Regulating Systems
Life is maintained by coordination of the functions of various body
systems. Both the internal coordination and proper changes in the
external environment are regulated by the endocrine and nervous systems,
which influence the activity of each other.
1. Endocrine system
It is formed of the endocrine glands. The endocrine glands secrete
chemical messengers called hormones, which are carried by blood stream
and influence the activity of some distant organs. In general, the
endocrine system acts slowly but has a prolonged action.
2. Nervous system (NS)
It is responsible for the rapid regulation of the functions of the
various systems of the body, according to changes in the external and
internal environments. The regulating orders are sent in the form of nerve
impulses, transmitted along nerves at a rate, which may reach 120 meters
/ sec. Changes in the external and internal environments are transmitted to
the nervous system along afferent nerves. At the terminations of the
afferent nerves, there are specialized structures, called the receptors. Each
receptor is most sensitive to a specific stimulus, such as:
* Cutaneous receptors: They are present in the skin such as temperature
(cold and heat), touch (crude and fine) and pain receptors. They inform
the CNS about changes in the external environment.
* Distant receptors: They are present in the retina (visual receptors), in
the internal ear (auditory receptors) and in the nose (smell receptors).
They inform the CNS about changes in the external environment, at a
distance from the body.
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* Proprioceptors: They are present in the muscles, tendons and joints.
They inform the CNS about the position and movements of the joints and
the degree of tension of the muscles.
* Interoceptors: They are present in the internal viscera. They are such as
receptors present in the carotid sinus, which inform the CNS about
changes in the arterial blood pressure.
According to the information reaching NS along afferent nerves, its
regulatory orders are sent in the form of nerve impulses to various body
systems along efferent nerves.
The nervous system consists of:
a) Central nervous system (CNS):
* Brain:
- Cerebral hemispheres: Cerebral cortex, white matter and basal ganglia.
- Brain stem: Diencephalon (thalamus and hypothalamus), midbrain,
pons and medulla oblongata.
- Cerebellum: Vermis and 2 cerebellar hemispheres.
* Spinal cord.
b) Peripheral nervous system (PNS):
They are the nerves, which connect the CNS to all body organs:
* Spinal nerves:
They are 31 pairs of nerves.
 8 cervical.
 12 thoracic.
 5 lumbar.
 5 sacral.
 1 coccygeal.
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* Cranial nerves:
They are 12 pairs of nerves.
No
Nerve
Type
Function
I
Olfactory
Sensory
Smell
II
Optic
Sensory
Vision
III
Occulomotor
Motor
Muscles of the eyeball
IV
Trochlear
Motor
One muscle of the eyeball
V
Trigiminal
Mixed
Face
VI
Abducent
Motor
One muscle of the eyeball
VII
Facial
Motor
Muscles of expression
VIII
Statoacustic (Auditory)
Sensory
Hearing
IX
Glosso-pharyngeal
Mixed
Mouth and pharynx
X
Vagus
Mixed
Widespread
XI
Accessory
Motor
Neck muscles
XII
Hypoglossal
Motor
Tongue
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Nerves
Neuron:
It is the unit of structure of the nervous system. It is formed of cell
body (soma) and cell processes.
 Cell body (soma):
It controls the metabolism of the whole neuron. It is surrounded
with the cell membrane, which extends over the cell processes. The cell
contains the nucleus with a well-developed nucleolus. The cytoplasm
contains:
a) Neurofibrils: They are fine fibrils, which extend into the cell
processes.
b) Nissil granules: They are rich in RNA, being important for metabolic
activity of the cell (protein synthesis).
c) Mitochondria.
d) Granular endoplasmic reticulum.
e) Golgi apparatus.
The cell body has no centriole, therefore nerve cells cannot divide.

Cell processes:
They contain:
a) Dendrites: They are short and branching processes. They carry
impulses toward the cell body. They form the receptive segment of the
neuron.
b) Axon (nerve fiber): It is a single non-branching long process. It arises
from a conical area in the cell body, free from Nissil granules, known as
axon hillock. It is surrounded with the plasma membrane, which is a
continuation of the cell membrane. The axon divides only near its
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termination. It carries impulses from the nerve cell. Two types of nerve
fibers are found:
- Myelinated (medullated) nerve fibers: The axon is covered with an
insulating lipid material, known as myelin sheath. The myelin sheath does
not form a complete layer but is interrupted at regular intervals of 1 mm,
by nodes of Ranvier.
- Non-mylineted (non-medullatied) nerve fibers: The axon is thin and has
no myelin sheath.
However, the nerve fiber, whether myelinated or non-myelinated,
is covered from outside by the neurolemmal sheath.
Degeneration and regeneration of nerve fiber:
If the nerve fiber (axon) is cut, the part distal to the cut degenerates
(Wallarian degeneration). Changes also occur in the cell body
(Retrograde degeneration).
a) Wallarian degeneration:
- The axon swells and breaks into short lengths.
- The myelin sheath breaks slowly into oily droplets.
- The Schwann cells divides.
- The nerurolemma becomes empty as macrophages remove the remnants
of the myelin sheath and axis cylinder.
b) Retrograde degeneration:
- The Nissil granules breakdown into fine dust, losing their staining
reaction (chromatolysis).
- The Golgi apparatus breaks into small fragments and then disappears.
- The cell swells and the dendrites become shorter and disappear.
- The nucleus is displaced to one side or may even extrude. In such a
case, the cell dies.
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Regeneration:
Repair of the cell begins about 20 days after nerve section, to be
complete in 80 days.
* The Nissil granules and Golgi apparatus gradually reappear.
* The cell regains its normal size.
* The nucleus returns to its central position.
If the gap between the two cut ends of the sectioned nerve fiber is
more than 3 mm, regeneration fails. Therefore, it is advised to
approximate the cut ends surgically.
Cross regeneration:
It is the growth of fibrils, belonging to one neuron into the
neurolemma of another neuron. If both are sensory, motor or autonomic,
we get functional recovery, but reeducation is necessary.
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Excitability
It is the ability of any living tissue to respond to changes in its
environment. Such changes are called stimuli. The most excitable tissues
in the body are nerves and muscles.
In order to study the functions of the nerve and muscle, electrical
stimuli are preferable because:
- They are similar to natural stimuli inside the body.
- They do not injure the tissue.
- Their amplitude and duration can be accurately regulated.
The physiochemical changes, produced by the stimulus in the
nerve are known as the nerve impulse. Conduction of the nerve impulse
along the nerve fiber is an active self-propagating process.
Types of electrical stimuli:
- Galvanic currents: They are long in duration and low in intensity, such
as currents obtained from a battery. They were used in the past to
stimulate the muscle.
- Faradic currents: they are short in duration and high in intensity, such
as induction currents. They are used to stimulate the nerve.
The effectiveness of an electrical stimulus depends on:
* Intensity of the stimulus: the electrical stimulus, of an intensity just
enough to excite the nerve fiber and produce a nerve impulse, is called
the threshold stimulus. Subthreshold or sub-minimal stimuli cause
localized changes called local response or local excitatory state (L.E.S).
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* Rate of rise of the stimulus intensity: If a subthreshold stimulus is
applied to the nerve and its intensity is increased slowly, the nerve will
not respond. This is called “accommodation“ to the passage of current.
So, the rate of rise of stimulus intensity should be very rapid.
* Duration of the stimulus: The duration of the current means the length
of time, it must be applied to the tissue to give a response. There is an
inverse relationship between the intensity and the duration of the
stimulus.
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