Three Functions of The Nervous System:
Sensory Input
Receptors are parts of the nervous system that sense changes
in the internal or external environments. Sensory input can be in many forms, including
pressure, taste, sound, light, blood pH, or hormone levels, that are converted
to a signal and sent to the brain or spinal cord.
Integration and Output
In the sensory centers of the brain or in the spinal cord,
the barrage of input is integrated and a response is generated. The response, a
motor output, is a signal transmitted to organs than can convert the signal
into some form of action, such as movement, changes in heart rate, release of
hormones, etc.
The neuron is the functional unit of
the nervous system. Humans have about 100 billion neurons in their brain alone!
While variable in size and shape, all neurons have three parts. Dendrites receive information
from another cell and transmit the message to the cell body. The cell body contains the
nucleus, mitochondria and other organelles typical of eukaryotic cells. The axon conducts messages away
from the cell body.
The Nerve Message
The plasma membrane of neurons, like all other cells, has an
unequal distribution of ions and electrical charges between the two sides of
the membrane. The outside of the membrane has a positive charge, inside has a
negative charge. This charge difference is a resting potential and is measured
in millivolts. Passage of ions across the cell membrane passes the electrical
charge along the cell. Resting potential results from differences between
sodium and potassium positively charged ions and negatively charged ions in the
cytoplasm. Sodium ions are more concentrated outside the membrane, while
potassium ions are more concentrated inside the membrane. This imbalance is
maintained by the active transport of ions to reset the membrane known as the
sodium potassium pump. The sodium-potassium pump
maintains this unequal concentration by actively transporting
ions against their concentration gradients.
Synapses
The junction between a nerve cell and another cell is called
a synapse. Messages travel within the
neuron as an electrical action potential. The space between two cells is known
as the synaptic cleft.
To cross the synaptic cleft requires the actions of neurotransmitters.
Neurotransmitters are stored in small synaptic vessicles
clustered at the tip of the axon.
The Central Nervous System
The central nervous
system is made up of the spinal cord and the brain.
The brain of all vertebrates develops from three swellings at
the anterior end of the neural canal of the embryo. From front to back these
develop into the forebrain, midbrain and the hindbrain. The brain receives
nerve impulses from the spinal cord and 12 pairs of cranial nerves.
The Hindbrain
The main structures of the hindbrain are the medulla oblongata,
pons and cerebellum. 
Medulla oblongata
The medulla looks like a swollen tip to the spinal cord.
Nerve impulses arising here
rhythmically stimulate the intercostal muscles and diaphragm
— making breathing possible, regulate heartbeat, and regulate the
diameter of arterioles thus adjusting blood flow.
Pons
The pons seems to serve as a relay station carrying signals
from various parts of the cerebral cortex to the cerebellum. Nerve impulses
coming from the eyes, ears,
and touch receptors
are sent on the cerebellum. The pons also participates in the reflexes that
regulate breathing. The reticular formation is a region running through the
middle of the hindbrain (and on into the midbrain). It receives sensory input
from higher in the brain and passes these back up to the thalamus. The
reticular formation is involved in sleep, arousal and vomiting.
Cerebellum
The cerebellum consists of two deeply-convoluted hemispheres.
Although it represents only 10% of the weight of the brain, it contains as many
neurons as all the rest of the brain combined. Its most clearly-understood function is to coordinate body
movements. People with damage to their cerebellum are able to perceive the
world as before and to contract their muscles, but their motions are jerky and
uncoordinated.
The midbrain acts as a relay station for tracts passing
between the cerebrum and the spinal cord or cerebellum. It also has reflex centers for visual,
auditory, and tactile responses.
Spinal
Cord
The Spinal Cord is connected to the brain and is about the
diameter of a human finger. From the brain the spinal cord descends down the
middle of the back and is surrounded and protected by the bony vertebral
column. 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
against damage from banging against the inside of the vertebrae.
The anatomy of the spinal cord itself, 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 brain and spinal
cord are referred to as the Central Nervous System, whilst the nerves
connecting the spinal cord to the body are referred to as the Peripheral Nervous
System. 
Cerebrospinal Fluid
Cerebrospinal fluid is a clear tissue fluid that forms a
protective cushion around and within the CNS.
Peripheral Nervous System
The PNS consists of: sensory neurons running from stimulus
receptors that inform the CNS of the stimuli, motor neurons running from the
CNS to the muscles and glands - called effectors - that take action.
The PNS is subdivided into the sensory somatic nervous system
and autonomic nervous system.
The Cranial Nerves
Nerves
Type
Function
I
Olfactory
sensory
II
Optic
sensory
(Contain 38% of all the axons connecting to the brain.)
III
Oculomotor
motor*
eyelid and eyeball muscles
IV
Trochlear
motor*
eyeball muscles
V
Trigeminal
mixed
Sensory: facial and mouth sensation
Motor: chewing
VI
Abducens
motor*
eyeball movement
VII
Facial
mixed
Sensory: taste
Motor: facial muscles and
salivary glands
VIII
Auditory
sensory
IX
Glossopharyngeal
mixed
Sensory: taste
Motor: swallowing
X
mixed
main nerve of the
parasympathetic nervous
system (PNS)
XI
Accessory
motor
swallowing; moving head and shoulder
XII
Hypoglossal
motor*
tongue muscles
The Spinal Nerves
All of the spinal nerves are "mixed"; that is, they
contain both sensory and motor neurons.
The Autonomic Nervous System
The autonomic nervous system consists of sensory neurons and
motor neurons that run between the central nervous system (especially the
hypothalamus and medulla oblongata) and various internal organs such as the:
heart, lungs, viscera, glands both exocrine and endocrine. It is responsible
for monitoring conditions in the internal environment and bringing about
appropriate changes in them. The contraction of both smooth muscle
and cardiac muscle is
controlled by motor neurons of the autonomic system.
The Sympathetic Nervous System
The preganglionic motor neurons of the sympathetic system
arise in the spinal cord. They pass into sympathetic ganglia
which are organized into two chains that run parallel to and on either side of
the spinal cord. The preganglionic neuron may do one of three things in the
sympathetic ganglion: synapse with postganglionic neurons which then reenter
the spinal nerve and ultimately pass out to the sweat glands and the walls of
blood vessels near the surface of the body. Pass up or down the sympathetic
chain and finally synapse with postganglionic neurons in a higher or lower
ganglion, leave the ganglion by way of a cord leading to special ganglia (e.g.
the solar plexus) in the viscera. Here it may synapse with postganglionic
sympathetic neurons running to the smooth muscular walls of the viscera.
However, some of these preganglionic neurons pass right on through this second
ganglion and into the adrenal medulla.
Here they synapse with the highly-modified postganglionic cells that make up
the secretory portion of the adrenal medulla.
The Parasympathetic Nervous System
The main nerves of the parasympathetic system are the tenth
cranial nerves, the vagus nerves. They originate in the medulla oblongata.
Other preganglionic parasympathetic neurons also extend from the brain as well
as from the lower tip of the spinal cord.
Each preganglionic parasympathetic neuron synapses with just
a few postganglionic neurons, which are located near - or in - the effector
organ, a muscle or gland. Acetylcholine (ACh) is the neurotransmitter at all
the pre- and many of the postganglionic neurons of the parasympathetic system.
However, some of the postganglionic neurons release nitric oxide (NO) as
their neurotransmitter.
Parasympathetic stimulation causes,slowing down of the
heartbeat,lowering of blood pressure, constriction of the pupils, increased
blood flow to the skin and viscera, peristalsis of the GI tract
In short, the parasympathetic system returns the body
functions to normal after they have been altered by sympathetic stimulation. In
times of danger, the sympathetic system prepares the body for violent activity.
The parasympathetic system reverses these changes when the danger is over.
The vagus nerves also help keep inflammation
under control. Inflammation stimulates nearby sensory neurons of the vagus.
When these nerve impulses reach the medulla oblongata, they are relayed back
along motor fibers to the inflamed area. The acetylcholine from the motor
neurons suppresses the release of inflammatory cytokines, e.g., tumor necrosis factor
(TNF), from macrophages in the inflamed tissue.
Although the autonomic nervous system is considered to be
involuntary, this is not entirely true. A certain amount of conscious control
can be exerted over it as has long been demonstrated by practitioners of Yoga
and Zen Buddhism. During their periods of meditation, these people are clearly
able to alter a number of autonomic functions including heart rate and the rate
of oxygen consumption. These changes are not simply a reflection of decreased
physical activity because they exceed the amount of change occurring during
sleep or hypnosis.
After the age of 60 the brain begins to lose thousands of
neurons a day. When these cells
die, they are not replaced. By the
age of 80 the brain ways about 10% less than when the person was a young adult. The cerebral cortex shrinks more than
other areas of the brain losing as much as 45% of its cells. Mental activities such as learning,
memory, and reasoning decline. Neurotransmitter
production decreases resulting in slower synaptic transmission. Neurological disorders especially
Alzheimer disease are more apt to occur in the elderly.
