Study Guide: Nerve Conduction, Neurotransmitters, CNS, Special Senses, and Autonomic Nervous System

Nerve Conduction

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the neuronal cell membrane when the cell is not actively transmitting a signal. It is essential for the excitability of neurons.

• Key ions: Sodium (Na+) is more concentrated outside the cell, while Potassium (K+) is more concentrated inside.

• Typical value: The resting membrane potential is usually around -70 mV.

• Mechanism: Maintained by the sodium-potassium pump and selective permeability of the membrane.

Equation:

$V_m = E_K$ (Nernst equation for a single ion)

$V_m = \frac{RT}{F} \ln \frac{[K^+]_{out}}{[K^+]_{in}}$

Action Potential Generation

An action potential is a rapid change in membrane potential that propagates along the neuron, allowing for signal transmission.

• Phases: Depolarization, repolarization, and hyperpolarization.

• Depolarization: Na+ channels open, Na+ enters the cell.

• Repolarization: K+ channels open, K+ leaves the cell.

• Propagation: The action potential travels along the axon, triggering neurotransmitter release at the synapse.

Neurotransmitter Release

Neurotransmitters are released from presynaptic cells in response to an action potential.

• Process: Action potential arrives at axon terminal, Ca2+ channels open, Ca2+ influx triggers vesicle fusion and neurotransmitter release.

• Removal: Neurotransmitters are removed by reuptake, enzymatic degradation, or diffusion.

Neurotransmitters

Types and Functions

Neurotransmitters are chemical messengers that transmit signals across synapses.

• Primary types: Excitatory (e.g., glutamate) and inhibitory (e.g., GABA).

• Metabotropic receptors: Activate second messenger systems (e.g., G-protein coupled receptors).

• Ionotropic receptors: Directly open ion channels.

Excitatory and Inhibitory Postsynaptic Potentials

Postsynaptic potentials determine whether a neuron will fire an action potential.

• EPSP (Excitatory): Depolarizes the postsynaptic membrane, increasing likelihood of firing.

• IPSP (Inhibitory): Hyperpolarizes the membrane, decreasing likelihood of firing.

• Temporal and spatial summation: Multiple EPSPs/IPSPs can combine to influence action potential generation.

Neurotransmitter Removal

• Reuptake: Neurotransmitters are taken back into the presynaptic neuron.

• Enzymatic degradation: Enzymes break down neurotransmitters (e.g., acetylcholinesterase for acetylcholine).

• Diffusion: Neurotransmitters diffuse away from the synaptic cleft.

Central Nervous System (CNS)

General Functions

The CNS consists of the brain and spinal cord, responsible for processing and integrating information.

• Components: Cerebral cortex, corpus callosum, cerebellum, spinal cord.

• Functions: Sensory processing, motor control, cognition, and homeostasis.

Special Senses

Special senses include vision, hearing, taste, smell, and equilibrium.

• Receptive fields: Area where a sensory neuron is activated by stimuli.

• Sensory adaptation: Decreased response to a constant stimulus over time.

• Perception vs. sensation: Sensation is the detection of stimuli; perception is the interpretation.

Autonomic Nervous System (ANS)

Overview and Divisions

The ANS regulates involuntary functions and is divided into sympathetic and parasympathetic systems.

• Sympathetic: Prepares the body for 'fight or flight' responses.

• Parasympathetic: Promotes 'rest and digest' activities.

• Origin: Sympathetic nerves originate from thoracolumbar regions; parasympathetic from craniosacral regions.

Neurotransmitters in the ANS

• Sympathetic: Releases norepinephrine (noradrenaline) at postganglionic synapses.

• Parasympathetic: Releases acetylcholine at both pre- and postganglionic synapses.

• Physiological effects: Norepinephrine increases heart rate; acetylcholine decreases heart rate and increases digestion.

Functional Diversity

Despite using only two main neurotransmitters, the ANS achieves functional diversity through receptor subtypes and tissue-specific responses.

• Adrenergic receptors: Respond to norepinephrine (e.g., alpha and beta receptors).

• Cholinergic receptors: Respond to acetylcholine (e.g., muscarinic and nicotinic receptors).

Table: Comparison of Sympathetic and Parasympathetic Divisions

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar (T1-L2)	Craniosacral (CN III, VII, IX, X; S2-S4)
Neurotransmitter (Postganglionic)	Norepinephrine	Acetylcholine
Main Function	Fight or flight	Rest and digest
Effect on Heart Rate	Increases	Decreases
Effect on Digestion	Decreases	Increases

Additional info:

• Neurotransmitter imbalances are implicated in mental illness, drug addiction, and other pathologies.

• Special senses also include equilibrium (balance), mediated by the vestibular system.

• Adaptation in sensory systems allows organisms to ignore constant stimuli and focus on changes in the environment.