Neurons & Nerve Impulses

Objectives

• Explain the function of the nervous system

• Contrast the three types of neurons

• Identify the 4 main parts of a neuron

• Identify where nerve impulses are transferred

Introduction

• The nervous system controls and coordinates functions throughout the body and responds to internal and external stimuli

• The nervous system is the communication system of the body

• Messages carried by the nervous system are electrical signals called impulses

Neurons

• Neurons – cells that transmit nerve impulses

• 3 Types of Neurons

– Sensory neurons carry impulses from sense organs to the spinal cord and brain

– Motor neurons carry impulses from the brain and spinal cord to muscles and glands

– Interneurons connect sensory and motor neurons and carry impulses between them

Parts of a Neuron

• Cell body – the largest part of a neuron that contains the nucleus and most of the cytoplasm

• Dendrites – short, branched extensions that carry impulses from the environment or from other neurons toward the cell body

• Axon – long fibers that carry impulses away from the cell body– the ends of the axon are called axon terminals

– Axons and dendrites are clustered into bundles of fibers called nerves

Parts of a Neuron

• Some neurons have a myelin sheath, which is an insulating membrane that surrounds an axon

• The myelin sheath has gaps in it called nodes which allow impulses to move quickly up the axon

The Nerve Impulse

• An impulse begins when a neuron is stimulated by another neuron or by the environment

• An impulse is a sudden reversal of the membrane potential• Sodium channels open to allow Na+ to flow into the cell• The inside of the membrane temporarily becomes more

positive than the outside• Action potential – the reversal of charges across the cell

membrane from negative to positive, also called a nerve impulse

• As the impulse passes, potassium channels open, allowing K+ to flow out, restoring resting potential

• Nerve impulses at one point trigger an impulse at the next point, like a row of falling dominoes

The Synapse

• Impulse reaches the end of a neuron at the axon terminal, where it makes contact with another cell– The neuron may pass the impulse

to the second cell

• Synapse – the location at which a neuron can transfer an impulse to another cell

• Axon terminals contain vesicles with neurotransmitters

• Neurotransmitter – chemicals used by a neuron to transmit an impulse across a synapse to another cell

Notes Review

• Explain the function of the nervous system

– The nervous system controls and coordinates functions throughout the body and responds to internal and external stimuli

Notes Review

• Contrast the three types of neurons

– Sensory neurons carry impulses from sense organs to the spinal cord and brain

– Motor neurons carry impulses from the brain and spinal cord to muscles and glands

– Interneurons connect sensory and motor neurons and carry impulses between them

Notes Review

• Identify the 4 main parts of a neuron

– Cell body, dendrites, axon, myelin sheath

Notes Review

• Identify where nerve impulses are transferred

– Nerve impulses are transferred at the synapse.