Synaptic transmission is the process by which a nerve impulse passes across the synaptic cleft from one neuron to another.
It is a method neurons use to communicate, enabling the transmission of information both within a neuron (via electrical signals) and between neurons (via chemical signals).
Signals between neurons occur via synapses, which are specialised connections with other cells.
Here’s a breakdown of the structures and processes involved:
The Structures of synaptic transmission
Neurons do not make direct contact with each other.
Instead, there is a very small gap between them called a synapse.
Within the synapse, there is a microscopic gap known as the synaptic cleft.
Information is transmitted across this gap from a transmitting neuron, known as the presynaptic neuron, to a receiving neuron, called the postsynaptic neuron.
The structures include:
- Presynaptic membrane: The nerve ending of the transmitting neuron.
- Postsynaptic membrane: The membrane of the receiving neuron.
- Synaptic cleft: The tiny gap between the presynaptic and postsynaptic membranes.
- Neurotransmitters: Chemical messengers stored in packets called vesicles within the presynaptic terminal (axon terminals).
- Postsynaptic receptor sites: These are specific molecules on the membrane of the postsynaptic neuron that neurotransmitters bind to.
The Process of Synaptic Transmission
Synaptic transmission is the process by which nerve cells (neurons) communicate with each other. It involves sending chemical signals across a tiny gap between neurons called the synapse.
- Action potential arrives at axon terminal: An electrical signal called an action potential travels along the neuron until it reaches the end part, known as the presynaptic terminal or button.
- Calcium channels open and enters the presynaptic neuron: When the action potential arrives at the presynaptic membrane, it causes depolarisation, opening voltage-dependent calcium ion channels and leading to an influx of calcium ions (Ca²⁺) to flow into the axon terminal.
- Calcium signals to neurotransmitter: The increase in calcium concentration sends a signal that triggers vesicles containing neurotransmitters. Neurotransmitter-filled vesicles move to the presynaptic membrane and prepare to release their contents.
- Diffuse across the synapse: The neurotransmitters move across the synaptic cleft (the small gap between neurons) by diffusion and attach to special sites on the next neuron called receptors. The neurotransmitter chemicals diffuse across the synaptic cleft, moving down a concentration gradient.
- Receptor binding and specificity: Neurotransmitters released from the presynaptic neuron attach to specific receptor molecules on the postsynaptic neuron. These receptors selectively bind only to their matching neurotransmitter chemicals.
- Channel opening: When neurotransmitters bind to receptors, they trigger ion channels to open in the membrane of the postsynaptic neuron. The open channels allow specific ions to enter or leave the postsynaptic neuron, creating a localized electrical change called a graded potential.
- Postsynaptic response type: The resulting electrical changes can be either excitatory (depolarization – moving the neuron closer to firing) or inhibitory (hyperpolarization – moving the neuron further from firing), depending on the neurotransmitter and receptor types.
- Signal propagation: If excitatory changes are strong enough, or if multiple excitatory inputs sum together effectively, they can reach threshold and trigger a new action potential in the postsynaptic neuron, continuing the neural signal transmission.
- Neurotransmitter clearance:
After the neurotransmitters have done their job, they must be removed from the synaptic cleft. This can happen in three ways:- They get broken down by enzymes.
- They are reabsorbed into the original presynaptic neuron (reuptake).
- They diffuse away from the synaptic cleft.
Neurotransmitters: Chemical Messengers
Neurotransmitters are brain chemicals that communicate information throughout our brain and body. They are released from the axon terminals.
The brain uses them for vital functions like telling your heart to beat or lungs to breathe, and they can also affect mood, sleep, and concentration.
Each neurotransmitter has a specific function. For example, acetylcholine is found where a motor neuron meets a muscle and causes the muscle to contract when released.
The action of dopamine at the synapse is linked to explanations for schizophrenia. Imbalances in neurotransmitters like serotonin are associated with conditions such as depression
Synaptic connections can be defined by the specific neurotransmitter they release, such as serotonin, dopamine, adrenaline, or GABA.
Excitation and Inhibition
Neurotransmitters can have one of two effects on the postsynaptic neuron:
1. Excitatory:
Increase the likelihood of the postsynaptic neuron firing an impulse. Excitatory neurotransmitters stimulate the brain.
They increase the positive charge inside the neuron, making it more likely to fire, an effect known as depolarisation.
Adrenaline is an example of a neurotransmitter (and hormone) that has an excitatory effect, making the neuron very positively charged, particularly important for fight-or-flight responses.
Dopamine is another excitatory neurotransmitter.
2. Inhibitory:
Decrease the likelihood of the postsynaptic neuron firing an impulse.
Inhibitory neurotransmitters calm the brain and help create balance.
They make the neuron more negatively charged, making it less likely to fire, an effect known as hyperpolarisation.
Examples include serotonin and GABA (Gamma-Aminobutyric Acid), which is purely inhibitory.
Summation
Normal brain function relies on a balance between excitatory and inhibitory influences.
The postsynaptic neuron receives both excitatory and inhibitory influences from multiple presynaptic neurons.
These influences are summed together (summation) If the net effect is inhibitory, the neuron will be less likely to fire.
If the net effect is excitatory, it will be more likely to fire.
If the net effect reaches a certain threshold, a new action potential forms and travels down the axon.
Inhibitory inputs can cancel out excitation, preventing an action potential.
Unidirectional Transmission
Information travels in one direction at a synapse. This is because the synaptic vesicles containing neurotransmitters are only present on and released from the presynaptic membrane.
Correspondingly, the receptors for these neurotransmitters are only located on the postsynaptic membrane.
The signal is transmitted when the neurotransmitter binds to these receptors.
Furthermore, the diffusion of neurotransmitters means they can only travel from the high concentration area (the presynaptic terminal after release) to the low concentration area (the synaptic cleft and then the postsynaptic membrane).
Related Concepts
Synaptic transmission is part of the broader function of neurons and the nervous system.
Neurons are electrically excitable cells that process and transmit information through electrical and chemical signals. The electrical signal within a neuron is the action potential.
The nervous system, including the central nervous system (brain and spinal cord) and peripheral nervous system (somatic and autonomic), transmits messages throughout the body.
The structure and function of different neuron types (sensory, relay, motor) are also relevant as they transmit information via these synaptic processes.
Therapeutic and Clinical Links
Understanding synaptic transmission is crucial in psychopathology and treatment.
For example, in depression, low levels of serotonin are suggested.
Drugs like Selective Serotonin Reuptake Inhibitors (SSRIs), used to treat conditions like OCD, work by preventing the reuptake and breakdown of serotonin by the presynaptic neuron, thus increasing its concentration in the synapse and prolonging its effect on the postsynaptic neuron.
The dopamine hypothesis for schizophrenia suggests excessive dopamine activity in certain brain regions, leading to symptoms. Drugs can target neurotransmitter systems to alleviate symptoms.
Exam Questions
1. Briefly outline how excitation and inhibition are involved in synaptic transmission (4 marks).
Diagrams can effectively describe structures, but text is necessary to explain the processes. Knowledge of both presynaptic and postsynaptic processes should be included.
2. Outline the structures and processes involved in synaptic transmission (6 marks).
3. Explain why information can only transmit information in one direction at a synapse.
