THE BRAIN FROM TOP TO BOTTOM

Anatomy by Level of Organization

Function by Level of Organization

By far the most common types of synapses are those between an axon and a dendrite (axodendritic synapses) and those between an axon and the cell body of a neuron (axosomatic synapses).

Other, less common kinds of synapses include axoaxonic synapses (involved in presynaptic inhibition phenomena) and dendrodendritic synapses.

SYNAPSES

A synapse is the junction point between two neurons.

However, a nerve impulse can also be transmitted from a sensory receptor cell to a neuron, or from a neuron to a set of muscles to make them contract, or from a neuron to an endocrine gland to make it secrete a hormone. In these last two cases, the connection points are called neuromuscular and neuroglandular junctions.

In a typical chemical synapse between two neurons, the neuron from which the nerve impulse arrives is called the presynaptic neuron. The neuron to which the neurotransmitters (chemical messengers) bind is called the postsynaptic neuron.

A presynaptic neuron has several specialized structures that distinguish it from a postsynaptic neuron.

The terminal button of the presynaptic neuron’s axon contains mitochondria as well as microtubules that transport the neurotransmitters from the cell body (where they are produced) to the tip of the axon.

(click on 2. Axonal Transport)

This terminal button also contains spherical vesicles filled with neurotransmitters. These neurotransmitters are secreted into the synaptic gap by a process called exocytosis, in which the vesicles’ membranes fuse with that of the presynaptic button.

The synaptic gap that the neurotransmitters have to cross is very narrow–on the order of 0.02 micron.

Across the gap, the neurotransmitters bind to membrane receptors: large proteins anchored in the cell membrane of the post-synaptic neuron. At this location, under an electron microscope, you can observe an accumulation of opaque material which consists of the cluster of receptors and other signalling proteins that are essential for chemical neurotransmission.

Any given neurotransmitter has several sub-types of receptors that are specific to it. It is the presence or absence of certain of these sub-types that causes a cascade of specific chemical reactions in the postsynaptic neuron. These reactions result in the excitation or inhibition of this neuron.

A neurotransmitter’s agonist is a molecule that has the same effect on the postsynaptic neuron as the neurotransmitter itself does.

An antagonist is a molecule that blocks the effect that the neurotransmitter normally has on the post-synaptic neuron.

It was long thought that a given neuron released only one kind of neurotransmitter. But today, many experiments show that a single neuron can produce several different neurotransmitters.

Neurons that use GABA and glutamate as neurotransmitters are used by more than 80% of the neurons in the brain and constitute the most important inhibition and excitation systems, respectively, of the substantia nigra pars compacta (SNc).

NEUROTRANSMITTERS

This section describes a few of the best known neurotransmitters that are involved in many functions in both the central and the peripheral nervous systems. Apart from acetylcholine, they all belong to the family of amines or amino acids.

Neurotransmitter

Example of Disorder Involving It

Molecular Structure

Acetylcholine is a very widely distributed excitatory neurotransmitter that triggers muscle contraction and stimulates the excretion of certain hormones. In the central nervous system, it is involved in wakefulness, attentiveness, anger, aggression, sexuality, and thirst, among other things.

Alzheimer’s disease is associated with a lack of acetylcholine in certain regions of the brain.

Dopamine is an inhibitory neurotransmitter involved in controlling movement and posture. It also modulates mood and plays a central role in positive reinforcement and dependency.

The loss of dopamine in certain parts of the brain causes the muscle rigidity typical of Parkinson’s disease.

GABA (gamma-aminobutyric acid) is an inhibitory neurotransmitter that is very widely distributed in the neurons of the cortex. GABA contributes to motor control, vision, and many other cortical functions. It also regulates anxiety.

Some drugs that increase the level of GABA in the brain are used to treat epilepsy and to calm the trembling of people suffering from Huntington’s disease.

Glutamate is a major excitatory neurotransmitter that is associated with learning and memory.

It is also thought to be associated with Alzheimer’s disease, whose first symptoms include memory malfunctions.

Norepinephrine is a neurotransmitter that is important for attentiveness, emotions, sleeping, dreaming, and learning. Norepinephrine is also released as a hormone into the blood, where it causes blood vessels to contract and heart rate to increase.

Norepinephrine plays a role in mood disorders such as manic depression.

Serotonin contributes to various functions, such as regulating body temperature, sleep, mood, appetite, and pain.

Depression, suicide, impulsive behaviour, and agressiveness all appear to involve certain imbalances in serotonin.

Scientists have now identified some 60 different molecules that meet the criteria for being regarded as neurotransmitters.