Chapter 33-1 The Nervous System

NWRC BIO 30

NeuronsWatch Video Dendrites –

receive signals and conduct impulses

Axon – carries the nerve impulse from the cell body to other neurons and muscles

Cell body – contains nucleus and other cell organelles

The cell body

– Round, centrally located structure

– Contains DNA– No role in neural

signaling

• Contains the cell’s Nucleus

Dendrites• Information

collectors

• Receive inputs from neighboring neurons

• Inputs may number in thousands

Dendritic Growth

• Mature neurons generally can’t divide

• But new dendrites can grow

• Provides room for more connections to other neurons

• New connections are basis for learning

Axon

• The cell’s output structure

• One axon per cell, 2 distinct parts– tubelike structure

branches at end that connect to dendrites of other cells

Neurons

Types of Neurons

Sensory Motor Interneurons

Sensory (incoming) Motor (outgoing)

SensoryNeuron

Sensory Neurons

• INPUT From sensory organs to the brain and spinal cord.

• Some sensory neurons respond to tactile stimuli and can activate motor neurons in order to achieve muscle contraction.

SensoryNeuron

Sensory Neurons• Such connections

between sensory and motor neurons underlie motor reflex loops and several forms of involuntary behavior, including pain avoidance.

• In humans, such reflex circuits are commonly located in the spinal cord..

Motor Neurons

• OUTPUT From the brain and spinal cord To the muscles and glands which results in a response. These transmit impulses from the central nervous system to the

• muscles and glands• that carry out the response.

Most motor neurons are stimulated by interneurons, although some are stimulated directly by sensory neurons.

SpinalCord

BrainSensoryNeuron

MotorNeuron

SpinalCord

BrainSensoryNeuron

MotorNeuron

Interneurons• Interneurons carry

information between other neurons. These are found exclusively within the spinal cord and brain. They are stimulated by signals reaching them from

• sensory neurons • other interneurons or • both.• Watch clip on reflex arc

How do neurons conduct electrical impulses?

(watch clip)

• Neurons conduct electrical impulses by using the Action Potential. This phenomenon is generated through the flow of positively charged ions across the neuronal membrane.

Begin Notes Here

• A Neuron at rest is polarized- this means that the fluids outside the cell membranes

have an electrical charge that is different than the charge inside the cell

Notes

• The difference in electrical charge is potential energy that is released when the neuron receives stimulus.

• Polarization is caused by the concentration of Na+ outside the cell and K+ inside the cell

Notes

• The Na+ /K+ pump is made of proteins in the cell membrane

• These pumps move Na+ and K+ in and out of the cell by Active Transport

• (remember from chapter 7 that this requires energy- see pg. 206)

Notes

• When the pump is open to the inside K+ no longer binds and Na+ becomes active

Notes

• When the pump is open to the outside Na+ no longer binds and K+ becomes active

Notes

• The cell membrane is slightly permeable to K+ so K+ can diffuse out of the cell. The loss of potassium ions by diffusion adds to the more positive charge outside the cell membrane

• The difference btw the inside and the outside of a resting neuron is called a resting potential

Notes

• The word potential means voltage (which is a measure of electrical energy)

Notes

• When a neuron is excited by a stimulus (such as light , heat, pressure, pain etc) changes in the resting potential take place.

notes

• The Stimulus alters the permeability of the cell membrane

• The change is permeability is what allows impulses to move along the neuron , carrying messages through the nervous system

Na+

Na+

Na+Na+

Na+

Na+

K+

K+

K+

notes

• In an action potential first the membrane becomes highly permeable to Na+

• When a neuron is excited by a stimulus gates permeable to Na+ are opened and ions rush into the cell

+

+

K

Notes

The inward rush of Na+ reverses the polarity of the cell. Now the inside has a greater positive charge than the outside

NotesOnce again this reversal of polarity takes place, the sodium gates close and the potassium gates open

K+ rush out of he cell because there is now a less positive charge outside the cell and they are attracted.

Polarity is reversed again.

Notes

• The flow of ions and the changes in the polarity caused by the stimulus creates an electrical current called the action potential

• A series of action potentials sweeping down an axon is a nerve impulse

Notes

• Restoring resting potential• After an action potential has swept along

the neuron some of the Na+ that were originally outside the cell are now inside it and some of the K+ moved from outside to inside

• BUT there are still more K+ inside than outside and more Na+ outside than inside

Notes

• As the action potential continues to move along the neuron a point is reached where there is balance of K+ and Na+ inside and out.

• At this point the sodium potassium pump operates to once again create a resting potential Na+ ions move into the cell and K+ move out

End of Notes

• For every 2 K+ ions pumped in 3 Na+ ions are pumped out. His results in a positive charge outside the neuron and a negative charge inside the cytoplasm

Action Potential

• Website I am going to go through this online tutorial quickly so please return to it on your own and work through it

The synapseWhen the wave of Action Potentials reach the end of the axon the electrical signal is converted into a chemical signal.  This chemical or neurotransmitter crosses the space (Synapse) between adjacent neurons and initiates an Action Potential on another neuron.

The Synapse

Assessment questions

• 1. Both are Vast networks capable of sending and receiving vast amounts of information

Assessment

• 2. The ions would naturally move from an area of high concentration to an area of low concentration but forcing ions to concentrate on one side of the cell membrane takes energy

Assessment

• 3. No. If the sensory neurons do not send impulses to the brain the person would not feel pain.

• The end

For further study

• View this tutorial with animations