lll. Somatic and Autonomic Nervous System

frontal lobes - programming and coordination of voluntary muscle movement

parietal lobes - receive sensory input

temporal lobes - speech comprehension; associated with language both written and

spoken; primary center for hearing

Wernicke’s area - in the posterior part of the temporal lobe; associated wih the ability

to speak intelligibly and to understand spoken and written language

occipital lobes - vision; primary visual area

cerebral hemispheres - control both the motor and sensory reception of the opposite

side of the body

left hemisphere - associated verbal and written language, as well as analytical ability

Broca’s area - primary speech area located in left hemisphere; lesions in this part

of the brain interfere with speech.

right hemisphere - emotional behavior; the processing of information that has to do

with the whole; for example, the identification of someone we

know from a picture of that person

medulla- controls cardiovascular and respiratory function

pons - contains nerve tracts that connect the spinal cord to other areas of the brain

cerebellum - receives information from the spinal cord relating to the position of

muscle groups in many parts of the body; controller of muscle tone

thalamus - receives input from sensory fibers; relay station of the brain

hypothalamus - regulates many of the homeostatic mechanisms of the body; regulates

biological rhythms such as sleeping and waking, body temperature,

blood pressure, salt and water balance, heart rate and behavioral

drives such as thirst, hunger and sex.

limbic system - ring of structures that plays an important role in emotion, behavior and

motivation.

reticular formation - promotes arousal, alertness and attention

A neurotransmitter is a chemical messenger released by a neuron to influence another nearby neuron or other effector cell, such as a muscle cell.

A receptor is a protein in the cell membrane that functions to transmit information across the cell membrane.

The major neurotransmitters we talked about earlier, acetycholine, epinephrine, norepinephrine and dopamine are also important for communication between neurons in the brain.

However, in addition to these chemicals, others play important roles in transferring information from one neuron to another.

Monoamines are modified amino acids including epinephrine, norepinephrine, serotonin, histamine and dopamine.

Serotonin is very important neurotransmitter in the brain. It has several different receptors.

Low levels of serotonin are considered to be an important factor in depression.

Drugs that increase the levels of serotonin in the brain have been successfully used for the treatment of depression.

This class of neurotransmitters is composed of short chains of amino acids.

Some of these neuropeptides, such as arginine, vasopressin and angiotensin ll, are also hormones.

Within this class of neurotransmitters are the enkephlins and endorphins, which are sometimes called opioid neurotransmitters. They bind the same receptor sites as does morphine.

Endorphins create a general sense of well-being ranging from reduced perception of pain to euphoria.

The well-known runner’s high is believed to result from the release of endogenous opioid neurotransmitters.

In addition to these neuropeptides many others that have been isolated and shown to affect neurotransmission and their number gets larger every year.

The brain makes many polypeptide neurotransmitters that play some still unknown role in neurotransmission and storage of information in the brain.

Nitric Oxide

Nitric Oxide (NO)

Nitric oxide is a new neuroactive compound that has been discovered and seems to have wide biological effects.

NO is a very reactive gas and is a highly toxic one.

NO is one of a class of highly reactive compounds known as free radicals; these chemicals, generated normally, are thought to be damaging to cellular molecules.

Fortunately, each cell possesses other chemicals or enzymes that rapidly destroy or inactivate free radicals.

Many neurons of the CNS, as well as other cell types, contain receptors for NO, and these receptors control many functions.

NO has been shown to play an important role in regulating blood pressure by its action on the cardiovascular center of the brain.

lll. Somatic and Autonomic Nervous System

efferent pathway - outgoing signal from the CNS to the periphery

afferent pathway - incoming pathway connects a sensory receptor to the CNS

effector or effector cell - a cell that carries out a response; for example, a muscle cell contracts in response to a signal from a motor neuron.

The cranial nerves that exit the brain through openings in the skull and the spinal nerves that leave the spinal cord make up the somatic nervous system.

The somatic nervous system is the voluntary nervous system.

Sensory nerves supply information that is generally experienced at the conscious level through vision, hearing, and smell, as well as through muscles, joints and skin.

The output of the somatic nervous system is primarily to skeletal muscle, that is under voluntary control.

The flow information in spinal nerves and the spinal cord

The dorsal roots carry information to the CNS via nerves called afferent nerves.

The ventral roots carry information away from the CNS via efferent nerves.

In addition, within the spinal cord, branches of the afferent and efferent nerves, may travel up or down the spinal cord, activating other neurons at higher or lower levels.

Sensory information from receptors that are consciously felt, such as pain and temperature, travel along a spinal nerve but shortly before entering the spinal cord diverge from that nerve to form the dorsal root.

After diverging, the fibers enter a small swelling in the dorsal root called the dorsal root ganglion.

The dorsal root ganglion contains the cell bodies of sensory fibers, which cause the swelling in the nerve root.

The axon of the sensory fiber continues out of the dorsal root ganglion into the spinal cord, where it may take one of many routes.

In the spinal cord the sensory fiber can:

(1) synapse directly with a motor fiber

(2) synapse with another fiber called an interneuron which, in turn, synapses with a

motor fiber. The interneuron may synapse with a motor fiber at the same level as

entry, or travel up or down the cord to synapse with a motor fiber in another level of

the spinal cord.

3) Alternatively, the sensory fiber might travel up the spinal cord to the brain, where a

particular sensation is registered.

Cell bodies of the motor fibers lie within the spinal cord and their axons leave the cord at the same level as the cell body.

They then travel through the ventral roots, joining with the dorsal roots to form a spinal nerve.

The axon continues along the spinal nerve to specific muscle groups served by that spinal nerve.

From this description, it is evident that the spinal nerves are mixed nerves containing both efferent and afferent fibers..

The division in the spinal cord between the areas containing cell bodies, the gray area, and the area containing axons is easily seen.

The innermost part of the spinal cord is butterfly-shaped, containing a high density of cell bodies, which are not covered with myelin.

Surrounding the gray matter is white matter, containing the axons of the myelinated nerve fibers.

Ventral horn is the region of the spinal cord that contains the cell bodies of motor neurons..

Dorsal horn is the region of the spinal cord that contains the cell bodies of sensory neurons.

A final set of axon bundles must be added to complete the wiring diagram of the motor nerves of the spinal cord.

Voluntary movement begins with a conscious effort originating in cell bodies in the cortex of the brain, signaling cells in the motor cortex.

Axons from these cells travel through the lower parts of the brain where they cross to the opposite side at the level of the medulla.

Another system, the pyramidal motor system travels down the spinal cord to the level at which they synapse with a motor neuron.

They exit the spinal cord at the level of this synapse.

It is this crossing that accounts for the fact that right side of the brain controls movement on the left side of the body and the left motor cortex controls muscle movement on the right side of the body.

What is important to understand is that motor fibers originate in the cortex, follow discrete paths through the spinal cord, and synapse with the final motor neuron called the lower motor neuron.

The axons of the pyramidal fibers extend from the brain to the level of the spinal cord at which postsynaptic muscle fiber exits the cord. The axons of pyramidal cells may be three or more feet in length.

Although the pyramidal motor system is responsible for innervating muscle groups, other motor neurons also contribute to control of muscle movement.

An extrapyramidal system of neurons, arising from the cerebellum and other structures in the brain, monitors the activity of the pyramidal system and coordinates overall muscle movement so that it is smooth and well-directed.

In the previous section you saw that the somatic nervous system receives sensory input from the skin, muscle, viscera, and sense organs, all of which are perceived at the conscious level.

The efferent side of the somatic system is also under conscious control and sends fibers to voluntary muscles.

However, there is another set of nerves that are called autonomic, as they are generally involved in activity that does not reach the conscious level.

The sympathetic nervous system prepares the individual for flight-or-flight responses.

Activation of the sympathetic nervous system causes increased heart rate, decreased blood flow to the gastrointestinal tract, and decreased motility, reduced blood flow to the skin, and other responses that prepare an individual for a potentially traumatic experience.

The parasympathetic nervous system in many ways acts in the reverse fashion of the sympathetic system.

In general, the nerve fibers of the autonomic nervous system regulate the internal environment.

They convey sensory (afferent) input from internal organs such as the glands and the heart and detectors throughout the body that sense blood pressure, gas concentrations in the blood, ion concentrations in the blood, and a host of other detectors that monitor the internal environment.

The efferent nerves of the autonomic nervous system regulate glandular and visceral activity as well as heart and smooth muscle of the circulatory system.

Although the autonomic nervous system functions as a unit in that it controls the homeostatic balance of the body, two distinct divisions of the autonomic nervous exist exist: the sympathetic and parasympathetic divisions.

The anatomy of these divisions is different from that of the somatic nervous system, as some of the cell bodies of the sympathetic and parasympathetic nervous systems are located outside of the spinal column and brain.

Small areas containing cell bodies are called ganglia. In the autonomic nerves located outside the brain they are seen as small swellings.

The autonomic nerves that exit the spinal cord and enter ganglia synapse with cell bodies within the ganglia, and so are called preganglionic fibers.

Almost all preganglionic fibers release acetylcholine from their nerve endings and thus are called cholinergic fibers.

The axons running from the ganglia to the organs of innervation are called postganglionic fibers.

The relationship of the ganglia to the target organ, as well as the type of neurotransmitter released, separates the sympathetic division from the parasympathetic division.

The sympathetic nervous system is also functionally different from the parasympathetic nervous system.

Sympathetic Nervous System

Preganglionic sympathetic fibers exit the spinal cord and enter ganglia near the cord, but relatively far from the target organ.

Most sympathetic ganglia lie in a series along the spinal column called the sympathetic chain.

The sympathetic fibers exit the spinal cord from the posterior horn and travel in the nerves of the posterior root.

They then enter a ganglion at the same spinal level, or the nerve may travel up or down the sympathetic chain to enter a ganglion at another location.

Within the ganglion the fiber synapses with a postganglionic neuron, which in turn may travel some distance before terminating in adrenergic endings the release the neurotransmitter norepinephrine.

General excitation of the sympathetic nerves occurs during situations that require flight-or-fight reactions.

This is the experience you have all encountered when suddenly startled, or when you have some reason to fear for your safety.

Your heart rate speeds up, your pupils dilate so that more light reaches the inner eye, and blood flows away from the skin, making it look pale.

Non-vital functions slow down, such as intestinal digestion and flow of blood to internal organs.

It is as if the sympathetic nervous system were designed to prepare an individual either to flee from some threatening situation or to fight for survival.

Preganglionic fibers of the parasympathetic division exit the CNS from some of the cranial nerves and from the lowest level of the spinal cord, the sacral elements.

Like the sympathetic nervous system, the nerve endings of the preganglionic fibers release acetylcholine.

However, the ganglia containing the nerve bodies of the postganglionic fibers are far removed from the CNS, and are close to the organ they innervate.

Postganglionic fibers release acetylcholine at their nerve endings and, in general, have physiological effects that are opposed to those of the sympathetic fibers.

For example, stimulation of the parasympathetic nerve to the heart, the vagus nerve, causes the heart to slow down.

From the above description you can see that many organs are dually innervated, receiving both sympathetic and parasympathetic fibers.

Generally, both systems are active simultaneously in opposing directions, and depending on the circumstances, one becomes more active as the other is inhibited.

This dual, tonic influence allows for a more finely regulated control system.

It is as if both a brake and an accelerator are applied to the heart simulateously, allowing for a much finer control and more rapid response than if only one system were operative.

Actions of an involuntary type are called reflexes.

Many of them are obvious, such as the blinking of an eye when an object suddenly approaches it, withdrawal of a hand that touches a hot object, and extension of a leg when the patella tendon of the knee is slightly hit in a doctor’s office.

None of these responses require conscious effort.

They are hard-wired into our nervous system and cause movement without conscious intervention.

In addition, many reflexes occur constantly within our bodies and we are never even conscious of them.

Autonomic reflexes control our heart rate, blood pressure, and many other functions that are beyond our ability to detect at the conscious level, although at times we may be aware of a racing heart.

Of all the skeletal muscle reflexes, the simplest ones are those that are monosynaptic.

The neural pathway of the reflex involves only a single synapse.The sensory neuron enters the spinal cord, where it synapses directly with a motor neuron in the ventral horn.

This pathway in which an impulse follows from a receptor to effector, is called a reflex arc.

The patellar reflex is an excellent example of a monosynaptic reflex.

A tap on the patella tendon causes the extensor muscle to stretch suddenly.

Receptors in the muscle sense the sudden, passive stretch, and set up an action potential in the sensory neuron.

Activation of that innervates the stretched extensor muscle causes it to contract, resulting in the knee-jerk response.

Note in this reflex only one sensory neuron and one effector or motor neuron is involved.

The entire reflex arc is contained in the spinal cord, without any involvement of the brain.

This is not to say that the individual is unaware of being tapped on the knee with a rubber hammer. Rather it means the movement at the knee joint does not require nervous input from higher centers.

Many complex spinal reflexes also control many of our responses to various stimuli. A good example is the crossed-extensor reflex withdrawal of a leg when one steps on a tack.

You may not be aware of this, but simultaneously the opposite leg is caused to straighten to allow you to take weight off the injured leg.

The painful stimulus to one foot sends te message to the spinal cord via a sensory nerve. The sensory neuron sends one branch to the same side of the cord and another to the opposite side of the cord, where they synapse with association neurons in the gray matter.

The association neurons, in turn, synapse with motor neurons, innervating both flexor and extensor muscles of both legs.

The flexor muscles of the injured leg are stimulated as the extensor muscles are inhibited, causing withdrawal of the leg.

On the opposite side, the extensor muscles are stimulated and the flexor muscles are inhibited, causing straightening of the opposite leg.

In addition, pain fibers also convey information to the brain and you become consciously aware of the thorn and the pain it causes. But that information is not necessary for reflex withdrawal of the injured leg, which is controlled entirely at the level of the spinal cord; the reflex occurs even when the spinal cord is severed.

The reflexes discussed are built right into the nervous system and require no learning.

There are however other reflexes that may be developed with practice, such as those learned by athletes.

A basketball player who is able to dribble a ball down court while avoiding a defensive players moves reflexly much of the time.

They learn to switch hands or spin around to get around an opposing player.

The player does not need a lot of thinking while making these movements.

Long hours of practice and repetitive movements set up neural circuitry that allows an athlete to perform complex, rapid movements without conscious effort.

An awake state in which we are aware of our thoughts, our surroudings, and our own physical and mental condition. Consciousness resides in well structured electrochemical happenings in the brain.

We are able to measure alterations in brain activity that correlate well with our ability to see, hear, feel and respond to stimuli in terms of altered neuronal activity.

When recording electrodes are placed on the skull one is able to measure very small cyclical alterations in the electrical voltages at the surface of the skull.

The voltage changes reflect the activity of many millions of brain cells. These changes in electrical voltages may be recorded and are called an electroencephalogram or EEG.

An alert person with eyes wide open generates electrical waves. Closing the eyes alters the pattern of the electrical waves.

If an individual is asleep, the EEG activity changes in a well ordered way.

If a person has suffered permanent brain damage and is comatose, the tracing of electrical activity becomes a flat line and is called a flat-line EEG.

Sleep

We said the EEG pattern correlates well with the state of consciousness.

As a person moves from an alert to a less alert state, the overall electrical activity of the brain changes in a well defined pattern.

Thus, there is an electrical correlate of the conscious state.

Additionally, the EEG indicates that sleep is not an absence of brain activity. The brain is very active even in a very deep sleep.

The pattern of the EEG changes during sleep. As an individual changes from drowsinesss to a deep sleep, the EEG slowly changes.

The period of sleep, including dreaming, increased respiration and increased brain metabolism is characterized by rapid eye movements. Although the eyes are closed, they move rapidly almost as if a person is watching a fast tennis match. Because of the rapid eye movements this period of sleep is called REM sleep.

It is also called paradoxical sleep, as it is a paradox that at a time when there seems to be great brain activity, the person is sound asleep.

We have come to accept tha sleep is necessary fro proper brain function, although the mechanism by this happens is not understood.

Sleep may also play an important role in maintaining the immune system in a functional state.

The body contains specific receptors for pain, and the pain fibers can be traced anatomically. However, in contrast to the other senses, no cortical localization may be assigned to pain. Regardless of where the brain is stimulated electrically no pain is experienced.

Unlike other systems, the brain appears to have a built-in system for the suppression of pain. CNS neurons may release opioid neurotransmitters, called endogenous opioids. They have the ability to reduce the perception of pain, apparently by blocking a pain- producing substance called substance P.

The intensity of pain felt is a function not only of the strength of the stimulus but also of psychological factors. Should one sustain an injury during a football game, the pain may not be felt until long after the injury, when the person is no longer intensely involved in the game. It is also evident that there is a large placebo effect. About 30% of people with chronic pain report a lessening of pain if told they are getting morphine but are given an injection of salt water instead.

All learning depends on the train of neurochemical events called memory.

Memory is spoken of as having two types: short-term memory and long-term memory.

Although there is no precise definition of the two terms, it is clear that remembering a name for a day is clearly different from remembering your mother’s birthday.

The neural circuits accounting for memory may be very large. Multiple synapses are involved in both short-term and long-term memory.

The difference between short-term and long-term memory seems to reside in the ease with which the neural circuits may be activated.

It is not possible to define the neurological events that make up any individual memory, but it is possible to understand some of the events that must take place in the CNS for memory to be stored.

In brief, a new event imposes itself on the CNS by causing a particular set of neurons to interact in a unique manner.

The event begins when receptors such as visual, auditory and pain are stimulated and the neural events connected to the receptors carry information to the brain.

A new set of synapses is stimulated to release the neurotransmitter substance of the synapses.

Immediately following such an event, recall is easy. If you looking quickly at a series of seven numbers you will be able to remember them for a time and recite their order easily.

By tomorrow, or even a few minutes from now you probably will not be able to recall those numbers, yet you can recall other sets of seven numbers readily.

You must store in your head many telephone numbers you use regularly, including those of people you care about, over time.

Those long-remembered numbers are evidence of long-term memory.

The difference between which numbers you remember and which numbers you forget must reside in the permanence of the synaptic circuits involved in wiring those numbers into your brain.

Evidence from simple animals having very few brain cells has shown that as the stimulus is repeatedly presented to the animal, the synapses affected by the stimulus may fire more readily in response to later stimuli.

Repetition changes some characteristic of synaptic transmission.

Short-term memory is not simply a result of repeated exposures to the same experience.

For short-term memory there seems to be a requirement for a neural circuit that allows recording of each new experience.

There is a record of a man who suffered a stroke that affected both temporal lobes. He recovered quite well physically, and his long term memory was intact. However, his short-term memory was essentially gone. He could remember a conversation for about 15 minutes and then it would disappear. The patient was not able to recount events that took place during the day. In essence, he was living only in the immediate present, having not recall of the moste recent events.

It was as if the neural circuit necessary for recording and storing new events was missing.

How do the changes in synaptic transmission differ between short-term and long-term memory?

In the example of telephone numbers the neural circuitry must be the sameor very similar, in that the neurons involved are the same for both short-term and long-term memory.

What then is the difference?

Short-term memory seem to affect only the sensitivity of only some synapses in releasing its neurotransmitter perhaps increasing the number of receptors on a given cell.

Long-term memory traces appear to be a different character, and may actually be visualized with the aid of electron microscopy.

Again, researchers using simple animals and repeatedly stimulating a nerve so that long term memory takes place are able to demonstrate anatomical changes in the affected synapse.

The total area at the presynaptic terminal of the vesicles containing neurotransmitter increases. If the synapse remains inactive for a time, the area of the vesicles decreases.

Continued stimulation of the same set of synapses causes an increase not only in the vesicular area, but also in the number of vesicles in the presynaptic terminal.

The increase in the size and the number of synaptic vesicles indicates that more transmitter is released for each action potential and so makes the receptor cell fire more easily.

If we carry this information over to a newborn, it is reasonable to consider the long-term memory, or learning, the accumulation of new synaptic circuits that fire more easily than non-stimulate circuits.

Storage of Memories

In the human brain, the quality of the information to be stored determines the location to which it is assigned.

We said last time that verbal memories have their neural circuits located in the temporal and parietal lobes. If either Broca’s or Wernicke’s area sustains trauma, memories having to do with speech and language are impaired.

Our brains have memory traces assigned to specific areas, depending on the character of the memory.

Localized damage to the brain may leave the individual deprived of one type of memory but in the posession of all other types

Human memories are not simply the retrieval of a single fact. Rather, they are complex, consisting of auditory, visual, tactile, and even emotional events.

You have seen that each of the above experiences is recorded in different parts of the cortex; visual events in the occipital lobe and tactile events in the parietal lobe.

Recall of those memories requires accurate retrieval of those traces from many parts of the brain.

With the passage of time some of the memory bits may be lost, leaving others intact. This means parts of the memory may be retrieved, but with some of the context lost. The time or place of the memory may not be remembered as it actually happened.

As with memory, specific areas of the brain are associated with specific emotions.

Many of our drives arise from small, well-defined areas in the brain, rather than from a widespread network of neuronal connections.

The hypothalamus controls many of the endocrine and vital functions of our body including thirst, pleasure and sexual drives. If stimulating electrodes are placed in appropriate positions in the hypothalamus, any one of above drives may be stimulated.

Conversely, if any discrete, well-defined areas of the hypothalamus are destroyed, the opposite drive may be felt.

If the ventromedial nucleus of the hypothalamus is destroyed, an animal will go on eating continuously. Its hunger cannot be satisfied.

The sensation of thirst appears to arise from the hypothalamic nucleus. Stimulation of that nucleus by neurotransmitters or by an increase in the concentration of blood solutes bathing the nucleus causes the animal to drink.

Most drugs that affect the CNS have a common mode of action: they resemble in some way a normal occuring compound and attach to the receptor for that compound.

In this way, the drug mimics the action of a normal neuroactive compound, but does so in an exaggerated manner or by modulating normal function so as to produce an unusal effect.

Nicotine, the only drug isolated from tobacco that acts on the CNS, stimulates receptors sensitive to the neurotransmitter acetylcholine.

An opioid drug is closely related to morphine and it may be naturally occuring or a synthetic drug.

Morphine produces a feeling of euphoria, well-being and contentment.

It is a powerful analgesic, reducing the perception of pain by blocking release of pain producing substances. Higher doses of morphine cause sedation and depress the respiratory center profoundly, which may result in death from respiratory failure.

Cocaine causes an intense euphoric experience and powerful psychostimulation. These properties account for the compulsive use of the drug. At low doses it increases alertness and reduces fatique. At higher doses muscular coordination is progressively lost and tremors appear, followed by seizures. Death may occur as a result of heart failure or respiratory failure. Cocaine is a highly addictive and dangerous.