The Nervous System
The nervous system is one of the most complex of all body systems. More than 10 billion nerve cells are operating constantly all over the body to coordinate the activities we do consciously and unconsciously.
Nerve cells collected into bundles called nerves carry electrical messages all over the body. External and internal stimuli activate the cell membranes of nerve cells so as to release stored electrical energy within the cells. This released energy is called the nervous impulse. Body receptors receive and transmit impulses to the complex nerwork of nerve cells in the brain and spinal cord. Within this central part of the nervous system impulses are recognized, interpreted, and relayed to other nerve cells which extend out to all parts of the body.
The nervous system is classified into two major divisions: the central nervous system (CNS) and the peripheral nervous system. The CNS consists of the brain and spinal cord. The peripheral nervous system consists of cranial nerves which carry impulses between the brain and the head and neck, and spinal nerves which carry messages between the spinal cord and the chest, abdomen, and extremities.
In addition, the peripheral nervous system consists of nerves which function involuntarily without conscious control. These peripheral nerves are those of the autonomic nervous system. This system carries impulses from the CNS to the glands, heart, blood vessels, and the involuntary muscles. These nerves are called efferent, since they carry impulses away from the CNS to organs which produce responses, e.g., muscles and glands. Peripheral nerves which carry impulses to the brain and spinal cord from receptors like the skin, eye, ear, and nose are called afferent nerves.
Some of the autonomic nerves are called sympathetic nerves and others are called parasympathetic nerves. The sympathetic nerves stimulate the body, i.e., increase heart rate and blood pressure, dilate airways, stimulate the adrenal glands.The parasympathetic nerves act as a balance for the sympathetic nerves. They slow down heart rate, lower blood pressure, etc.
Ganglia, which are collections of nerve tissue outside the brain and spinal cord, and plexuses, which are larger networks of nerves, are prevalent in the autonomic nervous system.
A neuron is an individual nerve cell. Impulses are passed along the parts of a nerve cell in a definite manner and direction.
A stimulus begins a wave of excitability in the receptive branching fibers of the neuron which are called dendrites. The impulse next reaches the cell body. Extending from the cell body is the axon which carries the impulse away from the cell body. The space where the impulse jumps from one neuron to another is called the synapse.
While a neuron is a microscopic structure, a nerve is macroscopic, able to be seen with the naked eye. A nerve consists of a bundle of dendrites and axons which travel together like strands of rope.
The brain is the primary center for regulating and coordinating body activities. It has many different parts, all of which control different aspects of body functions.
The largest part of the brain is the cerebrum. The outer nervous tissue of the cerebrum, known as the cerebral cortex, is arranged in folds to form elevated portions known as gyri and grooves known as sulci. The cerebrum has many functions. All thought, judgment, memory, association, and discrimination take place within it.
Within the middle region of the cerebrum are spaces, called ventricles. They contain a watery fluid which flows throughout the brain and around the spinal cord. This fluid is called cerebrospinal fluid and protects the brain and spinal cord from shock.
Two other important parts of the brain, the thalamus and hypothalamus, are below the cerebrum in the interbrain and midbrain. The thalamus monitors the sensory stimuli we receive. The hypothalamus controls body temperature, sleep, appetite, some emotions. Also it regulates the release of hormones from the pituitary gland.
The following structures within the brain lie below the posterior portion of the cerebrum and connect the cerebrum with the spinal cord: the cerebellum, pons, and medulla oblongata.
The function of the cerebellum is to aid in the coordination of voluntary movements and to maintain balance and muscular tone.
The pons contains nerve fiber tracts which connect the cerebellum and cerebrum with the rest of the brain. Nerve tracts “cross over” in the pons. E.g., nerve cells which control the left side of the body are found in the right half of the brain.
The medulla oblongata, located at the base of the brain contains centers which regulate internal activity of the body. These are:
Respiratory center which controls muscles of respiration.
Cardiac center which tends to slow heart rate.
Vasomotor center which affects (constricts or dilates) the muscles in the blood vessels.
The spinal cord is a column of nervous tissue extending from the medulla oblongata to the 2nd lumbar vertebra. It ends as cauda equina, a fan of nerve fibers. It carries all the nerves which affect the limbs and lower part of the body, and is the pathway for impulses going to and from the brain.
The meninges are three layers of connective tissue membranes that surround the brain and spinal cord. The outermost membrane is called the dura mater. It is a thick membrane and contains channels for blood to come into the brain tissue. The second layer is called the arachnoid membrane. The arachnoid membrane is loosely attached to the other meninges by weblike fibers so there is a space for cerebrospinal fluid . The third layer, closest to the brain and spinal cord, is called the pia mater. It is made of delicate connective tissue with a rich supply of blood vessels.