The nervous system is the complex arrangement throughout the body of highly specialized cells, the

nerve or neurons, w/c act as a communication system, enabling the organism to respond to external and
internal stimuli.

We approach the study of the nervous system in terms of three mechanism of behavior, namely:

1. The Receiving Mechanism

Sense organs are sensitive nerve endings located in certain body parts. They are receptors of
stimulus. Its attributes are:

a. Sensitivity and irritability: the power to react stimulus.

b. Conductivity: the power to conduct nerve impulses.
c. Specificity: the attribute of reacting to particular stimulus, i.e., light waves stimulate the eyes
etc.
d. Adaptability: the power to become used to stimulus.

Types of sense organs according to location are:

a. Exteroceptors: found in the eyes, ears, nose, mouth, skin and are sensitive to external stimulus.
b. Interoceptors: sense organs in the respiratory tract, digestive and genitor-urinary tract.
c. Proprieoceptors are embedded in muscles, tendons and joints.

2. The Reacting Mechanism

The muscles and glands comprise this reacting mechanism.

A. Muscles are of three kinds:

1) Voluntary, stripe, skeletal or unstriated
2) Involuntary, visceral smooth or unstriated
3) Cardiac or heart

B. Glands are special secreting organs which pour their secretions either directly or indirectly into
the bloodstream.

Kinds of Glands

a) Duct glands have ducts or canals thru which they pour out their secretions. These include
lachrymal (tear) glands, sebaceous (sweat); salivary, gastric sex, mammary glands
b) Ductless or endocrine glands pour their secretions called hormones directly into the
bloodstream.

3. The Connecting Mechanism – The Nervous System

A. The Neuron is the basic conducting unit of the nervous system. It has three principal parts: the
dendrites are specialized signal-receiving structures where the impulse (input) enters; the cell body (the
central region) which integrates input and output and the axon where the output is discharged.
 a) Sympathetic System consists of nerves and ganglia on the cervical, thoracic lumbar segments of
gray matter of the spinal cord. This division tends to act as a unit. It is active in emotion – activating the
organism in pain, anger or fear thru the controlled section of epinephrine (adrenalin). For example, in
emotional excitement, it simultaneously speeds up the heart, dilates the pupils, the arteries of the
muscles and the heart while constricting those of the skin and the digestive organs.

b) Parasympathetic System is composed of fibers from the cranial region, some below the
sympathetic system. This division acts or affects one organ at a time. If the sympathetic system is
dominant in excited activity, the parasympathetic system tends to act in quiescent activity. It maintains
the functions that conserve and protect the bodily reserves illustrating nature’s homeostatic device, i.e.,
of maintaining equilibrium.

The functions of these systems can also be described in terms of competition and cooperation, the
rule being, competition when fibers are connected to the same muscle or gland. For example, the
sympathetic system speeds up the heart; the parasympathetic system slows it; the SS inhibits digestion,
the PS facilities it. At other times, they cooperate – both being active at once or acting in sequence. For
example, the SS is dominant in fear and PS cooperate with an involuntary discharge of bowels or the
bladder.

The peripheral nervous system, or PNS, consists of the nerves and ganglia outside of the brain and
the spinal cord. The main function of the PNS is to connect the central nervous system (CNS) to the limb
and organ. Unlike the CNS, the PNS is not protected by bone or by the blood-brain barrier, leaving it
exposed to toxins and mechanical injuries. The peripheral nervous system is divided into the  somatic
nervous system and the autonomic nervous system; some textbooks also include sensory systems.

The somatic nervous system (SNS) is the part of the peripheral nervous system associated with

the voluntary control of body movements through the action of skeletal muscles, and with reception of
external stimuli, which helps keep the body in touch with its surroundings (e.g.,touch, hearing,
and sight).

The system includes all the neurons connected with skeletal muscles, skin, and sense organs.

The somatic nervous system consists of different nerves responsible for sending brain signals for muscle
contraction.
The autonomic nervous system (ANS or visceral nervous system) is the part of the peripheral nervous
system that acts as a control system functioning largely below the level of consciousness, and
controls visceral functions.

The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the

pupils, micturition (urination), and sexual arousal. Whereas most of its actions are involuntary, some,
such as breathing, work in tandem with the conscious mind. It is classically divided into two subsystems:
the parasympathetic nervous system and sympathetic nervous system.
 Sympathetic Nervous System is the part of the autonomic nervous system that is active during
stress or danger and is involved in regulating pulse and blood pressure, dilating pupils, and changing
muscle tone.
Parasympathetic Nervous System is one of the two divisions in the part of the nervous system
that controls involuntary and unconscious bodily functions autonomic nervous system. Its actions
include slowing the heart, constricting the pupils, and relaxing the bowels.

Sympathetic and parasympathetic divisions typically function in opposition to each other. But

this opposition is better understood as complementary in nature rather than antagonistic. For an
analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division
as the brake. The sympathetic division typically functions in actions requiring quick responses. The
parasympathetic division functions with actions that do not require immediate reaction. A useful
acronym to summarize the functions of the parasympathetic nervous system is SLUDD
(salivation, lacrimation, urination, digestion and defecation).

A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in
response to a stimulus. In most contexts, in particular those involving humans, reflex actions are
mediated via the reflex arc that is a nerve pathway that is responsible for triggering a reflex action.
(caused by sensory neurons)
Example is when tapping the tendon below the kneecap causes the leg to jerk involuntarily
because the impulse provoked by the tap, after traveling to the spinal cord, travels directly back to the
leg muscle. Such a response is called an involuntary reflex action.

Glands are special secreting organs w/c pour their secretions either directly or indirectly into the
bloodstream. Also it is an organ in an animal's body that synthesizes a substance for release such as
hormones or breast milk, often into the bloodstream (endocrine gland) or into cavities inside the body
or its outer surface (exocrine gland).

Endocrine glands are glands of the endocrine system that secrete their products, hormones, directly into
the blood rather than through a duct. The main endocrine glands include the pituitary
gland, pancreas, ovaries, testes, thyroid gland, and adrenal glands. The hypothalamus is
a neuroendocrine organ.

Exocrine glands are glands that secrete their products (excluding hormones and other chemical
messengers) into ducts (duct glands) which lead directly into the external environment. They are the
counterparts to endocrine glands, which secrete their products (hormones) directly into
the bloodstream (ductless glands) or release hormones (paracrines) that affect only target cells nearby
the release site.

Typical exocrine glands include sweat glands, salivary glands, mammary glands, stomach, liver, pancreas.

(Example of an endocrine gland is the adrenal gland, which is found on top of the kidneys and secretes
the hormone adrenaline, among others).
In physiology, the endocrine system is a system of glands, each of which secretes a type of
hormone into the bloodstream to regulate the body. The endocrine system is an information signal
system like the nervous system. Hormones regulate many functions of an organism,
including mood, growth and development, tissue function, and metabolism. The field of study that deals
with disorders of endocrine glands is endocrinology, a branch of internal medicine.

The endocrine system is made up of a series of ductless glands that produce chemicals called hormones.
A number of glands that signal each other in sequence is usually referred to as an axis, for example,
the hypothalamic-pituitary-adrenal axis. Typical endocrine glands are the pituitary, thyroid,
and adrenal glands. Features of endocrine glands are, in general, their ductless nature, their vascularity,
and usually the presence of intracellular vacuoles or granules storing their hormones. In
contrast, exocrine glands, such as salivary glands, sweat glands, and glands within the gastrointestinal
tract, tend to be much less vascular and have ducts or a hollow lumen.

In addition to the specialised endocrine organs mentioned above, many other organs that are part of
other body systems, such as the kidney, liver, heart and gonads, have secondary endocrine functions.
For example the kidney secretes endocrine hormones such as erythropoietin andrenin.