The Cranial Nerves

Cranial nerves are nerves that emerge directly from the brain in contrast to spinal nerves which emerge from segments of the spinal cord. Although thirteen cranial nerves in humans fit this description, twelve are conventionally recognized. The nerves from the third onward arise from the brain stem. Except for the tenth and the eleventh nerve, they primarily serve the motor and sensory systems of the head and neck region. However, unlike peripheral nerves which are separated to achieve segmental innervation, cranial nerves are divided to serve one or a few specific functions in wider anatomical territories.

Whereas most major nerves emerge from the spinal cord, the 12 pairs of cranial nerves project directly from the brain. All but 1 pair relay motor or sensory information (or both); the tenth, or vagus nerve, affects visceral functions such as heart rate, vasoconstriction, and contraction of the smooth muscle found in the walls of the trachea, stomach, and intestineCorporation.

Cranial Nerves:

1. olfactory nerves - smell

2. optic nerve - vision

3. ocumulator - eye movements

4. trochlear - eye movements

5. abducent nerves - eye movements

6. trigeminal-Facial sensation and jaw movements

7. Acoustic (vestibulocochlear nerve) - hearing and balance

8. Glossopharyngeal nerve -Taste and throat sensations

9. Vagus nerve - Breathing,circulation and digestion

10. Spinal accessory nerve -Movements of neck and back muscles

11. hypoglossal nerve- Tonge movements

12. Facial nerve - facial expressions and taste

courtesy of: 3D science.com.

source: wiki/encarta encyclopedia

The Brain

The human brain has three major structural components: the large dome-shaped cerebrum (top), the smaller somewhat spherical cerebellum (lower right), and the brainstem (center). Prominent in the brainstem are the medulla oblongata (the egg-shaped enlargement at center) and the thalamus (between the medulla and the cerebrum). The cerebrum is responsible for intelligence and reasoning. The cerebellum helps to maintain balance and posture. The medulla is involved in maintaining involuntary functions such as respiration, and the thalamus acts as a relay center for electrical impulses traveling to and from the cerebral cortex. Lack of blood flow to any part of the brain results in a stroke, permanent damage that interferes with the functions of the affected part of the brain

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Movement may occur also in direct response to an outside stimulus; thus, a tap on the knee causes a jerk, and a light shone into the eye makes the pupil contract. These involuntary responses are called reflexes. Various nerve terminals called receptors constantly send impulses into the central nervous system. These are of three classes: exteroceptors, which are sensitive to pain, temperature, touch, and pressure; interoceptors, which react to changes in the internal environment; and proprioceptors, which respond to variations in movement, position, and tension. These impulses terminate in special areas of the brain, as do those of special receptors concerned with sight, hearing, smell, and taste.

courtesy of: 3D science.com.

source: encarta encyclopedia

The nervous system range in complexity.

earthworm and Jellyfish

Neural Organization Nervous systems range in complexity from the jellyfish’s network of nerve cells to the central and peripheral systems of humans. Common to many animals is the nervous structure of the earthworm, which consists of a cerebral ganglion, a main nerve cord, and branching pairs of lateral nerves. In some cases, as in insects, the cerebral ganglion acts as a primitive brain, controlling and coordinating various basic functionsCorporation.
Voluntary movement of head, limbs, and body is caused by nerve impulses arising in the motor area of the cortex of the brain and carried by cranial nerves or by nerves that emerge from the spinal cord to connect with skeletal muscles. The reaction involves both excitation of nerve cells stimulating the muscles involved and inhibition of the cells that stimulate opposing muscles. A nerve impulse is an electrical change within a nerve cell or fiber; measured in millivolts, it lasts a few milliseconds and can be recorded by electrodes.
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The Human Nervous System

Blue is PNS while red is CNS.

The peripheral nervous system (PNS)

resides or extends outside the central nervous system.

The central nervous system (CNS),

which consists of:

The brain

The spinal cord

To serve the limbs and organs. Unlike the central nervous system, however,

The PNS is not protected by bone, leaving it exposed to toxins and mechanical injuries.

The peripheral nervous system is divided into:

a. The somatic nervous system

b. The autonomic nervous system

the Central Nervous System

The central nervous system (CNS) of the vertebrate nervous system which is enclosed in meninges. It contains the majority of the nervous system, and consists of the brain (in vertebrates which have brains), and the spinal cord. Together with the peripheral nervous system, it has a fundamental role in the control of behavior. The CNS is contained within the dorsal cavity, with the brain within the cranial cavity, and the spinal cord in the spinal cavity. The brain is also protected by the skull, and the spinal cord is, in vertebrates, also protected by the vertebrae.

Nervous System Organization

The nervous system is composed of the central nervous system and the peripheral nervous system. The central nervous system which includes the brain and spinal cord, processes and coordinates all incoming sensory information and outgoing motor commands, and it is also the seat of complex brain functions such as memory, intelligence, learning, and emotion.
The peripheral nervous system includes all neural tissue outside of the central nervous system. It is responsible for providing sensory, or afferent, information to the central nervous system and carrying motor, or efferent, commands out to the body’s tissues. Voluntary motor commands, such as moving muscles to walk or talk, are controlled by the somatic nervous system, while involuntary motor commands, such as digestion and heart beat, are controlled by the autonomic nervous system. The autonomic nervous system is further divided into two systems. The sympathetic nervous system, sometimes called the “fight or flight” system, increases alertness, stimulates tissue, and prepares the body for quick responses to unusual situations. In contrast, the parasympathetic nervous system, sometimes called the “rest and repose” system, conserves energy and controls sedentary activities, such as digestion.
source: wiki/encarta encyclopedia

What is the Nervous System?

The nervous system
is a highly specialized network whose principal components are nerves called neurons. Neurons are interconnected to each other in complex arrangements and have the property of conducting, using electrochemical signals, and a great variety of stimuli both within the nervous tissue as well as from and towards most of the other tissues. Thus, neurons coordinate multiple functions in organisms. Nervous systems are found in many multicellular animals but differ greatly in complexity between species.
The nervous system has two divisions: the somatic, which allows voluntary control over skeletal muscle, and the autonomic, which is involuntary and controls cardiac and smooth muscle and glands. The autonomic nervous system has two divisions: the sympathetic and the parasympathetic. Many, but not all, of the muscles and glands that distribute nerve impulses to the larger interior organs possess a double nerve supply; in such cases the two divisions may exert opposing effects. Thus, the sympathetic system increases heartbeat, and the parasympathetic system decreases heartbeat. The two nervous systems are not always antagonistic, however. For example, both nerve supplies to the salivary glands excite the cells of secretion. Furthermore, a single division of the autonomic nervous system may both excite and inhibit a single effector, as in the sympathetic supply to the blood vessels of skeletal muscle. Finally, the sweat glands, the muscles that cause involuntary erection or bristling of the hair, the smooth muscle of the spleen, and the blood vessels of the skin and skeletal muscle are actuated only by the sympathetic division.
source: wiki/encarta encyclopedia