Study Guide: Nervous System Structure and Function

Nervous System Overview

Functions of the Nervous System

The nervous system is responsible for coordinating the body's responses to internal and external stimuli. It integrates sensory information, controls motor functions, and maintains homeostasis.

• Sensory Input: Detects changes inside and outside the body.

• Integration: Processes and interprets sensory input to determine an appropriate response.

• Motor Output: Activates effector organs (muscles and glands) to cause a response.

Organs of the Central Nervous System (CNS)

• Brain: The control center for processing information and directing responses.

• Spinal Cord: Conducts signals to and from the brain and controls reflex activities.

Divisions of the Nervous System

• Central Nervous System (CNS): Brain and spinal cord.

• Peripheral Nervous System (PNS): All neural tissue outside the CNS, including cranial and spinal nerves.

• Somatic Motor Division: Controls voluntary movements of skeletal muscles.

• Autonomic Motor Division: Controls involuntary actions of smooth muscle, cardiac muscle, and glands.

Classification of Neurons

• Structural: Based on the number of processes extending from the cell body (multipolar, bipolar, unipolar).

• Functional: Based on the direction of impulse conduction (sensory/afferent, motor/efferent, interneurons/association).

Neuronal Structure and Function

Neuron Anatomy

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

• Dendrites: Receive incoming signals.

• Axon: Transmits electrical impulses away from the cell body.

• Axon Terminals (Synaptic Knobs): Release neurotransmitters to communicate with target cells.

Types of Neurons

• Multipolar Neurons: Many dendrites, one axon; most common in CNS.

• Bipolar Neurons: One dendrite, one axon; found in sensory organs (e.g., retina).

• Unipolar Neurons: Single process; mainly sensory neurons in PNS.

• Interneurons (Association Neurons): Connect neurons within the CNS for integration.

Neuroglia (Glial Cells)

• Astrocytes, Oligodendrocytes, Microglia, Ependymal Cells (CNS)

• Schwann Cells, Satellite Cells (PNS)

• Support, protect, and nourish neurons.

Neural Communication

Membrane Potentials

Neurons communicate via changes in membrane potential, primarily through the movement of ions across the plasma membrane.

• Resting Membrane Potential: The voltage difference across the membrane of a resting neuron, typically about -70 mV.

• Maintained by: Sodium-potassium pump ($3 \text{Na}^+$ out, $2 \text{K}^+$ in), and differential permeability of the membrane to ions.

• Depolarization: Membrane potential becomes less negative (more positive).

• Repolarization: Return to resting membrane potential after depolarization.

• Hyperpolarization: Membrane potential becomes more negative than resting.

Action Potential

An action potential is a rapid, temporary change in membrane potential that travels along the axon.

1. Local depolarization opens voltage-gated sodium channels; $\text{Na}^+$ enters the cell.

2. Membrane potential rises toward zero and becomes positive.

3. At peak, sodium channels close and voltage-gated potassium channels open; $\text{K}^+$ leaves the cell.

4. Repolarization occurs as the membrane returns to resting potential.

5. Hyperpolarization may occur before returning to resting state.

Refractory Periods:

• Absolute Refractory Period: No new action potential can be initiated.

• Relative Refractory Period: A stronger-than-normal stimulus can initiate another action potential.

Propagation of Action Potentials

• Continuous Conduction: Occurs in unmyelinated axons; action potential moves along every segment.

• Saltatory Conduction: Occurs in myelinated axons; action potential jumps from node to node (Nodes of Ranvier), increasing speed.

• Fiber Types: A (fastest, myelinated), B (intermediate), C (slowest, unmyelinated).

Synaptic Transmission

Synapses

A synapse is a junction between two neurons or between a neuron and an effector cell.

• Presynaptic Neuron: Sends the signal.

• Postsynaptic Neuron: Receives the signal.

• Synaptic Cleft: The gap between the two neurons.

• Neurotransmitters: Chemical messengers released from synaptic vesicles in response to calcium influx.

Types of Synapses

• Cholinergic Synapses: Use acetylcholine as the neurotransmitter.

• Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic membrane, bringing it closer to threshold.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane, moving it further from threshold.

Neuropeptides

• Small protein-like molecules used by neurons to communicate.

• Modulate synaptic transmission and can act as neurotransmitters or neuromodulators.

Neural Circuits

Converging Circuits

In a converging circuit, multiple presynaptic neurons synapse onto a single postsynaptic neuron, allowing integration of information from various sources.

• Example: Sensory input from different body regions converging on a single motor neuron.

Key Terms and Definitions

• Tract: Bundle of axons in the CNS.

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

• Axon Hillock: Region where the axon originates from the cell body; site of action potential initiation.

• Telodendria: Fine extensions at the end of the axon.

Table: Comparison of Neuron Types

Type	Structure	Location	Function
Multipolar	Many dendrites, one axon	CNS	Motor and interneurons
Bipolar	One dendrite, one axon	Retina, olfactory epithelium	Sensory
Unipolar	Single process	PNS (sensory neurons)	Sensory

Key Equations

• Nernst Equation (for equilibrium potential):

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]_{outside}}{[\text{ion}]_{inside}} \right)$

• Sodium-Potassium Pump: 3 $\text{Na}^+$ out, 2 $\text{K}^+$ in per ATP hydrolyzed.

Additional info:

• Some questions referenced specific processes (e.g., propagation of action potentials, synaptic transmission) that have been expanded with academic context for clarity.

• Definitions and examples have been added to ensure the notes are self-contained and suitable for exam preparation.