Neurons and Synaptic Transmission

Neurons are the fundamental units of the nervous system. They are specialised cells that transmit electrical impulses and communicate with one another via chemical signals at junctions called synapses. Understanding neuronal structure, electrical transmission, and synaptic transmission is essential for explaining how information flows through the nervous system and how drugs, neurotransmitters, and mental disorders affect behaviour.

Key Definition: A neuron is a specialised nerve cell that receives, processes, and transmits information through electrical and chemical signals.

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Types of Neurons

There are three main types of neurons in the human nervous system, each with a distinct function:

Neuron Type	Function	Direction of Impulse	Location
Sensory neuron	Carries nerve impulses from sensory receptors to the CNS	From receptors → CNS	PNS (connecting receptors to spinal cord/brain)
Relay neuron (interneuron)	Connects sensory and motor neurons within the CNS	Within the CNS	Brain and spinal cord
Motor neuron	Carries nerve impulses from the CNS to effectors (muscles or glands)	From CNS → effectors	CNS to PNS

Key Definition: An effector is a muscle or gland that carries out a response when stimulated by a motor neuron.

Structural Differences

• Sensory neurons have a long dendron and a short axon. Their cell body is located to one side of the main fibre (in the dorsal root ganglion of spinal nerves).
• Relay neurons have a short axon and many dendrites, forming extensive connections within the CNS.
• Motor neurons have a long axon extending from the spinal cord to the effector, and many short dendrites emerging from the cell body.

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Structure of a Neuron

A typical motor neuron has the following components:

Structure	Function
Cell body (soma)	Contains the nucleus and most organelles; metabolic centre of the neuron
Dendrites	Short, branching extensions that receive signals from other neurons
Axon	A long, thin fibre that carries the nerve impulse (action potential) away from the cell body
Myelin sheath	A fatty, insulating layer formed by Schwann cells; speeds up electrical transmission
Nodes of Ranvier	Gaps between sections of myelin sheath where ion exchange occurs
Terminal buttons (synaptic knobs)	Swollen endings at the axon terminal that contain vesicles of neurotransmitter

The myelin sheath is composed of layers of the cell membrane of Schwann cells wrapped around the axon. Myelinated neurons can transmit impulses at speeds of up to 120 metres per second, whereas unmyelinated neurons transmit at approximately 2 metres per second.

Exam Tip: If asked to draw and label a motor neuron, include: cell body with nucleus, dendrites, axon, myelin sheath, nodes of Ranvier, and terminal buttons. Label each clearly and indicate the direction of impulse transmission.

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Electrical Transmission

Neurons communicate over long distances by transmitting electrical impulses called action potentials along the axon. This process involves the movement of ions (charged particles) across the neuronal membrane.

Resting Potential

When a neuron is not transmitting an impulse, it is said to be at its resting potential. The inside of the neuron is negatively charged relative to the outside, with a potential difference of approximately −70 mV. This is maintained by:

• The sodium-potassium pump (Na⁺/K⁺ ATPase), which actively pumps 3 Na⁺ ions out of the neuron for every 2 K⁺ ions pumped in, using ATP.
• Potassium leak channels — some K⁺ ions diffuse back out of the neuron down their concentration gradient, further increasing the negative charge inside.
• Large, negatively charged proteins and organic ions are trapped inside the cell.

The result is a polarised membrane: negative inside, positive outside.

Key Definition: The resting potential is the electrical charge across the neuronal membrane when the neuron is not firing, typically around −70 mV.

Action Potential

When a neuron receives a stimulus above a critical threshold, an action potential is generated. This involves a rapid reversal of the resting potential:

1. Stimulus — the neuron receives sufficient stimulation (from a sensory receptor or another neuron).
2. Depolarisation — voltage-gated sodium (Na⁺) channels open; Na⁺ ions rush into the neuron down their concentration gradient. The membrane potential rises rapidly from −70 mV towards +40 mV.
3. Repolarisation — Na⁺ channels close; voltage-gated potassium (K⁺) channels open. K⁺ ions flow out of the neuron, restoring the negative charge inside.
4. Hyperpolarisation — the membrane briefly becomes more negative than the resting potential (approximately −80 mV) because K⁺ channels are slow to close.
5. Recovery — the sodium-potassium pump restores the original ion concentrations, returning the membrane to −70 mV (resting potential).

The All-or-Nothing Principle

Key Definition: The all-or-nothing principle states that a neuron either fires a full action potential or does not fire at all. There is no partial firing.