Nervous System



Nervous System









Functions of the Nervous System


Possibly the most complex and poorly understood system, the nervous system plays a major role in homeostasis (hoh mee oh STAY sis), keeping the other body systems coordinated and regulated to achieve optimum performance. It accomplishes this goal by helping the individual respond to his or her internal and external environments.



The nervous and endocrine systems are responsible for communication and control throughout the body. There are three main neural functions, which are as follows:



For example, the sensory function begins with a stimulus (e.g., the uncomfortable pinch of tight shoes). That information travels to the brain, where it is interpreted. The return message is sent to react to the stimulus (e.g., remove the shoes).




Anatomy and Physiology


Organization of the Nervous System


To carry out its functions, the nervous system is divided into two main subsystems. (See Fig. 12-1 for a schematic of the divisions.) The central nervous system (CNS) is composed of the brain and the spinal cord. It is the only site of nerve cells called interneurons (in tur NOOR ons), which connect sensory and motor neurons. The peripheral nervous system (PNS) is composed of the nerves that extend from the brain and spinal cord to the tissues of the body. These are organized into 12 pairs of cranial nerves and 31 pairs of spinal nerves. The PNS is further divided into voluntary and involuntary nerves, which may be afferent (or sensory), carrying impulses to the brain and spinal cord, or efferent (or motor), carrying impulses from the brain and spinal cord to either voluntary or involuntary muscles.



PNS nerves are further categorized into two subsystems:








Cells of the Nervous System


The nervous system is made up of the following two types of cells:





Neurons


The basic unit of the nervous system is the nerve cell, or neuron (Fig. 12-2). Not all neurons are the same, but all have the following features in common. Dendrites (DEN drytes), projections from the cell body, receive neural impulses, also called action potentials, from a stimulus of some kind. This impulse travels along the dendrite and into the cell body, which is the control center of the cell. This cell body contains the nucleus and surrounding cytoplasm.




From the cell body, the impulse moves out along the axon (AX on), a slender, elongated projection that carries the nervous impulse toward the next neuron. The terminal fibers result from the final branching of the axon and the site of the axon terminals that store the chemical neurotransmitters. In neurons outside the CNS, the axon is covered by the myelin (MY uh lin) sheath, which is a substance produced by Schwann (shvahn) cells that coat the axons.


From the axon’s terminal fibers, the neurotransmitter is released from the cell to travel across the space between these terminal fibers and the dendrites of the next cell. This space is called the synapse (SIN aps) (see Fig. 12-2). The impulse continues in this manner until its destination is reached.







The Central Nervous System


As stated previously, the CNS is composed of the brain and the spinal cord.




The Brain


The brain is one of the most complex organs of the body. It is divided into four parts: the cerebrum (suh REE brum), the cerebellum (sair ih BELL um), the diencephalon (dye en SEF fuh lon), and the brainstem (Fig. 12-3).


image
Fig. 12-3 The brain.



Cerebrum

The largest portion of the brain, the cerebrum, is divided into two halves, or hemispheres (Fig. 12-4). It is responsible for thinking, reasoning, and memory. The surfaces of the hemispheres are covered with gray matter and are called the cerebral cortex. Arranged into folds, the valleys are referred to as sulci (SULL sye) (sing. sulcus), and the ridges are gyri (JYE rye) (sing. gyrus). The cerebrum is further divided into sections called lobes, each of which has its own functions:


image
Fig. 12-4 The cerebrum.







The Spinal Cord


The spinal cord extends from the medulla oblongata to the first lumbar vertebra (Fig. 12-5). It then extends into a structure called the cauda equina (KAH dah eh KWY nah). The spinal cord is protected by the bony vertebrae surrounding it and the coverings unique to the CNS called meninges (meh NIN jeez). The spinal cord is composed of gray matter, the cell bodies of motor neurons, and white matter, the myelin-covered axons or nerve fibers that extend from the nerve cell bodies. The 31 pairs of spinal nerves emerge from the spinal cord at the nerve roots.





Meninges

Meninges act as protective coverings for the CNS and are composed of three layers separated by spaces (Fig. 12-6). The dura mater (DUR ah MAY tur) is the tough, fibrous, outer covering of the meninges; its literal meaning is hard mother. The space between the dura mater and arachnoid membrane is called the subdural space. Next comes the arachnoid (uh RACK noyd) membrane, a thin, delicate membrane that takes its name from its spidery appearance. The subarachnoid space is the space between the arachnoid membrane and the pia mater, containing cerebrospinal fluid (CSF). CSF is also present in cavities in the brain called ventricles. Finally, the pia mater (PEE uh MAY tur) is the thin, vascular membrane that is the innermost of the three meninges; its literal meaning is soft mother.


image
Fig. 12-6 The meninges.



The Peripheral Nervous System


The peripheral nervous system is divided into 12 pairs of cranial nerves that conduct impulses between the brain and the head, neck, thoracic, and abdominal areas, and 31 pairs of spinal nerves that closely mimic the organization of the vertebrae and provide innervation to the rest of the body. If the nerve fibers from several spinal nerves form a network, it is termed a plexus (PLECK sus). Spinal nerves are named by their location (cervical, thoracic, lumbar, sacral, and coccygeal) and by number (Fig. 12-7). Cranial nerves are named by their number and also their function or distribution.


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Fig. 12-7 Spinal nerves.


Dermatomes (DUR mah tomes) are skin surface areas supplied by a single afferent spinal nerve. These areas are so specific that it is actually possible to map the body by dermatomes (Fig. 12-8). This specificity can be demonstrated in patients with shingles, who show similar patterns as specific peripheral nerves are affected (see Fig. 12-16).




The autonomic nervous system (ANS) consists of nerves that regulate involuntary function. Examples include cardiac muscle and smooth muscle. The motor portion of this system is further divided into the sympathetic nervous system and the parasympathetic nervous system, two opposing systems that provide balance in the rest of the body systems:



Here is an example of a sensory response:


“Eight-year-old Joey is hungry. He decides to sneak some cookies before dinner. Afraid his mother will see him, he surreptitiously takes a handful into the hall closet and shuts the door. As he begins to eat, the closet door flies open. Joey’s heart begins to race as he whips the cookies out of sight. When he sees it’s only his sister, he relaxes and offers her a cookie as a bribe not to tell on him.”


Joey’s afferent (sensory) somatic neurons carried the message to his brain that he was hungry. This message was interpreted by his brain as a concern, and the response was to sneak cookies from the jar and hide himself as he ate them. When the closet door flew open, his sensory neurons perceived a danger and triggered a sympathetic “fight-or-flight” response, which raised his heart rate and blood pressure and stimulated his sweat glands. When the intruder was perceived to be harmless, his parasympathetic nervous system took over and reduced his heart rate, bringing it back to normal. The same afferent fibers perceived the intruder in two different ways, with two different sets of autonomic motor responses (sympathetic and parasympathetic).




image Exercise 3: Central and Peripheral Nervous System


Match the following parts of the brain with their functions.



Match the CNS part with its combining form.



Decode the terms.




image Exercise 4: Central and Peripheral Nervous System


Label the drawings below with the correct anatomic labels.


image

image





Pathology


The signs and symptoms for this system encompass many systems because of the nature of the neural function: communicating, or failing to communicate, with other parts of the body.




Terms Related to Signs and Symptoms







































































































Term Word Origin Definition
amnesia   Loss of memory caused by brain damage or severe emotional trauma.
am NEE zsa
aphasia a- without
phas/o speech
-ia condition
Lack or impairment of the ability to form or understand speech. Less severe forms include dysphasia (dis FAY zsa) and dysarthria (dis AR three ah); dysarthria refers to difficulty in the articulation (pronunciation) of speech.
ah FAY zsa
athetosis   Continuous, involuntary, slow, writhing movement of the extremities.
ath uh TOH sis
aura   Premonition; sensation of light or warmth that may precede an epileptic seizure or the onset of some types of headache.
OR uh
dysphagia dys- difficult
phag/o eat
-ia condition
Condition of difficulty with swallowing.
dis FAY zsa
dyssomnia dys- difficult
somn/o sleep
-ia condition
Disorders of the sleep-wake cycles. Insomnia is the inability to sleep or stay asleep. Hypersomnia is excessive depth or length of sleep, which may be accompanied by daytime sleepiness.
dih SAHM nee ah
fasciculation   Involuntary contraction of small, local muscles.
fah sick yoo LAY shun
gait, abnormal   Disorder in the manner of walking. An example is ataxia (uh TACK see uh), a lack of muscular coordination, as in cerebral palsy.
hypokinesia hypo- deficient
kinesi/o movement
-ia condition
Decrease in normal movement; may be due to paralysis.
hye poh kih NEE sza
neuralgia neur/o nerve
-algia pain
Nerve pain. If described as a “burning pain,” it is called causalgia.
noor AL jah
paresthesia para- abnormal
esthesi/o feeling
-ia condition
Feeling of prickling, burning, or numbness.
pair uhs THEE zsa
seizure   Neuromuscular reaction to abnormal electrical activity within the brain (see Fig. 12-21). Causes include fever or epilepsy, a recurring seizure disorder; also called convulsions.
SEE zhur
spasm   Involuntary muscle contraction of sudden onset. Examples are hiccoughs, tics, and stuttering.
SPAZ um
syncope   Fainting. A vasovagal (VAS soh VAY gul) attack is a form of syncope that results from abrupt emotional stress involving the vagus nerve’s effect on blood vessels.
SINK oh pee
tremors   Rhythmic, quivering, purposeless skeletal muscle movements seen in some elderly individuals and in patients with various neuro-degenerative disorders.
TREH murs
vertigo   Dizziness; abnormal sensation of movement when there is none, either of oneself moving, or of objects moving around oneself.
VUR tih goh

Oct 6, 2017 | Posted by in GENERAL SURGERY | Comments Off on Nervous System
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