The Neuron

Special Characteristics of Neurons

Neurons are highly specialized for their job of conductionThe transmission of nerve impulses along neurons. electricity, also called an action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission..  Neurons are long lived in your body, and thank goodness because they do not like to do mitosis. So, if a neighboring neural cell dies, its neighbor won’t do mitosis and replace it.  Neurons use  a lot of energyThe capacity to do work or cause change..  They need a lot of ATPThe energy currency of cells used for muscle contraction. and their little mitochondria need glucoseA simple sugar that is the main source of energy for cells. and oxygen to make it.  This is why you have to eat when you study.  No joke.

Neurons

Neurons can be divided up into three areas: receiving, propagating, and sending.  Part of the neuron is called the cell bodyThe central part of a neuron containing the nucleus and organelles..  It contains the nucleusThe control center of the cell that contains DNA and directs cellular activities. and all the other organellestructures within a cell that perform specialized functions. that would be needed by a cell that makes proteinsLarge molecules made of amino acids with various functions in the body..  In the case of a neuron, its proteins are neurotransmittersChemicals released by neurons to transmit signals across a synapse..  Specific to neuronsThe functional cells of the nervous system that transmit signals. are these histological structures called Nissl bodies.  These are just a specific endoplasmic reticulum that takes up a dark stain.  Once made in the cell body (or soma or perikaryon), the neurotransmitters are produced. They are then transported to the axon terminalThe endpoint of an axon where neurotransmitters are stored and released into a synapse. where they are stored.  These axon terminals are like the sending portion of the neuron.  It is here that an action potential in this neuron will transfer to another neuron.

These branches coming off the cell body are called dendritesBranch-like extensions from a neuron that receive signals from other neurons..  These are like antenna that are made for receiving signals from other neurons.  On these dendrites are tons of ligand-gated channelsProtein passages in the cell membrane that allow specific molecules to pass through. that open with neurotransmitters from another neuron.  This area between the receiving and sending portions of the neuron is called the conducting or the propagation area.  This is the axon, which is similar to an electrical cord, wrapped in insulation.  These little yellow pillow-looking things are myelin sheaths that basically smother the axon.  This little bump here is the cell the nucleus of the cell that makes these sheaths.  Between the sheaths are exposed areas of the axon called Nodes of Ranvier.

Information or action potentials are supposed to flow from cell body to axon terminals.  This type of movementA fundamental property of life involving motion of the body or its parts. is called anterograde.  Neurotransmitters made in the cell body move anterograde to the axon terminals.  If something moves in the other direction, that type of movement is called retrograde movement.  If you get bit by a rabid animal, you could become infected with the rabies virus.  This virus will enter into neurons nearby to the bite.  It could eventually make its way to the brain by traveling retrograde in a motor neuron.  Rabies is a central nervous systemComposed of the brain and spinal cord; integrates and processes information.  disease as its most damaging effects are done on the brain and spinal cordThe central nervous system structure that relays signals between the brain and body..

Neuron Shapes

Neurons come is many shapes.  The most common type of neuron is the one on the left which is called a multipolar neuronA neuron with one axon and multiple dendrites; the most common type in the CNS..  I feel as though it looks like a tree with roots and branches.  We will use this neuron as our example neuron always in this class.  These types of neurons are the most common in the brain and are also motor neuronsNeurons that carry outgoing information from the CNS to effector organs. in the PNS.

This next neuron is called a unipolar neuronA neuron with one process that splits into peripheral and central branches, common in sensory neuron.  In the PNS many sensory neuronsNeurons that carry information from sensory receptors to the CNS. are unipolar.  The cell body of these types of neurons is located in the middle of the axon. It does not hang off one end of the axon.

These last three neurons here are called bipolar neurons.  They have no correlation to the condition of bipolar disease.  The word bipolar is applied to these neurons because of their shape.  Bipolar refers to the orientation of the cell body that is continuous with the axon.  The cell body isn’t hanging off the axon, it’s smack in the middle of it.

Ion Channel Distribution

Now that you are familiar with the terms that describe the anatomyThe study of the structure of the human body. of a neuron, it’s important for us to understand the types of membrane channels. We also need to learn about their distribution in the membrane and of the neuron.  Leak channels are everywhere.  So are the sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function./potassium pumps that return the leaked ionsCharged atoms or molecules. to their original place.  Ligand gated channels are only located at points of contact. This is specifically where this neuron meets the axon terminal of another neuron. For our purposes with this multipolar neuron, that would be all these dendrites here.  They will receive the neurotransmitters that another neuron puts out into the synapseThe junction between two neurons where communication occurs..  We are somewhat familiar with that process. We learned about the movement of acetylcholinealso know as ACh A neurotransmitter that stimulates muscle contraction. across the synapse. This movement results between a neuron and a skeletal muscle.  With neurons, there’s a lot more going on.  Voltage gated channels are present along the axon hillockCone-shaped region of the neuron where the axon originates and where action potentials begin. and the axon. They are also located between the yellow myelin sheaths and down the axon terminals.  These voltage channels all depolarize in a coordinated fashion.  If we generate enough depolarizationThe loss of electrical charge across a membrane, triggering an action potential. with these ligand-gated channels, the action potential can hop, skip, and jump down the axon.

Nodes of Ranvier

Yes, action potentials, or moving electricity, hops skips and jumps along the axon.  These purplish pillows are myelin sheaths made by Schwann cellsPNS glial cells that form the myelin sheath around peripheral nerve fibers..  So this neuron is in peripheral nervous systemPNS All nervous tissue outside the CNS; includes nerves and ganglia., most likely a motor neuron, which are commonly multipolar.  These myelin sheaths smother the axon and the membrane channels.  But, there are these gaps between them were the axon membrane channels are exposed.  These exposed areas are called Nodes of Ranvier.  This histological picture on the right shows some Nodes of Ranvier.  These axons are oriented from the top left to the bottom right of the picture.  The open light pink spaces are the myelin sheaths, cut in the same direction as the diagram on the left.  These pinched spaces are the Nodes of Ranvier.