Download Organization of the Human Nervous System

Nerve Impulses
Neurons are nerve cells that are specialized for
conducting information. Bundles of neurons form thin
cables called nerves.
Nerves form a communication network that travels
throughout the body including organs like your skin, eyes,
and stomach.
All neurons have the same basic structure which includes
the cell body, dendrites and axons.
The enlarged area of the cell body
contains the nucleus and organelles.
 Nerve responses enter the cell on the
short extensions called the dendrites.
 Information is passed on through the cell
body to the long fibers called the axons.
 Each neuron has many dendrites, but only
one axon.

Nerve impulses are actually electrical
charges.
 The key to the transmission of nerve
impulses lies in the neuron’s cell
membrane.

◦ The membrane is studded with protein
channels called voltage-gated channels. These
channels open and close in response to
changes in electrical charge across the
membrane.
◦ Some allow only Na+ ions and some allow only
K+ ions to pass through the channels.
Transmission of Nerve Impulses

The unequal distribution of ions across the
nerve cell membrane results in a voltage
difference and that difference propels the
stimulus along the nerve.
◦ At resting potential, the concentration of K+
ions are higher inside the membrane and the
Na+ ions are higher outside the membrane.
◦ When the impulse causes the exchange of the
ions across the membrane, the impulse is
propelled down the nerve.
Myelin sheath is an insulating layer made
up of fatty acids. This sheath allows
nerve impulses to travel quickly.
 The importance of the myelin sheath
becomes apparent in patients with
multiple sclerosis. Their myelin sheath
disappears and they become weak and
disabled.
 At the end of a nerve axon there is a
small gap to the next nerve, called a
synapse. The impulse has to cross the
synapse by jumping to the other side.

The impulse is carried across the synapse
by chemical neurotransmitters. They are
found in tiny sacs at the end of the axon.
 After carrying the impulse across the
synapse, the neurotransmitter must be
removed or the synapse would continue
to fire.
 Different neurotransmitters produce
different effects.

Organization of the
Human Nervous System
Central Nervous System
Peripheral Nervous System
The nervous system has two main parts:
 The central nervous system – consisting of
the brain and spinal cord. It is your
control center that organizes all incoming
and outgoing information.
 The peripheral nervous system – the
nerves that connect to the rest of the
body. The sensory neurons and motor
neurons act as independent
communications pathways that monitor
different parts of the body.

The Central Nervous System

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
The delicate parts of the central nervous
system are protected by the skull and the
vertebral column.
The CNS sorts out a vast amount of
information and issues commands to
muscles, glands, and organs.
The cerebrum is the control center of the
brain. The brain weighs about 3 pounds and
is one of the largest organs in the body. It is
divided into two halves that are connected by
a large band of nerve fibers.
Each hemisphere has an outer gray layer
called the cerebral cortex. This 2-4mm thick
layer handles the most sophisticated
functions of the brain.
The cerebellum is a cauliflower-shaped
structure essential for coordination of
movement. Damage to the cerebellum
causes a lack of coordination and tremors.
• Many nerves that control muscles in the
body cross in the brain stem, therefore
damage to one side of the brain above the
crossover causes weakness or paralysis on
the opposite side of the body.
• The brain stem contains the hypothalamus
which is the center for emotions and
instincts controlling hunger, thirst, body
temperature, water balance, blood
pressure, pleasure, hostility, and pain.
•
The spinal cord shuttles information
between the body and the brain. Like the
brain, the spinal cord contains gray
matter.
 The spinal cord is surrounded and
protected by the backbone. Injuries to
the spinal cord can result in pain,
numbness, or paralysis in limbs and
extremities.

New technologies are allowing scientists
to learn more about brain functions.
 Learning occurs when two or more pieces
of information are linked into a pattern of
connections between neurons.
 Language processing is very complex and
involves several regions of the left
hemisphere.
 Researchers study the brain using CAT
scans (computerized axial tomography).
Patients lay on a table and go through an
intense magnetic
field in a tube. Radio
Mapping
the Brain
frequencies are transformed into images.

All of the nervous system outside the spinal cord
and brain is known as the peripheral nervous
system. This system carries all the messages
sent back and forth between the central nervous
system and the rest of the body.
 Sensory neurons tell the central nervous system
what is happening.
 Motor neurons are partners of sensory neurons.
Motor neurons carry information from the central
nervous system to a muscle or gland.
 Sensory nerves go into the spinal cord and motor
nerves come out of the spinal cord.

Peripheral Nervous System
A reflex is a sudden, involuntary
movement.
 A reflex produces a rapid motor response
to a stimulus because the sensory neuron
synapses directly with a motor neuron.
 A reflex occurs before the brain is aware
there is a problem.
 The autonomic nervous system carries
messages to muscles and glands that
usually work without notice. Examples:
muscles and glands that control blood
pressure and movement of food through
the digestive system even during sleep.
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