LESSON 12 THE NERVOUS SYSTEM

INTRODUCTION

The nervous system is one of the most complexes of all human body systems. It contains billions of nerve cells that are operating constantly all over the body to coordinate the activities we do consciously as well as those that occur unconsciously. We speak, we move, we hear, we taste, we see, we think, our glands secrete hormones, we react to danger, pain, temperature, and touch, and we have memory association.

Some nerve fibers carry impulses from the central nervous system to the glands, heart, blood vessels, and the involuntary muscles found in the walls of tubes, like the intestines, and hollow organs, like the stomach and urinary bladder. These nerves are called afferent or sensory nerves. Other nerve fibers carry impulses from the body towards the central nervous system. These nerves are called efferent or motor nerves.

Some of the automotive nerves are called sympathetic nerves and others are called parasympathetic nerves. The sympathetic nerves stimulate the body in times of stress and crisis. They increase heart rate and strength, dilate airways so more oxygen can enter, increase blood pressure, stimulate the adrenal glands to secrete epinephrine and inhibit intestinal contractions so that digestion is slower. The parasympathetic nerves normally act as a balance for the sympathetic nerves. Parasympathetic nerves slow down heart rate, contract the pupils of the eye, lower blood pressure, stimulate peristalsis to clear the rectum, and increase the quantity of secretions like saliva.

NEURONS

A neuron is an individual nerve cell, a microscopic structure. Impulses move along the parts of a nerve cell in a definite manner and direction. A stimulus begins a wave of excitability in the receptive branching of fibers of the neuron known as dendrites. A change in the electrical charge of the dendrite membranes cause nervous impulse waves to move along the dendrites like the movement of falling dominoes. The impulse, traveling in only one direction, reaches the cell body which contains the cell nucleus. Extending from the cell body is the axon, which carries the impulse away from the cell body. Axons are covered with a fatty tissue called a myelin sheath. The myelin sheath gives a white appearance to the nerve fiber so it is called the white matter. The gray matter of the brain and spinal cord refers to the collections of cell bodies and dendrites that appear gray because they are not covered by a myelin sheath.

The neurilemma is a membranous sheath outside the myelin sheath on the nerve cells of peripheral nerves. The nervous impulse passes through the axon to leave the cell via the terminal end fibers of the neuron. The space where the nervous impulse jumps from one neuron to another is called the synapse. The transfer of the impulse across the synapse depends upon the release of a chemical substance. This chemical substance is known as neurotransmitters or neurohormones because most function as hormones. The neuron releases the neurotransmitter into the synapse. Tiny sacs containing the neurotransmitter are located at the ends of neurons and they release the neurotransmitter into the synapse. Acetylcholine, epinephrine, serotonin, and dopamine are examples of neurotransmitters.

Neurons and nerves are the parenchymal tissue of the nervous system; that is, they do the essential work of the system by conducting impulses throughout the body. The stromal tissue of the nervous system consists of other cells called neuroglia. Neuroglial cells are supportive and connective in function, as well as phagocytic, and are able to help the nervous system ward off infection and injury. Neuroglial cells do not transmit impulses.

There are three types of neuroglial cells. Astrocytes (astroglia), as their name suggests, are star-like and are believed to be responsible for transporting water and salts between capillaries and nerve cells. Microglial cells (microglia) are very small and have many branching processes. These cells are phagocytes. Oligodendroglia has few dendrites. They are responsible for the production of myelin that covers nerve fibers in the central nervous system. They are also associated with blood vessels and can help regulate the passage of potentially harmful substances from the blood into the nerve cells of the brain. This protective barrier between the blood and the brain cells is called the blood-brain barrier.

THE BRAIN

The brain is the main center for regulating and coordinating body activities. In the human adult it weighs about 3 pounds and has many different parts, all of which control different aspects of body functions.

The largest part of the brain is the cerebrum. The outer nervous tissue of the cerebrum, known as the cerebral cortex, is arranged in folds to form elevated portions known as convolutions and depressions or grooves known as fissures. The cerebral hemispheres are the paired halves of the cerebrum. Each hemisphere is subdivided into four major lobes named for the cranial bones that cover them.

The cerebrum has many functions. All thought, judgment, memory, association,

and discrimination take place within it. In addition, sensory impulses are received through afferent cranial nerves and, when registered in the cortex, is the basis for perception. Efferent cranial nerves carry motor impulses from the cerebrum to muscles and glands, and these produce movement of body parts.

Within the middle region of the cerebrum are spaces, or canals, called ventricles. They contain a watery fluid that flows throughout the brain and around the spinal cord. This fluid is called cerebrospinal fluid (CSF) and protects the brain and spinal cord from shock as might a cushion. CSF is usually clear and colorless and contains lymphocytes, sugar, chlorides, and protein. Spinal fluid can be withdrawn for diagnosis or relief of pressure on the brain; this is called a spinal puncture or lumbar puncture. A hollow needle is inserted in the lumbar region of the spinal column below the region where the nervous tissue of the spinal cord ends, and fluid is withdrawn.

Two other important parts of the brain, the thalamus and hypothalamus, are located below the cerebrum. The thalamus acts as a circuit broad relay switch for impulses that travel from receptors such as the eye, ear, and skin to the cerebrum. The thalamus controls and monitors these sensory impulses, suppressing some and magnifying others. Pain sensation is controlled by this area of the brain.

The hypothalamus contains neurons that control body temperature, sleep, appetite, sexual desire, and emotions such as fear and pleasure. The hypothalamus regulates the release of hormones from the pituitary gland at the base of the brain and controls the activities of the sympathetic and parasympathetic nervous systems.

The following structures connect the cerebrum with the spinal cord: the cerebellum pons, and medulla oblongata. The pons and medulla are part of the brain stem.

The cerebellum is located under the posterior part of the cerebrum. Its function is to aid in the coordination of voluntary movements as well as to maintain balance and posture for bicycle riding, piano playing, walking, running, and regular muscular tone.

SPINAL CORD

The spinal cord is a column of nervous tissue extending from the brain stem to the second lumbar vertebra within the vertebral column. It ends at the cauda equina or horse’s tail, a ray of nerve fibers found inferior to the second lumbar vertebra of the spinal column. It carries all the nerves that affect the limbs and lower part of the body and it is the pathway for impulses going to and from the brain. A cross

section of the spinal cord reveals an inside area of gray matter containing cell bodies and dendrites of peripheral nerves, and an outside area of white matter containing the nerve fiber tracts with myelin sheaths, transmitting impulses to and from the brain.

MENINGES

The meninges are three layers of connective tissue membranes that surround the brain and spinal cord. The outermost membrane of the meninges is called the dura mater. It is a thick and tough membrane and contains channels for blood to come into the brain tissue. The subdural space is a space below the dura membrane and contains many blood vessels. The second layer around the brain and spinal cord is called the arachnoid membrane. The arachnoid (meaning spider-like) membrane, is loosely attached to the other meninges by fibers so there is a space for fluid between the fibers and the third membrane. This space is called the subarachnoid space and it contains the cerebral spinal fluid. The third layer of the meninges, closest to the brain and spinal cord, is called the pia mater. It is made of connective tissue with a rich supply of blood vessels.

LESSON 12 GRAPHICS

TERMS FOR LESSON 12

THE NERVOUS SYSTEM

Organs of Central Nervous System to Knowcerebrumventriclescerebral spinal fluidthalamushypothalamuscerebellumbrain stemponsmedulla oblongataspinal cordmeningesdura materarachnoidpia mater

Autonomic Nervous System Words to Knowsympatheticparasympathetic

Organs of Peripheral Nervous System to Know:nerveganglionneurondendritescell nucleuscell bodyaxonmyelin sheathneurilemmaafferent nerve or sensory nerve

efferent nerve or motor nerveneurogliaastrocyte cellsmicroglial cellsoligodendroglia cellsWord Roots to Know: Nervous Systemcerebell/ocerebr/odur/oencephal/ogangli/oganglion/omening/i, mening/omyel/oneur/oradic/o, radicul/oesthesi/omon/ophas/opoli/opsych/oment/ophren/oquad/i

Prefixes and Suffixes to Know: Nervous Systemhemi-tetra--iatry-paresis

Diagnostic Terms to Know: Nervous Systemcerebellitis

cerebral thrombosisduritisencephalitisencephalomalaciaencephalomyeloradiculitisgangliitismeningitismeningocelemeningomyeloceleneuralgianeuroarthropathyneuroasthenianeuritisneuroblastneuromapoliomyelitispolyneuritisradiculitissubdural hematomaalzheimer’s diseasecerebral palsycerebrovascular accidentepilepsyhydrocephalusmultiple sclerosisneurosisParkinson’s diseasepsychosissciaticashingles

Surgical Terms to Know: Nervous Systemganglionectomy

neurectomyneurolysisneuroplastyneurorrhaphyneurotomyradicotomy

Diagnostic Procedural Terms to Know: Nervous Systemechoencephalographyelectroencephalogramelectroencephalographelectroencephalographymyelogrampneumoencephalogramventriculogramcomputed tomography of the brainlumbar punctureAdditional Terms to Know: Nervous System:anesthesiaaphasiacephalalgiacerebralcraniocerebraldysphasiaencephaloschlerosishemiparesishemiplegiahyperesthesiamonoplegiamyelomalacianeuroidneurologistneurology

phrenicphrenopathypsychiatrypsychogenicpsychologistpsychologypsychopathypsychosomaticquadriplegiasubduraltetraplegiacomaconcussionconsciousconvulsionparaplegiaseizureshuntunconsciousness

PRACTICE EXERCISES FOR LESSON 12THE NERVOUS SYSTEM

DEFINE:cerebral thrombosisduritisencephalomalacianeurastheniasubdural hematomapolyneuritisAlzheimer’s diseasecerebral palsyepilepsymultiple schlerosis

Parkinson’s diseasepsychosisshingleslumbar puncturedysphasispsychologisttetraplegiaconcussion

MATCHING:---- maintains balance a spinal cord---- connects cerebrum to cerebellum b cerebellum---- spaces within cerebrum c sensation---- contains respiration controls d pons---- carries impulses e nerve root---- conduct impulses to brain f brain word root---- group of nerve cells g nerve word root---- cerebell/o h nerve---- cerebr/o i spinal cord word root---- dur/o j ventricles---- encephal/o k ganglion---- gangli/o l cerebrum---- mening/i m ganglion---- myel/o n cerebrum---- neur/o o cellebellum word root---- radic/o p hard---- esthesi/o q meninges

ASSIGNMENT FOR LESSON 12

Medical Terminology, HS 280The Nervous System

MATCHING: May Be Used More Than Once---- 1 mon/o a treatment---- 2 phas/o b gray matter---- 3 poli/o c speech---- 4 psych/o d part---- 5 ment/o e slight paralysis---- 6 phren/o f one---- 7 quadr/i g four---- 8 hemi- h half---- 9 tetra- i mind---- 10 -iatry j hard---- 11 -paresis k soft

MATCHING:---- 12 duritis a inflamed dura mater---- 13 encephalomalacia b inflamed ganglion---- 14 gangliitis c blood filled tumor in dura mater---- 15 neuralgia d soft brain---- 16 inflamed nerve e neuritis---- 17 subdural hematoma f brain damage caused by a loss of oxygen at or near birth---- 18 cerebral palsy g nerve pain---- 19 multiple schlerosis h viral disease of inflamed nerve---- 20 shingles i smallpox

j degenerative disease of spinal cord

DEFINE:

21 sciatica:

22 hydrocephalus:

23 cerebrovascular accident:

24 poliomyelitis:

Assignment for Lesson 12, Nervous System, pg. 2

DEFINE:

25 neuritis:

26 paraplegia:

27 convulsion:

28 psychogenic:

29 hyperesthesia:

30 cerebral thrombosis:

31 encephalitis:

32 neurasthenia:

33 Alzheimer’s disease:

34 Parkinson’s disease:

35 neurorrhaphy:

36 electroencephalogram:

37 myelogram:

38 ventriculogram:

39 lumbar puncture:

40 aphasia: