1. Nerve Impulses
Neurons are nerve cells that are specialized for conducting information. Bundles of neurons form thin cables called nerves.
Nerves form a communication network that travels throughout the body including organs like your skin, eyes, and stomach.
All neurons have the same basic structure which includes the cell body, dendrites and axons.

2. The enlarged area of the cell body contains the nucleus and organelles.
Nerve responses enter the cell on the short extensions called the dendrites.
Information is passed on through the cell body to the long fibers called the axons.
Each neuron has many dendrites, but only one axon.

3. Transmission of Nerve Impulses
Nerve impulses are actually electrical charges.
The key to the transmission of nerve impulses lies in the neuron’s cell membrane.
The membrane is studded with protein channels called voltage-gated channels. These channels open and close in response to changes in electrical charge across the membrane.
Some allow only Na+ ions and some allow only K+ ions to pass through the channels.
Transmission of Nerve Impulses

4. The unequal distribution of ions across the nerve cell membrane results in a voltage difference and that difference propels the stimulus along the nerve.
At resting potential, the concentration of K+ ions are higher inside the membrane and the Na+ ions are higher outside the membrane.
When the impulse causes the exchange of the ions across the membrane, the impulse is propelled down the nerve.

5. Myelin sheath is an insulating layer made up of fatty acids
Myelin sheath is an insulating layer made up of fatty acids. This sheath allows nerve impulses to travel quickly.
The importance of the myelin sheath becomes apparent in patients with multiple sclerosis. Their myelin sheath disappears and they become weak and disabled.
At the end of a nerve axon there is a small gap to the next nerve, called a synapse. The impulse has to cross the synapse by jumping to the other side.

6. The impulse is carried across the synapse by chemical neurotransmitters. They are found in tiny sacs at the end of the axon.
After carrying the impulse across the synapse, the neurotransmitter must be removed or the synapse would continue to fire.
Different neurotransmitters produce different effects.

7. Organization of the Human Nervous System
Central Nervous System
Peripheral Nervous System

8. The nervous system has two main parts:
The central nervous system – consisting of the brain and spinal cord. It is your control center that organizes all incoming and outgoing information.
The peripheral nervous system – the nerves that connect to the rest of the body. The sensory neurons and motor neurons act as independent communications pathways that monitor different parts of the body.

9. The Central Nervous System
The delicate parts of the central nervous system are protected by the skull and the vertebral column.
The CNS sorts out a vast amount of information and issues commands to muscles, glands, and organs.
The cerebrum is the control center of the brain. The brain weighs about 3 pounds and is one of the largest organs in the body. It is divided into two halves that are connected by a large band of nerve fibers.
Each hemisphere has an outer gray layer called the cerebral cortex. This 2-4mm thick layer handles the most sophisticated functions of the brain.

10. The cerebellum is a cauliflower-shaped structure essential for coordination of movement. Damage to the cerebellum causes a lack of coordination and tremors.
Many nerves that control muscles in the body cross in the brain stem, therefore damage to one side of the brain above the crossover causes weakness or paralysis on the opposite side of the body.
The brain stem contains the hypothalamus which is the center for emotions and instincts controlling hunger, thirst, body temperature, water balance, blood pressure, pleasure, hostility, and pain.

11. The spinal cord shuttles information between the body and the brain
The spinal cord shuttles information between the body and the brain. Like the brain, the spinal cord contains gray matter.
The spinal cord is surrounded and protected by the backbone. Injuries to the spinal cord can result in pain, numbness, or paralysis in limbs and extremities.

12. New technologies are allowing scientists to learn more about brain functions.
Learning occurs when two or more pieces of information are linked into a pattern of connections between neurons.
Language processing is very complex and involves several regions of the left hemisphere.
Researchers study the brain using CAT scans (computerized axial tomography). Patients lay on a table and go through an intense magnetic field in a tube. Radio frequencies are transformed into images.
Mapping the Brain

13. Peripheral Nervous System
All of the nervous system outside the spinal cord and brain is known as the peripheral nervous system. This system carries all the messages sent back and forth between the central nervous system and the rest of the body.
Sensory neurons tell the central nervous system what is happening.
Motor neurons are partners of sensory neurons. Motor neurons carry information from the central nervous system to a muscle or gland.
Sensory nerves go into the spinal cord and motor nerves come out of the spinal cord.
Peripheral Nervous System

14. A reflex is a sudden, involuntary movement.
A reflex produces a rapid motor response to a stimulus because the sensory neuron synapses directly with a motor neuron.
A reflex occurs before the brain is aware there is a problem.
The autonomic nervous system carries messages to muscles and glands that usually work without notice. Examples: muscles and glands that control blood pressure and movement of food through the digestive system even during sleep.