The Function & Anatomy of Neurons

What is a Neuron? It is the cell of nerve tissue that is

responsive and conducts impulses within the Nervous System at high rates of speed.

They are the primary structural and functional unit of the nervous system.

All Neurons are Made of Three Basic Parts

Cell Body Dendrites A Single Axon

Neuron Parts Cell Body- Main part of neuron, cytoplasm

with organelles, a nucleus, and a plasma membrane.

Processes: Axon-Conducts impulses away from the cell

body, only one per neuron (may have side branches), often has many small branches at terminal end.

Dendrite- Conducts impulses towards the cell body, thin branching extensions.

Parts Continued..

Nucleus- control center of the cell.Neurofibrils- Cytoskeleton of the

cell.Nissl Bodies-Type of rough ER

that performs metabolic activities.Collaterals- Side branches of

axon.Axon Hillock- Enlargement at

beginning of axon.

Parts Continued Again…Axon Terminals- branching ends of

axon.Myelin Sheath- Insulated covering

over axon, increases impulse speed.Schwann Cells- Cells that wrap

around axon to make myelin sheath.Nodes of Ranvier- Unmyelinated

gaps on axon.

Neurons Structures

Impulse Propagation of a Myelinated Neuron

The insulation provided by the myelin sheath forces the impulse to jump the myelin to each node of Ranvier.

This speeds up the propagation and is called saltatory conduction.

Different Types of Neurons

Multipolar- Has many dendrites surrounding the cell body has a single axon (CNS Skeletal Muscle).

Bipolar-Has a single dendrite and single axon arising from the cell body (found in eyes, ears, nose).

Unipolar- Single nerve fiber extending from the cell and that splits in two branches, one extending to the spinal cord (axon), the other to the P.N.S. (dendrite).

Neuroglia and Associated Structures

Neuroglia- Cell that supports and maintains neuron structure AKA (glial cell).

Astrocytes- Star shaped cell that attaches neurons to their blood vessels.

Oligodendrocytes- Support neurons and produce a fatty myelin sheath around axon in CNS.

Microglia- Also called brain macrophages, engulf and destroy microbes.

Ependymall Cells- Single layer ciliated cells that line the ventricles of the brain and central canal of the spinal column.

Satellite Cells- Surround cell body and aid in controlling chemical environment.

Neurons Have Three Different Types of Function Sensory(Afferent)- Carry impulses

from the peripheral to the central nervous system (unipolar).

Association- Carry impulses between neurons within the central nervous system(multipolar).

Motor(Efferent)- Carry impulses from the central nervous system to any part of the body capable of responding. (most are multipolar).

Impulse Propagation The Neuron is at rest. At rest the neuron has a resting membrane

potential (+ outside, - inside). An impulse comes from the brain, another

neuron etc. Ion channels open and move Na+ in to the cell. This changes the balance of charges on the

neuron and causes the electrical charge on the inside of the neuron to become more positive.

Action Potential and the conduction of nerve impulse The wave of ionic reversals create the

action potential which conducts the nerve impulse along the neuron.

Soon after the depolarization the membrane begins to repolarize.

The rapid depolarization followed be repolarization creates the Action Potential and conducts the impulse.

Transmission of Nerve Impulse Can happen neuron to neuron, neuron to

muscle etc. In nerve to nerve transmission the impulse

crosses the nerve junction (synapse). Presynaptic neuron that which sends the

impulse. Postsynaptic neuron that which receives

the impulse across the synapse. The impulse carries down the presynaptic

neuron to the synaptic end bulb.

Transmission of a Nerve Impulse Continued The postsynaptic neuron has a

concaved surface that creates a gap (synaptic cleft) between the synaptic bulb and the postsynaptic neuron.

Once the impulse reaches the bulb the synaptic vesicles of the bulb move toward the bulb membrane.

At the membrane the vesicles open and release the neurotransmitters.

These chemicals travel across the cleft and attach to the receptor sites of the postsynaptic membrane.

Impulse Transmission

This figure is a graphic representation of nerve to nerve impulse transmission