The Central and Peripheral Nervous Systems

[The Central Nervous System - PNGKEY]

- The Central and Peripheral Nervous Systems

• The brain and the spinal cord make up the central nervous system.  
• Cranial nerves, spinal nerves, Sensory organs (eyes, nose, tongues, ears, and skin) make up the peripheral nervous system.

Like other systems in the body, the nervous system is composed of organs, principally the brain, spinal cord, nerves, and ganglia. These, in turn, consist of various tissues, including nerve, blood, and connective tissue. Together these carry out the complex activities of the nervous system. 

The various activities of the nervous system can be grouped together as three general, overlapping functions: Sensory, Integrative, Motor. 

Millions of sensory receptors detect changes, called stimuli, which occur inside and outside the body. They monitor such things as temperature, light, and sound from the external environment. Inside the body, the internal environment, receptors detect variations in pressure, pH, carbon dioxide concentration, and the levels of various electrolytes. All of this gathered information is called sensory input. 

Sensory input is converted into electrical signals called nerve impulses that are transmitted to the brain. There the signals are brought together to create sensations, to produce thoughts, or to add to memory; Decisions are made each moment based on the sensory input. This is integration. 

Based on the sensory input and integration, the nervous system responds by sending signals to muscles, causing them to contract, or to glands, causing them to produce secretions. Muscles and glands are called effectors because they cause an effect in response to directions from the nervous system. This is the motor output or motor function.

- Neurons in Nervous Tissue Relay Rapid-Fire Signals

All nervous tissue, from the brain to the spinal cord to the furthest nerve branch, includes cells called neurons. Neurons are charged cells: they conduct electrical signals to pass information through the body. A typical neuron consists of a cell body, dendrites, and an axon with an axon terminal. The dendrites receive signals from body tissues or other neurons and pass them into the cell body. If an outgoing signal is produced, it zips down the axon to the axon terminal and passes to the next neuron or target cell. This conductive capability sends information up and down nerve pathways and through the central nervous system at incredible speed. Some 100 billion neurons give the brain its awesome processing power 

- Neurotransmitters Are the Activators of the Nervous System

Nervous system messages travel through neurons as electrical signals. When these signals reach the end of a neuron, they stimulate the release of chemicals called neurotransmitters. Neurotransmitters travel across synapses, spaces between neurons or between neurons and other body tissues and cells. Neurotransmitters can be classified as two types: excitatory or inhibitory. Excitatory neurotransmitters stimulate electrical signals in other neurons and encourage responses from body cells. Inhibitory transmitters discourage signals and cellular responses. Through these chemicals, the nervous system regulates the activity of muscles, glands, and its own nerve pathways.

- The Spinal Cord Transmits Signals to and from the Brain and Commands Reflexes

The spinal cord is an elongated cylinder of neuron cell bodies, bundles of axons and other cells, protected by connective tissue and bone. It connects to the brain at the medulla oblongata and runs down the vertebral column, the hollow tunnel enclosed within the vertebrae of the spine. The spinal cord is part of the central nervous system and serves as a kind of superhighway. Sensory information and motor commands travel up and down, heading to and from the brain. These signals speed in and out of the spinal cord via spinal nerves—the “on-ramps and off-ramps” that branch out to supply the limbs, torso, and pelvis. Some incoming signals demand a simple, immediate response. The spinal cord can shoot out a reflex command without bothering the brain.

- The Brain Connects Perceptions to Complex Thought, Memory, and Emotion

The nervous system does more than route information and process commands. Why do certain smells immediately raise particular memories? The answer appears to lie in the limbic system. The limbic system forms two paired rings within the brain, consisting of the hippocampus, the amygdala, the cingulate gyrus, and the dentate gyrus, along with other structures and tracts. As with other brain segments, the limbic system is involved in multiple nervous system functions and levels of activity. It helps to process both memory and olfaction—our sense of smell—and it manages a range of emotions. The aroma rising from a pot on the stove may send your hand reaching for a spoon. It may also call up a dinner from earlier times, and make you happy, regretful, or nostalgic.