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The Nervous System

The Nervous System

The nervous system is the communications network that controls and coordinates the muscle, organ and sensory function that takes place within our bodies.

The nervous system has two main parts:

The central nervous system (CNS), made up of the brain and spinal cord.

The peripheral nervous system (PNS), consisting of the network of nerves that branch out from the brain and spinal cord. The function of the peripheral nervous system is to transmit information back and forth between the CNS and the rest of the body. The PNS is subdivided into the somatic nervous system and the autonomic nervous system.

  • Nerves in the somatic nervous system control voluntary movement and activity, transmitting information between the CNS and the skeletal muscles and external sensory organs.
  • Nerves in the autonomic nervous system regulate involuntary movement, transmitting information between the CNS and the smooth muscles of the body’s internal organs and the cardiac muscle of the heart.

Brain

The brain is the control center of the nervous system. It enables us to think, feel, and move. The brain constantly receives information and sends out instructions to the body through the spinal cord and the body’s vast network of nerves.

Some of the functions of the nervous system – breathing, body temperature and heart rate, for example – are involuntary, meaning they’re not under our conscious control; others, such as movement, speech and thought, are voluntary, meaning they are under our control.

There are 12 pairs of cranial nerves branching off the brain. These nerves relay impulses from the sensory organs, such as the eyes or ears. Thirty-one pairs of spinal nerves branch off the spinal cord, exiting between each level of vertebrae. These nerves relay impulses to and from the rest of the body.

The largest part of the brain is the cerebrum, which controls the most sophisticated functions, such as thought, imagination, memory, emotion, speech and sensory perception. The human cerebrum is quite large. It has two halves, or hemispheres. A band of nerve tissue, called the corpus callosum, links two halves to allow them to exchange information. Each hemisphere is covered by a layer of gray tissue, called the cerebral cortex, which is responsible for the higher functions of the brain, including conscious thought. The cortex is composed of sulci (folds) and gyri (bulges), which together provide a large surface area in the limited space inside the skull.

The cortex of each hemisphere has four lobes. The occipital lobe controls vision. The temporal lobe controls sound and speech. The parietal lobe controls movement, touch and recognition. The frontal lobe controls thinking and planning.

The brain stem and hypothalamus control automatic processes, such as breathing and heartbeat.

The cerebellum acts as a “mini brain” that coordinates body balance, posture and movement.

Brain tissue is soft and delicate, and requires protection. In addition to the protection provided by the skull, the brain is surrounded by: the connective tissue membranes, or meninges: the dura mater; the arachnoid mater and the pia mater. Beneath the arachnoid mater is a wide space filled with cerebrospinal fluid. This fluid forms a liquid cushion that reduces the weight of the brain and protects it from knocks and jolts. It also helps the brain receive nourishment.

Spinal Cord

The spinal cord extends from the base of the brain to the area between the bottom of the first lumbar vertebra and the top of the second lumbar vertebra. It is enclosed in and protected by the bony vertebral column, and its main function is the transmission of neural inputs between the peripheral nervous system and the brain.

Like the brain, the spinal cord is covered by a protective membrane called the dura mater, which forms a watertight sac around the spinal cord and nerves. Inside this sac is spinal fluid, which surrounds the spinal cord.

The nerves in each area of the spinal cord are connected to specific parts of the body. Those in the cervical spine, for example, extend to the upper chest and arms; those in the lumbar spine the hips, buttocks and legs. The nerves also carry electrical signals back to the brain, creating sensations.

The spinal cord ends by diverging into individual nerves that travel out to the lower body and the legs. Because of its appearance, this group of nerves is called the cauda equina – the Latin name for “horse’s tail.” The nerve groups travel through the spinal canal for a short distance before they exit the neural foramen.

Neurons

Neurons are the “building blocks” of the nervous system. They’re long, thin cells that transmit electrical impulses, and have many branched endings, called dendrites, which receive impulses from other neurons. An axon, or nerve fiber, carries nerve impulses to other neurons or to muscle. Neurons do not touch, but are separated by a tiny gap called a synapse. When an impulse arrives at the end of an axon, it releases chemicals that generate an impulse in the dendrites of the neighboring neuron.

There are three types of neurons: sensory, motor and association.

  • Sensory neurons transmit nerve impulses from sense organs (eyes, ears, nose, tongue and touch) to the brain. They also carry nerve impulses to the brain and spinal cord.
  • Motor neurons transmit nerve impulses from the brain and spinal cord to a specific area of the body. A nerve impulse to a muscle, for example, may cause it to contract.
  • Association neurons make up 90% of all neurons and are found only in the brain and spinal cord.

Nerves are made up of bundles of both sensory and motor neurons.