Nervous System Structure and Function: Organization, Neurons, and Electrical Signaling

Nervous System Organization

Diversity of Nervous System Patterns

The nervous system is responsible for coordinating responses to internal and external stimuli. Its structure varies widely among animal groups, reflecting evolutionary adaptations.

• Nerve Nets: The simplest nervous systems, found in cnidarians (e.g., Hydra), consist of interconnected nerve cells forming a diffuse network without a central brain.

• Radial Nerves and Nerve Rings: Echinoderms (e.g., sea stars) possess a nerve ring and radial nerves, allowing coordination of movement without a true brain.

• Cephalization: The development of a head region with a concentration of nervous tissue (brain) leads to more complex nervous systems, as seen in flatworms, annelids, arthropods, mollusks, and chordates.

Example: Insects have a brain and ventral nerve cord with segmental ganglia, while vertebrates (e.g., salamanders) have a brain, spinal cord, and sensory ganglia.

Central and Peripheral Nervous Systems

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Includes all neurons and projections outside the CNS; transmits signals between the CNS and the rest of the body.

• In some invertebrates, the distinction between CNS and PNS is not clear or absent.

Neurons: Structure and Function

Definition and Distribution

• Neurons: Specialized cells that send and receive electrical and chemical signals throughout the body.

• All animals except sponges possess neurons.

• The number of neurons varies with organism size and behavioral complexity.

Neuron Structure

• Cell Body (Soma): Contains the nucleus and organelles; metabolic center of the neuron.

• Dendrites: Extensions of the plasma membrane; receive incoming signals; may be single or highly branched.

• Axon: Single, long extension; transmits signals away from the cell body; may be insulated by a myelin sheath for faster conduction.

• Axon Hillock: Region where the axon joins the cell body; site where action potentials are initiated.

• Terminal Branches (Synaptic Terminals): Endings of the axon that release neurotransmitters to communicate with other cells.

Synapses and Neurotransmitters

• Synapse: Junction between a presynaptic neuron and a postsynaptic cell (neuron, muscle, or gland).

• Neurotransmitters: Chemical messengers stored in synaptic terminals; released to transmit signals across the synaptic cleft.

Clusters of Neuron Cell Bodies

• Ganglion: Cluster of nerve cell bodies in the PNS.

• Nucleus: Cluster of nerve cell bodies in the CNS.

Glial Cells

Types and Functions

Glia are non-neuronal cells that support and protect neurons. They are more numerous than neurons and perform various essential functions.

• Oligodendrocytes (CNS) and Schwann Cells (PNS): Produce myelin sheath, which insulates axons and increases signal speed.

• Astrocytes: Provide metabolic support to neurons.

• Microglia: Remove cellular debris and act as immune cells in the CNS.

• Radial Glia: Guide neuronal migration during embryonic development; can function as stem cells.

• Radial glia and astrocytes can generate new glial cells and neurons.

Types of Neurons and Neural Pathways

Classification of Neurons

• Sensory Neurons (Afferent): Detect external or internal stimuli and transmit information to the CNS.

• Motor Neurons (Efferent): Carry signals away from the CNS to effectors (muscles or glands) to elicit a response.

• Interneurons: Connect neurons within the CNS; integrate sensory input and coordinate motor output.

Reflex Arc

A reflex arc is a simple neural pathway that mediates a rapid, involuntary response to a stimulus.

• Sensory neuron detects stimulus and sends signal to CNS.

• Signal is transmitted directly (with little or no interpretation) to motor neurons.

• Motor neurons elicit a response in effectors (e.g., muscle contraction).

Electrical Properties of Neurons

Membrane Potential

• Membrane Potential: The difference in electrical charge across the plasma membrane, due to unequal distribution of ions.

• The plasma membrane acts as a barrier, separating charges and creating a potential difference.

• Typical resting membrane potential in neurons is about -70 mV (inside more negative than outside).

Factors Contributing to Resting Potential

1. Na+/K+-ATPase (Sodium-Potassium Pump): Actively transports 3 Na+ ions out and 2 K+ ions into the cell per ATP hydrolyzed. Equation: $3\,\mathrm{Na}^+_{\text{in}} + 2\,\mathrm{K}^+_{\text{out}} + \mathrm{ATP} \rightarrow 3\,\mathrm{Na}^+_{\text{out}} + 2\,\mathrm{K}^+_{\text{in}} + \mathrm{ADP} + \mathrm{P}_i$

2. Ion-Specific Channels: Allow passive movement of ions; K+ channels are more frequently open at rest, making the membrane more permeable to K+.

3. Negatively Charged Molecules: Proteins and other anions are more abundant inside the cell, contributing to the negative resting potential.

Electrochemical Gradient

• The combined effect of the electrical gradient (charge difference) and chemical gradient (ion concentration difference) drives ion movement across the membrane.

Neuronal Signaling: Graded and Action Potentials

Graded Potentials

• Small changes in membrane potential; can be depolarizing or hyperpolarizing.

• Magnitude varies with stimulus strength; occurs locally and decays with distance.

Depolarization and Hyperpolarization

• Depolarization: Membrane potential becomes less negative (e.g., Na+ influx).

• Hyperpolarization: Membrane potential becomes more negative (e.g., K+ efflux).

Action Potential: All-or-Nothing Response

• If graded potentials sum to reach the threshold potential (about -50 mV), an action potential is triggered.

• Action potentials are rapid, large changes in membrane potential that propagate along the axon.

Phases of the Action Potential

1. Resting State: Both Na+ and K+ channels are closed; membrane at resting potential.

2. Threshold: Stimulus causes depolarization to threshold; some Na+ channels open.

3. Depolarization: Many voltage-gated Na+ channels open; Na+ rushes in, membrane potential becomes positive.

4. Repolarization: Na+ channels inactivate; K+ channels open, K+ exits, membrane potential returns toward negative.

5. Undershoot (Hyperpolarization): K+ channels remain open longer, membrane potential becomes more negative than resting.

Refractory Period

• During the undershoot, Na+ channels are inactivated; neuron cannot fire another action potential (ensures one-way propagation).

• Limits the frequency of action potentials.

Evolutionary Note

• K+ channels evolved to open more slowly than Na+ channels, allowing proper action potential formation.

Conduction of Action Potentials

Propagation Along the Axon

• Action potentials are regenerated at each segment of the axon, ensuring signal does not diminish with distance.

• The refractory period prevents backward movement of the action potential.

Factors Affecting Conduction Speed

• Axon Diameter: Larger diameter reduces resistance, increasing speed.

• Myelination: Myelin sheath insulates axon; action potentials jump between nodes of Ranvier (saltatory conduction), greatly increasing speed.

Summary Table: Key Nervous System Components

Component	Location	Function
Neuron	CNS & PNS	Transmit electrical and chemical signals
Glia	CNS & PNS	Support, insulate, and protect neurons
Ganglion	PNS	Cluster of neuron cell bodies
Nucleus	CNS	Cluster of neuron cell bodies
Myelin Sheath	Axons (CNS: oligodendrocytes; PNS: Schwann cells)	Insulation, increases conduction speed

Additional info: This guide covers the structure and function of nervous systems, neuron anatomy, glial cell roles, and the mechanisms of electrical signaling, as outlined in General Biology chapters on nervous system organization and function.