The Nervous System: Neurons
Nervous and Endocrine systems
Neurons are cells that transfer stimuli to other cells.
- contains nucleus and organelles
Dendrites - receive input
Axon - conducts impulses away from the cell body
Axon hillock - an enlarged region where an axon attaches to the cell body
Synaptic terminal - Neurotransmitters are manufactured in the cell body but released from synaptic terminals. The neurotransmitters stimulate other neurons.
Synapse - A synapse is the junction between the synaptic terminal and another cell. The other cell is called a postsynaptic cell.
Nerves and Ganglia
Axons and dendrites are bundled with axons or dendrites from other neurons to form nerves. Clusters of neuron cell bodies are called ganglia.
Central and Peripheral Nervous Systems
Neuroglia (also called glia) are cells within the nervous system that are not neurons.
There are different kinds of neuroglia, and they provide neurons with insulation, physical support, metabolic assistance, and protection.
MyelinationSome neuroglia function to provide insulation for axons or dentrites. They do so by wrapping around the long fibers.
The insulation properties come from myelin contained within the cells.
The layer of insulation is referred to as a myelin sheath.
If these insulating cells are located in the peripheral nervous system, they are called Schwann cells.
Terms that are used to describe structures found in both the CNS and PNS
You will be responsible for learning the terms used for the peripheral nervous system in the table below. Be aware that these structures have a different name if they are located within the central nervous system.
Membrane potentials were first demonstrated using the giant axons of a squid (1mm dia). An oscilloscope measured the electrical difference by placing one electrode outside the neuron and the other inside the neuron.
The sodium-potassium pump pumps out 3 sodium ions (Na+) for each 2 potassium ions (K+) pumped into the neuron. This results in more potassium ions inside and more sodium ions on the outside.
Unequal pumping (3 Na+ out to 2K+ in) results in more positive charge on the outside compared to the inside. The membrane is polarized.
Some K+ channels are open so K+ tends to leak out. This contributes to the negative charge inside. The charge difference prevents further leakage.
The charge difference is measured in millivolts.
The membrane contains channels that open or close, allowing the polarity of the membrane to change as ions pass through the channel.
Ligand-gated channels are found in the synapses on postsynaptic cells. They open when bound to specific ligands (molecules or ions) such as specific neurotransmitters.
Voltage-gated channels open when the membrane becomes depolarized. For example, sodium gates open and then close slowly when the membrane is depolarized but remain closed when it is polarized. When the sodium channel is open, sodium can pass through.
In a resting (polarized) neuron, sodium gates are closed. A slight depolarization will not cause the gates to open but if the depolarization is greater than a threshold value, the gates will open.
Propagation of an Action Potential
As mentioned earlier, stimulation of the neuron causes Na+ gates open allowing Na+ to rush in. This results in depolarization of the membrane in the area where the stimulation occurred. If the depolarization is sufficient, it will depolarize adjacent areas of membrane causing more Na+ gates to open, thus spreading the depolarization.
Immediately after depolarization, Na+ channels close and K+ channels open causing K+ to flow out. This process returns positive charge to the area just outside the membrane, thus restoring the resting polarity.
The depolarization and repolarization events described above are called an action potential. During an action potential, the depolarization spreads to neighboring areas of the neuron, regenerating the action potential. Depolarization continues to spread all the way to the action terminal where the axon joins another cell.
Action potentials are "all or nothing." The intensity of an action potentials does not diminish as depolarization spreads along an axon.
Action potentials are initiated when depolarization reaches a threshold level. In typical mammalian neurons, a depolarization to -55 mV produces an action potential.
The sodium-potassium pump operates continuously to restore the ionic gradient.
In the diagram below, depolarization caused by the influx of sodium can be seen spreading to the right.
The diagram below shows the voltage difference across the membrane during an action potential. Initially, the inside of the membrane is approximately -65 or -70 millivolts compared to the outside. When sodium gates open and sodium ions rush in, the inside temporarily becomes positively charged. Potassium gates then open and potassium ions rush out, restoring the negative charge.
Saltatory Conduction and Neuron Diameter
The gap between the Schwann cells in the myelin sheath is called a node of Ranvier. Gated channels are concentrated in this area and not in the area under the myelin sheath. Action potentials are regenerated at the nodes but not in the area underneath the myelin sheath. The result is that the depolarization events spread farther, increasing the speed at which they spread.
The action potential spreads from node to node (saltatory conduction) causing it to spread faster.
Sodium-potassium pumps require a substantial amount of energy to pump the ions, so the presence of insulation reduces the amount of membrane that requires active sodium-potassium pumps, thus saving energy.
The diameter of the neuron also is related to the speed of conduction. Larger diameter axons conduct faster. Example: squid axons are 500 microns dia.
A synapse is a junction between a neuron and another cell. It is separated by a synaptic cleft.
In most synapses, the axon terminal of the presynaptic cell contains numerous synaptic vesicles with neurotransmitter stored within them.
The action potential causes calcium channels to open in the plasma membrane of the presynaptic cell. The calcium ions (Ca++) diffuse into the neuron and activate enzymes, which in turn, promote fusion of the neurotransmitter vesicles with the plasma membrane. This process releases neurotransmitter into the synaptic cleft.
Neurotransmitter molecules diffuse across the cleft and stimulate the postsynaptic cell, causing Na+ channels to open and depolarization of the postsynaptic cell.
The depolarization of the postsynaptic cell is referred to as a synaptic potential.
The magnitude of a synaptic potential depends on:
After the neurotransmitter is released into the synaptic cleft, it must be quickly removed or inactivated to prevent the postsynaptic cell from being continuously stimulated and to allow another synaptic potential.
Excitatory and inhibitory postsynaptic potentials
A synaptic potential can be excitatory (they depolarize) or inhibitory (they polarize). Some neurotransmitters depolarize and others polarize. The diagram below shows that a hyperpolarized membrane requires more stimulation to initiate an action potential.
There are more than 50 different neurotransmitters.
In the brain and spinal cord, hundreds of excitatory potentials may be needed before a postsynaptic cell responds with an action potential.
Synaptic integration is the combining of excitatory and inhibitory signals acting on adjacent membrane regions of a neuron.
In order for an action potential to occur, the sum of excitatory and inhibitory postsynaptic potentials must be greater than a threshold value.
Temporal and Spatial Summation