The basic unit of the nervous system is the cell it is referred to as a neuron. Some common characteristics of all neurons are that they are sensitive to oxygen deprivation, they are specialized for the conduction of impulses and they do not multiply. Billions of brain cells are formed in the first months of fetal development, half of them die as hormones and other stimuli eliminate and organize them to form the brain’s basic scaffolding male female etc. After birth trillions of brain cell connections are established to form the brain’s basic maps or systems governing such things as vision hearing and sense of smell.
From age 4 to 10 new learning re-organizes and reinforces connections between brain cells. After age 10 a process of mylenisation begins.
The typical cell body has a nucleus and projecting from that many dendrites that convey impulses to the cell body. It also has a longer process called the axon that conducts impulses away from the cell body and toward the dendrites of other neurons and to muscles and glands. The site of contact between the axon and the dendrite is called the synapse. The site of contact between the axon and a muscle fiber is the motor end plate. In most cases the electro chemical impulse does not pass directly from neuron to neuron but is transmitted across the synaptic gap by neurotransmitters. The most common of these is acetylcholine, others include dopamine, serotonin, and gammaaminobutyric acid referred to as GABA.
There are many different types of neurons. A motor or efferent neuron transmits impulses to muscles whereas a sensory or afferent neuron takes impulses from the peripheral tissue toward the CNS. Projection neurons have long axons that take information from one area of the brain to another.
When a neuron is activated by other neurons an action potential is initiated. This electrical storm begins at the trigger zone where the nucleus and the axon join. Neurons are at rest when there is not sufficient stimulation for them to fire. They require many inputs from other dendrites in order to fire.
When enough stimulation is received an action potential begins which means that the charge progressively changes along the axon from negative to positive. When the positive charge reaches the terminal a chemical neurotransmitter is released.
The site of contact between the axon of one neuron and the dendrite of another neuron is the synapse.
The contact between the axon and the dendrite occurs across the synaptic gap. This takes place by neurotransmitters. The neurotransmitters are either excitatory or inhibitory. They are discharged across the synaptic gap to receptacles in the dendrite.
This in turn creates a change in electrical current in the dendrite that moves toward the postsynaptic cell body.
The primary sequence of events in neuronal firing is electrical-chemical-electrical. However that does not fully explain the process as there are secondary messengers that also play a significant role in the creation of neural pathways. However much of what the brain does is translate experience into electro-chemical sequences.
Neurotransmitters are the chemicals that allow neurons to communicate across the synaptic gap. There are numerous neurotransmitters, among them serotonin, dopamine and acetycholine. Two of the most important actors at the synaptic gap are glutamate and gammaaminobutyric acid. These are of great importance because they act as inhibitory or excitatory factors in neuronal circuits.
Neurons are grouped into circuits and systems for instance there are many circuits in the visual system. There are two principal types of neuron: The interneuron and the projection neuron. The projection neuron tends to have long axons and often takes impulses from one system to the next. The interneuron tends to have shorter axons and is generally found within circuits.
Glutamate is the principal excitatory neurotransmitter in the brain. GABA is the principal inhibitory neurotransmitter. GABA cells control the flow of information passing through a given area of the brain.
They are the fastest acting neurotransmitters and work by either bridging the synaptic gap for other chemicals to cross or preventing it from being bridged.
If glutamate has opened a synaptic circuit other forms of neurotransmitters can cross the synaptic gap. These neurotransmitter modulators include monoamines, peptides and hormones. Serotonin, dopamine, epinephrine and norepinephrine are monoamines. The cells that produce monoamines are only found in the brain stem, yet their axons extend to many areas of the brain. Monoamines tend to be involved in general changes of state within the brain rather than alteration in small circuits. Many drugs that affect mood changes alter monoamine levels. For instance SSRI medications work by preventing the reuptake of Serotonin at the synaptic gap.
Acetylcholine is a crucial neurotransmitter involved in nerve control and muscle movement. Peptides are primarily amino acids and are often opiates. There are a great many Peptides functioning in the brain. Endorphins are peptides.
Hormones are typically released from body organs for example adrenaline from the adrenal gland. Estrogen and Testosterone from the sex glands. These hormones are secreted into the blood stream where they travel to the brain. They have direct affect on the transmission of glutamate or GABA.
—Peter Nadin
