Learning Objectives
1. Name and describe the parts of a neuron and how neurons transmit impulses.
2. Give an example of a sensory neuron, motor neuron, and interneuron impulse.
3. List primary functions of each of the brain’s four cerebral lobes.
4. Explain how an injury to the cerebellum might manifest itself in an individual.
5. Identify the role of the limbic system in maintaining a person’s level of awareness.
6. State the functions of the medulla, pons, and midbrain. Describe two nursing considerations appropriate for a client with brainstem dysfunction.
7. Identify the three meninges, and explain the functions of the spinal cord and cerebrospinal fluid.
8. List the 12 cranial nerves and the function of each. Describe the functions of spinal nerves.
9. Compare and contrast the functions of the parasympathetic and sympathetic nervous systems.
10. Explain nerve transmission, including definitions for resting potential, action potential, and neurotransmitter.
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IMPORTANT TERMINOLOGY |
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action potential |
hypothalamus |
parietal lobe |
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afferent |
interneuron |
plexus |
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axon |
limbic system |
pons |
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brainstem |
medulla |
receptors |
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cerebellum |
meninges |
reflex |
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cerebrum |
midbrain |
reflex arc |
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cranial nerves |
myelin sheath |
spinal cord |
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decussation |
nerve |
spinal nerves |
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dendrites |
neuroglia |
sympathetic |
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effectors |
neurology |
synapse |
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efferent |
neuron |
temporal lobe |
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frontal lobe |
neurotransmitter |
thalamus |
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ganglion |
occipital lobe |
vagus nerve |
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glial cells |
parasympathetic |
ventricle |
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Acronyms |
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ANS |
PNS |
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CNS |
RAS |
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CSF |
SNS |
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EEG |
In this topic, we will study one of the two major control systems of the body. The nervous system coordinates and makes rapid responses to external (and internal) stimuli. In the next topic,we will study the endocrine system, which controls slower, longer-lasting responses to internal stimuli. The functions of these two systems are closely interrelated and have major influence on all other body systems. The nervous system is director of many body activities and is responsible for much of the communication between body systems.
The nervous system takes information from the outside world (external stimuli) and selectively stores this information for future reference and application. It also coordinates messages from internal body systems (internal stimuli), making it possible for the body to readjust constantly to changing internal and external environments. The nervous system may be compared to a complex hardwired computer network. Through a system of wires (nerves), messages come into a central processor (brain and spinal cord), where connections are made to send messages out to the body via outgoing nerves. Neurology is the study of the nervous system.
BOX 19-1.
Functions of the Nervous System
Communication
♦ Monitors impressions and information from external stimuli
♦ Monitors information from internal stimuli
♦ Responds to danger; pain, and other situations
♦ Responds to internal and external changes
♦ Helps to maintain homeostasis
♦ Responds to conscious decisions and thoughts
♦ Coordinates processing of new learning
♦ Stores and retrieves memories, including previous learning
♦ Facilitates judgment, reasoning, and decision making Control
♦ Directs all body activities
♦ Maintains blood pressure, respiration, and other vital functions
♦ Regulates body systems (in coordination with endocrine system)
♦ Coordinates reflex actions
♦ Controls instinctual behaviors
♦ Controls conscious movement and activities
♦ Stores unconscious thoughts
The basic functions of the nervous system are to receive sensory input (stimuli), to integrate and interpret stimuli, and to respond to the stimuli.
Structure and Function
The functions of the nervous system are communication and control (Box 19-1). To understand these functions, the nurse must first learn about the specialized cells of the nervous system (neurons and neuroglia), and the structures that are made up of these cells.
CELLS OF THE NERVOUS SYSTEM
The nervous system is made up of neurons (the nerve cell) and neuroglia. The neuron is the basic structural and functional cell of the nervous system. Neurons are specialized to respond to electrical, chemical, and physical stimuli, and messages are conducted and transferred through them. The human brain regulates more than 10 billion neurons throughout the body at all times. Neuroglia outnumber neurons in the body by a ratio of five to one. Neuroglia support and connect nervous tissue, but do not transmit impulses. Neuroglia are composed of cells, three of which are called astrocytes, oligodendrocytes, and microglia.
Neurons
The neuron is the functional unit of the nervous system. Neurons perform many functions. In the brain, they influence thinking, affect memory, and regulate other organs and glands. Although neurons are microscopic, they vary greatly in size and length. A neuron is composed of three basic parts: one cell body and two processes (consisting of an axon and numerous dendrites). Figure 19-1 illustrates the structure of a neuron.
FIGURE 19-1 · A myelinated motor neuron. (The break in the axon shows that the axon is longer than pictured.) The arrows show the direction of the nerve impulse.
Each neuron has only one cell body, which contains the nucleus, mitochondria, and other organelles. Neurons do not divide and reproduce by mitosis as do other cells of the body. If a nerve cell body is destroyed, it is lost forever. Protein synthesis occurs within the cell body in specialized organelles called Nissl bodies (chromatophilic substance), which are unique to the neuron. The cell body may be quite a distance from its axon or dendrites.
Key Concept All neurons have the same basic structure, but differ in size and shape, depending on their function.
Axons. An axon is an extension that carries impulses away from the neuron cell body. An axon may be as short as a few millimeters, or it may be longer than a meter. It may be myelinated (covered in a protective layer), or it may be bare. This structural difference is important.
An axon surrounded by a myelin sheath (a fatty covering) is said to be myelinated. The myelin sheath electrically insulates one nerve cell from another. Without this sheath, these particular nerve cells would short circuit. Myelin is formed by the plasma membranes of specialized glial (neuroglial) cells called Schwann cells, which provide nutrition and support. The gap between Schwann cells is called the node of Ranvier (Fig. l9-2). These nodes provide points along a neuron where a signal is generated. Signals jumping between these nodes travel hundreds of times faster than signals traveling on the bare surface of an axon. Thus, myelinated axons conduct impulses faster than nonmyelinated axons. The axons within a nerve fiber are in bundles and they are surrounded by connective tissue and blood vessels.
Dendrites. Dendrites are short, often highly branched extensions of the cell body. Dendrites receive impulses from the axons of other neurons and transmit these impulses toward the cell body. Dendrites respond to chemical messages sent across the synapse, which is the tiny space that separates neurons from each other. This very narrow gap, called the synaptic cleft, is about 20 nm (nanometers) in width.
The synapse or synaptic cleft is the junction, or space, between the axon of one neuron and the dendrites of the next (Fig. 19-3). A nerve can transmit impulses in only one direction because of the location of neurotransmitters. A neurotransmitter is a chemical released by the axon that enables nerve impulses to cross the synapse and reach the dendrites. Cell bodies and dendrites do not have neurotransmitters. The transmission of a nerve impulse and the action of neurotransmitters are discussed later in this topic.
FIGURE 19-2 · The formation of the myelin sheath. The outermost layer of the Schwann cell forms the neurolemma. The space between the cells is the node of Ranvier
Classification of Neurons. Neurons can be classified according to their shape, but they are more commonly remembered by their functions: sensory, motor, or as interneurons.
• Sensory (afferent) neurons receive and transmit messages to the central nervous system from all parts of the body. These messages include such factors as hormone levels, blood pH, touch, sound, light, and pressure. Sensory neurons usually have long dendrites and a short axon.
• Motor (efferent) neurons receive and transmit messages from the central nervous system to muscles and glands in all parts of the body. These messages are transmitted as motor output, which causes an action. The signals carried by motor neurons alter muscle activity or cause glands to secrete. Motor neurons usually have short dendrites and a long axon.
• Interneurons (connectory, association neurons, or integrators) can be thought of as a link between the two other types of neurons. They are interconnecting neurons. Interneurons are located only within the central nervous system.
Key Concept The three types of neurons are sensory neurons, motor neurons, and interneurons.
Sensory neurons make up sensory nerves. Motor neurons make up motor nerves. Mixed nerves, which are discussed later in this topic, contain both sensory and motor neurons. In a sensory center, input is integrated and a response is generated.
Neuroglia
Neuroglia (glial cells) are much more numerous than neurons (10:1). They can multiply to fill spaces that have previously been occupied by neurons. Some brain tumors are called gliomas because they are caused by rapid growth of glial cells. The four types of neuroglia in the central nervous system are astrocytes, oligodendrocytes, microglia, and ependymal cells. The two types of neuroglia in the peripheral nervous system are the neurolemmocytes and Schwann cells. These cells help form the blood-brain barrier, cerebrospinal fluid, and the myelin sheath. They also obtain nutrients (specifically, glucose and oxygen) for the neurons, and support and protect the central and peripheral nervous systems by maintaining homeostasis. The neuroglia surround neurons and hold them in place, insulate between neurons, form myelin, destroy pathogens, and remove dead neurons.
Special Considerations: LIFESPAN
Infant Blood-Brain Barrier
Infants have an immature blood-brain barrier This allows substances to pass into the brain of an infant that would not normally enter an adult’s brain.
FIGURE 19-3 · The synapse.
