The human nervous system is made up of the central nervous system and the peripheral nervous system.
Human Nervous System
• What Is The Nervous System?
|What Is The Nervous System?
The nervous system is a network of nerves that link the brain to every part of the body. The nerves carry instant messages from the brain to every muscle and organ of the body. And they send back a constant stream of information to the brain about what is going on inside and outside of the body (such as pain or the sensation of danger) so that the body can respond and remain in what is known as homeostasis: a stable, physiological state. In short, it is our body's communication highway system.
|What Is The Structure Of The Nervous System?
The nervous system is composed of the brain, spinal cord and nerves. It is divided into two 2 parts - the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
|The Central Nervous System
The CNS consists of the brain and the spinal cord.
The Brain Stem
The brain stem consists of 3 parts: the midbrain, pons Varolii and the medulla oblongata.
The Spinal Cord
The spinal cord is the other main part of the central nervous system.
Other important parts of the central nervous system
|The Peripheral Nervous System
The peripheral nervous system (PNS) concerns all the nervous system outside the central nervous system and contains motor and sensory nerves which transmit information to and from the body and brain. It consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves.
The Cervical Plexus
The Thoracic (intercostal) Nerves
The Sacral Plexus
The Coccygeal Plexus
The motor division of the PNS is divided into the Somatic Nervous System and the Autonomic Nervous System.
Somatic Nervous System
The Somatic Nervous System conducts impulses from the CNS to the skeletal muscle fibers. This is the voluntary branch of the PNS and allows conscious control over the contraction of skeletal muscles.
Autonomic Nervous System
The Autonomic Nervous System conducts impulses from the CNS to cardiac and smooth muscles. This is an involuntary system controlled by the hypothalamus. Its nerves arise from the medulla oblongata. The ANS is further divided into Sympathetic and Parasympathetic divisions. Every organ in the body has a sympathetic and parasympathetic nerve supply with one division generating the opposite effect to the other.
Structure: Consists of nerves that arise from the spinal cord at the thoracic and lumbar region, form ganglia (bundles of nerve fibers) just outside the CNS and then extend to the organ or tissue they supply.
Reflexes are mostly protective and designed to stimulate the fast motor responses (movements). They are reflexes which are automatic and do not require supervision, like the secretion of gastric juices when food reaches the stomach. Your doctor can test your nervous system by tapping your knee with a small hammer. With a healthy system your leg will automatically kick up.
|What Are Reflexes?
A reflex is the automatic (not controlled by the brain) movement produced by a sensory stimulus. It is instant and involuntary e.g. a finger touching boiling hot water will immediately move away.
Several structures are involved in the production of a reflex and together they constitute the reflex arc:
Structure Of Nerve Cells
What Is A Nerve Cell?
Nerve cells are the basic unit of the system on which everything else is built. Like all cells, they have a membrane containing a nucleus and a cytoplasm but they have a particular shape: long and narrow. Some are very long (up to a meter). Nerve cells are easily damaged by toxins and lack of oxygen. Unlike other cells in the body, they are not usually replaced when they die, however, current research suggests that some may have the ability to regenerate.
The main parts of a nerve cell are:
|What Do Nerve Cells Do?
Nerve cells act as links in a chain, like relay runners, each one passing the 'baton' (information or instruction) to the next until it reaches the brain or the part of the body in question. The axon end feet of one cell are close to the dendrites of the next but they don't actually touch. The 'baton' of nerve impulses jumps across the gap via neurotransmitters, chemicals released by the nerve endings.
Tip: How do you remember which are afferent and which are efferent nerves? Efferent exit the brain, afferent arrive in the brain.
Nerve cells transmit and receive impulses throughout the body. Impulses do not continually run along each nerve but are created in response to internal or external stimuli - including changes in temperature, pressure or chemicals. Positively charged sodium and potassium ions are present inside and outside the cell. In a resting axon, the concentration of sodium ions is lower inside the cell than in the tissue fluid outside, but the concentration of potassium ions is higher inside than outside. This is maintained by differences in membrane permeability to these ions, and the sodium-potassium pump. The overall result is that the inside of the cell has a more negative charge than the outside.
|How Do Nerve Cells Communicate?
Nerve impulses only travel in one direction. So the movement of nerve impulses in a single neurone is as follows: the impulse crosses the synapse from the end feet of cell A into the dendrites of cell B. The impulse travels from the dendrites to the cell body and then out again along the axon to cell B's end feet. It then jumps across the synapse, helped by the chemical messengers. This process continues until the impulse reaches either the brain or the muscle/organ concerned.
|Diseases And Disorders Of The Nervous System
What It Links To
Nervous system links to:
The nervous system: has two parts, the central and peripheral (including autonomic) nervous systems informs and warns the body of environmental changes, sensations, pain and danger and initiates responses to stimuli.
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