Download The Nervous System

The Nervous System
Functions of the Nervous System
 The nervous system is the communication and control center of our
body.
 Our nervous system controls all of our body’s actions and functions.
 It originates and coordinates physical reactions to the environment
and controls involuntary muscles and organs such as the heart and
lungs.
 It maintains a balanced state within the body (homeostasis).
Structure of the Nervous System
Two main divisions of the nervous system
1) Central Nervous System (CNS)
2) Peripheral Nervous System (PNS)
The PNS gathers information from inside and outside the body.
The CNS receives and analyzes this information and initiates responses.
PNS then picks up and carries the response signals. The information is
transmitted throughout our body by means of electrical charges called
impulses.
(up to 248 mph)
The messengers and receivers of these transmissions are neurons (nerve
cells)
Functions of Neurons
Classified according to their functions. Three main types of neurons:
sensory neurons, motor neurons, and interneurons.
Sensory Neurons – carry signals from sense receptors (eyes, ears,
skin) into the CNS.
Motor Neurons – carry signals from the CNS to muscles or
glands.
Interneurons – form all the electrical connections within the CNS
itself.
How all of this works:
Nerve cells (neurons), unlike other cells, cannot replace themselves.
If the cell body is damaged or degenerates, the cell dies!
The three basic parts of a neuron: cell body, dendrites, axons
(refer to hand-out)
Cell body – consists of a nucleus and cytoplasm enclosed by the cell
membrane. The nucleus receives and sends nerve impulses.
Regulates amount and type of proteins made.
Dendrites – are branching projections of the cell body. They receive
and carry impulses toward the cell body.
Axons – threadlike extension that carries impulses away from the
cell body.
 Branches at its end forming terminals, signals are sent to target
cells (other neurons, muscle cells, or glands)
 Length can vary from ~ mm to > 1 meter
 Most are coated (sheath) with myelin (fatty material). Myelin
insulates the nerve fiber, speeds transmission of impulses.
Central Nervous System
Consists of two main parts:
The brain and spinal cord
Every bodily function involves the CNS.
Messages from outlying receptors and sensors arrive at the CNS, where they
are interpreted; the CNS then sends out reaction impulses.
The Spinal Cord
o Cylinder of nerve tissue about 43 cm long (17-18 inches) and as thick
as index finger.
o Downward extension of brain.
o Runs down canal in spine, protected by vertebrae
o Also protected by cerebrospinal fluid (shock absorber), three layers of
connective tissue called spinal meninges.
o Shaped like a flattened cylinder, slightly winder than a pencil for most
of its length
The center region of the cord is white matter made up of numerous
sheathed nerve fibers that run lengthwise through the spinal cord.
 The interior is gray matter. The gray matter contains the
cell bodies of neurons and supporting cells. Some are
interneurons.
 Others are motor neurons whose axons pass out of the
spinal cord in bundles and extend to muscles and glands.
 Axons of sensory neurons enter the gray matter and
connect with motor neurons and interneurons.
Thirty-one pairs of spinal nerves branch out from the vertebrae to the right
and left sides of the body. The nerves are large cables of sensory and motor
fibers.
Each nerve pair controls a particular area of the body and is identified by the
number of the vertebrae over which it leaves the spinal cord.
 Cervical nerves (8 pairs) for neck, shoulders, and arms
 Thoracic nerves (12 pairs) for ribs, back, and abdomen
 Lumbar nerves (5 pairs) for lower back and thighs
 Sacral nerves (5 pairs) for buttocks, genitals, legs, and feet
 Coccygeal (1 pair) for hips, tailbone, buttocks
Nerve signals can only travel one way along a nerve, so some nerves carry
signals down the spine (the descending pathway) and some carry
signals up (the ascending pathway).
Spinal cord reaches full length at about 4 to 5 years of age.
The Brain
Your brain is the largest most complex part of the nervous system.
 Brain helps you receive and process messages
 To think, remember, and reason
 Coordinates your muscle movement
 Involved in your emotions and everything you sense
The brain weighs about three pounds and contains almost 100 billion
neurons. At birth, the brain weighs about one pound. The brain is fully
grown by your sixth birthday.
It makes up only about two percent of total body weight, but use more than
20 percent of the oxygen you inhale.
Without oxygen the brain can only last four to five minutes before suffering
serious and irreversible damage.
The brain has three main divisions: the cerebrum, the cerebellum, and the
brain stem.
The Cerebrum
This is the largest, most complex part of the brain. This is the site of the
most conscious and intelligent activities. Conscious thought occurs in the
outer layer (cortex).
Cerebrum is divided into two halves called cerebral hemispheres.
Hemispheres are connected to the rest of the CNS in a cross-over fashion.
That means that the right hemisphere controls the muscular activity of and
receives sensory input from the left half of the body.
The left hemisphere does the same for the right half of the body.
In humans: the left hemisphere is mainly responsible for language, logic,
arithmetic calculation, analysis, and critical thinking.
The right hemisphere is concerned with imagination and visual
thinking—spatial relationships, form artistic activities, and emotional
responses.
Each hemisphere has four lobes. Each lobe is named after the bone in the
skull that protects it.
The frontal lobe controls voluntary movements and also has a role in the use
of language. The prefrontal areas of the lobe are believed to be involved
with intellect and personality.
The parietal lobe is involved with a wide variety of sensory information—
heat, cold, pain, touch, and body position in space.
The occipital lobe contains the sense of vision.
The temporal lobe contains the senses of hearing and smell as well as
memory, thought, and judgment.
The Cerebellum
Cerebellum is the second largest part of the brain. It’s a rounded structure
located beneath the occipital lobes of the cerebrum. The cerebellum is
divided into two hemispheres.
The cerebellum:
 Concerned mainly with maintaining posture and balance and
coordinating skeletal muscle movement.
 Receives impulses from balance organs of the inner ear and muscles.
 After receiving information, cerebellum refines orders being sent to
muscles from motor cortex in cerebrum, ensures smooth coordinated
movements
The Brain Stem
The brain stem is a 3-inch long (7.6 cm) stalk of nerve cells and fibers that
connects the spinal cord to the rest of the brain.
 It acts partly as a pathway for messages traveling between other parts
of the brain and the spinal cord.
 It also connects with 10 and the 12 pairs of cranial nerves.
 Controls our basic functions: breathing heartbeat, and eye reflexes.
 Activities of the brainstem are not under conscious control
The brain stem consists of three main parts:
medulla oblongata
pons
midbrain
Medulla oblongata—
 Lowest part of the brain stem (top end of the spinal cord)
 Regulates/controls heartbeat, breathing blood pressure, and digestion
 Is also control center for swallowing, vomiting, sneezing, and
coughing
 Receives and relays taste sensations from the tongue and is involved
in speech and tongue movements
Pons
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Located just above the medulla
Is the pathway for nerve impulses passing to and from the cerebrum
Connects the cerebrum with the cerebellum
Joins the hemispheres of the cerebellum
Relays sensory information from the ear, face, teeth, signals that move
the jaw, and adjust our facial expressions.
Midbrain
Shortest part of the brain stem
Located just above the pons
Contains two pairs of cranial nerves that control eye movements and
size and reactions of the pupils.
Closely associated to the brain stem are the thalamus and hypothalamus.
Thalamus
 Located above the midbrain, two lobes that are egg-shaped (walnutsized).
 Important relay center for incoming sensory impulses.
 Clusters of nerve cells, receive information from different sense
organs such as eyes and ears.
 Through the spine, the thalamus also receives information from touch
and pressure receptors in the skin.
Hypothalamus
 Located behind the eyes and under the thalamus
 Have nerve centers that control various body processes and keep the
conditions balanced.
 Nerve cells regulate body temperature, stimulate appetite for food and
drink, and regulate sleep.
 Also controls secretions ( hormones ) from the pituitary gland that
control many processes such as metabolism and sexual development
and emotional responses.
Peripheral Nervous System
Peripheral = away from center
The peripheral nervous system (PNS) consists of nerves that
fan out from the CNS to the muscles, skin, internal organs,
and glands.
PNS carries messages between the CNS and the rest of the
body.