What are the structures of dendrites that receive the neurotransmitters?

Information from one neuron flows to another neuron across a synapse. The synapse contains a small gap separating neurons. The synapse consists of:

1. a presynaptic ending that contains neurotransmitters, mitochondria and other cell organelles
2. a postsynaptic ending that contains receptor sites for neurotransmitters
3. a synaptic cleft or space between the presynaptic and postsynaptic endings.

Hear IT!: Axon | Dendrite | Myelin | Neuron | Neurotransmitter | Soma | Synapse | Vesicle

Electrical Trigger for Neurotransmission

For communication between neurons to occur, an electrical impulse must travel down an axon to the synaptic terminal.

Neurotransmitter Mobilization and Release

At the synaptic terminal (the presynaptic ending), an electrical impulse will trigger the migration of vesicles (the red dots in the figure to the left) containing neurotransmitters toward the presynaptic membrane. The vesicle membrane will fuse with the presynaptic membrane releasing the neurotransmitters into the synaptic cleft. Until recently, it was thought that a neuron produced and released only one type of neurotransmitter. This was called "Dale's Law." However, there is now evidence that neurons can contain and release more than one kind of neurotransmitter.

Diffusion of Neurotransmitters Across the Synaptic Cleft

The neurotransmitter molecules then diffuse across the synaptic cleft where they can bind with receptor sites on the postsynaptic ending to influence the electrical response in the postsynaptic neuron. In the figure on the right, the postsynaptic ending is a dendrite

(axodendritic synapse), but synapses can occur on axons (axoaxonic synapse) and cell bodies (axosomatic synapse).

When a neurotransmitter binds to a receptor on the postsynaptic side of the synapse, it changes the postsynaptic cell's excitability: it makes the postsynaptic cell either more or less likely to fire an action potential. If the number of excitatory postsynaptic events is large enough, they will add to cause an action potential in the postsynaptic cell and a continuation of the "message."

Many psychoactive drugs and neurotoxins can change the properties of neurotransmitter release, neurotransmitter reuptake and the availability of receptor binding sites.

Types of Synapses

"You are your synapses. They are who you are."
--- Joseph LeDoux, 2002 (in Synaptic Self)

See some synapses "Up Close and Personal".

Play the Interactive Word Search Game on the neuron and neurotransmitters. Play an Outside Game to reinforce what you have learned about the synapse. Color the synapse online: Picture 1| Picture 2

The brain contains many billions of neurons that work together to produce sensation, thought, learning, movement, emotion, and many other processes. The coordination of these activities requires rapid and extensive communication among individual neurons and tissues (e.g. muscles). In order to achieve this, neurons use electrical signals to transmit information within a single cell and chemical signals between cells. These unique functions have forced the neuron to adopt a cell structure unlike that of other cells.

Neurons comprise a , , and an  that ends at a terminal. The cell body contains the nucleus and the machinery necessary to synthesize proteins. The cell body is also the region of the neuron in which an electrical impulse is generated. Extending from the cell body are short, branched dendrites which receive chemical signals from other neurons or stimuli that initiate an electrical signal. This electrical impulse (or ) propagates from the cell body, along the axon toward its terminal. The axon is an elongated fiber that transmits the impulse by altering the flow of sodium and potassium ions across the neuronal membrane. Many axons are surrounded by a  composed of lipids and proteins. Like insulation coating an electrical wire, this fatty layer greatly increases the speed of electrical impulses down the axon.

Though the nerve terminal of one neuron is in close proximity to the dendrites of an adjacent cell, the cells are actually separated by a small space; this connection between the two cells is called a . The synapse represents a true gap between cells; there is no sharing of cytoplasm or cell structures between the pre-synaptic and post-synaptic cells. Communication between neurons is a chemical process that uses neurotransmitters in a process called synaptic transmission.

The neuron consists of a cell body, dendrites, and an axon. Information flows from the dendrites to the cell body, and then on down the axon to its terminal.

Neurotransmission

When an electrical impulse travels down the axon to the nerve terminals, it triggers the movement of vesicles in the terminal to release their contents, chemicals known as . After release, the neurotransmitters diffuse across the synaptic space and bind to  on the dendrites of post-synaptic cells. The binding of a neurotransmitter to its receptor is specific. Just as a key fits only a certain lock, a neurotransmitter binds only to a certain type of receptor.

There are many types of neurotransmitters in the brain, each having a unique function. The interaction between the receptor and the neurotransmitter produces chemical and/or electrical changes in the post-synaptic cell depending on the exact neurotransmitter bound. Excitatory neurotransmitters promote the propagation of the electrical signal in the receiving cell whereas inhibitory neurotransmitters dampen the transmission of the electrical signal. If the neurotransmitter triggers an action potential in the post-synaptic neuron, the communication process continues. Just a fraction of a second after binding to their receptors, neurotransmitters may be broken down by enzymes or recycled back into the pre-synaptic cell.

An example of neurotransmission is shown for the neurotransmitter acetylcholine binding to acetylcholine receptors. Used with permission from “Animated Neuroscience and the Actions of Nicotine, Cocaine, and Marijuana in the Brain” (www.films.com)

What is the structure that receives neurotransmitters?

What is the structure at a dendrites?