Download m5zn_363798b57fd4c88

Nervous system: is the system of communications ,the
aim of communications is to keep body homeostasis,
N.S does its functions through a special steps:
1. Detection of the changes
2. Evaluation of the information
3. Responding property
Classification of the nervous system:
we can classify N.S according to its location or
according to its functions.
1. Location of the NS: there are two types:
a. central NS.(CNS)which is formed of the brain and
the spinal cord.
b. perpheral NS. (PNS) or the nerve, they are the
axons of two types of neurons:
1. cranial nerves where their cell bodies are
present in the brain, they are 12 nerve.
2. spinal nerves: their cell bodies are present in
the spinal cord.
Afferent and efferent fibers:
afferent: are all the incoming sensory fibers from periphery
to the central nervous system.
Efferent: are the outcoming motor fibers, carrying the orders
from the CNs to the periphery .
2. according to the function: there are two types:
a. somatic NS: which controls the skeletal muscles , one
type is ordering the muscle movements (efferent)
, and the other is carrying the sensory impulses (afferent).
b. autonomic NS: which is the system of viscera and is
formed of two divisions.
Sympathetic system: is the system of fight and flight
Parasympathetic NS. The system of autonomic
activity of the viscera
 The nervous system is the most highly developed and
perhaps the most important of all the system of the
body. Not only does it correlate the activities of the
other system but also in the brain is situated the site of
conciousness, thought, memory , speech and the will
to carry out purposeful actions. These factors all
contribute to the formation of the personality of the
individual.
 The basic unit of the nervous system is the individual
nerve cell, or neuron
. Neurons can be devided into three functional classes:
1. Afferent (sensory)neurons
afferent neurons transmit information into the
central nervous system from receptors at their
peirpheral ending. They are mostly (that is, the cell
body and the long peirpheral process of the axon)
outside the central nervous system, only the short
central process of the axon enters the central nervous
system. Afferent neurons have no dendrites.
1. Efferent (motor) neurons
efferent neurons transmit information out of the
central nervous systemto effector cells, particularly
muscles, glands. They are mostly (that is, the cell
body, dendrites, and a small segment of the axon) in
the central nervous system, most of the axon is
outside the central nervous system.
3. Interneurons (connecting neurons)
interneurons function as integrators and signal
changers. They integrate groups of afferent and
efferent neurons into reflex circuits. They lie entirely
within the central nervous system, and account fo 99%
of all neurons.
Neurons have two characteristics:
1. Excitability
ability of the plasma membrane to generate action
potential, if a strong enough stimulus affect on it.
2. Conductivity
the ability of the neuron to move the action potential
from point to the next to it point. Conduction
velocities range from about 0.5 m/s for unmyelinated
fibers to about 100 m/s for myelinated fibers.
Action potential propagation
once generated, one particular action potential does not
travel along the membrane. Rather, the local current
produced by one action potential serves as the stimulus
that depolarizes the adjacent membrane to its threshold
potential.
The new action potential then produces local currents of its
own, which depolarize the region adjacent to it, producing
yet another action potential at the next site, and so on to
cause action – potential a propagation along the length of
the membrane. The velocity with which an action potential
propagates along a membrane depends upon fiber
diameter and whether or not the fiber is myelinated.
The larger the fiber diameter, the faster the action
potential propagates. In the myelinated fiber action
potentials occur only at the nodes of ranvier where the
myelin coating is interrupted and the concentration of
sodium channels is high. Thus, action potentials
literally jump from one node to the next as they
propagate along a myelinated fiber, and for this reason
such propagation is called saltatory conduction.
Propagation via saltatory conduction is faster than
propagation in nonmyelinated fibers of the same axon
diameter.
If the fiber continuity is disrupted the action potential
will not transmitted. Action potential normally
transmitted only in one direction.
synapsis
Neurons are connected with each other and from chains
of neurons, the axon of one cell extending to the
dendrite or body of another cell. The point at which an
impulse (action potential) passes from an axon of one
neuron to the dendrite or cell body of another is called
a synapse
The signal from a presynaptic to a postsynaptic neuron is
the neurotransmitter stored in synaptic vesicles in the
presynaptic axon terminal. Depolarization within the
terminal, which raises the calcium concentration
within the terminal, causes the release of
neurotransmitter into the synaptic cleft.
The neurotransmitter diffuses across the synaptic cleft
and binds to receptors on the postsynaptic cell, the
activated receptors usually open ion channels. In the
synapses the propagation is slower than in the fibers,
and may even be delayed in them
Synaptic effectiveness are influenced by presynaptic and
postsynaptic events, drugs, and diseases.
Neurotransmitters are chemical substances, released
from axon terminals of presynaptic. At most synapses ,
the signal is transmitted from one neuron to another
by neurotransmitters. These chemical messengers
diffuse across an extracellular gab to the cell opposite
the terminal. Neurotransmitters bind to receptors on
the plasma membrane of the postsynaptic cell. The
result of the binding of neurotransmitter to receptor is
the causing of the action potential
Neurotransmitter binding to the receptor is a transient
event, and the ion channels in the postsynaptic
membrane return to their resting state when the
neurotransmitter is no longer bound. Unbound
neurotransmitters are removed from the synaptic cleft
when they enzymatically transformed into ineffective
substances.
Control systems
In order to synchronize the functions of the trillion of
cells of the human body, two control systems exist.
One, the endocrine system, a collection of bloodborne messengers, that works slowly, and for long
term. The other is the nervous system, which is a rapid
and short term control system.
Together they regulate many internal functions and
organize and control the activities we know as human
behavior.
Parts of nervous system
The various structures of the nervous system are
intemately interconnected, but for convenience they
are divided into:
1. Central nervous system (CNS)
2. Peripheral nervous system
3. Autonomic nervous system
Central nervous system
central nervous system composed of the brain and
spinal cord . Inside the skull and vertebral column, the
brain and spinal cord are enclosed in and protected by
the meninges.
Spinal cord
Spinal cord is divided into two areas: central gray matter,
which contains nerve cell bodies and dendrites, and
white matter, which surrounds the gray matters and
contains myelinated axons organized into ascending
(sensory) or descending (motor)tracts. The axons of
the afferent and efferent neurons from the spinal
nerves
Function of the spinal cord
The main functions of the spinal cord are:
1. The spinal cord communicates through nerve fibers,
its nervous pathways, with various parts of the brain
and through spinal nerves with organs. The spinal
cord contains two kinds of nervous pathway:
ascending (sensory) and descending (motor). The
spinal nerve also contain two types of nerve fiber –
sensory and motor.
Nerve impulses are transmitted to spinal cord
from the periphery, from organs, along sensory fibers
of the spinal nerves, then conducted along the
ascending nervous pathways to brain.
Nerve impulses are transmitted from the brain to the
spinal cord along the descending pathways and thence
along motor fibers of the spinal nerves to the
periphery, the organs. These impulses alter the state of
various organs
2. Reflex activity
the spinal cord contains the reflex centers of
various functions. Reflex is an involuntary,
unpremeditated, unlearned ‘’built – in ‘’ response to
astimulus.
Examples of such reflexes include pulling one’s hand
away from a hot object or shutting one’s eyes as an
object rapidly approaches the face.
The pathway mediating a reflex is known as the reflex
arc.
A stimulus is a detectable change in the internal or
external environment, such as a change in
temperature, or blood pressure.
A receptor detects the environmental change. A
stimulus acts upon a receptor to produce a signal that
is relayed to an integrating center. The pathway
traveled by the signal between the receptor and the
integrating center is known as the afferent pathway
(the general term afferent means ‘’to carry to’’ in this
case, to carry to integrating center) .
An integrating center often receives signals from many
receptors, some of which may be responding to quite
different types of stimuli. Thus, the output of an
integrating center reflects the net effect of the total
afferent input, that is, it represents an integration of
numerous bits of information.
The output of an integrating center is sent to the last
component of the system, a device whose change in
activity constitutes the overall response of the system.
This component is known as an effectors. The
information going from an integrating center to an
effector is like a command directing the effector to
alter its activity.
The pathway along which this information travels is
known as the efferent pathway (the general term
efferent means ‘’ to carry away from ‘’ , in this case,
away from the integrating center).
Characteristics of reflexes:
1. Many reflexes are protective in character.
2. Reflexes are designed to obtain the quickest possible
motor response
3. Some reflexes are concerned with automatic control of
functions which do not require the supervision of
consciousness.
4. Reflexes are specific: each stimulus has its own
response.
5. Most reflexes are subject to alteration by learning.
The brain
Anatomically , the brain is composed of four
subdivisions: cerebrum, diencephalon, brainstem, and
cerebellum. The cerebrum and diencephalons together
constitute the forebrain. The brainstem consists four
interconnected cavities, the cerebral ventricles, which
are filled with circulating cerebrospinal fluid.
Brainstem
brainstem is composed midbrain, pons, and medulla
oblongata.
Function of the midbrain :
1. Regulation of the cerebrospinal fluid circulation.
2. The gray matter forms the nuclei of third and fourth
cranial nerves.
3. Joins the cerebral hemispheres above to the pons
below.
Function of the pons:
1. It acts a bridge between the two lobes of the
cerebellum.
2. Nerve fibers pass up and down between the midbrain
above and the medulla oblongata below.
3. The gray matter forms the nuclei of the 5th ,6th ,and 7th
cranial nerves.
Function of medulla oblongata:
Connects the brain with the spinal cord.
2. Contains fibers passing from spinal cord, forebrain,
and cerebellum.
3. It contains also collection of gray matter known as
vital centers. The most important of these are:
a. the respiratory center which controls the rate and
depth of respiration.
b. the vasomotor center which controls the caliber of
the blood vessels.
1.
c. the cardiac center which influences the rate of the
heart.
d. special centers such as the swallowing, vomiting
centers, centers for the movement of the stomach and
the secretion of saliva and gastric juice.
4. The gray matter forms the nuclei of 9th , 10th, 11th .12th ,
cranial nerves.
cerebellum
Function of the cerebellum
1. Coordinates movements, including those for posture
and balance.
2. participates in some forms of learning.
3. it helps to maintain balance and equilibrium.
cerebrum
 The cerebrum consists of the right and left cerebral
hemispheres.
Functions of the cerebral hemispheres:
1. Contain the cerebral cortex, which participates in
perception, the generation of skilled movements,
reasoning, learning, and memory.
2. Contain subcortical nuclei, which participata in
coordination of skeletal-muscle activity.
3. Contain interconnecting fiber pathways
Diencephalon (between brain)
 Diencephalon contains two major parts: the thalamus
and hypothalamus.
Functions of thalamus
1. Is a synaptic relay station for sensory pathways on their
way to the cerebral cortex.
2. Participates in control of skeletal-muscle coordination
3. Plays a key role in awareness.
Function of hypothalamus:
1. Regulates the anterior pituitary gland.
2. Regulates water balance.
3. Participates in regulation of autonomic nervous
system.
4. Regulates eating and drinking behavior.
5. Regulates reproductive system.
6. Reinforces certain behaviors.
7. Generates and regulates circadian rhythms.
8. Regulates body temperature .
9. Participates in generation of emotional behavior.
The physiology of the brain
The brain is the control center of the whole human body.
Physiologically it may be divided into the higher
centers which are the seat of consciousness, mind,
memory and will; and the lower centers which control
many important unconscious acts.
The higher centers are situated in the cerebral
hemispheres while the lower ones are found in the
cerebellum and brainstem as well as in the basal
ganglia of the cerebrum .
 The following mechanisms enable the brain to exercise
this power of control:
1. It receive sensory or afferent impulses from all parts
of the body, through the sensory pathway.
2. It is able to send out motor or afferent impulse from
all parts of the body, through the motor pathway ,
which has two tracts: pyramidal(corticospinal) tract,
which may be two kinds : straight or crossed, and
extrapyramidal tract.
3. There is a complicated system of connection between
all parts of the brain with each other. Projection fibers
transmits the impulse from the brainstem to the
cerebral cortex. Association fibers which are situated
in the cerebral cortex transmits the impulse from one
area to another within the same hemisphere.
Commissural fibers transmit the impulse from one
hemisphere to another one (from right to the left for
example)
Functional regions
 All areas of the cortex are interconnected and the
activity of each depends on the state of the entire
cortex.
however, the different regions differ in function and
structure. These areas are:
1. The motor cortex area
the motor cortex area is located in the frontal
lobe from the cell of this area, voluntary motor
impulses arises and are transmitted to the various
groups of muscles in the body are represented in this
area. The right cerebral hemisphere controls the left
side of the body and vice versa