I want to make a few posts that will help psychology students or other people interested in the brain and how it works. These are just basic so don't expect them to be extremely detailed because they are only meant for beginners (like me); also, they are not meant for learning brain surgery :) When I will finish the neuron series I will also post a map that will help you even more with your recap.
The neuron is the core component of the nervous system. It is an electrically excitable cell that processes and transmits the information through chemical and electrical signaling. It is formed of a cell body and protoplasmic processes that extend from that cell body.
There are three types of neuron: unipolar, bipolar, and multipolar.
Unipolar neurons (sensory neurons which conduct impulses into the central nervous system) have one process from the cell body which splits into longer processes. They are found in the spinal ganglia and cranial nerves.
Bipolar neurons (also sensory neurons) have two processes: one axon and one dentrite. For example, bipolar neurons are found in the retina of the eye, the ganglia of the vestibulocochlear nerve and bipolar cells that transmit motor signals to control muscles.
Multipolar neurons have multi processes and they have many dentrites and a single axon. Their function is motor or as associations (they conduct impulses and permit communications between neurons in the central nervous system)
The cell body contains
the nucleus (with nuclear envelope containing chromatin, nucleolus, nucleoplasm, ribosomes and nuclear pore)
- cytoplasm (a thick liquid between cell membrane holding all the cell’s internal sub structures which are called organelles) which contains mitochondria, Golgi apparatus, lysosome, rough and smooth endoplasmic reticulum, vacuole, vesicle and cytoskeleton.
- cell membrane
The protoplasmatic processes have dentrites and axons. Dentrites are short and transport the impulses to the cell body.
Axons have varied length to a few nanometers to 1 meter (like the axons of the sciatic nerve); they make contact with other cells like other neurons, but also muscles or gland cells through synapses.
There are two types of axons:
- myelinated axons and unmyelinated axons
Myelin is a dielectric material composed of 40 % water; however the dry mass is about 70-85% lipids and 15%-30% proteins. If the myelin layer is think the transport is faster.
Axons contain two types of axoplasmic transport:
- anterograde transport (from the cell body to the synapse)
- retrograde transport (sends chemical messages and endocytosis products headed to endolysosomes from the axon back to the cell) is practically the way toxins and vitamins are transported to the central nervous system
Neuron Part 2 – Synapses
A synapse is a junction that permits a neuron to pass an electrical or chemical signal to another cell.
Two neurons are connected through a synapse. The synaptic cleft is what separates the presynaptic neuron and the postsynaptic one. Synthesized neurotransmitters are sent in the synaptic cleft and this produces feedback at a postsynaptic level. When there is an action potential at the synaptic terminal (a bulb at the end of an axon in which neurotransmitter molecules are stored and released), it releases neurotransmitters into the synaptic cleft. The receptors in the membrane or the cell body of the postsynaptic neuron receive the neurotransmitters from the synaptic cleft.
The axon terminal contains synaptic vesicles (about 39.5 nanometers) that play an important role for propagating nerve impulses between neurons and are constantly recreated by the cell.
In most synapses the presynaptic part is located on the axon but there are some presynaptic parts located on a dendrite or soma.
There are many types of synapses and here you can see them grouped into 5 types:
Excitatory Ion Channel Synapses – synapses with neurorecepters that are sodium channels.
Inhibitory Ion Channel Synapses – synapses with neuroreceptors with chloride channels/ typical neurotransmitters are glucine or GABA
Non Channel Synapses – neuroreceptors that are not channels but are membrane- bound enzymes
Neuromuscular Junctions – synapses formed between motor neurons and muscle cells
Electrical Synapses – the membranes of the two cells actually touch and share proteins in these synapses so the action potential passes directly from one membrane to the next (can only be found in the heart and eye)
Tomorrow we'll learn about neurotransmitters and I will post the final recap map.