The Nervous System

A great example of cell to cell communication and multicellular coordination…

Anatomy 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 The main cells in the nervous system are called Neurons. They consist of three main parts Cell body = contains the organelles, nucleus Dendrites = short branches off the cell body where a signal is received Axon = long branch off the cell body that leads to the next neuron

Continued Anatomy of a Neuron Myelin = insulating layer that surrounds the axon to speed up the signal transfer Synaptic Terminal = end of the neuron Synapse = gap/space between neurons where neurotransmitters transfer

How do neurons communicate? Signals transfer through neurons using synaptic signaling. It follows this general pathway Dendrites  cell body  Axon  Synaptic Terminals (then into the synapse to get to the next neuron or other cell)

Talking Cells The “signal” sent through a neuron is actually an electrical signal Electricity is generated anytime an ion moves. By using the dam metaphor, the cells create ion movement cascades called action potentials The movement of the ions cause changes in the + and – charges inside the cell

A Neuron at Rest A neuron at rest (unstimulated) is slightly negatively charged thanks to the careful arrangement of ions by the cell. This is called the resting potential This means that it is more negative inside than outside The cell controls this potential by moving ions in or out as needed to adjust the charges

A Neuron at Rest Resting potential and proper location of ions is maintained mostly by the Sodium-Potassium Pump Pumps Na+ out of cell Pumps K+ into the cell Active transport More sodium outside than potassium inside = negative cell

Ion Concentrations at Rest Inside neuron Outside neuron Na+ Lower Higher Cl- K+ Leak channels allow some K+ to escape out of the cell in case it gets too positively charged on the inside Leaves negatively charged molecules behind (Cl- ions, etc.)  more negative on the inside than on the outside.

Sending a Signal: Action Potential GATED Na+ channels are embedded in the membrane of the neuron, but they are normally CLOSED. These channels can be triggered to open by a stimulus such as a touch or smell or ligand When the gate opens, the sodium come rushing into the cell (dam metaphor) and cause the voltage to change.

Sending a Signal: Action Potential More sodium channels that are voltage gated exist in the nearby membrane. These channels open in response to a change in voltage or charge near them. When that first gate opens by the stimulus and lets in the sodium, the other gates are triggered to open in a chain, adding more sodium and triggering the next in line.

The Cascade of Opening Sodium Gates is called an 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

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 through the individual neurons 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 must be “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 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 getting too far away from the cell.

How does the signal jump the gap? Remember that a gap exists between neurons that the action potential cannot “jump”. They are just too far apart. When the signal reaches the end of the axon and wants to go to the next cell in line, it must change to a chemical messenger instead of an electrical impulse. These chemical messengers are called neurotransmitters

How does the signal jump the gap? The last voltage gated channel triggers the release of Calcium ions. This triggers exocytosis and vesicles that contain neurotransmitter molecules fuse with the plasma membrane and expel the neurotransmitters into the synaptic cleft (space between neurons)

Communication between Neurons The neurotransmitters diffuse across the cleft and bind to receptors on the next neuron This triggers a Na+ chemical gated channel to open on that neuron, creating a new action potential

Communication between Neurons After the signal has been sent, neurotransmitters must be eliminated. This is done by Diffusion = diffuse away Reuptake (proteins that reabsorb the neurotransmitters for recycling) Enzyme degradation (proteins that break down the neurotransmitters completely)

Question… Why is it important that our bodies / medications control nerve communication? So that signals are only sent to neurons when needed.