Neurons and Neurotransmission

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OBJECTIVES:

Summarize the mechanisms of neurotransmission

Distinguish between antagonist and agonist mechanisms

Identify the Function and Dysfunction of Neurotransmitters

Anatomy of the Neuron = Nerve Cell

At birth, the human brain consists of an estimated 100 billion neurons

Neurons vary in structure, function, and genetic makeup. Given the sheer number of neurons, there are thousands of different types, much like there are thousands of species of living organisms on Earth.

In terms of function, scientists classify neurons into three broad types: sensory, motor, and interneurons.

Communication Pathways

Sensory/Afferent neurons

Sensory neurons are triggered by physical and chemical inputs from your environment affecting the sensory receptors. Sound, touch, heat, and light are physical inputs. Smell and taste are chemical inputs.

Two Paths

Spinal-Reflex Arc

Interneurons

Interneurons are neural intermediaries found in your brain and spinal cord. They’re the most common type of neuron. They pass signals from sensory neurons and other interneurons to motor neurons and other interneurons. Often, they form complex circuits that help you to react to external stimuli.

For instance, when you touch something hot, sensory neurons in your fingertips send a signal to interneurons in your spinal cord. Some interneurons pass the signal on to motor neurons in your hand, which allows you to move your hand away. Other interneurons send a signal to the pain center in your brain, and you experience pain.

Chemical-Electrical Neuronal Communication

Neurons send signals using action potentials. An action potential is a shift in the neuron’s electric potential caused by the flow of ions in and out of the neural membrane.

Action potentials can trigger both chemical and electrical synapses.

Chemical synapses

In a chemical synapse, action potentials affect other neurons via a gap between neurons called a synapse. Synapses consist of a presynaptic ending, a synaptic cleft, and a postsynaptic ending.

When an action potential is generated, it’s carried along the axon to a presynaptic ending. This triggers the release of chemical messengers called neurotransmitters. These molecules cross the synaptic cleft and bind to receptors in the postsynaptic ending of a dendrite.

Neurotransmitters can excite the postsynaptic neuron, causing it to generate an action potential of its own. Alternatively, they can inhibit the postsynaptic neuron, in which case it doesn’t generate an action potential.

Electrical synapses

Electrical synapses can only excite. They occur when two neurons are connected via a gap junction. This gap is much smaller than a synapse, and includes ion channels which facilitate the direct transmission of a positive electrical signal. As a result, electrical synapses are much faster than chemical synapses. However, the signal diminishes from one neuron to the next, making them less effective at transmitting.

Communication: Membrane & Action Potential

Neurotransmission Animation

Neurotransmitter - Function and Dysfunction

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