The Nervous System

The nervous system is a complex network of nerves and cells that transmit signals between different parts of the body. The components of the nervous system are the central nervous system and the peripheral nervous system.

As stated by the National Institutes of Health, the central nervous system is made up of the brain and spinal cord. The peripheral nervous system consists of nerves that branch off from the spinal cord and extend to all parts of the body.

The nervous system plays a role in nearly every aspect of our health and well-being. It guides everyday activities such as waking up; automatic activities such as breathing; and complex processes such as thinking, reading, remembering, and feeling emotions.

The basic unit of the nervous system is a nerve cell, or neuron. The human brain contains about 100 billion neurons. A neuron has a cell body, which includes the cell nucleus, and special extensions called axons (pronounced AK-sonz) and dendrites (pronounced DEN-drahytz). Bundles of axons, called nerves, are found throughout the body. Axons and dendrites allow neurons to communicate, even across long distances. Axons are also called nerve fibres.

Different types of neurons control or perform different activities. For instance, motor neurons transmit messages from the brain to the muscles to generate movement. Sensory neurons detect light, sound, odor, taste, pressure, and heat and send messages about those things to the brain. Other parts of the nervous system control involuntary processes. These include keeping a regular heartbeat, releasing hormones like adrenaline, opening the pupil in response to light, and regulating the digestive system.

When a neuron sends a message to another neuron, it sends an electrical signal down the length of its axon. At the end of the axon, the electrical signal changes to a chemical signal. The axon then releases the chemical signal with chemical messengers called neurotransmitters into the synapse (pronounced SIN-aps)—the space between the end of an axon and the tip of a dendrite from another neuron.

The neurotransmitters move the signal through the synapse to the neighboring dendrite, which converts the chemical signal back into an electrical signal. The electrical signal then travels through the neuron and goes through the same conversion processes as it moves to neighboring neurons.

The nervous system also includes non-neuron cells, called glia (pronounced GLEE-uh). Glia perform many important functions that keep the nervous system working

Functions of Gilia

Help support and hold neurons in place
Protect neurons
Create insulation called myelin, which helps move nerve impulses
Repair neurons and help restore neuron function
Trim out dead neurons
Regulate neurotransmitters

Central Nervous System (CNS)

The central nervous system is divided into two major parts: the brain and the spinal cord. It is responsible for integrating sensory information and responding accordingly.

The Brain

The brain lies within the skull and is shaped like a mushroom. The brain consists of four principal parts:

The brain stem
The cerebrum
The cerebellum
The diencephalon

The brain weighs approximately 1.3 to 1.4 kg. It has nerve cells (neurons) and supporting cells (gilia).

There are two types of matter in the brain: grey matter and white matter. Grey matter receives and stores impulses. Cell bodies of neurons and neuroglia are in the grey matter. White matter in the brain carries impulses to and from grey matter. It consists of the nerve fibers (axons).

The Brain Stem

The stem-like part of the base of the brain that is connected to the spinal cord. The brain stem controls the flow of messages between the brain and the rest of the body, and it also controls basic body functions such as breathing, swallowing, heart rate, blood pressure, consciousness, and whether one is awake or sleepy. The brain stem consists of the midbrain, pons, and medulla oblongata.

The Cerebrum

The largest part of the brain. It is divided into two hemispheres. Each hemisphere controls the activities of the side of the body opposite that hemisphere.

The hemispheres are further divided into four lobes:

Frontal lobe
Temporal lobes
Parietal lobe
Occipital lobe

The Cerebellum

The portion of the brain that is in the back of the head, between the cerebrum and the brain stem. It is involved in the control of voluntary and involuntary movement as well as balance.

The Diencephalon

It includes the thalamus and hypothalamus. The thalamus is where sensory and other impulses go and coalesce. The hypothalamus is a smaller part of the diencephalon

The Spinal Cord

The major column of nerve tissue that is connected to the brain and lies within the vertebral canal and from which the spinal nerves emerge. Thirty-one pairs of spinal nerves originate in the spinal cord: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

The spinal cord consists of nerve fibers that transmit impulses to and from the brain. Like the brain, the spinal cord is covered by three connective-tissue envelopes called the meninges. The space between the outer and middle envelopes is filled with cerebrospinal fluid (CSF), a clear colorless fluid that cushions the spinal cord against jarring shock. Also known simply as the cord.

The Peripheral Nervous System (PNS)

The part of the nervous system that is made up of the nerves outside of the brain and spinal cord. It transmits information from the brain and spinal cord to the rest of the body

The Peripheral nervous system is made up of two parts:

Somatic nervous system
Autonomic nervous system

Somatic Nervous System

The somatic nervous system consists of peripheral nerve fibers that pick up sensory information or sensations from the peripheral or distant organs (those away from the brain like limbs) and carry them to the central nervous system.

These also consist of motor nerve fibers that come out of the brain and take the messages for movement and necessary action to the skeletal muscles. For example, on touching a hot object the sensory nerves carry information about the heat to the brain, which in turn, via the motor nerves, tells the muscles of the hand to withdraw it immediately.

The whole process takes less than a second to happen. The cell body of the neuron that carries the information often lies within the brain or spinal cord and projects directly to a skeletal muscle.

Autonomic Nervous System

A part of the nervous system that regulates key involuntary functions of the body. It controls the nerves of the inner organs of the body on which we have no conscious control. This includes the heartbeat, digestion, breathing (except conscious breathing) etc.

It is divided into three parts: