SEC 31.1 THE NEURON

THINK ABOUT IT How do we know about the world outside our bodies? When you reach for a ball at a ballgame, how does your body make this happen? How do the sights, sounds, and smells at the ballgame get into your mind? The answers to all these questions are found in the nervous system.

Functions of the Nervous System collects information about the body’s internal and external environment processes that information responds to it.

Parts of the Nervous System Central nervous system (CNS) consists of the brain and spinal cord processes information and creates a response

Parts of the Nervous System Peripheral nervous system (PNS) consists of nerves and supporting cells collects information about the body’s external and internal environment creates a response in glands or muscles

Neurons The messages carried by the nervous system are electrical signals called impulses. Nervous system impulses are transmitted by cells called neurons.

Types of Neurons Grouped according to the direction an impulse travels Sensory neurons carry impulses from the sense organs, such as the eyes and ears, to the CNS (spinal cord and brain). Motor neurons carry impulses from the CNS (brain and the spinal cord) to muscles and glands. Interneurons process information from sensory neurons and then send commands to other interneurons or motor neurons.

Structure of Neurons The largest part of a typical neuron is the cell body, which contains the nucleus and much of the cytoplasm.

Structure of Neurons Spreading out from the cell body are short, branched extensions called dendrites.

Structure of Neurons Dendrites receive impulses from other neurons and carry impulses to the cell body.

Structure of Neurons The long fiber that carries impulses away from the cell body is the axon.

Structure of Neurons An axon ends in a series of small swellings called axon terminals.

Structure of Neurons Neurons may have dozens of dendrites, but usually they have only one axon. In most animals, axons and dendrites of different neurons are grouped into bundles called nerves.

Structure of Neurons In some neurons, the axon is surrounded by an insulating membrane called the myelin sheath. Nodes are gaps in the myelin where the axon membrane is exposed.

Structure of Neurons As an impulse moves along the axon, it jumps from one node to the next. This causes an impulse to travel faster than it would without a myelin sheath.

The Nerve Impulse An impulse begins when a neuron is stimulated by another neuron or by the environment.

The Resting Neuron Neurons have a charge, or electrical potential, across their cell membranes. The inside of a neuron is negative compared to the outside. This difference is known as the resting potential.

The Resting Neuron The movement of sodium ions (Na+) and potassium ions (K+) is responsible for the charge difference.

The Moving Impulse A neuron remains in its resting state until it receives a stimulus large enough to start a nerve impulse. The impulse travels quickly down the axon toward the axon terminals.

The Moving Impulse The impulse will cause the nerve cell to become positively charged. This is called a nerve impulse, or an action potential.

The Moving Impulse When the impulse passes, the neuron becomes negatively charged and the resting potential is restored.

The Synapse The point at which a neuron transfers an impulse to another cell is called a synapse. A space, called the synaptic cleft, separates the axon terminal from the next cell.

The Synapse The axon terminal at a synapse contains tiny structures filled with chemicals called neurotransmitters. These chemicals travel across a synapse to cause an impulse in another cell.

The Synapse Then neurotransmitters are broken down or recycled by the axon terminal.