Neuron

last authored:
last reviewed:

 

 

Introduction

The perhaps 10¹¹ neurons in the human brain are specialized for receiving, integrating, and sending signals.

 

Neurons can be classified according to location, target, geometry, and number of processes.

Prinicpal neurons have long axons that connect to othe rparts of the nervous system.

 

Neurons receive information from up to tens of thousands of other neurons, processing and integrating them into a single new message. Neurons also have the capacity to remember information that flows through them.

return to top

 

 

 

Structure

The cell body, or perikaryon, contains the nucleus, protein-synthesizing machinery, and microtubules for their transplrt.

 

Dendrites are the primary site for a cell to receive information. They are full of receptors that respond to neurotransmitters. Excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) combine in a graded manner. The sum of these signals is communicated to the cell body, where other inputs are communicated. Once threshold is reached, action potentials fire. Their timing is used to communicate a cell's thoughts.

Some dendrites, such as those of Purkinje cells in the cerebellum, have voltage-gated Ca2+ channels and can produce action potentials.

 

Axons are long (up to 1m) projections that leave the cell body. They begin in an unmyelinated initial segment.

The typical axon contains up to 1000x cytoplasm as the cell body and thus has special mechanisms to sustain it. It is packed with microtubules and microfilaments that provide stability and a means to rapidly transport materials to and from the cell body.

Some axons are covered with myelin, greatly increasing signal speed.

 

Axon transport

Materials are transported down axons in vesicles along microtubules with the help of a motor protein called kinesin at up to 1m/day, in an ATP-dependent manner. Soluble proteins move down neurofilament and microtubules much more slowly, at 1-5 mm/day.

Dyenin moves material from the axon terminal back to the cell body.

 

 

 

Synapses

There is bidirectional communication at almost every synapse to maintain or modulate synaptic activity.

1. Na influx during depolarization
2. Ca influx (how?) at the synapse
3. vesicle fusion
4. vesicle recycling

 

Synapse Locations

Synapses are located on the cell body and dendrites.

 

At the Neuromuscular Junction (NMJ) the whole thing is encased by Schwann cells.

ACh esterase is attached to the basal lamina in synaptic invaginations.

 

Pre-synaptic terminal

Calcium release induces vesicle fusion with the membrane, mediated by SNARE proteins. Botulinum and tetanus toxins paralyze SNARE proteins (check)

 

presynaptic inhibition (often GABAB receptors) can occur via phosphorylation of calcium channels, preventi

 

Post-synaptic terminal

The post-synaptic membrane is frequently amplified through invagination or outfolding of the membrane. This significantly increases the receptor surface area.

 

ACh Receptors are affected in the autoimmune disease Myasthenia gravis; curare also affects the AChR.

 

Neurotransmitter removal

• degradation (ie AChE), principally at the NMJ
• diffusion
• uptake, principally by glial cells and pre-and post-synaptic cells. This is the most common

 

Post-synaptic responses

a fast epsp is 10 ms

 

ionotropic mechanisms (fast)

the receptor is the channel

• nicotinic ACh
• glutamate (AMPA, NMDA, kainate)
• GABA_{A , C}
• glycine
• 5-HT3

metabotropic mechanisms

coupled to a G protein and any number of signaling mechanisms (typically IP3, cAMP, and the alpha subunit of the G protein)

• catacholamines (dopamine, norepinephrine)
• serotonin
• muscarinic ACh
• peptides
• metabatropic glutamate
• GABA_B

drugs or toxins that enhance transmission

• L-dopa
• amphetamine
• benzodiazapines
• morphine
• fluoxetine
• cocaine

 

drugs or toxins that depress transmission

• interfere with neurotransmitter synthesis or packaging
• interfere with neurotransmitter release
• by blocking neurotransmitter-gated ion channels
• by affecting GPCRs

 

 

Gap Junctions

6 connexins form a hemichannel, while a hemichannel in each cell form a gap junction channel

 

 

 

Neuronal Migration

Neurons migrate using cell adhesion molecules, incouding N-CAMs and cadherins.

Matrix molecules such as liminin and fibronectin also assist migrating cells.

Radial glial cells span from ventricles through the length of the developing cortex, and neurons migrate along them.

 

 

 

Regeneration in the Nervous System

Most neurons are formed in the first three months of life.

After birth, neurons do not divide, except for olfactory bulb neurons, which are constantly renewed through a population of neural stem cells.

 

Glial cells can proliferate if needed. The typical reaction to brain injury is formation of an astrocytic glial scar, mediated primarily by hypertrophy and to a lesser extent proliferation. Microglial cells proliferate a fair amount at sites of injury.

Axons in the peripheral nervous system can slowly regenerate and reconnect to sensory and muscle targets.

CNS axons, on the other hand do not recover effectively. This appears due to the neuronal microenvironment, including the inhibitory myelin-associated glycoprotein.

 

 

 

Resources and References

return to top