Topic 25: The Nervous System

The nervous system is built up of specialized cells, which we dealt with when we studied histology (the study of tissues).  You can review it here.  Scroll down to "nervous tissue".

Information is carried through the nervous system along neurons.

The direction of information flow in a neuron is always from the dendrite to the axon.

We say that the "nerve impulse" carries the information.  An impulse is actually a change in the cell's charge (an electrical signal).  Neurons have a resting potential of -70mV.  This is the charge of the cell when no information is being sent along it.  When a strong enough impulse arrives, the voltage gated sodium channels found in the cell's membrane open.  This causes the inside of the axon to become more positive than the outside.

Na-channel opening

The positive charge causes the neighbouring sodium channels to open, allowing the charge change to move along the axon.  Once the initial change in charge has occurred, the sodium channels close and potassium gates open.  Potassium is pumped out of the cell to bring the charge back to -70mV.  The region where this occurs is called the refractory region and a new impulse cannot occur there until it has returned to the resting potential.

Impulse conduction along the axon

At this point, there are more sodium ions inside the cell and more potassium outside the cell.  The sodium-potassium pump plays a role in balancing the difference and also in maintaining a resting potential of -70mV

All of what has just been explained is called the ACTION POTENTIAL, which is shown below in its graphical form.

On the graph, numbers 1 and 4 represent the resting potential, when the cell is not transmitting any information.  When the sodium channels open, the charge of the cell rapidly becomes positive as sodium ions rush in.  This is called depolarization.  Once the sodium channels close and the potassium channels open, the potassium rushes out, causes the charge to become negative again.  This is repolarization.  The return from hyperpolarization (the dip below -70mV) and the maintance of the resting potential is carried out by the sodium-potassium pump.

This video is a nice explanation of the action potential.  

Saltatory conduction refers to the rapid conduction of the impulse down a myelinated axon, where channels opening and closing only occurs in the nodes of Ravier.

Neurons have spaces between them.  They never actually touch the neighbouring cell.  The axon comes up close to the neighbouring cell's dendrite, but a space remains, which is the synapse.

Information crosses across the synapse in chemical form.  These chemicals are called neurotransmitters.  The neurotransmitter is released from the axon, it crosses across the cleft (space) and binds to a receptor on the dendrite.

The attachment to the receptor causes the action potential to begin and pass along the next cell, transmitting the impulse further.