The Synapse and Neurotransmitters

Synapse and Neurotransmitters

Synapse Definition

• The synapse is the point where the synaptic end bulb of a neuron comes into close proximity with another neuron, muscle, or gland.

• There is a small gap between the synaptic end bulb and the adjacent neuron, muscle, or gland, which is called the synaptic cleft.

• A synapse is a functional junction between two neurons, a neuron and a muscle, or a neuron and a gland.

Neurotransmitters Role

• Neurotransmitters facilitate the transmission of information across the synaptic cleft.

• The neuron before the synapse is called the presynaptic neuron, and the neuron on the other side is called the postsynaptic neuron.

Action Potential and Calcium

• When an action potential reaches the synaptic end bulb, voltage-gated calcium channels open.

• Calcium ions (Ca^{2+}) flood into the cell, stimulating synaptic vesicles to bind with the plasma membrane and undergo exocytosis.

• Exocytosis is the process where a vesicle binds with the plasma membrane and releases its contents into the extracellular fluid.

Neurotransmitter Release

• Calcium stimulates synaptic vesicles to undergo exocytosis, releasing neurotransmitters into the synaptic cleft.

• These neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic neuron, relaying the message.

Variety of Neurotransmitters

• There are over a hundred different types of neurotransmitters.

• The effect of a neurotransmitter on the postsynaptic neuron depends on the type of neurotransmitter released and the receptor it binds to.

Inhibitory Neurotransmitters

• Some neurotransmitters can inhibit the postsynaptic neuron.

• Inhibitory neurotransmitters move the membrane potential further away from the threshold of -55 mV, making the neuron less likely to fire.

• This is achieved by making the membrane potential more negative, such as around -75 mV.

Excitatory Neurotransmitters

• Other neurotransmitters are excitatory, making the membrane potential less negative and closer to the -55 mV threshold.

• Multiple excitatory stimuli may be needed collectively to reach the threshold and generate an action potential.

Graded Potentials

• Excitatory graded potentials are short-lived and make the membrane potential slightly less negative but may not be sufficient to send an action potential.

• Inhibitory graded potentials make the membrane potential more negative and are also short-lived, returning to the resting membrane potential.

Neurotransmitter Effects

• Besides stimulating or inhibiting action potentials, neurotransmitters can also:

  • Change gene expression.

  • Influence the production of proteins.

  • Alter the metabolism within cells.

Examples of Neurotransmitters

• Hormones, particularly those in the fight or flight response, can act as neurotransmitters.

• Examples of neurotransmitters include:

  • Noradrenaline.

  • Dopamine.

  • Serotonin.

  • Glutamate.

  • GABA.

  • Nitric oxide (a gas).

  • Acetylcholine (well-known, especially in muscle function).

Removal of Neurotransmitters

• After a nerve impulse, neurotransmitters need to be removed to prevent perpetual stimulation. Mechanisms include:

  • Diffusion away from the synaptic cleft.

  • Reabsorption and recycling by the neuron and neuroglia.

  • Enzymatic degradation within the synaptic cleft.

  • Reuptake back into the presynaptic neuron.

Summary of Key Concepts

• Resting membrane potential is generated by the separation of ions, creating electrical charges; negatively charged proteins contribute to this charge difference.

• Neurons are electrically excitable and use action potentials for rapid communication.

• Action potentials are generated by the flow of ions like sodium and potassium through voltage-gated channels.

• Action potentials are all-or-nothing.

• Myelin increases propagation speed through saltatory conduction.

• Neurotransmitters transmit information across the synaptic cleft by altering the membrane potential.