Neural Signaling and Synaptic Transmission: Study Notes for Anatomy & Physiology

Divisions of the Nervous System

Central and Peripheral Nervous Systems

The nervous system is divided into central and peripheral components, each with distinct structures and functions.

• Central Nervous System (CNS): Comprises the brain and spinal cord. Responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Consists of all neural tissue outside the CNS. Divided into afferent (input) and efferent (output) divisions.

• Interneurons: Neurons that connect sensory and motor pathways within the CNS.

Afferent vs. Efferent Pathways

• Afferent (Input): Sensory neurons that carry information from sensory receptors to the CNS.

• Efferent (Output): Motor neurons that transmit signals from the CNS to effectors (muscles, glands, etc.).

• Efferent Output Types:

  • Somatic: Controls skeletal muscles (voluntary).

  • Autonomic: Controls glands, heart, smooth muscle (involuntary). Subdivided into sympathetic ("fight or flight") and parasympathetic ("rest and digest").

Structure and Types of Neurons

Basic Structure of a Neuron

Neurons are specialized cells for communication, consisting of several key parts:

• Dendrites: Receive incoming signals.

• Soma (Cell Body): Contains the nucleus and organelles.

• Axon: Conducts electrical impulses away from the soma.

• Axon Hillock: Site where action potentials are initiated.

• Axon Terminal: Releases neurotransmitters to communicate with other cells.

• Axon Collaterals: Branches of the axon that allow communication with multiple targets.

Types of Neurons

• Bipolar: One dendrite and one axon (e.g., retina).

• Multipolar: Multiple dendrites, one axon (most common in CNS).

• Pseudounipolar: Single process that splits into two branches (e.g., sensory neurons).

Transport in Neurons

• Anterograde Transport: Moves materials from soma to axon terminal.

• Retrograde Transport: Moves materials from axon terminal to soma.

• Utilizes motor proteins along microtubule tracks.

Ion Channels and Membrane Potentials

Types of Ion Channels

• Leak Channels: Always open, contribute to resting membrane potential.

• Ligand-Gated Channels: Open in response to binding of a chemical messenger (e.g., neurotransmitter).

• Voltage-Gated Channels: Open/close in response to changes in membrane potential (e.g., Na+, K+).

Key Terminology

• Nuclei: Groups of neuron cell bodies in the CNS.

• Tracts: Bundles of axons in the CNS.

• Ganglia: Groups of neuron cell bodies outside the CNS.

• Nerves: Bundles of axons outside the CNS.

Glial Cells

• Oligodendrocytes: Myelinate axons in the CNS.

• Schwann Cells: Myelinate axons in the PNS.

• Astrocytes: Regulate extracellular ion concentrations, form blood-brain barrier.

• Microglia: Immune cells of the CNS.

Resting Membrane Potential and Action Potentials

Resting Membrane Potential

Neurons maintain a resting membrane potential (typically around -70 mV) due to the differential distribution of ions and selective permeability of the membrane.

• High permeability to K+, low permeability to Na+.

• Na+/K+ ATPase pump maintains gradients by moving 3 Na+ out and 2 K+ in.

Equilibrium Potential

• Calculated using the Nernst equation:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

• For K+, equilibrium potential is typically around -90 mV.

• For Na+, equilibrium potential is typically around +60 mV.

Graded Potentials

• Small changes in membrane potential in response to a stimulus.

• Can be depolarizing (more positive) or hyperpolarizing (more negative).

• Strength decreases with distance from the source.

• Can summate to trigger an action potential if threshold is reached.

Action Potentials

• All-or-none electrical signals that travel along the axon.

• Phases: Depolarization, Repolarization, Hyperpolarization.

• Initiated when membrane potential reaches threshold, usually due to opening of voltage-gated Na+ channels.

• Refractory periods prevent immediate re-firing:

  • Absolute Refractory Period: No action potential can be generated.

  • Relative Refractory Period: Stronger stimulus required for action potential.

Propagation of Action Potentials

• Unidirectional due to refractory period.

• Saltatory conduction in myelinated axons (jumps between nodes of Ranvier) increases speed.

• Larger axon diameter also increases conduction velocity.

Synaptic Transmission and Neural Integration

Electrical vs. Chemical Synapses

• Electrical Synapses: Gap junctions allow direct, rapid communication between neurons.

• Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft.

Chemical Synapse Structure

• Presynaptic Terminal: Contains synaptic vesicles with neurotransmitter, voltage-gated Ca2+ channels.

• Synaptic Cleft: Space between pre- and postsynaptic membranes.

• Postsynaptic Membrane: Contains receptors for neurotransmitter.

Termination of Neurotransmitter Signal

• Diffusion away from the synaptic cleft.

• Enzymatic degradation (e.g., acetylcholinesterase).

• Reuptake into presynaptic cell.

Chemical Synapse Receptor Types

• Ionotropic Receptors: Ligand-gated ion channels, fast response.

• Metabotropic Receptors: G-protein coupled, slower response, can activate second messenger systems.

Excitatory and Inhibitory Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic membrane, often due to Na+ influx.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane, often due to Cl- influx or K+ efflux.

Neural Integration

• Divergence: One neuron synapses on many others.

• Convergence: Many neurons synapse on one neuron.

• Summation: EPSPs and IPSPs can summate temporally (over time) or spatially (from multiple synapses).

• Frequency Coding: The rate of action potentials encodes stimulus intensity.

Neurotransmitters

Major Neurotransmitters and Their Functions

• Acetylcholine (ACh): Most abundant in PNS, acts on nicotinic (ionotropic) and muscarinic (metabotropic) receptors. Broken down by acetylcholinesterase.

• Biogenic Amines: Derived from amino acids, include catecholamines (dopamine, norepinephrine, epinephrine), serotonin, and histamine.

• Amino Acid Neurotransmitters: Glutamate and aspartate (excitatory); GABA and glycine (inhibitory).

• Neuropeptides: Short peptides with modulatory roles (e.g., endorphins, oxytocin, substance P).

Table: Major Neurotransmitters and Their Effects

Neurotransmitter	Main Location	Receptor Type	Main Effect
Acetylcholine	PNS, CNS	Nicotinic (ionotropic), Muscarinic (metabotropic)	Excitatory or inhibitory, muscle activation
Dopamine	CNS	Metabotropic	Movement, reward
Norepinephrine	PNS, CNS	Metabotropic (adrenergic)	Alertness, sympathetic response
Serotonin	CNS	Metabotropic	Mood, sleep
Glutamate	CNS	Ionotropic, Metabotropic	Major excitatory neurotransmitter
GABA	CNS	Ionotropic, Metabotropic	Major inhibitory neurotransmitter
Glycine	Spinal cord	Ionotropic	Inhibitory
Endorphins	CNS	Metabotropic	Pain modulation

Summary

• The nervous system is organized into central and peripheral divisions, with specialized cells (neurons and glia) for communication.

• Neural signaling relies on ion gradients, membrane potentials, and the generation and propagation of action potentials.

• Synaptic transmission can be electrical or chemical, with neurotransmitters mediating communication between neurons.

• Neural integration involves the summation of excitatory and inhibitory inputs, determining whether a neuron will fire an action potential.

• Major neurotransmitters have specific roles in modulating neural activity and behavior.