THE NERVOUS SYSTEM

All organisms are endowed the ability to control their environments (external and internal environments). In the external environment, the organism must be able to notice an unfriendly or inhospitable surrounding and to protect itself. The nervous system (NS) has the most complex organization of cells that receive information from the outside and inside environments and integrate these activities in the central nervous system (CNS). By coordinating these activities in conjunction with chemical regulation, the nervous system maintains homeostasis, balance and protects the organism.

COMPONENTS OF THE NERVOUS SYSTEM

· The nervous system is similar to a computer.

The CNS functions similar to the central processing unit (CPU) of the computer.

· The main function of the CNS and CPU is processing and integration of information. Information is sent to CNS through the sensory nervous system (SNS) similar to typing information using the keyboard.

· After processing, the information is sent out through another pathway called the motor nervous system (MNS).

· This unit of the nervous system is similar to an output device (printer, monitor).

· The NS has three main components: The central nervous system (CNS) is the central processing center. Information from all parts of the body are received, processed and commands are issued for the response.

· The motor nerve system (MNS) deals with sending out information. The CNS uses this branch to send out nerve signals to muscles (voluntary and involuntary) and to glands for hormone secretion.

· Sensory nerve system (SNS) is the branch that acts as an input device, transmitting information or signals from all parts of the body to the CNS.

· The end organs are not essentially a fourth component, but they are an integral part of the system. The end organs include muscles (voluntary and involuntary), glands, specialized senses and the skin.

· Nerves from the motor neurons are also called efferent fibers which carry signals to the end organs. Sensory or afferent neurons carry sensory signals from these organs to the CNS for processing, integration and relay to the MNS for output.

Central Nervous System

( Brain and Spinal cord

Motor Nervous Syst (Efferent Nerves)		Sensory Nervous System (Afferent Nerves)

End Organ

(Muscle, Glands

Skin and Senses)

EVOLUTION OF THE NERVOUS SYSTEM

The nervous system evolved from simple nerve-net-like arrangements to a complex, organized network of computer-like systems. The level of development, adaptive changes and sophistication depend on the activities of the organisms and its development in the evolutionary ladder.

In simple invertebrate animals such as cnidarians, the nervous system is a simple net-like arrangement. In hydra for example, the nervous system is a nerve net scattered all over the organism without any control center. This arrangement is appropriate for simple animals without a head or tail. The organism has no sense of direction as it moves. However, evolutionary changes began to emerge as the animals became more sophisticated and structurally more complex as in flat worms, insects and human beings.

In bilateral and symmetrical animals such as flatworms with a clearly defined head and tail regions, the central nervous system appears more organized. The head contains a concentrated mass of nerve tissue called the brain. Two parallel nerve cords project from the brain to the tail region. Small nerve cords radiate from the main cord and interconnect other cords, thus forming a ladder-shaped network, which form the peripheral nervous system (PNS). Thus, central nervous system in bilateral animals consists of the brain and nerve cords. Which form the PNS. Adaptive and structural changes emerged as clearly evident in insects and segmented worms.

The nervous system in insects (arthropods) and earthworms (annelids) among a host of others is more advance probably because of evolutionary changes. The brain center consists of ganglia (group of fused nerve tissues) from which emerged a single ventral nerve cord. Nerve branches radiate from the ventral nerve cord to each body segment as ganglia. The ganglia in the various segments of the organism form the PNS. In vertebrates, the nervous is developed into clearly defined areas.

Humans and higher animals represent the highest evolutionary advancement in structure and development. In humans, for example, the CNS consists of the brain in a protective skull and the spinal cord also in vertebral column of bones. The brain directs and controls the activities of the NS. The spinal cord processes information related to motor controls. Nerve branches radiate from the spinal cord and run down the upper and lower extremities. These nerved connections form the PNS.

CELLS OF THE NERVOUS SYSTEM

• The basic functional cells of the nervous system are the neurons (nerve cells) and support cells. What is a neuron, what is the function of a neuron and what is the structure of a neuron?

Neurons are the functional units of the nervous system.

• Neurons are the basic units of the nervous system. They function primarily in conducting or transferring and receiving signals (nerve impulses) from all parts of the body to the CNS.
• Our body contains millions of neurons in various shapes and sizes. However, all neurons essentially consist of three parts: dendrites, cell body and an axon.
• Dendrites are hair-like projections on the cell body and are used to conduct impulses into the cell body. A cell body is that part of the neuron that contains the nucleus and the cytoplasm. It also forms part of the dendrites.
• An axon is a long process that projects from the cell body. It is often myelinated. That means it is covered with myelin sheath, which provides insulation for the axon. The myelin insulation has interruptions. These uncovered spaces or interruptions are called nodes of Ranvier. The axon often terminates in a knob on an effector organ.
• Neurons differ in structure and come in different shapes. Their dendrites and axons also differ in length. Based on their structural differences, neurons are classified as unipolar, bipolar and multipolar.

1. Unipolar neurons

These are neurons that have a single nerve fiber extending from its cell body. The fiber immediately divides into two branches. One branch enters the brain and serves as an axon and the other branch enter the periphery of the body as dendrites.

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2.Bipolar neurons

Bipolar neurons have two nerve fibers extending from their cell body, one arising from each end. One of the fibers serves as an axon and the other serves as a dendrite. Such neurons are found in the nose, ears and specialized parts of the eye.

3.Multipolar

Multipolar neurons have many dendrites arising from their cell bodies. There is only one axon in multipolar neurons and the remaining are dendrites. Most of the neurons with cell bodies within the brain or spinal cord are multipolar neurons.

• A nerve is a unit of fiber (within a bundle of fibers) that extends from the neuron. A neuron is a bundle of nerve fibers and each bundle is wrapped in a fascia.
• Each neuron in a bundle has been described. There are three types of nerves: sensory nerves conduct impulses into the brain or spinal cord. Motor nerves carry impulses away from the brain or spinal nerves to the muscles and glands. Mixed nerves both sensory and motor fibers carry signals back and forth.

Support cells provide many functions including nourishment, insulation, and structural framework.

• Neurons might not perform their function efficiently were it not for the services provided by the support cells.
• Support cells are a vital unit in facilitating the activities of the neuron. What are the support cells and what are their functions? There are four types of support cells.

1. Astrocytes.

These cells are mostly found in blood vessels and nervous tissues. They provide structural, maintain support; maintain nutrients; and help form scar tissue, which fills spaces after injury.

2. Oligodendrocytes.

These cells are usually found in rows along nerve fibers and they provide myelin insulation for neurons in the brain and spinal cord.

3. Microglia cells.

These cells function in phagocytosis cleaning debris and bacterial cells. They are found scattered all over the CNS.

4. Ependymal cells

Ependymal cells form the inner lining of the ventricles (spaces) found within the brain and central canal of the spinal cord. They also form epithelia-like membranes such as choroid plexuses that cover specialized brain parts.

WHAT ARE NEURONAL SIGNALS

• The CNS receives thousands of signals, which are processed and relayed for motor response.
• All of these activities and more are dependent on impulses or signals received and fed to the CNS, processed and returned by way of sensory and motor neurons.
• What are these signals, where do they originate and what is the final outcome (effects) of the signals?

Signals are electrical messages

Information or nerve signals are transmitted in two ways: electrical and chemical signals. What make up these signals and how are they transmitted and stored: To answer these questions let us first look at what make up signals. Signals are electrical ions or charges. These signals are cationic electrolytes such as sodium ions (Na⁺), potassium ions (k⁺) and anions such as chloride ions (Cl^-), hydroxyl ions (OH^-) or sulfate ions (SO₄^-).

Resting membrane potential, action potential and refractory period of neurons.

• Nerve impulse is information transmitted across the membrane of a neuron to a target organ. Under experimental conditions, a nerve impulse is the voltage (the electrical difference across a surface) recorded in millivolts (mV), when a voltmeter is used.
• A voltmeter then is equipment that records the flow of electrical ions across the surface of a membrane. In order to observe this voltage difference across the surface, the voltmeter is attached to another equipment called an oscilloscope. The screen of the oscilloscope deflects or shows the signal pattern made by the voltmeter.
• A nerve cell membrane at rest measures a negative value (-70); however, a threshold value (-50 mV) is required before action potential can take place.
• There are three phases of a nerve signal: a resting membrane potential (RMP), action potential (AP) and refractory period.

Resting membrane potential

• When a cell membrane is unstimulated, the electrical charge outside the membrane is positive (+) relative to the inside, which maintains negative charges

(-). This charge difference across the cell membrane is called the resting membrane potential.

• The differences in electrical charges across the membrane are due to differences in the concentration of ions and permeability characteristics of the cell membrane.
• The concentration of sodium and potassium ions is maintained by an active transport process, which uses the sodium and potassium ion gates or channels regulated by proteins.
• The proteins that regulate the gates are called Na-K ATPases or channel proteins.
• There is a differential concentration of sodium and potassium ions inside and outside of the cell membrane.
• The concentration of Na⁺ ions is high outside the membrane relative to the inside. In contrast, potassium ion concentration is higher inside the cell membrane relative to the outside.
• It is the channel proteins (Na-K ATPases) that maintain the difference in Na-K ion concentration in the cell membrane. In a neuron, the resting membrane potential measures approximately (-70 mV).
• An action potential starts when nerve impulse cause the Na⁺ ion gates to open, allowing the entry of Na ions.

Action Potential (AP)

What happens when the Na⁺ ions gates open and how does it affect the movement of ions?

A synapse may be electrical or chemical

• A neuronal synapse is a very important component of a nerve cell. It functions as an interchange between relay stations.
• When the message or signal carried by the neuron gets to the end of the axon containing the synaptic knob, it is released to the next station via the synapse.
• A synapse is a small gap between the axonal knob and the organ (another neuronal cell body or organ).
• Synapses can be either electrical or chemical in nature. Impulse transmission in electrical synapses is rapid. These types of electrical synapses are found in humans (e.g., heart, digestive tract), and in small organisms such as lobsters and crayfish. Synapses with chemical transmitters contain synaptic cleft, a space between the synaptic knob and the organ. When the action potential arrives at the end of the synaptic knob, it is converted into neurotransmitter chemical stored in vesicles of the synaptic knobs. The neurotransmitter later elicits the action potential in the end organ. What is a neurotransmitter chemical?
• A neurotransmitter is a chemical substance (a peptide or molecule), which carries electrical or chemical signals across a nerve synapse.
• The origin where the neurotransmitter is released from the neuron is called the presynaptic knob or membrane and the target to which it is released is the postsynaptic membrane. The target organs that receive the neurotransmitters have receptors that recognize the molecules, and therefore, facilitate binding to the receptors.
• There are several types of neurotransmitters. Acetylcholine (Ach) for example is the neurotransmitter that stimulates muscle contraction).
• Other transmitter molecules include monoamines (e.g., epinephrine and norepinephrine, dopamine, serotonin) amino acids (e.g., glycine, glutamic acid, aspartic acid and gama-aminobutyric acid, GABA) and some peptides.
• Neurotransmitters are stored in storage vesicles at the presynaptic knobs as electrical signals. How are neurotransmitters released from their storage vesicles?
• As a nerve impulse or action potential reaches the presynaptic membrane, the excitation causes an increase in membrane permeability by allowing calcium ions to flow inwards into the cell membrane. The presence of calcium ions causes the vesicles to discharge the neurotransmitter molecules such as acetylcholine (Ach) across the synaptic cleft.
• When a neurotransmitter substance is released, it must be removed after it has performed its function. Acetylcholinesterase (ACE), an enzyme, degrades Ach after it has been released.
• Monoamine oxidases (MAO) are another group of enzymes that degrade the neurotransmitters of the brain (NEP, EP and others)..

Neurotransmitters may cause excitation or inhibition.

· How do we accept good or bad news? We are elated or may even shout for joy or show our outward emotion. We may show the same feeling for bad news or events that we consider unappealing.

• Information (good or bad) is received in two ways: we read about it, therefore, our eyes pick up the information through the optic sensory fibers and send it to the brain for analysis.
• Another way information is received is through the ear. Spoken words travel by sound waves through the ear.
• The waves strike the eardrum, which transmits the message through the ear bones (ossicles) to the cochlear bearing the auditory nerve (#8). The information is then transmitted to the brain for analyses.
• Following analysis, the brain sends back the impulse through different tracts of neurons. For example, the response to the message may stimulate excitatory nerve transmitters such as NE, and Ach.
• Release of NE may increase our heartbeat, increase our breathing and make us feel elated.
• Ach causes muscle stimulation so that we may even dance or shout and express our emotions outwardly. Therefore, excitatory transmitters produce multiple effects because multiple neurotransmitters are released.
• When sad news is received, the pathways are similar, however, the neurotransmitters released are inhibitory transmitters such that few of them are released.

Table 15. Major Neurotransmitters and their locations

Group Transmitter Possible

Name substance function Comments

Acetylcholine Acetylcholine Excitatory/ In skeletal muscles

(Ach) Stimulatory

Amino acids Gama-amino- Inhibitory Maj. transmitter in

butyric acid brain

(GABA)

Glutamate Excitatory Found in brain

Glycine Inhibitory/

Excitatory

Biogenic amines Dopamine Evokes Degeneration leads to

(Monoamines) excitation Parkinson’s disease

Norepinephrine Excitation/ Cocaineblocks the site

(NE) inhibitory amphetamines release

Serotonine Excitation Found in brain

(5-TH)

Neuroactive Somatostatin Inhibit secretion Controlled by GHRH

Peptides of growth

Hormone

Endorphins/ Has opiate-like

Enkephalins Analgesic properties

NERVOUS SYSTEM AND DRUGS

Chemical molecules play a major role in the nervous system. We have examined neurons and how endogenous chemicals such as Ach, EP, NEP and others influence how we react (e.g., movements, respiration, heartbeats and more). We believe that neurotransmitters cause excitation and also lead to depression. That means, impulses may selectively release excitatory or inhibitory transmitter molecules. Now, we want to examine the influence of the intake of exogenous stimulants or depressants.

Chemical stimulants may produce excitatory or inhibitory neurotransmitters.

Some of our household drinks are stimulants. Here is a list of examples. Coffee contains caffeine, herbal tea contains thiobromine and thiophilline, Coca cola, Pepsi and Sprite contain caffeine. Some individuals on medication receive sedatives or tranquilizers (depressants), while others are on amphetamines (stimulants). How do these chemicals affect our mood and performance? Chemicals we take into our body may function at several levels: they may stimulate, inhibit or produce both as combined effects.

1. Stimulation.

The intake of coffee, amphetamines, or nicotine might produce more excitatory transmitter molecules. Coffee intake might also produce synergistic effects (attenuate the effects of the neurotransmitter). These effects may be the exact opposite in some individuals.

2. Inhibition.

The intake of coffee might also produce inhibitory neurotransmitters (e.g., GABA, glycine); also it may attenuate the effects of GABA. While coffee keeps most awake, it makes others sleep or relaxes them probably because in these individuals, inhibitory rather than excitatory transmitters are released.

3. Combination effects.

The intake of some stimulants does not produce any stimulation even in individuals that are sensitive to the stimulatory effects. These individuals are not sad or happy. It appears the stimulants produced both excitatory and inhibitory transmitters in such individuals. It may have something to do with the dose of caffeine (it is either low or too high).

With this in mind, let us examine why we become addicted to some chemicals, depressed or hyperactive sometimes. It is believed that the effect of many drugs in our system depends on individual susceptibility. This may explain why some individuals may find caffeine or nicotine relaxing to the extent that it induces sleep; while others find it stimulatory therefore it keeps them awake. Let us now examine chemical dependence (drug addiction), depression and hyperactivity.

Chronic use of a chemical substance produces chemical dependence.

Drug potency

The response of a chemical substance is dependent primarily on dose (strength or potency) in addition to other factors. When a chemical substance is consumed, the dosage is probably sufficient to produce the desired effect (e.g., excite, inhibit). In most cases, the desired effect of the drug is achieved based on the recommended dose and conditions of administering the drug. If the desired effect is not produced, the strength may be increased. An individual abuses a drug when it is used continuously with increasing dosage over an extended period of time. The chronic use of the drug results in drug addiction or chemical dependence. It means the individual uses the chemical to perform daily chores (physical dependence) and for mood elevation (emotional stability). There are many individuals who use prescription drugs such as morphine for pain relief, but have then become addicted to the drug.

Controlled and addictive substances

Morphine and other psychoactive drugs such as diazepam (Valium) and imipramine are controlled substances and can only be prescribed by physicians. Morphine is used for pain relief and also used during surgery. Valium is an antianxiety drug used to control anxiety disorders and Imipramine is an antidepressant drug used in the treatment of mood changes and depression. These substances are addictive because diseases such as depression, anxiety disorder a chronic pains problems related chemical imbalance or symptoms of other preexisting conditions. Consequently the individuals continue to take the drug until tolerance level is reached. It means that the drug dose is no longer effective in producing desired responses therefore a stronger dose is needed.

Drugs and neurotransmitters

These chemical compounds affect the release of neurotransmitter molecules. Morphine appears to block pain reception at the nerve endings, therefore, it may be associated with blocking neurotransmitter molecules, or may be enhancing the secretion of inhibitory neurotransmitters such as GABA, glutamic acid and glycine. It may also promote the residence time of these inhibitors by preventing their degradation, or delaying the sensitivity of degradative enzymes such as monoamine oxidases. Similarly, Valium (antihypertensive) may work by inhibiting excitatory neurotransmitters associated with mood such as serotonin (5-hydroxy trypthamine) or suppressing their release at the presynaptic knobs. Imipramine also functions similarly. It is an antidepressant drug therefore its administration may stimulate excitatory neurotransmitters (EP, NEP, Ach, 5-HT). It may also suppress the release of inhibitory neurotransmitter chemicals.

Depression may be associated with frequent release of inhibitory neurotransmitter molecule.

Why do some individuals feel depressed? Depression may be defined as a state of melancholy, sadness or unhappiness most of the time. Accidents and the loss of a loved one may evoke unhappy feelings, but some individuals remain depressed almost always. Such individuals have mood depression as a disease. What happens to those individuals?

Excitatory and inhibitory neurotransmitters are constantly released in every individual depending on what turns us on. Good new evokes excitatory stimulants or neurotransmitters and bad new elicits inhibitory transmitters. Sometimes both inhibitory and excitatory transmitters may be release but to different targets. This is often the case in some women who give birth to a baby and soon afterwards become sad or depressed. They call it postpartum depression. Since these neurotransmitters are secreted independently, there is a chemical balance.

What happens if the balance is lost and the nerve impulse results in the production of more inhibitory neurotransmitter molecules? Let us suppose that the excitatory neurotransmitter molecule receptors are degenerated, that means less receptors and more neurotransmitters. This would result in chemical imbalance. It appears depression may be related to imbalance of brain neurotransmitters. Inhibitory neurotransmitter chemicals are released more frequently than normal. This is similar to the case in Parkinson’s disease where instability in nerve firing occurs because of the degeneration of the opposing nerves. Antidepressant drugs such as Valium and others are used in the management of individuals with depression.

Anxiety or hyperactivity may be associated with overactive excitatory neurotransmitter molecules.

You must have heard about anxiety attacks. What does it mean anyway? What about nervousness? They are all related, if not similar. Some individuals are overly anxious or appear anxious in anticipation of an event, performance, and public speaking and display such emotions. These nuances are considered normal; however, they are sometimes extreme. What is happening to such individuals?

Similar to the conditions in depression, hyperactivity may be related to excessive secretion of excitatory neurotransmitter molecules. Excessive firing of neurons perhaps causes the release of excitatory neurotransmitter molecules (Ach, Serotonin, NEP, EP, glutamate). These molecules, depending on the target would produce various effects ranging from nervousness (excessive firing of the nerves), excessive motor activities (pacing up and down, wring the fingers, sweating and more). It is also possible that in some individuals, the normal amount of excitatory neurotransmitters is secreted. However, endogenous molecules or other chemicals that were used by the individual may increase the residence time of the molecules in the target organ, or prevent disintegration by enzymes. Antianxiety drugs (e.g. Imipramine) are used to treat anxiety disorders and related problems.

SENSORY and MOTOR NERVOUS SYSTEMS

The sensory nervous system (SNS) is the second arm of the nervous system responsible for collecting information and sending it to the CNS. The SNS informs the CNS about what is going on within the body. The sense organs (eyes, nose, mouth, ear) form part of the highways accessibility through which information is delivered. They serve as special conduits for information vital to our body. The senses referred to are: the somatic senses of touch, pressure, temperature, pain, the special sense of vision, smell, equilibrium, hearing and taste. Both the specialized senses and somatic senses deal with environmental stimuli and how we are able to feel the sensations of the stimuli and how the stimuli are transmitted to the CNS.

The motor nervous system receives processed information from the CNS and sends the signals through efferent motor fibers (nerves). The motor fibers send the signal to voluntary skeletal muscles. In contrast, the autonomic nervous system (ANS) controls the glands and the smooth involuntary muscles. The glands secret hormones and other chemical mediators after receiving commands from the CNS, however, these hormonal secretions are not under our conscious effort.

Let us look at these aspects of the sensory nervous and motor nervous systems in the CNS and Spinal cord.

II. THE CENTRAL NERVOUS SYSTEM

In humans, the brain consists of cerebrum (center for thought and memory association); hypothalamus-thalamus (center for processing information), cerebellum and brain stem (center for locomotion and coordination) and the spinal cord, an extension of the brain consists of motor and sensory nerves in the cervical, thoracic and lumber regions of the spine.

A. Embryological Development

During the development stages, the brain starts out as a tube in most mammals. The tube has three bulges or balloon-like structures at one end of the third week of conception.

· The first part is prosencephalon (forebrain); mesencephalon (midbrain) and rhobemencephalon (hindbrain).

· After 5wks, telencephalon and diencephalons form develop from the forebrain (prosencephalon); the hind brain forms metencephalon (afterbrain) and myelin-cephalon (form spinal cord).

· These structures form the brain and the other end forms the spinal cord.

· The cerebrum develop from telencephalon; diencephalons forms the hypothalamus, thalamus and epithalamus.

· Mesencephalon, metencephalon and myelencephalon develop into cerebellum and midbrain or brain stem (pons and medulla oblongata).

B. Characteristics of the CNS

· The entire CNS is protected by a bony capsule – the cranium and the vertebral column protects the spinal cord.

· The cranium is covered with membrane called meminges of which there are three: dura mater (out layer), arachnoid (middle) and pia mater (interior layer).

· The surface of brain contains ridges called gyri (gyrus, singular) and are separated by valleys called sulci and deeper valleys or grooves are called fissure.

· There are five lobes: frontal, parietal, occipital, temporal and the insula lobe.

· Two cerebral hemispheres exist which are mirrow images and are connected in the middle by a bundle of nerves called corpus collasum.

C. Functions of the Brain

1. Cerebrum

· Cerebrum is the center for control, thought and associations.

• Command center: It serves as the command center of the brain. All information from the periphery nervous system (PNS), motor functions and sensory activities are received, processed, analyzed and relayed or commands are issued for their respective responses.
• Other functions include development, vision, speech, memory, thought, and association.
• The cerebrum is divided into left and right hemispheres and these hemispheres contain fibers or tracts, which cross over each half of the hemispheres
• The association of the tract with each half of the cerebral hemispheres is very important. During an accident or removal of tumor, the tracts can be cut accidentally. When the cut is not repairable, it may leave permanent damage that could lead to loss of function or paralysis of the functions related to that area.

2. Thalamus and hypothalamus

· Thalamus and hypothalamus are centers for processing information.

• The thalamus and hypothalamus are collectively called diencephalons.
• These organs regulate bodily functions and motor activities (motion), therefore, process information related to bodily functions and motor activities.
• The thalamus and hypothalamus are located immediately below the corpus callosum.
• The thalamus represents the last center where all sensory signals (except those associated with the sense of smell) are processed before they are passed on to the cerebrum.
• The hypothalamus maintains homeostasis (that is maintenance of constant body temperature) and contains centers for regulating hunger, sleep, thirst, temperature, water balance and blood pressure.
• It also regulates the pituitary gland, which secretes hormones, thereby maintaining regulation of hormones.

3. Cerebellum

· Cerebellum coordinates motor functions.

• The cerebellum is located at the back of the brain below the cerebrum and appears partially separated from the cerebrum by the fourth ventricle.
• The surface of the cerebellum is made of gray matter and the interior is white matter.
• It functions mainly in muscle coordination and integrating all motor activities.
• It also maintains normal muscle tone and posture and balance.

4. The brainstem