Study Guide: Nervous System Structure and Function

Nervous System Overview

Main Divisions of the Nervous System

The nervous system is divided into two primary components: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). Each division has distinct structures and functions essential for body regulation and response.

• CNS: Composed of the brain and spinal cord; responsible for processing and integrating information.

• PNS: Consists of all neural tissue outside the CNS; subdivided into the somatic and autonomic nervous systems.

• Somatic Nervous System: Controls voluntary movements and transmits sensory information.

• Autonomic Nervous System: Regulates involuntary functions; further divided into sympathetic and parasympathetic divisions.

General Components of CNS and PNS

• CNS: Brain (cerebrum, cerebellum, brainstem) and spinal cord.

• PNS: Cranial nerves, spinal nerves, ganglia, and sensory receptors.

Neurons and Neuroglia

Main Parts of a Neuron and Their Functions

Neurons are specialized cells for transmitting electrical and chemical signals.

• Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals.

• Dendrites: Receive signals from other neurons.

• Axon: Conducts impulses away from the cell body.

• Axon Terminals: Release neurotransmitters to communicate with other cells.

Synapses and Neurotransmitters

A synapse is the junction between two neurons or between a neuron and another cell. Neurotransmitters are chemicals released at synapses to transmit signals.

• Presynaptic Cell: Releases neurotransmitter.

• Postsynaptic Cell: Receives neurotransmitter and responds.

• Function: Enables communication and integration of neural signals.

Neuronal Regeneration

Most CNS neurons cannot divide or regenerate due to the absence of certain growth factors and inhibitory environment. PNS neurons have a greater capacity for regeneration.

Classification of Neurons

• Structural: Multipolar, bipolar, unipolar.

• Functional: Sensory (afferent), motor (efferent), interneurons.

Neural Tissue Organization

Connective Tissue Sheaths

Nerves are organized by connective tissue layers:

• Endoneurium: Surrounds individual axons.

• Perineurium: Encloses bundles (fascicles) of axons.

• Epineurium: Surrounds the entire nerve.

Neuroglia of the CNS

There are four main types of neuroglia in the CNS, each with specific functions:

Type	Function
Astrocytes	Support neurons, maintain blood-brain barrier
Oligodendrocytes	Form myelin sheaths in CNS
Microglia	Phagocytosis of debris and pathogens
Ependymal cells	Produce and circulate cerebrospinal fluid

Myelination and Signal Transmission

Myelinated vs. Unmyelinated Axons

Myelination increases the speed of action potential transmission by enabling saltatory conduction.

• Myelinated Axons: Action potentials jump between nodes of Ranvier, increasing speed.

• Unmyelinated Axons: Action potentials propagate continuously, slower transmission.

Types of Glia in the PNS

• Schwann Cells: Form myelin sheaths in the PNS.

• Satellite Cells: Support neuron cell bodies in ganglia.

Membrane Potentials and Action Potentials

Resting Membrane Potential

The resting membrane potential is the voltage difference across the cell membrane when the cell is at rest, typically around -70 mV in neurons.

• Maintained by ion gradients and selective permeability.

• Key factors: sodium-potassium pump, leak channels.

Chemical and Electrical Gradients

• Chemical Gradient: Difference in ion concentration across the membrane.

• Electrical Gradient: Difference in charge across the membrane.

• Electrochemical Gradient: Combined effect of chemical and electrical gradients.

Gated Channels

There are three main types of gated channels:

• Voltage-gated: Open in response to changes in membrane potential.

• Chemically-gated (ligand-gated): Open in response to binding of a specific molecule.

• Mechanically-gated: Open in response to physical deformation of the membrane.

Action Potential Formation

Action potentials are rapid changes in membrane potential that propagate along axons.

• Depolarization: Membrane potential becomes less negative.

• Repolarization: Membrane potential returns to resting value.

• Hyperpolarization: Membrane potential becomes more negative than resting.

Steps in Action Potential:

1. Resting state: All channels closed.

2. Depolarization: Voltage-gated Na+ channels open.

3. Repolarization: Voltage-gated K+ channels open, Na+ channels close.

4. Hyperpolarization: K+ channels remain open briefly.

5. Return to resting potential.

Equation:

$V_m = V_{inside} - V_{outside}$

Refractory Periods

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

• Relative Refractory Period: Action potential possible with a stronger stimulus.

Propagation of Action Potentials

• Continuous Propagation: Occurs in unmyelinated axons; slower.

• Saltatory Propagation: Occurs in myelinated axons; faster due to jumping between nodes.

Synaptic Transmission and Summation

EPSP, IPSP, and Summation

• EPSP (Excitatory Postsynaptic Potential): Depolarizes the postsynaptic membrane, increasing likelihood of action potential.

• IPSP (Inhibitory Postsynaptic Potential): Hyperpolarizes the postsynaptic membrane, decreasing likelihood of action potential.

• Summation: Integration of multiple EPSPs and IPSPs; can be temporal (rapid succession) or spatial (simultaneous inputs).

Temporal vs. Spatial Summation

• Temporal Summation: Multiple signals from one presynaptic neuron in rapid succession.

• Spatial Summation: Signals from multiple presynaptic neurons at the same time.

Information Transfer at Synapses

Neurotransmitters released from the presynaptic cell bind to receptors on the postsynaptic cell, causing changes in membrane potential and cellular response.

Summary Table: Key Nervous System Concepts

Concept	Definition	Example/Application
CNS	Brain and spinal cord	Processing sensory input
PNS	Nerves outside CNS	Transmitting signals to muscles
Neuron	Signal-transmitting cell	Motor neuron activating muscle
Synapse	Junction between neurons	Neurotransmitter release
Action Potential	Rapid change in membrane potential	Nerve impulse conduction
EPSP/IPSP	Excitatory/Inhibitory postsynaptic potentials	Integration in postsynaptic neuron