The Nervous SystemThe Nervous System

AP BiologyUnit 6

Branches of the Nervous System

• There are 2 main branches of the nervous system

• Central Nervous System– Brain– Spinal Cord

• Peripheral Nervous System– All nerves leading to rest of

body

Anatomy of a Neuron• Dendrites = where a signal is received by the

neuron

• Cell body = contains the organelles, nucleus of the cell

• Axon = signal travels down this to get to the other end of the neuron

Anatomy of a Neuron• Myelin = surrounds the axon to speed up

the signal

• Synaptic Terminal = end of the neuron

• Synapse = gap/space between neurons

Question…

• What is the general pathway of a signal through a neuron?– Dendrites cell body Axon Synaptic

Terminals (then into the synapse to get to the next neuron or other cell)

Sending Signals

• The “signal” sent through a neuron is an electrical signal

• Based on the movement of ions into and out of the cell– Causes changes in the + and – charges inside

the cell

A Neuron at Rest• A neuron at rest (unstimulated) has a

difference in charge (voltage) across the plasma membrane = -70 mV = resting potential – This means that it is more negative inside than

outside

• The resting potential is caused by the distribution of ions on either side of the membrane

A Neuron at Rest

• Resting potential (-70 mV) is maintained by the Sodium-Potassium Pump – Pumps Na+ out of cell – Pumps K+ into the cell– Active transport

Ion Concentrations at Rest

• At rest, there are also open K+ channels in the membrane Allows some K+ to escape

• Leaves negatively charged molecules behind (Cl- ions, etc.) more negative on the inside than on the outside.

Ion Inside neuron Outside neuron

Na+ Lower Higher

Cl- Lower Higher

K+ Higher Lower

Sending a Signal: Action Potential

• Na+ channels are embedded in the membrane of the neuron

• Usually, these Na+ channels are closed, but can be triggered to open when the correct stimulus is received– Voltage gated channels = open in response to a

particular change in voltage (charge)– Chemical gated channels = open in response to a

chemical binding to them

Action Potential• STEP 1: To start an

action potential, some kind of stimulus (light, pressure, chemical, etc.) causes Na+ channels in the dendrite to open.

• This causes Na+ to flood into the neuron from outside DEPOLARIZATION

Questions…

• Why does Na+ diffuse in from the outside? – Higher concentration on the outside

• When depolarization occurs, how is the charge inside the neuron affected? – Becomes more positively charged inside

• What would happen if Cl- channels are also opened? – Cl- would also flow in– makes the inside more

negative (cancels out the charge from Na+ coming in) -- HYPERPOLARIZED

Action Potential• STEP 2: The change in

voltage triggers the next Na+ channel (voltage gated channel) to open.

• STEP 3: As Na+ diffuses down the neuron, it continues to trigger voltage gated Na+ channels to open. – This is what sends a signal

down the neuron towards the axon terminal.

Action Potential

• STEP 4: Na+ voltage gated channels only open temporarily. After a short period of time, they close and an inactivation gate opens to prevent them from opening again for a little while REFRACTORY PERIOD

Action Potential• STEP 5: The neuron is “reset”

(REPOLARIZED) by the opening of voltage gated K+ channels.

• K+ flows OUT of the neuron, making the inside more negative again. – Why does K+ flow out?

– Higher K+ concentrations inside neuron

• The Na+/K+ pump also helps reestablish resting potential.

Saltatory Conduction

• Depolarization & Repolarization happens over and over down the axon, so the nerve impulse travels.

• Myelin sheaths insulate the axon, keeping ions from flowing out except at Nodes of Ranvier.

Saltatory Conduction

• Wider axons yield faster conduction because there is less resistance.

• Action potentials jump from one Node of Ranvier (space between myelin sheaths) to the next, speeding up the signal.

Communication between Neurons

• When the signal reaches the axon terminal, it triggers voltage gated Ca2+ channels to open.

• This causes vesicles that contain neurotransmitter molecules to fuse with the plasma membrane and expel the neurotransmitters into the synaptic cleft (space between neurons)

Communication between Neurons• The neurotransmitters will diffuse across the

cleft and bind to receptors on the next neuron (postsynaptic neuron).

• This triggers a Na+ chemical gated channel to open on the postsynaptic neuron, triggering an action potential in that neuron.

Communication between Neurons

• After the signal has been sent, neurotransmitters are eliminated from the synaptic cleft by – Diffusion = diffuse away– Reuptake – Enzyme degradation

Communication between Neurons

• Reuptake– Neurotransmitters are actively transported back

into the presynaptic neuron repackaged into vesicles to be released again

– Recycling neurotransmitters

• Enzyme degradation– Enzymes in the synaptic cleft break down the

neurotransmitter

Question…

• Why is it important that our bodies / medications control nerve communication? – So that signals are only sent to neurons when

needed.

Control of Communication

• How can nerve communication be controlled?– Change conduction of impulse – Change synaptic cleft size – Change volume of neurotransmitters or vesicles – Change number of ligand-gated ion channels on

post synaptic neuron– Add a chemical that binds to ligand-gated ion

channels to block them or always keep them open– Add a chemical that binds to neurotransmitters, so

they cannot bind to the ligand-gated ion channels

Brain Regions• Brain = organ where nervous

processes are centralized• Cephalization = evolutionary

trend in which most nervous processes are at head (anterior) region

• Different regions of the vertebrate brain have different functions – Ex. Vision, Movement,

Memory

Overview of the Nervous System• The neuron is the basic structure/cell of the nervous

system.

• Action potentials are what send messages down a neuron.

• Pathway: Dendrites Cell body Axon Synapse

• In order for a signal to be tranmitted across the synapse, neurotransmitters are used.

• Transmission along neurons and synapses results in response.– Responses can be stimulatory or inhibitory.