Neuronal Action Potentials and Synaptic Transmission

Resting Potential

The resting potential is the electrical charge difference across the neuronal membrane when the neuron is not actively transmitting an impulse, approximately -70mV.

Action Potential

An action potential occurs when the resting potential is altered, leading to membrane depolarization. Key phases include:

Depolarization: Membrane becomes less negative due to sodium ion influx (Na⁺).
Repolarization: Membrane potential returns to a more negative value as potassium ions (K⁺) exit the cell.
Hyperpolarization: Membrane potential becomes more negative than the resting potential.
Refractory Period: Time following an action potential during which a neuron is less responsive to stimuli.

Transmission of Action Potential

Action potentials are rapid depolarizations traveling in one direction along the neuron.
All action potentials are identical in strength; variations in stimulus intensity are represented by impulse frequency.
Impulse conduction speed is approximately 8m/s, while saltatory conduction in myelinated neurons can increase speed to about 100m/s.

Refractory Period

There are two types of refractory periods:

Absolute Refractory Period: Sodium channels are inactivated, preventing depolarization regardless of stimulus strength.
Relative Refractory Period: Some sodium channels have reopened, but a stronger stimulus is required to reach the threshold for action potential generation.

Depolarization and the All-or-None Principle

During depolarization, sodium ions rush into the neuron, causing the membrane to reach a critical threshold level, triggering an action potential. The All-or-None Principle states that if the threshold is not reached, no action potential will occur.

Saltatory Conduction

Saltatory conduction refers to the "jumping" of action potentials between nodes of Ranvier in myelinated neurons, significantly speeding up impulse transmission.

Synapses

A synapse is the junction between two neurons, characterized by a narrow gap known as the synaptic cleft (approximately 20nm wide). Action potentials cannot cross this gap; they are transmitted via neurotransmitters.

Components of a Synapse

Pre-synaptic Neuron: The neuron that sends the impulse.
Post-synaptic Neuron: The neuron that receives the impulse.

Neurotransmitters

Neurotransmitters are synthesized by the pre-synaptic neuron and stored in synaptic vesicles. Common neurotransmitters include:

Acetylcholine: Involved in the parasympathetic nervous system.
Noradrenaline: Associated with the sympathetic nervous system.
Serotonin: Linked to feelings of calmness.
GABA: A typical inhibitory neurotransmitter.

Cholinergic Synapses

Cholinergic synapses utilize acetylcholine as a neurotransmitter. The process involves:

An action potential reaches the pre-synaptic membrane, opening voltage-gated calcium channels.
Calcium ions enter the pre-synaptic neuron, causing synaptic vesicles to fuse with the membrane and release acetylcholine into the synaptic cleft.
Acetylcholine diffuses across the cleft and binds to neuroreceptor sites on the post-synaptic membrane.
This binding opens sodium channels, allowing sodium ions to enter the post-synaptic neuron, leading to depolarization and potential action potential initiation.
Acetylcholinesterase breaks down acetylcholine into choline and ethanoic acid, which are reabsorbed by the pre-synaptic neuron for resynthesis.

Effects of Drugs on Synapses

Drugs can mimic or inhibit neurotransmitter action at synapses:

Agonists: Stimulate the nervous system.
Antagonists: Inhibit the nervous system.

Examples of drug effects include:

Drug	Action	Effect on Synapse
Endorphin	Binds to receptor	Inhibits impulses
Morphine, Codeine, Heroin	Bind to receptor	Inhibits impulses, blocking pain
Valium	Increases binding of GABA	Inhibits impulses
Ecstasy (MDMA)	Binds to receptors	Stimulates impulses
Cocaine	Prevents noradrenaline breakdown	Stimulates impulses
Magic Mushrooms	Combine with excitatory transmitters	Creates inappropriate impulses
Strychnine	Inhibits breakdown of acetylcholine	Stimulates more impulses, causing muscle spasms
Curare & Atropine	Block acetylcholine receptors	Inhibits impulses
DDT (insecticide)	Inhibits breakdown of acetylcholine	Stimulates impulses

Functions of Synapses

Transmit information between neurons.
Ensure impulses travel in one direction only.
Facilitate spatial summation by allowing multiple neurons to communicate with a single neuron.
Filter out low-level stimuli by not transmitting insufficient neurotransmitter levels.

Summary

A synapse is the connection point between two nerve cells, characterized by a tiny gap known as the synaptic cleft. Neurotransmitters released from the pre-synaptic neuron diffuse across this gap and bind to receptors on the post-synaptic membrane, potentially causing depolarization and initiating an action potential. Various drugs can significantly affect synaptic transmission.

Control Systems in Humans

This section discusses the control systems in humans, focusing on the roles of electrical signals, hormones, and the nervous system in regulating bodily functions.

Electrical Signals

Reflexes are automatic responses to stimuli, illustrated by scenarios such as:
- A girl burning her hand on a hot oven door.
- A boy sneezing.
- A short-sighted boy squinting at the board.
- A long-sighted girl looking at a piece of paper.
Key concepts include:
- Comparison of hormonal and nervous control features.
- Stages of a nervous reflex arc.
- Structure of neurones: motor, sensory, and reflex.
- Mechanism of a nervous impulse.
- Mechanisms of a synapse.
- Effects of drugs on synapse function.

Hormones

Hormones are chemical messengers produced by endocrine glands, transported in blood plasma, affecting only target cells with medium to long-lasting effects.

Nervous System Overview

Divided into two main parts:
- Central Nervous System (CNS): Comprises the brain and spinal cord.
- Peripheral Nervous System (PNS): Includes nerves connecting the CNS to receptors and effectors.
Comparison of control systems:
- Hormonal Control System:
  - Chemical signals transported in blood plasma.
  - Slower response with longer-lasting effects.
  - Affects target tissues and can influence the whole body.
  - Coordinates long-duration activities involving multiple organs (e.g., growth, reproduction, homeostasis).
- Nervous Control System:
  - Uses electrical impulses transmitted by neurones.
  - Very fast response with short-lived effects.
  - Affects specific tissues and coordinates rapid, precise responses.

Organisation of the Nervous System

Components:
- CNS: Brain and spinal cord.
- PNS: Connects CNS to the rest of the body.
Subdivisions of the PNS:
- Somatic Nervous System: Controls voluntary movements.
- Autonomic Nervous System: Controls involuntary functions, further divided into:
  - Sympathetic Nervous System: Activates fight or flight responses.
  - Parasympathetic Nervous System: Promotes rest and digest activities.

Sympathetic Nervous System

Comprises a series of ganglia with motor neurone cell bodies located outside the spinal cord, using noradrenaline to stimulate activity and prepare the body for stressful situations.

Parasympathetic Nervous System

Neurones originate in the brain or spinal cord and synapse within target organs, releasing acetylcholine to inhibit organ activity and promote relaxation and digestion.

Pupil Dilation and Constriction

The pupil's diameter is controlled by the iris muscles:
- Sympathetic Action: Radial muscles contract, dilating the pupil in response to dim light, excitement, or fear.
- Parasympathetic Action: Circular muscles contract, constricting the pupil in response to bright light.

Reflexes

Reflexes are fast, automatic responses to stimuli, linking a stimulus to a response. Common reflexes include sneezing and focusing the eyes, with some being learned (conditioned responses).
Reflex actions occur involuntarily, as the CNS sends electrical signals to muscles before the brain processes the information.

Reflex Arc

Sequence of events in a reflex action:
- Stimulus → Receptor → Coordinator → Effector → Response
Components:
- Stimulus: Can be internal or external (e.g., sound, light, pressure).
- Receptor: Specialized cells that detect stimuli (e.g., photoreceptors in the eye).
- Coordinator: The CNS processes the information and decides on a response.
- Effector: Muscles or glands that carry out the response.
- Response: Muscles contract or relax; glands secrete or do not secrete chemicals.

Nervous Impulse

Neurones transmit electrical impulses using ion movement rather than electrons. Resting potential is established by the sodium-potassium pump, creating a potential difference across the membrane (approximately -70mV). Action potential occurs when the resting potential changes, leading to depolarization.
Impulses are rapid depolarizations that travel in one direction. All impulses are uniform; differences in stimuli are detected by frequency. Saltatory conduction increases impulse speed significantly.
Refractory Period:
- Absolute Refractory Period: Na+ channels are inactive, preventing depolarization regardless of stimulus.
- Relative Refractory Period: Some Na+ channels are open, but a stronger stimulus is required to trigger depolarization.

Neuronal Action Potentials and Synaptic Functionality

Neuronal Action Potentials and Synaptic Functionality