The Nervous System & Neurons Unit 9 Chapter 35-2

Overview of the Nervous System STRUCTURES: brain, spinal cord, & peripheral nerves (1) FUNCTION: Recognizes and coordinates the body’s response to changes in its internal and external environments (2)

Overview of the Nervous System The ultimate control of all the organ systems is done by the nervous system.

A. Neurons NeuronsNeurons are nerve cells that carry electrical impulses through the body (3) Impulse = message (4) Neurons are classified according to the direction an impulse travels.

3 Types of Neurons Sensory –1. Sensory: carry impulses from sense organs to the brain (5) Motor –2. Motor: carry impulses from brain to muscles & glands (6) Interneurons –3. Interneurons: connect sensory and motor neurons and process impulses (7)

InterneuronSensory NeuronMotor Neuron

Reflex Arc Sensory Neuron Interneuron Motor Neuron

Structure of a Neuron (8) Axon terminals Myelin sheath Cell body Nodes Axon Dendrites Nucleus

Dendrites –1. Dendrites: receive impulses (9)

Cell body –2. Cell body: contains nucleus & cytoplasm, largest part of cell

Axon –3. Axon: transmit impulses away from cell body (10)

Myelin Sheath –4. Myelin Sheath: covering that insulates the axon, sending the impulse faster and give it a whitish appearance (11) –Neurons with axons that have myelin make up “white matter” in the brain, while neurons without myelin are called “gray matter”

Nodes –5. Nodes: gaps in the myelin sheath where membrane is exposed –Impulses jump from one node to the next

Axon terminals –6. Axon terminals: transmits impulse (message) to next cell (12)

II. Transmission of an Impulse A nerve impulse is an electrical message. At rest, no impulse is being sent. The neuron must be stimulated (receive input) to send a impulse.

A. Resting Neuron When resting, neuron is not transmitting an impulse –inside of cell has a net negative charge – –outside of cell has a net positive charge + Membrane PotentialThe cell membrane has Membrane Potential, the potential to carry an electrical current because there is a difference in charge sodium (Na + )potassium (K + ) –The ions sodium (Na + ) and potassium (K + ) cause the potential

(K + )

Sodium-potassium pumps in the nerve cell membrane pumps sodium (Na + ) ions out of the cell and potassium (K + ) ions into the cell by means of active transport. As a result, the inside of the cell contains more K + ions and fewer Na + ions than the outside.

B. Stimulated Neuron stimulated by another neuron or the environment An impulse begins when a neuron is stimulated by another neuron or the environment The stimulated impulse must reach thresholdThe stimulated impulse must reach threshold, the minimum impulse strength needed to create a new impulse (all or none)

At the leading edge of the impulse, gates in the sodium channels open allowing positively charged sodium (Na+) ions to flow into the cell.

This reversal of charges is called the nerve impulse or ACTION POTENTIAL (13)

As the action potential passes, gates in the potassium channels open, allowing potassium (K+) ions to flow OUT of the cell This restores the negative potential in the axon.

The action potential continues to move along the axon in one direction toward the axon terminals At Rest Action Potential

Action Potential

hill.com/sites/ /student_view0/chapter45/animations.html# hill.com/sites/ /student_view0/chapter45/animations.html#

C. Synapse The synapseThe synapse is at the end of the axon terminals where a neuron can transfer an impulse to another cell (14) The small space between cells is the synaptic cleft Terminals contain vesicles filled with neurotransmitter

NeurotransmittersNeurotransmitters are chemical signals of neurons that transmit an impulse across the synapse to another cell (15) Ex: epinephrine, norepinephrine, acetylcholine, histamine, seratonin, dopamine, melatonin

Steps of Transmission 1.Action Potential reaches axon terminal 2.Causes Neurotransmitter to be released into synaptic cleft 3.Neurotransmitter binds to receptors on next cell. 4.Channels open, which can create a new action potential hill.com/sites/ /st udent_view0/chapter45/ani mations.html#

Major Neurotransmitters & Their Effects