Human Anatomy & PhysiologyNERVOUS SYSTEM

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Nervous System FunctionsControl & coordinate functions throughout the body

Respond to internal & external stimuli

Provides fast communication between body systems and parts

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Nervous system cells

dendrites

cell body

axon

synaptic terminal

Neuron a nerve cell

Structure fits function many entry points

for signal one path out transmits signalsignal direction

signaldirection

dendrite cell body axon synapse

myelin sheath3

Neuron Basic Structure

Messages sent by the nervous system are electro-chemical signals (impulses) carried by neurons (nervous system cells)Impulses always travel from dendrites, through the cell body to the axon terminal 4

Neuron AnatomyDendrites – short, branched extensions that receive impulses from the environment or other neurons toward the cell bodyCell Body – the largest part of a neuron, containing most of the cytoplasm and the nucleusAxon – the long extension that carries an impulse away from the cell bodyMyelin (myelin sheath) – insulating membrane surrounding most axons (roduced by Schwann cells) separated by small gaps (Nodes of Ranvier = “nodes”)Axon terminals – branches at the end of an axon Neurons may have many dendrites and axon terminals, but only one axon Nerve – bundle of axons and dendrites from many neurons

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Fun facts about neurons• Most specialized cell in

animals• Longest cell

– blue whale neuron• 10-30 meters

– giraffe axon• 5 meters

– human neuron• 1-2 meters

Nervous system allows for 1 millisecond response timeNervous system allows for 1 millisecond response time 6

Neuron Types

Sensory Neuron – carries impulse from sense organs to brain & spinal cordMotor Neuron – carries impulse from brain to muscles & glandsInterneuron – connects sensory and motor neurons

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Nerve ImpulseHOW does a signal travel along an neuron?

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Transmission of a signal• Think dominoes!

– start the signal • knock down line of dominoes by tipping 1st one

trigger the signal– propagate the signal

• do dominoes move down the line? no, just a wave through them!

– re-set the system• before you can do it again,

have to set up dominoes again reset the axon

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Transmission of a nerve signal• Neuron has similar system

– protein channels are set up – once first one is opened, the rest open in

succession• all or nothing response

– a “wave” action travels along neuron – have to re-set channels so neuron can react

again

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Resting NeuronWhen a neuron is resting (NOT transmitting an impulse), the outside of the cell has a net positive ion charge, while the inside has a net negative charge.

Na+ ions are pumped out of the cell while K+ ions are pumped into the cell, using active transport

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Na+ K+ pumps (active transport) and leakage channels

This creates a difference in charge between the inside and the outside of a neuron = resting potential

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Measuring cell voltage

unstimulated neuron = resting potential of -70mV13

Moving ImpulseThe neuron remains in a “resting state” until it receives a stimulus large enough to start an impulse (“All or nothing response”) from the environment or another neuron.The electrical imbalance between the inside and the outside of the neuron is reversed as the impulse travels along the axon toward the axon terminals = ACTION POTENTIAL (nerve impulse) 14

Synapse

Impulse has to jump the synapse!– junction between neurons– has to jump quickly from one cell to next

What happens at the end of the axon?

How does the wavejump the gap?

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The SynapseThe action potential travels along the axon (“jumping” from node to node) until it reaches an axon terminal and the gap (synapse) between it and another cell

Chemicals (neurotransmitters) travel between the axon terminal and the next cell through the synaptic cleft (synapse)

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1. Resting potential2. Stimulus reaches threshold

potential3. Depolarization

Na+ channels open; K+ channels closed

4. Na+ channels close; K+ channels open

5. Repolarizationreset charge gradient

6. UndershootK+ channels close slowly

Action potential graph

–70 mV

–60 mV

–80 mV

–50 mV

–40 mV

–30 mV

–20 mV

–10 mV

0 mV

10 mV DepolarizationNa+ flows in

20 mV

30 mV

40 mV

RepolarizationK+ flows out

ThresholdHyperpolarization(undershoot)

Resting potential Resting1

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Divisions of the Nervous SystemCentral Nervous System (CNS) – brain and spinal cord

“Coordination/communication center”Receives most impulses and determines “next steps”Sends communication along spinal cord

Peripheral Nervous System (PNS) – all the nerve cells not part of the CNS

Receives internal and external sensory inputReceives and sends impulses to muscles or glandsReflex arcs

Autonomic Nervous System – regulates activities that are involuntary 18

CNS and PNS

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Reflex ArcImpulse pathway travels directly from sensory neuron to spinal cord, interneuron and back to muscle or gland

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Summary – Brain Basics & Neuroscience

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