Nervous system
The nervous system is a complex network of neurons specialized to carry
messages.
Consists of two divisions:
1. Central nervous system
containing the brain and spinal cord,
2. Peripheral nervous system which is
a network of nerves and neural tissues branching out throughout the body.
Neurons
Neurons are the nerve cells, the structural and functional units of the
nervous system.
They conduct impulses that enable the body to interact with its internal
and external environments. There are various types of neurons. The tissues that
support the nerve cells is called neuroglia.
Types of
neurons
Mostly 4 type of neurons find in human body:
1. Motor
neurons
2. Sensory
neurons
3. Interneurons
4. Neurons in
the brain
Motor Neurons
The motor neurons usually have one axon and several
dendrites. The axon is covered with a
insulating fatty layer called a myelin sheath, and transmits signals a long
distance from the neuron to the area to be activated. Dendrites are short and
unsheathed.
Motor neurons cause muscle contractions and control secretions from
glands and organs… controlling body functions.
Sensory
neurons
Sensory
neurons do not have true dendrites. They
are attached to sensory receptors and transmit impulses to the central nervous
system, which then stimulate the interneurons, and then motor neurons.
Interneurons
Interneurons are located entirely within the central nervous system. They intercept the impulses from the sensory neurons and transmit the signals to the motor neurons.
Nerve
fibers…
Schwann cells, only found on peripheral nerves which can regenerate
Myelin sheath, a fatty layer of insulation on some nerve fibers
The axon that transmits the message
There are no Schwann cells on nerve fibers in the central nervous system,
therefore damage to those nerve fibers is not reversible. A bundle of nerve fibers is simply called ‘a
nerve’. AFFERENT nerves conduct impulses to the central nervous system;
EFFERENT nerves conduct impulses to the muscles, organs, and glands.
Synapses…
Nerve impulses are transmitted via branches called synapses. The synapses
are connectors… hooking dendrites and axons from one neuron to another.
The number of synapses influences transmission. That number can decrease with disease, lack
of stimulation, drug use, etc.
Central nervous system
Central nervous system (CNS) is called the central processing unit (CPU) of
the body.
It consist of the brain and spinal card.
Gray and white matter of the central nervous system…
The brain and spinal cord of the central nervous system receive impulses,
process the information, and respond with the appropriate action.
Gray matter of the brain and spinal cord consists of unsheathed nerve
fibers (cannot be regenerated if damaged) in the cortex or surface layer. The
white matter makes up the internal structure, and consists of myelinated nerve
fibers.
Brain and spinal cord…
The brain and spinal cord are enclosed by 3 membranes, collectively known
as the meninges.
The outside membrane is the dura mater; the middle membrane is the
arachnoid; the inside membrane is the pia mater.
Parts of the brain…
Cerebrum
Taking up 7/8ths of the brain’s weight, the cerebrum governs all sensory
and motor activity. This includes sensory perception, emotions, consciousness,
memory, and voluntary movements.
It is divided into the left and right hemispheres. The surface or cortex
is arranged in bulging folds, each one called a gyrus, and furrows, each one
called a sulcus.
The cortex is also divided into 4 lobes that correspond to the overlying
bones of the skull: the frontal lobe
specializes in motor activity, personality, and speech; the parietal lobe is
where language, temperature, pressure, touch are interpreted;
The temporal lobe contains centers for hearing, smell, and language
input; the occipital lobe specializes in vision.
Cerebellum
The cerebellum is the second largest part of the brain. It contains nerve
fibers that connect it to every part of the central nervous system.
It coordinates voluntary and involuntary patterns of movements. It also
adjusts muscles to automatically maintain posture.
Diencephalon
The diencephalon refers to the thalamus and hypothalamus. The thalamus is the relay center for all
sensory impulses except olfactory (sense of smell) and motor areas of the
cortex.
The hypothalamus regulates behavior and emotional expression, body
temperature, and many metabolic activities.
Attached to the pituitary gland, it also controls hormonal secretions of
this gland.
Brainstem
The brainstem consists of 3 structures: the gray matter of the midbrain control
visual reflexes and sense of hearing. The white matter of the pons plays a role
in regulating visceral (internal organ) control.
The medulla oblongata connects the rest of the brain to the
spinal cord. It regulates breathing, swallowing, coughing, sneezing, vomiting,
heartbeat, and blood pressure.
The
spinal cord…
The spinal cord has 31 segments: 8
cervical segments that correspond to the C1-C8 vertebrae; 12 thoracic segments corresponding to the
T1-T12 vertebrae; 5 lumbar segments
corresponding to L1-L5 vertebrae, 5 sacral segments corresponding to S1-S5
vertebrae, and 1 coccygeal (cock SĬJ ee ul) segment.
The 44 cm long spinal cord is shorter than the spinal column, so segments
do not perfectly correspond to the vertebrae.
At each segment of the spinal cord, left and right pairs of sensory and
motor nerves branch out and connect to the peripheral nervous system. Impulses
travel back and forth to the brain and back to the muscles.
Cerebrospinal fluid…
A colorless fluid is produced in the ventricles of the brain; it surrounds the brain and spinal cord. It is called cerebrospinal fluid, and it cushions the brain and cord from shocks that could cause injury. It is maintained at a level around 1/2 - 2/3 cup.
Peripheral nervous system…
The network of nerves branching out throughout the body from the brain
and spinal cord is called the peripheral nervous system.
It’s include 2 type of nerve fibers:
1. Afferent nerve fibers – these
are responsible for transmitting message from tissue and organ to CNS.
2. Efferent nerve fibers –
responsible for conveying message from CNS to corresponding peripheral organ.
In addition to the 31 pairs of spinal nerves mentioned in the slides on
the spinal cord, there are 12 pairs of cranial nerves that attach to the brain:
I.
The olfactory nerve carries sensory input for smell
II.
The optic nerve carries sensory input for vision
III.
The oculomotor nerve controls muscles of the eye and
eyelid
IV.
The trochlear nerve controls the eyeball
V.
The trigeminal nerve controls the face, nose, mouth,
forehead, top of head, and jaw.
VI.
The abducens nerve also controls the eyeball
VII.
The facial nerve controls muscles of the face and
scalp, and part of the tongue for sense of taste.
VIII.
The auditory or cochlear nerve provides sensory
input for hearing and equilibrium.
IX.
The glossopharyngeal nerve controls saliva,
swallowing, and taste.
X.
The vagus nerve is the longest cranial nerve,
extending to and controlling the heart, lungs, stomach, and intestines.
XI.
The accessory nerve permits movement of the head and
shoulders.
XII.
The hypoglassal nerve controls the muscles of the
tongue.
Classification of peripheral nervous system
Peripheral nervous system divided into two types
1.
Somatic
nervous system (SNS).
2.
Autonomic
nervous system (ANS)
1.
Somatic nervous system (SNS)…
Control the
voluntary action in the body by transmitting impulses from CNS to Skeletal
muscles cell.
It consist of
somatic nerves.
2. Autonomic nervous system…
The autonomic nervous system is a part of the peripheral nervous system.
It controls the involuntary bodily functions such as sweating, gland
secretions, blood pressure, and the heart.
It is divided into the ‘sympathetic’ and ‘parasympathetic’ divisions.
The sympathetic nervous system is responsible for the ‘flight or fight’
responses… increased alertness, metabolic rate, respiration, blood pressure,
heart rate, and sweating AND a decrease in digestive and urinary function. The
parasympathetic nervous system counteracts the responses of the sympathetic
system… restoring homeostasis.
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