Synapses

A synapse is the specialized junction between two neurons or between a neuron and an effector (a muscle or gland). It allows nerve impulses to be transmitted from one cell to another through either chemical or electrical signaling. A typical chemical synapse consists of:

1. Presynaptic terminal (synaptic knob)
2. Synaptic cleft
3. Postsynaptic membrane

Remember: Most synapses in humans are chemical synapses.

Properties and Functions of Synapses

Synapses do much more than simply connect neurons. They regulate how information is transmitted throughout the nervous system, ensuring signals reach the correct destination while preventing unnecessary stimulation.

Key functions

• Ensure one-way transmission of nerve impulses
• Connect neurons into neural pathways
• Integrate information from multiple neurons
• Filter weak or unimportant stimuli
• Prevent overstimulation of the nervous system
• Play an important role in learning and memory

Mechanism of Synaptic Transmission (Cholinergic Synapse)

Transmission across a cholinergic synapse follows a precise sequence of events. Each step triggers the next, eventually producing a new action potential in the postsynaptic neuron.

Step-by-step mechanism

1. An action potential reaches the presynaptic terminal.
2. The presynaptic membrane depolarizes.
3. Voltage-gated Ca²⁺ channels open.
4. Ca²⁺ ions diffuse into the synaptic knob.
5. Calcium causes synaptic vesicles containing acetylcholine (ACh) to move toward the membrane.
6. Vesicles fuse with the membrane by exocytosis.
7. Acetylcholine is released into the synaptic cleft.
8. ACh diffuses across the cleft.
9. ACh binds to receptors on the postsynaptic membrane.
10. Chemically gated Na⁺ channels open.
11. Na⁺ enters the postsynaptic neuron.
12. The postsynaptic membrane depolarizes.
13. A new action potential is generated if threshold is reached

Explain the mechanism of transmission of a nerve impulse across a cholinegic synapse

Logical sequence of events is very important when explaining a mechanism

• arrival of an impulse
• Depolarization of presynaptic membrane
• opening if calcium channels
• increase in mebrane permeability to Ca²⁺
• rushing of Ca²⁺ into presynaptic knob
• synaptic vesicle fuse with presynaptic cleft(by exocytosis)
• acetylecholine released into synaptic cleft
• attaches to a receptor on the post synaptic membrane
• entry of Na⁺
• depolarisation of membrane

Recycling of Acetylcholine

Acetylcholine must be removed quickly to prevent continuous stimulation of the postsynaptic neuron.

Process

1. Acetylcholinesterase breaks down acetylcholine.
2. Acetylcholine → Acetate + Choline
3. Choline is reabsorbed into the presynaptic neuron.
4. Choline combines with acetyl-CoA.
5. New acetylcholine is synthesized.
6. Acetylcholine is packaged into synaptic vesicles.

Purpose: Stops continuous nerve stimulation and prepares the synapse for the next impulse.

Functions of synapses

• ensures one-way transmission: neurotransmitters are only released on one side of synapse; receptors on the other side of synapse
• allow connection of nerve pathways
• integration of impulses
• involved in memory making and learning.
• Decreases the overload of information in the brain

Factors affecting the speed of transmission

The following factors affect the speed of transmission

• Temperature – Increases in temperature increase kinetic energy and therefore speeds up the transport of ions; this speeds up nerve transmission.
• Diameter of the axon – The greater the diameter of the axon the lower the resistance to the movement of ions. Giant squids have axons up to 1mm in diameter allowing them to react quickly in low temperatures.
• Myelination – Schwann cells wrap around the axon and secrete a fatty myelin sheath which is an electrical insulator. Only the Nodes of Ranvier, which are gaps in the myelin sheath exposing the cell membrane, can become depolarized (only these regions have voltage gated Na+ channels). The action potential jumps from node to node and speeds up the rate of transmission; this is called saltatory transmission. The greater the distance between the nodes the greater the rate of transmission. Myelination is only found in vertebrates.

Explain the roles of synapses in the nervous system.

• ensure one-way transmission;
• receptor (proteins) only in postsynaptic, membrane / neurone;
• vesicles only in presynaptic neurone;
• ref. adaptation;
• increased range of actions;
• due to interconnection of many nerve pathways;
• ref. inhibitory synapses;
• involved in memory / learning;
• due to new synapses being formed;
• e.g. summation / discrimination

The effect of chemicals on the synapse

Many drugs act at the synapse and can either amplify or inhibit synaptic transmission. Psychoactive drugs act on the CNS by affecting neurotransmitters or their receptors.

Amplification	Inhibition
Accelerating neurotransmitter production in the synaptic knob (cocaine).	Inhibiting neurotransmitter production in the synaptic knob.
Opening calcium channels in the pre-synaptic membrane	Closing calcium channels in the pre-synaptic membrane
Accelerating the release of neurotransmitter from the synaptic knob by exocytosis.	Inhibiting the release of neurotransmitter from the synaptic knob by exocytosis

Amplification	Inhibition
Making the post-synaptic receptors more sensitive to the neurotransmitter.	Making the post-synaptic receptors less sensitive to the neurotransmitter.
Opening the sodium channels on the post synaptic membrane	Closing sodium channels on the post-synaptic membrane.
Inhibiting cholinesterase activity in the synaptic cleft.	Inhibiting cholinesterase activity in the synaptic cleft.
Mimicking the neurotransmitter substance (cannabis).	Masking the effect of the neurotransmitter substance

Nervous Control questions and answers