Neurons and Synaptic Transmission

The brain and nervous system are made up of specialised cells called neurons. Neurons are the fundamental units of the nervous system — they transmit information throughout the body using a combination of electrical impulses and chemical signals. Understanding how neurons work is essential for understanding brain function, neurotransmitters, and psychological processes.

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Structure of a Neuron

All neurons share certain basic features:

Part	Function
Cell body (soma)	Contains the nucleus and other organelles; keeps the neuron alive
Dendrites	Branch-like extensions that receive signals from other neurons
Axon	A long fibre that carries electrical impulses away from the cell body towards the axon terminals
Myelin sheath	A fatty insulating layer around the axon that speeds up the transmission of electrical impulses
Axon terminals (synaptic knobs)	The ends of the axon, which contain vesicles filled with neurotransmitter chemicals

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Types of Neuron

There are three main types of neuron, each with a different function:

Type	Function	Direction of Signal	Location
Sensory neurons	Carry signals from sensory receptors (eyes, ears, skin, etc.) to the central nervous system (brain and spinal cord)	Receptor → CNS	Peripheral nervous system
Relay neurons (interneurons)	Connect sensory and motor neurons within the CNS; process information	Within CNS	Brain and spinal cord
Motor neurons	Carry signals from the CNS to effectors (muscles and glands) to produce a response	CNS → Effector	Peripheral nervous system

The Reflex Arc

A simple example of how the three types of neuron work together is the reflex arc:

1. A stimulus (e.g. touching a hot surface) is detected by a sensory receptor
2. A sensory neuron carries the signal to the spinal cord
3. A relay neuron in the spinal cord processes the signal
4. A motor neuron carries the signal to the effector (muscle)
5. The effector produces a response (pulling the hand away)

This process is automatic and rapid — it does not involve conscious thought, which is why reflexes are so fast.

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Synaptic Transmission

Neurons do not physically touch each other. There is a tiny gap between them called the synapse (or synaptic cleft). Information crosses this gap through a process called synaptic transmission.

Steps of Synaptic Transmission

1. An electrical impulse travels along the axon of the pre-synaptic neuron (the neuron sending the signal)
2. When the impulse reaches the axon terminal, it triggers the release of neurotransmitter molecules from vesicles into the synaptic cleft
3. The neurotransmitter molecules cross the synaptic cleft and bind to receptors on the post-synaptic neuron (the neuron receiving the signal)
4. This binding triggers a new electrical impulse in the post-synaptic neuron, continuing the signal
5. After transmission, the neurotransmitter is either reabsorbed (reuptake) by the pre-synaptic neuron or broken down by enzymes in the synapse

Synaptic transmission step by step

flowchart LR
    A["Dendrites<br/>receive signal"] --> B[Cell body / soma]
    B --> C["Axon<br/>myelin sheath"]
    C --> D[Axon terminal]
    D --> E["Vesicles release<br/>neurotransmitter"]
    E --> F["Synaptic cleft<br/>~20 nm gap"]
    F --> G["Receptors on<br/>post-synaptic neuron"]
    G --> H{"Excitatory<br/>or inhibitory?"}
    H -- Excitatory --> I[Neuron more likely to fire]
    H -- Inhibitory --> J[Neuron less likely to fire]
    G --> K["Reuptake or<br/>enzyme breakdown"]

Excitatory and Inhibitory Signals

Neurotransmitters can have one of two effects:

Effect	Description
Excitatory	Makes the post-synaptic neuron more likely to fire (produce an electrical impulse)
Inhibitory	Makes the post-synaptic neuron less likely to fire

Whether a neuron fires depends on the balance of excitatory and inhibitory signals it receives. If excitatory signals outweigh inhibitory signals past a certain threshold, the neuron fires.

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The Importance of Synaptic Transmission

Understanding synaptic transmission is crucial because:

• Drugs work by affecting synaptic transmission (e.g. increasing or decreasing neurotransmitter levels)
• Mental health conditions such as depression are linked to imbalances in neurotransmitters
• Learning involves changes in the strength of synaptic connections
• Addictive substances often affect the brain's reward system by altering neurotransmitter activity at synapses

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Key Points

• Neurons are specialised cells that transmit information via electrical impulses and chemical signals.
• Three types: sensory (receptor to CNS), relay (within CNS), motor (CNS to effector).
• Synaptic transmission: electrical impulse triggers neurotransmitter release, which crosses the synapse to the next neuron.
• Neurotransmitters can be excitatory (promote firing) or inhibitory (prevent firing).
• Understanding synaptic transmission is essential for understanding drugs, mental health, and learning.

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Worked study: Loewi (1921) — chemical transmission at the synapse

Aim: To test whether nerve impulses are transmitted between cells electrically or chemically. Before Loewi's work, scientists hotly debated whether nerves communicated directly (via electrical sparks) or via chemical messengers.

Procedure: Otto Loewi famously came up with the experiment in a dream. He dissected two frog hearts and placed them in separate chambers filled with saline solution. The first heart still had its vagus nerve (part of the parasympathetic system) attached. The chambers were connected so fluid could flow from the first to the second heart, but no nerve connection existed between them. Loewi electrically stimulated the vagus nerve of the first heart, slowing its beat. He then transferred the fluid from the first chamber into the second heart's chamber.

Findings: The second heart also slowed even though no nerve stimulation had been applied to it. Something in the fluid, released by the first heart when its vagus nerve was stimulated, had slowed the second heart. Loewi named this substance "Vagusstoff" — later identified as the neurotransmitter acetylcholine.

Conclusion: Nerves communicate by releasing chemical messengers (neurotransmitters) at synapses, not only by electrical impulses. Loewi won the Nobel Prize in 1936 for this work, which founded the modern understanding of synaptic transmission.

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