Glenda Mendoza: THE NERVOUS SYSTEM

WHAT IS NERVOUS SYSTEM?

The nervous system is a complex network of nerves and cells that carry messages to and from the brain and spinal cord to various parts of the body.

What is its function?

There are three general functions of the nervous system:

Sensing specific information about external and internal conditions (In the example above, this is seeing your friend).
Integrating that information (This is the understanding of the information coming from the eyes).
Issuing commands for a response from the muscles or glands (This is the reaction of walking towards him).

The nervous system includes both the Central nervous system and Peripheral nervous system. The Central nervous system is made up of the brain and spinal cord and The Peripheral nervous system is made up of the Somatic and the Autonomic nervous systems.

The Central nervous system

The central nervous system is divided into two major parts: the brain and the spinal cord.

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 called the neurons and supporting cells called the glia.
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 brain stem is also known as the Medulla oblongata. It is located between the pons and the spinal cord and is only about one inch long.

It is the connection between the rest of the brain and the rest of the central nervous system. This part of the brain is primarily concerned with life support and basic functions such as movement, thus meaning that more advanced processes are left to the more evolved areas of the brain.

The cerebrum
The cerebrum forms the bulk of the brain and is supported on the brain stem. The cerebrum is divided into two hemispheres. Each hemisphere controls the activities of the side of the body opposite that hemisphere.

It is the largest part of the human brain, and the part responsible for intelligence and creativity, and also involved in memory. The 'grey matter' of the cerebrum is the cerebral cortex, the center that receives information from the thalamus and all the other lower centers in the brain.

The hemispheres are further divided into four lobes:

Frontal lobe
Temporal lobes
Parietal lobe
Occipital lobe

Frontal Lobe - Found at the front of the head, near the temples and forehead, the frontal lobe is essential to many of the advanced functions of an evolved brain. It deals with voluntary muscle movements and deals with more intricate matters such as thought and speech.

Parietal Lobe - Situated behind the frontal lobe, this section deals with spatial awareness in the external environment and acts as a receptor area to deal with signals associated with touch.

Temporal Lobe - The temporal lobes are situated in parallel with the ears, they serve the ears by interpreting audio signals received from the auditory canal.

Occipital Lobe - This is the smallest of the four lobe components of the cerebrum, and is responsible in interpreting nerve signals from the eye at the back of the brain.

The cerebellum

This is located behind and below the cerebrum. Consisting of two hemispheres, the cerebellum is primarily concerned with movement and works in partnership with the brainstem area of the brain and focuses on the well being and functionality of muscles.

The diencephalon

The diencephalon is also known as the forebrain stem. 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. It regulates or is involved directly in the control of many of the body's vital drives and activities, such as eating, drinking, temperature regulation, sleep, emotional behavior, and sexual activity.

Other parts of the brain

Other parts of the brain include the midbrain and the pons:

the midbrain provides conduction pathways to and from higher and lower centers. It contains major relay stations for neurons transmitting signals to the cerebral cortex, as well as many reflex centers—pathways carrying sensory (input) information and motor (output) commands.

the pons acts as a pathway to higher structures; it contains conduction pathways between the medulla and higher brain centers; it is located below the midbrain and directly in front of the cerebellum is a prominent bulge in the brain stem called the pons. The pons consists of large bundles of nerve fibers that connect the two halves of the cerebellum and also connect each side of the cerebellum with the opposite-side cerebral hemisphere. The pons serves mainly as a relay station linking the cerebral cortex and the medulla oblongata.

The Spinal Cord

The spinal cord is along tube like structure which extends from the brain. The spinal cord is composed of a series of 31 segments. A pair of spinal nerves comes out of each segment. The region of the spinal cord from which a pair of spinal nerves originates is called the spinal segment. Both motor and sensory nerves are located in the spinal cord.
The spinal cord is about 43 cm long in adult women and 45 cm long in adult men and weighs about 35-40 grams. It lies within the vertebral column, the collection of bones (back bone).

Other parts of the central nervous system

The meninges are three layers or membranes that cover the brain and the spinal cord. The outermost layer is the dura mater. The middle layer is the arachnoid, and the innermost layer is the pia mater. The meninges offer protection to the brain and the spinal cord by acting as a barrier against bacteria and other microorganisms.
The Cerebrospinal Fluid (CSF) circulates around the brain and spinal cord. It protects and nourishes the brain and spinal cord.

Neurons

The neuron is the basic unit in the nervous system. It is a specialized conductor cell that receives and transmits electrochemical nerve impulses. A typical neuron has a cell body and long arms that conduct impulses from one body part to another body part.

There are three different parts of the neuron:

the cell body
dendrites
axon

Cell body of a neuron

The cell body is like any other cell with a nucleus or control center.

Dendrites

The cell body has several highly branched, thick extensions that appear like cables and are called dendrites. The exception is a sensory neuron that has a single, long dendrite instead of many dendrites. Motor neurons have multiple thick dendrites. The dendrite's function is to carry a nerve impulse into the cell body.

Axon

An axon is a long, thin process that carries impulses away from the cell body to another neuron or tissue. There is usually only one axon per neuron.

Myelin Sheath

The neuron is covered with the Myelin Sheath or Schwann Cells. These are white segmented covering around axons and dendrites of many peripheral neurons. The covering is continuous along the axons or dendrites except at the point of termination and at the nodes of Ranvier.
The neurilemma is the layer of Schwann cells with a nucleus. Its function is to allow damaged nerves to regenerate. Nerves in the brain and spinal cord do not have a neurilemma and, therefore cannot recover when damaged.

Types of neuron

Neurons in the body can be classified according to structure and function. According to structure neurons may be multipolar neurons, bipolar neurons, and unipolar neurons:

Multipolar neurons have one axon and several dendrites. These are common in the brain and spinal cord
Bipolar neurons have one axon and one dendrite. These are seen in the retina of the eye, the inner ear, and the olfactory (smell) area.
Unipolar neurons have one process extending from the cell body. The one process divides with one part acting as an axon and the other part functioning as dendrite. These are seen in the spinal cord.

The Peripheral nervous system

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

Another part of the nervous system is the Autonomic Nervous System. It has three parts:

the sympathetic nervous system
the parasympathetic nervous system
the enteric nervous system

This nervous system controls the nerves of the inner organs of the body on which humans have no conscious control. This includes the heartbeat, digestion, breathing (except conscious breathing) etc.
The nerves of the autonomic nervous system enervate the smooth involuntary muscles of the (internal organs) and glands and cause them to function and secrete their enzymes etc.
The Enteric nervous system is the third part of the autonomic nervous system. The enteric nervous system is a complex network of nerve fibers that innervate the organs within the abdomen like the gastrointestinal tract, pancreas, gall bladder etc. It contains nearly 100 million nerves.

Neurons in the peripheral nervous system

The smallest worker in the nervous system is the neuron. For each of the chain of impulses there is one preganglionic neuron, or one before the cell body, that is like a central controlling body for numerous neurons going out peripherally.
The preganglionic neuron is located in either the brain or the spinal cord. In the autonomic nervous system this preganglionic neuron projects to an autonomic ganglion. The postganglionic neuron then projects to the target organ.
In the somatic nervous system there is only one neuron between the central nervous system and the target organ while the autonomic nervous system uses two neurons.

Glenda Mendoza

Monday, December 3, 2012

THE NERVOUS SYSTEM