PART 1

FUNCTIONS OF THE NERVOUS SYSTEM

The nervous system is involved in some way in nearly every body function. We can consider the nervous system as a commu-nication system, receiving signals from and sending commands to different areas of the body. By way of this communication, the nervous system helps to coordinate the body functions to maintain homeostasis. Some major functions of the nervous system are

1. Receiving sensory input.

- Sensory receptors monitor numerous external and internal stimuli. We are aware of sensations from some stimuli, such as vision, hearing, taste, smell, touch, pain, body position, and temperature. Other stimuli, such as blood pH, blood gases, and blood pressure, are processed at a subconscious level.

2. Integrating information.

The brain and spinal cord are the major organs for processing sensory input and initiating responses. The input may produce an immediate response, be stored as memory, or be ignored.

3. Controlling muscles and glands.

Skeletal muscles normally contract only when stimulated by the nervous system. Thus, by controlling skeletal muscle, the nervous system controls the major movements of the body. The nervous system also participates in controlling cardiac muscle, smooth muscle, and many glands.

4. Maintaining homeostasis.

The nervous system plays an important role in maintaining homeostasis. This function depends on the nervous system's ability to detect, interpret, and respond to changes in internal and external conditions. In response, the nervous system can stimulate or inhibit the activities of other systems to help maintain a constant internal environment.

5. Establishing and maintaining mental activity.

The brain is the center of mental activity, including consciousness, memory, and thinking.

DIVISIONS OF THE NERVOUS SYSTEM

The nervous system can be divided into two major divisions: (1) the central nervous system and the (2) peripheral nervous system.

Central nervous system (CNS)

- consists of the brain and spinal cord.

Peripheral nervous system (PNS)

- consists of all the nervous tissue outside the CNS, including nerves and ganglia.

The PNS is the communication link between the CNS and the various parts of the body. The PNS carries information about the different tissues of the body to the CNS and delivers commands from the CNS to other body tissues that alter body activities. Considering this two-directional flow of information, the PNS can be subdivided into two parts:

1. THE SENSORY DIVISION

- or afferent (toward) division, of the PNS conducts action potentials from sensory receptors to the CNS.

- The neurons that transmit action potentials from the periphery to the CNS are called sensory neurons.

2. THE MOTOR DIVISION

- or efferent (away) division, of the PNS conducts action potentials from the CNS to effector organs, such as muscles and glands. The neurons that transmit action potentials from the CNS toward the periphery are called motor neurons.

The effectors controlled by the motor division include muscle tissue and glands. Muscle tissue includes skeletal muscle, which is voluntarily controlled, and cardiac and smooth muscle, which are involuntarily controlled. Glands are also involuntarily controlled. The motor division can be further subdivided into two components, based on the type of effector being innervated: (1) somatic nervous system and (2) autonomic nervous system.

Somatic (bodily) nervous system

- transmits action potentials from the CNS to skeletal muscles

Autonomic (self-governing) nervous system (ANS)

- transmits action potentials from the CNS to cardiac muscle, smooth muscle, and glands. The autonomic nervous system, in turn, is divided into sympathetic and parasympathetic divisions.

Enteric nervous system (ENS)

- a unique part of the peripheral nervous system. The ENS has both sensory and motor neurons contained wholly within the digestive tract. The ENS can function without input from the CNS or other parts of the PNS, although it is normally integrated with the CNS by sensory neurons and ANS motor neurons.

CELLS OF THE NERVOUS SYSTEM

The two types of cells that make up the nervous system are (1) neurons and (2) glial cells.

Neurons

Neurons (nerve), or nerve cells, receive stimuli, conduct action potentials, and transmit signals to other neu-rons or effector organs. A neuron has three parts: (1) a cell body and two types of processes, called (2) dendrites and (3) axons.

Each neuron cell body contains a single nucleus. As with any other cell, the nucleus of the neuron is the source of information for gene expression. Extensive rough endoplasmic reticulum (rough ER), a Golgi apparatus, and mitochondria surround the nucleus. Large numbers of neurofilaments (intermediate filaments) and microtubules organize the cytoplasm into distinct areas.

Dendrites (DEN-drights; trees) are short, often highly branch-ing cytoplasmic extensions that are tapered from their bases at the neuron cell body to their tips. Most dendrites are extensions of the neuron cell body, but dendrite-like structures also project from the peripheral ends of some sensory axons. Dendrites usually receive

information from other neurons or from sensory teeptors and transmit the information toward the neuron cell body. Each neuron has an axon, a single long cell process extending

from the neuron cell body. The area where the aon leaves the en cell body is called the axon hillock. Each on bus a uniform diam eter and may vary in length from a few millimeters to more than a meter. Axons of sensory neurons conduct action potentials towards the CNS, and axons of motor neurons conduct action potentials away from the CNS. Axons also conduct action potentials from one part of the brain or spinal cord to another part. An azon may remain unbranched or may branch to form collateral (ko-LAT-er-chi) axones. Axons can be surrounded by a highly specialized insulating layer of bells called the myelin sheath (described in more detail in Myelin Sheaths, later in this chapter).

Types of Neurons

The body's neurons vary in function and in structure, As such, neurons can be classified on the basis of their function as well as their structure. For example, referring to a neuron as a "sensory neuron" indicates that it is carrying information to the CNS from a specific receptor in the body. Alternatively, referring to a neuron as a "motor neuron" indicates that it is sending information to an effector of the body from the CNS. However, neurons are also dis-tinguished from one another on the basis of their structure. In the structural classification, three categories of neurons exist, based on the arrangement of their processes (figure 8.4 and table 8.1).

1. Multipolar neurons have many dendrites and a single axon. Most of the neurons within the CNS and nearly all motor neurons are multipolar.

2. Bipolar neurons have two processes: one dendrite and one axon. Bipolar neurons are located in some sensory organs, such as in the retina of the eye and in the nasal cavity.

3. Pseudo-unipolar neurons have a single process extending from the cell body. This process divides into two extensions a short distance from the cell body. One extends to the periphery, and the other extends to the CNS. The two extensions function as a single axon with small, dendrite-like sensory receptors at the periphery. The axon receives sensory information at the periphery and transmits that information in the form of action potentials to the CNS. With the exception of the bipolar neurons described earlier, most sensory neurons are pseudo-unipolar.

Glial Cells

Glial cells (GLEE-al), or neuroglia (nyu-ROH-glee-ah; nerve glue), are the supportive cells of the CNS and PNS, meaning these cells do not conduct action potentials. Instead, glial cells carry out different activities that enhance neuron function and maintain normal conditions within nervous tissue. Glial cells are far more numerous than neurons. Most glial cells retain the ability to divide, whereas neurons do not. The types of glial cells are dif-ferent in the CNS versus the PNS. In the CNS, there are four types of glial cells. These include (1) astrocytes, (2) ependymal cells, (3) microglia, and (4) oligodendrocytes. The glial cells of the PNS include (1) Schwann cells and (2) satellite cells.

Astrocytes (ASS-troh-sites) serve as the major supporting cells in the CNS. In this role, astrocytes can stimulate or inhibit the signaling activity of nearby neurons. In addition, astrocytes participate with the blood vessel endothelium to form a perme-ability barrier, called the blood-brain barrier, between the blood and the CNS. Astrocytes help limit damage to neural tissue; however, the repair process can form a scar that blocks regen-eration of damaged axons. Ependymal (ep-EN-dih-mal) cells line the fluid-filled cavities (ventricles and canals) within the CNS. Some ependymal cells produce cerebrospinal fluid, and others, with cilia, help move the cerebrospinal fluid through the CNS.Microglia (my-KROH-glee-ah) act as immune cells of the CNS. They help protect the brain by removing bacteria and cell debris. Oligodendrocytes (OL-ih-goh-DEN-droh-sites) provide an insu-lating material that surrounds axons (figure 8.5 and table 8.1).

In the PNS, the glial cells known as Schwann cells provide insulating material around axons. Satellite cells are found around the cell bodies of certain neurons of the PNS. These cells provide support and nutrition to the neurons and protect the neurons from heavy-metal poisons, such as lead and mercury