allowing better perception of sensory stimuli and more concentration on performance.
• Glucose is released from the liver into the blood as an energy source.
• Functions not directly needed are slowed(e.g. renal function, digestion), conserving energy so that it can be used for action.

These basic alterations in bodily function facilitate motor responses, demonstrating the importance of the autonomic nervous system in preparing the both for and sustaining it during acute stress or physical activity.

Parasympathetic nervous system

The parasympathetic nervous system is the body’s house-keeping system. It has a major role in carrying out such processes as digestion, urination, glandular secretion, and conservation of energy. This system is more active when one is calm and at rest. Its effects tend to oppose those of the sympathetic system. The parasympathetic division causes decreased heart rate, constriction of coronary vessels, and bronchoconstriction.

The various effects of the sympathetic and parasympathetic divisions of the autonomic nervous system are summarized in the table below.

Effects of the sympathetic and parasympathetic nervous system on various organs
Target organ or system	Sympathetic effects	Parasympathetic effects
Heart muscle	Increases rate and force of contraction	Decreases rate of contraction
Heart: coronary blood vessels	Cause vasodilatation	Cause vasoconstriction
Blood vessels	Increase blood pressure; cause vasoconstriction in abdominal viscera and skin to divert blood when necessary; cause vasodilatation in the skeletal muscles and heart during exercise	Little or no effect
Liver	Stimulates glucose release	No effect
Cellular metabolism	Increases metabolic rate	No effect
Adipose tissue	Stimulates lypolysis	No effect
Sweat glands	Increase sweating	No effect
Adrenal glands	Stimulate secretion of epinephrine and norepinephrine	No effect
Digestive system	Decreases activity of glands and muscles; constricts sphincters	Increases activity of glands and muscles; relaxes sphincters
Kidney	Causes vasoconstriction, decreases urine formation	No effect

Central nervous system(CNS)

 To comprehend how even the most basic stimulus can cause muscle activity, we must next consider the complexity of the CNS.

Brain

The brain is composed of numerous parts. For our purposes, we subdivide it into the four major regions: the cerebrum, diencephalons, cerebellum, and brain stem.

Cerebrum

The cerebrum is composed of the right and left cerebral hemispheres. These are connected to each other by fiber bundles(tracts) reffered to as the corpus callosum, which allows the two hemispheres to communicate with each other. The cerebral cortex forms the outer portion of the cerebral hemispheres and has been referred to as the site of the mind and intellect. It is also called the gray matter, which simply reflects its distinctive color resulting from lack of myelin on the cell bodies located in this area. The cerebral cortex is the conscious brain.

It allows people to think, to be aware of sensory stimuli, and to voluntarily control their movements.

The cerebrum consists of five lobes – four outer lobes and the central insula. Its four outer lobes have the following general functions:

• Frontal lobe: general intellect and motor control ;
• Temporal lobe: auditory input and its interpretation ;
• Parietal lobe: general sensory input and its interpretation ;
• Occipital lobe: visual input and its interpretation.

The three areas in the cerebrum that are of primary concern to our discussion and that we discuss later in this chapter are the primary motor cortex, in the frontal lobe; the basal ganglia, in the white matter below the cerebral cortex; and the primary sensory cortex, in the parietal lobe.

Diencephalon

The region of the brain known as the diencephalons is composed mostly of the thalamus and the hypothalamus. The thalamus is an important sensory integration center. All sensory input(except smell) enters the thalamus and is relayed to appropriate area of the cortex. The thalamus regulates what sensory input reaches the conscious brain and thus is very important for motor control.

The hypothalamus, directly below the thalamus, is responsible for maintaining homeostasis by regulating almost all processes that affect the body’s internal environment. Neural centers here assist in the regulation of:

• Blood pressure, heart rate and contractility, respiration, and digestion;
• Body temperature;
• Fluid balance;
• Neuroendocrine control;
• Emotions;
• Thirst;
• Food intake;
• Sleep-wake cycles.

Cerebellum

The cerebellum is located behind the brain stem. It is connected to numerous parts of the brain and has a crucial role in coordinating movement.

Brain stem

The brain stem, composed of the midbrain, the pons, and the medulla oblongata; is the stalk of the brain, connecting the brain and the spinal cord. Sensory and motor neurons pass through the brain stem as they relay information between the brain and the spinal cord. This is the site of origin for 10 of the 12 pairs of cranial nerves. The brain stem also contains the major autonomic regulatory centers that control the respiratory and cardiovascular systems.

A specialized collection of neurons in the brain stem known as the reticular formation, is influenced by, and has an influence on, nearly all areas of the CNS. These neurons help:

• Coordinate skeletal muscle function;
• Maintain muscle tone;
• Control cardiovascular and respiratory functions;
• Determine our state of consciousness(both arousal and sleep).

The brain has a pain control system, called an analgesia system. The enkephalins and beta-endorphin are important opiate substances that act on the opiate receptors in the analgesia system to help reduce pain. Research has demonstrated that exercise of long duration increases the natural levels of these opiate substances.

Spinal cord

The lowest part of the brain stem, the medulla oblongata, is continuous with the spinal cord below. The spinal cord is composed of tracts of nerve fibers that allow two-way conduction of nerve impulses. The sensory(afferent) fibers carry neural signals from sensory receptors, such as those in the skin, muscles, and joints, to the upper levels of the CNS. Motor(efferent) fibers from the brain and upper spinal cord transmit action potentials to end organs.

Postsynaptic response

Once the neurotransmitter binds to the receptors, the chemical signs that traversed the synaptic cleft once again becomes and electrical signal. The binding causes a graded potential in the postsynaptic membrane. An incoming impulse may be either excitatory or inhibitory. An excitatory impulse causes depolarization, known as an excitatory postsynaptic potential(EPSP). An inhibitory impulse causes a hyperpolarization, known as an inhibitory postsynaptic potential(IPSP).

The discharge of a single presynaptic terminal generally changes the postsynaptic potential less than 1mV. Clearly this is not sufficient to generate an action potential, because reaching the threshold requires a change of at least 15 to 20 mV. But when a neuron transmits an impulse, several presynaptic terminals typically release their neurotransmitters so that they can diffuse to the postsynaptic receptors. Also, presynaptic terminals from numerous axons can converge on the dendrites and cell body of a single neuron. When multiple presynaptic terminals discharge at the same time, or when only a few fire in rapid succession, more neurotransmitter is released. With an excitatory neurotransmitter, the more that is bound, the greater the EPSP will be.

Triggering an action potential at the postsynaptic neuron depends on the combined effects of all incoming impulses from these various presynaptic terminals. A number of impulses are needed to cause sufficient depolarization to generate an action potential. Specifically, the sum of all changes in the membrane potential must equal or exceed the threshold. This summing of the individual impulses’ effect is called summation.

For summation, the postsynaptic cell must keep a running total of the neuron’s responses, both EPSPs and IPSPs, to all incoming impulses. This task is done at the axon hillock, which lies on the axon just past the cell body. Only when the sum of all individual graded potentials meets or exceeds the threshold can an action potential occur.

Individual neurons are grouped together into bundles. In the CNS(brain and spinal cord), these bundles are referred to as tracts, or pathways. Neuron bundles in the PNS are reffered to as nerves.

Top corner save!!!

Brian Rowe Last Second Jedi Save - UCLA Soccer 2011 Final Four

Neuromuscular junctions and neurotransmitters

Neuromuscular junction

Whereas neurons communicate with other neurons at synapses, an alpha-motor neuron communicates with muscle fibers at a site known as a neuromuscular junction. The function of the neuromuscular junction is essentially the same as that of a synapse. In fact, the proximal part of the neuromuscular junction is the same: It starts with the axon terminals of the motor neuron, which release neurotransmitters into the space between the motor nerve and the muscle fiber in response to an action potential. However, in the neuromuscular junction, the axon terminals protrude into motor end plates, which are troughlike segments on the plasmalemma. Picture below shows.

The motor end plate is invaginated(folded to form cavities). The cavity thus formed is called the synaptic gutter. As with synapses, the space between the neuron and the muscle fiber is the synaptic cleft.

Neurotransmitters released from the alpha-motor neuron axon terminals diffuse across the synaptic cleft and bind to receptors on the muscle fiber’s plasmalemma. This binding typically causes depolarization by opening sodium ion channels, allowing more sodium to enter the muscle fiber. As always, if the depolarization reaches the threshold, an action potential is formed. It spreads across the plasmalemma into the T-tubules, inititating muscle fiber contraction. As in the neuron, the plasmalemma, once depolarized, must undergo repolarization. During the period of repolarization, the sodium gates are closed and the potassium gates are open; thus, like the neuron, the muscle fiber is unable to respond to any further stimulation. This period is reffered to as the refractory period. Once the electrical conditions of the muscle fiber are restored to resting levels, the fiber can respond to another stimulus. Thus, the refractory period limits the motor unit’s firing frequency.

Now we know how the impulse is transmitted between two cells. But to understand what happens once the impulse is transmitted, we must first examine the chemical signals that accomplish transmission.

Neurotransmitters

More than 50 neurotransmitters have been positively identified or are suspected as potential candidates. These can be cathegorized as either (a) small-molecule, rapid-acting neurotransmitters or (b) neuropeptide, slow-acting neurotransmitters. The small-molecule, rapid-acting transmitters, which are responsible for most neural transmissions, are our main concern.

Acetylholine and norepinephrine are the two major neurotransmitters involved in regulating our physiological responses to exercise. Acetylholine is the primary neurotransmitter for the motor neurons that innervate skeletal muscle and for most parasympathetic neurons. It is generally an excitatory neurotransmitter, but it can have inhibitory effects at some parasympathetic nerve endings, such as in the heart. Norepinephrine is the neurotransmitter for the most sympathetic neurons, and it too can be either excitatory or inhibitory, depending on the receptors involved.

Once the neurotransmitters binds to the post-synaptic receptor, the nerve impulse has been successfully transmitted. The neurotransmitter is then either degraded by enzymes, actively transported back into the presynaptic terminals for reuse, or diffused away from the synapse.

Synapse

For a neuron to communicate with another neuron, first in action potential must occur. Once the action potential occurs, it travels the full length of the axon, ultimately reaching the axon terminals. How does the action potential move from the neuron in which it starts to another neuron?

Neurons communicate with each other across synapses. A synapse is the site of action potential transmission from one neuron to another. There are both chemical and mechanical synapses, but the most common type is the chemical synapse, which is our focus. As seen in the figure up, a synapse between two neurons includes:

• The axon terminals of the neuron sending the action potential;
• Receptors on the neuron receiving the action potential, and
• The space between these structures.

The neuron sending the action potential across the synapse is called the presynaptic neuron, so axon terminals are presynaptic terminals. Similarly, the neuron receiving the action potential on the opposite side of the synapse is called the postsynaptic receptors. The axon terminals and postsynaptic receptors are not physically in contact with each other. A narrow gap, the synaptic cleft, separates them.

The action potential can be transmitted across a synapse in only one direction: from the axon terminals of the presynaptic neuron to the postsynaptic receptors, usually on the dendrites, of the postsynaptic neuron. Impulses also can go directly to receptors on the cell body: about 5% to 20% of the axon terminals are adjacent to the cell body instead of the dendrites. Why can the action potential go in only one direction?

The presynaptic terminals of the axon contain a large number of saclike structures, called synaptic vesicles. These sacs contain neurotransmitter chemicals. When the impulse reaches the presynaptic axon 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. The postsynaptic receptors bind the neurotransmitter once it diffuses across the synaptic cleft. When this binding occurs, the impulse has been transmitted successfully to the next neuron and can be transmitted onward.

Nerve impulse

A nerve impulse – an electrical charge – is the signal that passes from one neuron to the next and finally to an end organ, such as a group of muscle fibers, or back to the CNS. For simplicity, think of the nerve impulse traveling through a neuron much as electricity travels through the electrical impulse is generated and how it travels through a neuron.

Resting membrane potential

The cell membrane of a neuron at rest has a negative electrical potential of about -70mV. This means that if one were to insert a voltmeter probe inside the cell, the electrical charges found there and the charges found outside the cell would differ by 70mV, and the inside would be negative relative to the outside. This electrical potential difference is known as the resting membrane potential(RMP). It is caused by a separation of charges across the membrane differ, the membrane is said to be polarized.

The neuron has a high concentration of potassium ions(K⁺) on the inside of the membrane and a high concentration of sodium ions(Na⁺) on the outside. The imbalance in the number of ions inside and outside the cell causes the RMP. The imbalance in the number of ions inside and outside the cell causes the RMP. This imbalance is maintained in two ways. First, the cell membrane is much more permeable to K⁺ than to Na⁺, so the K⁺ can move more freely. Because ions tend to move to establish equilibrium, some of the K⁺ will move to an area where it is less concentrated, outside the cell. The Na⁺ cannot move to the inside as easily. Second, sodium-potassium pumps in the neuron membrane, which contain Na⁺-K⁺ adenosine triphosphatase(ATPase), maintain the imbalance on each side of the membrane by actively transporting potassium ions in the sodium ions out. The sodium-potassium pump moves three Na⁺ out of the cell for each two K⁺ it brings in. The end result is that more positively charged ions are outside the cell than inside, creating the potential difference across the membrane. Maintenance of a constant RMP of about -70mV is primarily a function of the sodium-potassium pump.

Depolarization and hyperpolarization

If the inside of the cell becomes less negative relative to the outside, the potential difference across the membrane decreases. The membrane will be less polarized. When this happens, the membrane is said to be depolarized. Thus, depolarization occurs any time the charge difference becomes less than the RMP of -70mV, moving closer to zero. This typically results 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 value, then the membrane becomes more polarized. This is known as hyperpolarization. Changes in the membrane potential are actually signals used to receive, transmit, and integrate information within and between cells. These signals are of two types, graded potentials and action potentials. Both are electrical currents created by the movement of ions.

Graded potentials

Graded potentials are localized changes in the membrane potential. These changes can be either depolarizations or hyperpolarizations. The membrane contains ion channels with ion gates that act as doorways into and out of the neuron. These gates are usually closed, preventing ion flow; but they open with stimulation; allowing ions to move from the outside to the inside or vice versa. This ion flow alters the charge separation, changing the polarization of the membrane.

Graded potentials are triggered by a change in the neuron’s local environment. Depending on the location and type of neuron involved, the ion gates may open in response to the transmission of an impulse from another neuron or in response to sensory stimuli such as changes in chemical concentrations, temperature, or pressure.

Recall that most neuron receptors are located on the dendrites( although some are on the cell body), yet the impulse is always transmitted from the axon terminals at the opposite end of the cell. For a neuron to transmit an impulse, the impulse must travel almost the entire length of the neuron. Although a graded potential may result in depolarization of the entire cell membrane, it is usually just a local event such that the depolarization does not spread very far along the neuron. To travel the full distance, an impulse must generate an action potential.

Action potentials

An action potential is a rapid and substantial depolarization of the neuron’s membrane. It usually lasts about 1ms. Typically, the membrane potential changes from the RMP of about -70mV to a value of about +30mV and then rapidly returns to its resting value. This is illustrated in the picture below. How does this marked change in membrane potential occur?

All action potentials begin as graded potentials. When enough stimulation occurs to cause a depolarization of at least 15 to 20mV, an action potential results. In other words, if the membrane depolarizes from the RMP of -70mV to a value of -50 to -55mV, the cell will experience an action potential. The membrane voltage at which a graded potential becomes an action potential is called the depolarization threshold. Any depolarization that does not attain the threshold will not result in an action potential. For example, if the membrane potential changes from the RMP of -70mV to -60mV, the change is only 10mV and does not reach the threshold; thus, no action potential occurs. But any time depolarization reaches or exceeds the threshold, an action potential will result. This is the all-or-none principle.

When a given segment of an axon is generating an action potential and its sodium gates are open, it is unable to respond to another stimulus. This is reffered to as the absolute refractory period. When the sodium gates are closed, the potassium gates are open, and repolarizing is occurring, the segment of the axon can then respond to a new stimulus, but the stimulus must be of substantially greater magnitude to evoke an action potential. This is reffered to as the relative refractory period.

Propagation of the action potential

Now that we understand how a neural impulse, in the form of an action potential, is generated, we can look at how the impulse is propagated, or how it travels through the neuron. Two characteristics of the neuron become particularly important when we consider how quickly an impulse can pass through the axon: myelination and diameter.

Myelin sheath

The axons of many neurons, especially large neurons, are myelinated, meaning they are covered with a sheath formed by myelin, a fatty substance that insulates the cell membrane. This myelin sheath is formed by specialized cells called Schwann cells.

The sheath is not continuous. As it spans the length of the axon, the myelin sheath exhibits gaps between adjacent Schwann cells, leaving the axon uninsulated at those points. These gaps are reffered to as nodes of Ranvier. The action potential appears to jump from one node to the next as it transverses a myelinated fiber. This is reffered to as salutatory conduction, a much faster type of conduction than occurs in unmyelinated fibers.

Myelination of peripheral motor neurons occurs over the first several years of life, partly explaning why children need time to develop coordinated movement. Individuals affected by certain neurological diseases, such as MS as discussed in our chapter opening, experience degeneration of the myelin sheath and a subsequent loss of coordination.

Diameter of the neuron

The velocity of nerve impulse transmission is also determined by the neuron’s size. Neurons of larger diameter conduct nerve impulses faster than neurons of smaller diameter because larger neurons present less resistance to local current flow.