Structure and Function 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.

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.

Brain Byte

The human brain has about a 100 billion neurons that, if lined up, would be 600 miles long.

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.

Brain Byte

Neurons are the largest cells in the body and do not perform mitosis.

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.

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