Nervous System
Define at least 10 terms relating to the nervous system.
Describe the function of the nervous system.
Identify at least 10 structures of the nervous system.
Identify at least three methods used to assess the function of the nervous system.
(se-ree-bro-SPY-nal FLOO-id) Fluid contained in the brain’s ventricles, intracranial spaces, and central canal of the spinal cord
(is-KEE-mee-uh) Insufficient blood to a body part caused by a functional constriction or actual obstruction of a blood vessel
(noo-ro-TRANS-mit-er) Chemical messenger, released from the axon of one neuron, that travels to another nearby neuron
Term | Definition | Prefix | Root | Suffix |
Anesthesia | Without sensation | an | esthes | ia |
Cerebrospinal | Pertaining to the brain and spine | cerebro | spin | al |
Craniotomy | Incision into the skull | crani | otomy | |
Encephalotomy | Incision into the brain | encephal | otomy | |
Hypnotic | Pertaining to sleep | hypnot | ic | |
Insomnia | Lack of sleep | in | somn | ia |
Meningitis | Inflammation of the meninges | mening | itis | |
Microencephaly | Small brain | micro | encephal | y |
Neuralgia | Nerve pain | neur | algia | |
Neurology | Study of the nerve | neur | ology |
*A transition syllable or vowel may be added to or deleted from the word parts to make the combining form.
Structure and Function of the Nervous System
The nervous system is one of the most complex and interesting body systems. It is also one of the least understood. New discoveries are made almost daily about the capabilities of the nervous system.
The function of the nervous system is to sense, interpret, and respond to internal and external environmental changes to maintain a steady state in the body (homeostasis). The nervous system is divided into two major structures: the central nervous system (CNS) and the peripheral nervous system (PNS) (Fig. 19-1).
The CNS is made up of the brain and spinal cord (Fig. 19-2). It functions as the coordinator of the body’s full nervous system and contains the nerves that control connections between impulses coming to and from the brain and the rest of the body. The CNS plays a crucial role in maintaining a healthy, normally functioning body. Because nervous tissues are delicate and easily damaged, tough membranes called meninges surround the tissues. The nervous tissue and meninges are further protected by bones (vertebrae and cranium).
The PNS consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves (Table 19-1) that reach all parts of the body. The cranial nerves originate in the brain, and the spinal nerves emerge from the spinal cord. The spinal cord nerves can act independently from the brain in some reflex reactions (Fig. 19-3). Other reflex reactions of the nervous system may lead to the release of glandular secretions.
TABLE 19-1
Functions of the Peripheral Nervous System
Nerve | Function | |
Cranial Nerves | ||
I | Olfactory | Smell (S) |
II | Optic | Vision (S) |
III | Oculomotor | Raise eyelids, move eyes, focus lens, control pupil size (M) |
IV | Trochlear | Rotate eyes (M) |
V | Trigeminal | Facial and head sensation, control muscles in floor of mouth for chewing (B) |
VI | Abducens | Move eyes laterally (M) |
VII | Facial | Taste in anterior of mouth, control facial expression (B) |
VIII | Acoustic (auditory) | Hearing and balance (S) |
IX | Glossopharyngeal | Taste and swallowing (B) |
X | Vagus | Control muscles of speech, swallowing, and of thorax and abdomen; feeling from pharynx, larynx, and trachea (B) |
XI | Spinal accessory | Move neck and back muscles (M) |
XII | Hypoglossal | Move tongue (M) |
Spinal Nerves | ||
C1-8 Cervical (8 pair) | Neck and head movement; elevation of shoulders; movement of arms, hands, and diaphragmatic breathing | |
T1-12 Thoracic (12 pair) | Intercostal muscles of respiration and abdominal contractions | |
L1-5 Lumbar (5 pair) | Leg movement | |
S1-5 Sacral (5 pair) | Sphincter muscles of anus and urinary meatus; foot movement |
The organs of the PNS contain sensory (afferent) and motor (efferent) neurons (Fig. 19-4). Afferent neurons, or nerves, carry messages from the sensory cells of the body to the brain. Efferent, or motor, nerves carry messages from the brain to the body organs or parts. The connecting nerves (interneurons) of the CNS carry messages from afferent nerves to efferent nerves. Efferent nerves are classified as voluntary (somatic) or involuntary (autonomic).
The autonomic (involuntary) nervous system (ANS) is a part of the PNS (Fig. 19-5). It has two parts: the sympathetic system and the parasympathetic system. The sympathetic nerves are stimulated in situations that require action, such as the “fight or flight” reaction. The parasympathetic functions in response to normal, everyday situations. For example, the parasympathetic system would stimulate the digestion of food and slow the heart rate, whereas the sympathetic system would inhibit digestion and increase the heart rate.
Neuron
The basic structural unit of the nervous system is the nerve, which is a bundle of fibers that carries impulses. Nerve fibers consist of neuron cells, which are the functional unit of the nervous system. The three main types of neurons are afferent, efferent, and interneuron. Each carries messages, or impulses, to and from the body’s organs.
The neuron has several important parts (Fig. 19-6). The dendrites receive impulses and transmit them to the cell body. The cell body, which contains the nucleus of the neuron, transmits the impulse to the axon. The axon transmits the impulse away from the cell body to the dendrite of the next neuron. These impulse transmissions can travel more than 130 meters per second or 300 miles per hour.
Some neurons outside the CNS have a white, fatty substance covering the axon called myelin. Myelin, also called white matter, is arranged in bundles called Schwann cells. Layers of Schwann cells wrap around the axon forming the myelin sheath. The myelin sheath is covered with a membrane called the neurilemma. It is believed that neurilemma enables the axons to repair and regenerate themselves. Axons in the CNS, called gray matter, do not have neurilemma and therefore cannot repair or regenerate themselves. Another benefit of the myelin sheath is the microscopic spaces between the Schwann cells. These are called the nodes of Ranvier, and they greatly increase the speed of impulse transmission.
Neuroglia
Neuroglia, often called glia, are special nervous tissue cells that act as “glue” to support, bind, repair, and protect neurons. An estimated 900 billion neuroglia are in the body. They can be divided into four major types:
The astrocytes, star-shaped cells, are believed to help transfer substances from the blood to the brain. They make up what is known as the blood-brain barrier.
The oligodendroglia in the CNS and Schwann cells in the PNS help to develop the myelin sheath.
The microglia destroy and engulf bacteria and fight infection.
The ependymal cells line the cavities of the nervous system, producing and circulating fluid in the system.
Neuroglia may become cancerous when they divide to make new cells.
Synapse
A synapse is the space between two neurons. Neurons may be as close as one millionth of an inch to each other but still not touch. One neuron may send messages to up to 10,000 other neurons through the synapse. Impulses from one neuron are transmitted across the synapse to another neuron by a chemical called a neurotransmitter. The two most common neurotransmitters are acetylcholine and norepinephrine. One neurotransmitter may have different effects in varied synapses. More than 100 chemical messengers used by the nervous system have been identified.
Plexus and Dermatome
Four major networks of interwoven spinal nerves, called plexuses, provide impulses to specific regions of the body. They are called the cervical, brachial, lumbar, and sacral plexuses on the basis of the location of the spinal nerves that are involved in each group.
Sensations on the skin surface are controlled by specific spinal nerves. For example, the second cervical spinal nerve senses afferent messages on the top of the cranium. These areas are called dermatomes.