The Nervous System is largely divided into two components:
The Peripheral Nervous System connects the Central Nervous System to the limbs and organs, thus ensuring communication by impulses, to and from these components, especially where they are very distant from the CNS. Unlike the Central Nervous System that is protected by the bones of skull and spine as well as by a highly exclusionary blood-brain barrier, the Peripheral Nervous System is usually not protected beyond a sheath and is therefore more exposed to toxins and the possibility of mechanical injuries.
The brain is the most complex part of the human body. Weighing approximately 3 lbs, it is the seat of intelligence, interpreter of the senses, initiator of body movement, store of life experiences and controller of behavior. It is encased in a bony shell and is washed by protective fluid.
All the parts of the brain work together, but each part has its own special properties. It can be divided into three basic units: the forebrain; the midbrain, and the hindbrain.
The forebrain is the largest and most highly developed part of the human brain.
It consists primarily of the cerebrum and the structures hidden beneath it. It is the source of intellectual activities. It holds your memories, allows you to plan, enables you to imagine and think.
The cerebrum is split into two halves (hemispheres) by a deep fissure but they communicate with each other through a thick tract of nerve fibers that lies at the base of this fissure. Although the two hemispheres seem to be mirror images of each other, they are different. For instance, the ability to form words seems to lie primarily in the left hemisphere, while the right hemisphere seems to control many abstract reasoning skills.
For unknown reasons, nearly all of the signals from the brain to the body and vice-versa cross over on their way to and from the brain. This means that the right cerebral hemisphere primarily controls the left side of the body and the left hemisphere primarily controls the right side. When one side of the brain is damaged, the opposite side of the body is affected. For example, a stroke in the right hemisphere of the brain can leave the left arm and leg paralyzed.
The hindbrain includes the upper part of the spinal cord, the brain stem, and a wrinkled ball of tissue called the cerebellum. The hindbrain controls the body’s vital functions such as respiration and heart rate. The cerebellum coordinates movement and is involved in repetitively learned (rote) movements.
The uppermost part of the brain stem is the midbrain, which controls some reflex actions and is part of the circuit involved in the control of eye movements and other voluntary movements.
The Spinal cord is about the diameter of a human finger and descends from the brain, down the middle of the back. It consists of millions of nerve fibres which transmit electrical information to and from the limbs, trunk and organs of the body, back to and from the brain. The spinal cord is surrounded by a clear fluid called Cerebral Spinal Fluid (CSF), that acts as a cushion to protect the delicate nerve tissues from damage from banging against the inside of the vertebrae. The cord and fluid are all encased within a bony vertebral column
The anatomy of the spinal cord itself, The nerves which exit the spinal cord in the upper section, the neck, control breathing and the arms. The nerves which exit the spinal cord in the mid and lower section of the back, control the trunk and legs, as well as bladder, bowel and sexual function.
The nerves which carry information from the brain to muscles are called Motor Neurones. The nerves which carry information from the body back to the brain are called Sensory Neurones. Sensory Neurones carry information to the brain about skin temperature, touch, pain and joint position.
The nerves within the spinal cord are grouped together in different bundles called Ascending and Descending tracts:
Nerves called the spinal nerves or nerve roots, branch off the spinal cord and pass out through a hole in each of the vertebrae called the Foramen. There are four main groups of spinal nerves, which exit different levels of the spinal cord. These are in descending order down the vertebral column:
Nerves connecting the spinal cord to the body are referred to as the Peripheral Nervous System (PNS). The sensory division carries information from the body to the brain while the motor division transmits signals from the brain to the body.
The Autonomic nervous system largely operates at the subconscious level to control vital body functions including heart-rate, digestion, respiratory rate, salivation, perspiration, pupillary dilation, micturition (urination), and sexual arousal.
The somatic nervous system transmits voluntary action, mainly through skeletal muscles.
Common disorders of the Nervous System include Chronic pain (with diverse causes ranging from traumatic injury to underlying diseases such as diabetes and cancers), Epilepsy, Tremors, Age-related disorders including Alzeihmer's disease, Parkinson's disease and many others.