Action Potentials, Synaptic Transmission, and Neural Pathways: Study Notes for Anatomy & Physiology

Action Potentials

Definition and Phases of Action Potential

An action potential (AP) is a rapid, temporary change in a cell's membrane potential, essential for nerve impulse transmission. It involves depolarisation, repolarisation, and restoration of the resting membrane potential.

• Resting Membrane Potential: The typical resting potential of a neuron is -70mV, meaning the inside of the cell is negatively charged compared to the outside. This is maintained by the Na+/K+ pump and selective permeability of the membrane.

• Depolarisation: When a stimulus reaches threshold, voltage-gated Na+ channels open, allowing Na+ ions to enter the cell. The membrane potential becomes less negative, moving towards +30mV.

• Repolarisation: After depolarisation, voltage-gated K+ channels open, and K+ ions exit the cell, returning the membrane potential towards -70mV.

• Hyperpolarisation: Sometimes, the membrane potential briefly becomes more negative than the resting potential due to continued K+ efflux.

Equation:

$V_{m} = V_{rest} + ext{(change due to ion flux)}$

Additional info: The Na+/K+ ATPase pump maintains the resting potential by pumping 3 Na+ out and 2 K+ in.

Key Terms

• Threshold: The critical level to which a membrane potential must be depolarised to initiate an action potential.

• Voltage-gated channels: Ion channels that open or close in response to changes in membrane potential.

Synapses and Neurotransmitters

Synaptic Transmission

Synaptic transmission is the process by which one neuron communicates with another cell at a synapse, typically involving neurotransmitter release.

• Presynaptic neuron: The neuron sending the signal.

• Postsynaptic cell: The cell receiving the signal.

• Synaptic vesicles: Membrane-bound sacs in the presynaptic terminal containing neurotransmitters.

• Voltage-gated Ca2+ channels: Open in response to an action potential, allowing Ca2+ influx, which triggers vesicle fusion and neurotransmitter release.

Release and Action of Neurotransmitters

• Neurotransmitters are stored in vesicles in the axon terminal.

• When an AP arrives, Ca2+ enters the terminal, causing vesicles to fuse with the membrane and release neurotransmitters into the synaptic cleft.

• Neurotransmitters bind to receptors on the postsynaptic membrane, causing ion channels to open or close, leading to graded potentials.

Types of Neurotransmitters

• GABA: Main inhibitory neurotransmitter in the CNS.

• Glutamate: Main excitatory neurotransmitter in the CNS.

Removal of Neurotransmitters

• Neurotransmitters can be pumped back into presynaptic terminals (reuptake), broken down by enzymes (e.g., acetylcholine by acetylcholinesterase), or diffuse away from the synaptic cleft.

Table: Neurotransmitter Removal Mechanisms

Mechanism	Example
Reuptake	Serotonin
Enzymatic breakdown	Acetylcholine
Diffusion	General neurotransmitters

Neural Pathways

Sensory (Afferent) Pathway

Sensory pathways transmit information from receptors to the primary sensory cortex in the brain.

• First-order neuron: Receptor (mechanoreceptor, thermoreceptor, photoreceptor, chemoreceptor, or nociceptor) sends signals via the peripheral sensory neuron to the spinal cord.

• Second-order neuron: Interneuron in the dorsal horn crosses over to the opposite side and ascends to the thalamus.

• Third-order neuron: Cell body in the thalamus projects to the primary sensory cortex and other higher centers.

Additional info: Photoreceptors send information to the primary visual center.

Motor (Efferent) Pathway

Motor pathways transmit signals from the primary motor cortex to muscle fibers, enabling movement.

• Upper motor neuron: Cell bodies in the primary motor area (pyramidal cells); most axons cross over in the medulla and descend the spinal cord.

• Lower motor neuron: Cell body in the ventral horn of the spinal cord; axons exit to peripheral nerves and innervate muscle fibers.

Table: Comparison of Sensory and Motor Pathways

Pathway	Origin	Destination	Key Neurons
Sensory (Afferent)	Receptor (e.g., skin, eye)	Primary sensory cortex	First-order, second-order, third-order
Motor (Efferent)	Primary motor cortex	Muscle fiber	Upper motor neuron, lower motor neuron

Summary of Key Concepts

• Action potentials are essential for neural communication, involving rapid changes in membrane potential due to ion channel activity.

• Synaptic transmission relies on neurotransmitter release, receptor binding, and removal mechanisms to ensure proper signal propagation.

• Sensory and motor pathways involve multiple neurons and relay stations to transmit information to and from the brain.