The Nervous System 35-2

1. The Nervous System35-2 BIO 1004 Flora

2. Nervous System • Nervous system – controls and coordinates functions throughout the body and responds to internal and external stimuli • Neuron – cell that transmits electrical impulses

3. Neurons • Messages carried by the nervous system are electrical signals called impulses • 3 Types of Neurons • Sensory • Motor • Interneurons

4. 3 Types of Neurons • Sensory – carry impulses from the sense organs to the spinal cord or brain • Motor – carry impulses from brain and spinal cord to muscles and glands • Interneurons – connect sensory and motor neurons and carry impulses between them.

5. Features of Neurons • Cell body – largest part of the neuron • Contains nucleus and most of cytoplasm • Metabolic activity occurs here • Dendrites – carry impulses from environment or other neurons to the cell body. • Axon – long fiber tat carries impulses away from the cell body. • In animals, axons and dendrites are clustered into bundles called nerves.

6. Resting Neuron • When at rest, the outside of the cell has a net positive charge and the inside has a net negative charge. • Nerve cells pump Na+ out and K+ ions in. (sodium/potassium pump) • More K+ leak out of the cell than in, causing the cell to become negatively charged. • The electrical charge across the cell membrane in its resting state is known as its resting potential.

7. The Moving Impulse • Neurons remain resting until they receive a stimulus large enough to create an impulse • Once it begins, the stimulus travels down the axon opening protein gates allowing ions to pass in and out. • At the leading edge, Na+ ions flow in. • When the cell membrane becomes positively charged, it is called a nerve impulse or action potential.

8. Threshold • Strength of the impulse is always the same • The minimum level of stimulus that is required to activate a neuron is called a threshold.

9. The Synapse • At the end of the neuron terminal, the neuron makes contact with another cell, the location at which the neuron can transfer an impulse to another cell is called a synapse. • Synaptic cleft – gap that separates axon terminals and adjacent dendrites.

10. Synapse Cont. • Axon terminals contain vesicles filled with neurotransmitters (NT) • Neurotransmitters – chemicals used by neurons to transmit an impulse across the synaptic cleft. • The NT will attach to receptors on the membrane of the receptive neuron causing an action potential.