THE NERVOUS SYSTEM 35-2 BIO 1004 Flora

NERVOUS SYSTEM  Nervous system – controls and coordinates functions throughout the body and responds to internal and external stimuli  Neuron – cell that transmits electrical impulses

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

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.

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.

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.

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.

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.

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.

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.