Neurons, Synapses, Signaling, and the Central Nervous System (CNS)

Neurons, Synapses, Signaling, and the CNS

Overview and Objectives

This study guide covers the structure and function of the nervous system, focusing on neurons, synapses, and the mechanisms of neural signaling. Key objectives include understanding the types of nervous systems, neuron anatomy, ion channels and pumps, and the role of myelination and synapses in information propagation.

• Objective 1: Identify two types of nervous systems and their main components.

• Objective 2: Describe the parts of a neuron and their functions.

• Objective 3: Explain the gates and pumps involved in action potential propagation.

• Objective 4: Understand how myelination and synapses facilitate neural signaling.

Types of Nervous Systems

PNS versus CNS

The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

• CNS: Consists of the brain and spinal cord. Integrates sensory information and sends signals to muscles, glands, and other tissues via PNS neurons.

• PNS: Includes all nervous system components outside the CNS. PNS neurons transmit sensory information to the CNS and carry motor commands from the CNS to effectors (muscles, glands).

• Animal Movements: Triggered by electrical signals conducted by neurons, based on ion flows across plasma membranes.

Two Basic Nervous System Types

• Nerve Net: Diffuse arrangement of cells, found in Cnidarians and Ctenophores.

• Central Nervous System (CNS): Characterized by large clusters of aggregated neurons called ganglia.

Neural Pathways in the CNS

Sensory Receptors and Neuron Pathways

• Sensory Receptors: Located in skin, eyes, ears, nose, etc. Transmit data about the external environment via sensory neurons.

• Internal Sensory Cells: Monitor homeostatic conditions (e.g., blood pH, oxygen levels).

• Neural Pathways: Sensory neurons send information to the spinal cord and brain through nerves, which are bundles of thousands of neurons.

CNS and PNS Components

• CNS: Brain and spinal cord; integrates sensory neuron information.

• PNS: Sensory neurons and motor neurons outside the CNS.

• Motor Neurons: Elicit responses from effectors such as muscles and glands.

Neurons: Types and Structure

Types of Neurons

• Interneurons: CNS cells that connect sensory and motor neurons.

• Glia: Support cells that nourish and insulate neurons.

Parts of a Neuron

• Dendrite: Receives electrical signals from adjacent cells.

• Cell Body (Soma): Contains the nucleus, integrates incoming signals, and generates outgoing signals.

• Axon: Includes the axon hillock; sends signals to dendrites of other neurons or to effectors.

Neuron Connections

• Synapse: Junction between axon terminal and another cell; site of chemical messenger (neurotransmitter) release.

• Information Transmission: From presynaptic neuron to postsynaptic neuron, muscle, or gland cell.

Electrochemical Basis of Neural Signaling

Basics of Electrochemistry

• Electrical Potential (Voltage): Difference in electrical charge between two points.

• Membrane Potential: Electrical potential across the plasma membrane, typically expressed as inside relative to outside. For neurons, resting potential is about -60 to -80 mV.

Plasma Membrane Selective Permeability

• Ion Leak Channels: Allow ions to cross along electrochemical gradients.

• Cotransporter/Antiporter Proteins: Carry ions across the membrane.

• ATPase Pumps: Pump ions against electrochemical gradients using ATP.

Na+/K+ ATPase Pumps

• Cl-/Na+: Cross plasma membrane less readily than K+ due to fewer leak channels.

• Na+/K+ ATPase: Imports K+ ions and exports Na+ ions, maintaining higher K+ inside and higher Na+ outside the cell. This creates a negative resting potential inside the neuron.

Action Potentials

Neurotoxins and Neuron Function

• Neurotoxins: Poisons that affect neuron function, such as tetrodotoxin (from puffer fish), which blocks voltage-gated Na+ channels and prevents action potentials.

Stages of Action Potential

• Resting Potential: Most voltage-gated Na+ and K+ channels are closed.

• Initiation: Voltage-gated Na+ channels open, Na+ flows into the cell, depolarizing the membrane.

• Rising Phase: Threshold is crossed, membrane potential increases.

• Falling Phase: Na+ channels inactivate, K+ channels open, K+ flows out.

• Undershoot: Membrane permeability to K+ is higher than at rest, then K+ channels close and resting potential is restored via Na+/K+ ATPase pumps.

Refractory Period

• Refractory Period: Temporary inactivation of Na+ channels prevents initiation of a second action potential.

• All-or-None Response: Action potential is regenerated as it moves down the axon, with undiminished signal strength.

Action Potential Propagation

• One-Way Propagation: Due to refractory Na+ channels, action potentials only move forward along the axon.

Axon Diameter and Signal Propagation

• Large Diameter Neurons: Transmit action potentials faster due to fewer sodium channels per unit area and less current leakage.

Myelination and Signal Transmission

Schwann Cells and Myelin Sheaths

• Glia: Accessory cells (Schwann cells in PNS, oligodendrocytes in CNS) wrap around axons, forming myelin sheaths.

• Myelin Sheath: Acts as electrical insulation, preventing charge leakage and allowing smaller axons to transmit signals rapidly.

Myelin Sheaths and Nodes of Ranvier

• Nodes of Ranvier: Unmyelinated sections of axon with high concentrations of voltage-gated Na+ channels, enabling action potentials to occur.

• Saltatory Conduction: Electrical signals jump from node to node, increasing transmission speed.

• Multiple Sclerosis (MS): Degeneration of myelin impairs electrical signaling and coordination.

Synapses and Neurotransmission

Synapses

• Synapse: Interface between two neurons; presynaptic neuron sends signal, postsynaptic neuron receives.

• Synaptic Vesicles: Store neurotransmitters at the axon terminal.

Neurotransmitters

• Definition: Molecules that transmit information from presynaptic neuron to another neuron, muscle, or gland.

• Requirements: Must be present at synapse, released in response to action potentials, bind to postsynaptic receptors, and be taken up or degraded.

Mechanism of Synaptic Transmission

1. Action potential arrives at axon terminal.

2. Triggers Ca2+ entry into presynaptic cell via voltage-gated channels.

3. Synaptic vesicles fuse with presynaptic membrane.

4. Vesicles release neurotransmitters into synaptic cleft.

5. Neurotransmitters bind to ligand-gated ion channels on postsynaptic membrane.

6. If threshold is reached, action potential is initiated in postsynaptic cell.

7. Response ends as neurotransmitter is broken down or retaken up.

Ligand-Gated Channels

• Ligands: Molecules (e.g., neurotransmitters) that bind to specific receptor sites.

• Channel Opening: Binding allows ion flow along electrochemical gradient, transducing chemical signal to electrical signal and changing postsynaptic membrane potential.

Postsynaptic Potentials: IPSPs and EPSPs

Types of Postsynaptic Potentials

• Excitatory Postsynaptic Potentials (EPSPs): Cause membrane depolarization, increasing likelihood of action potential.

• Inhibitory Postsynaptic Potentials (IPSPs): Cause membrane hyperpolarization, decreasing likelihood of action potential.

Summation and Modulation

• Neurons receive hundreds to thousands of synaptic inputs.

• EPSPs and IPSPs are graded in size and can be summed (spatial and temporal summation).

• Simultaneous EPSPs and IPSPs can cancel each other out, modulating the neuron's response.

Axon Hillock: Strategic Location for Action Potential Initiation

• Voltage-gated sodium channels are concentrated at the axon hillock.

• Charges from EPSPs/IPSPs spread to the axon hillock, where action potentials are triggered if threshold is reached.

Peripheral Nervous System Functions

PNS Divisions

• Afferent Division: Transmits sensory information to the CNS.

• Efferent Division: Carries commands from CNS to body (motor functions).

Efferent Division Subsystems

• Somatic System: Controls voluntary movement.

• Autonomic System: Controls involuntary internal processes.

Autonomic Nervous System Divisions

• Parasympathetic: Promotes relaxation, digestion, and maintenance functions.

• Sympathetic: Corresponds to arousal and energy expenditure ("fight or flight").

Parts of the Brain and Their Functions

Major Brain Structures

• Spinal Cord: Conduit for information traveling to/from the brain; processes some reflexes.

• Cerebrum: Involved in conscious thought and memory; divided into hemispheres.

• Cerebellum: Coordinates complex motor patterns and balance.

• Diencephalon: Includes hypothalamus; relays sensory information and controls homeostasis.

• Brain Stem: Connects brain to spinal cord; regulates autonomic functions.

Summary Table: Nervous System Components

Component	Main Function	Location
CNS	Integration, processing	Brain, spinal cord
PNS	Sensory input, motor output	Outside CNS
Neuron	Signal transmission	CNS & PNS
Glia	Support, insulation	CNS & PNS
Myelin Sheath	Electrical insulation	Axons
Synapse	Signal transfer	Between neurons
Neurotransmitter	Chemical signaling	Synaptic cleft

Key Equations

• Membrane Potential: $V_m = V_{inside} - V_{outside}$

• Resting Potential (typical neuron): $V_{rest} \approx -60 \text{ to } -80 \text{ mV}$

Example: Tetrodotoxin Effect

Tetrodotoxin (from puffer fish) blocks voltage-gated Na+ channels, preventing action potentials and causing paralysis.

Additional info:

• Summation of EPSPs and IPSPs at the axon hillock determines whether an action potential is initiated.

• Saltatory conduction in myelinated axons greatly increases the speed of neural signaling.