Nervous System & Nervous Tissue: Study Guide

Nervous System Overview

Functions and Organization of the Nervous System

The nervous system is a complex network responsible for controlling and communicating information throughout the body. It gathers sensory input, processes information, and initiates motor output.

• Control and Communication: The nervous system is considered the body's controlling and communicating system, integrating sensory and motor functions.

• Sensory Input: Sensory receptors detect internal and external changes, sending information to the central nervous system (CNS).

• Integration: The CNS processes and interprets sensory input, determining appropriate responses.

• Motor Output: The nervous system initiates responses by activating effector organs (muscles and glands).

• Divisions: The nervous system is divided into the CNS (brain and spinal cord) and the peripheral nervous system (PNS), which includes sensory and motor divisions.

Example: Touching a hot surface activates sensory receptors in the skin, which send signals to the CNS for processing, resulting in a rapid motor response (withdrawal).

Neuroglia (Glial Cells)

Types and Functions of Neuroglia

Neuroglia, or glial cells, are non-neuronal cells that support, protect, and nourish neurons in the nervous system. They play vital roles in maintaining homeostasis, forming myelin, and providing support.

• Astrocytes: Most abundant glial cells in the CNS; regulate the blood-brain barrier, support neurons, and maintain extracellular ion balance.

• Microglia: Act as phagocytes, cleaning up debris and pathogens in the CNS.

• Ependymal Cells: Line the ventricles of the brain and central canal of the spinal cord; circulate cerebrospinal fluid (CSF) with their cilia.

• Oligodendrocytes: Form myelin sheaths around axons in the CNS, increasing the speed of nerve impulse transmission.

• Schwann Cells: Form myelin sheaths around axons in the PNS; vital for regeneration of damaged peripheral nerve fibers.

• Satellite Cells: Surround neuron cell bodies in the PNS, providing support and regulating the environment.

Example: Multiple sclerosis (MS) is a disease where oligodendrocytes are damaged, leading to loss of myelin in the CNS.

Neurons

Structure and Function of Neurons

Neurons are the functional units of the nervous system, specialized for rapid communication. They have extreme longevity and minimal ability to divide.

• Cell Body (Soma): Contains the nucleus and organelles such as Golgi apparatus and Nissl bodies.

• Dendrites: Receive input signals from other neurons.

• Axon: Transmits electrical impulses away from the cell body; axon hillock is the site where action potentials are initiated.

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

• Myelin Sheath: Insulates axons, increasing the speed of impulse transmission.

• Clusters: In the CNS, neuron cell bodies are grouped in nuclei; in the PNS, they are grouped in ganglia.

• Bundles: Axon bundles are called tracts in the CNS and nerves in the PNS.

• Axonal Transport: Movement of materials away from the cell body is called anterograde transport.

Example: Damage to myelin sheaths slows nerve impulse transmission, affecting motor and sensory functions.

Membrane Potentials & Action Potentials

Electrical Properties of Neurons

Neurons communicate via electrical signals, which depend on membrane potentials and the movement of ions across the membrane.

• Resting Membrane Potential: Typically about $-70\ \text{mV}$; maintained by the sodium-potassium pump ($3\ \text{Na}^+$ out, $2\ \text{K}^+$ in).

• Depolarization: Membrane becomes less negative as sodium ions enter the cell.

• Repolarization: Potassium channels open, allowing $\text{K}^+$ to leave the cell, restoring negativity.

• Hyperpolarization: Excess $\text{K}^+$ leaves, making the membrane more negative than resting potential.

• Action Potential: Initiated at the axon hillock; all-or-none response; saltatory conduction occurs in myelinated axons, increasing speed.

Key Equation:

$\text{Resting Membrane Potential} = -70\ \text{mV}$

Example: During an action potential, rapid depolarization and repolarization allow neurons to transmit signals efficiently.

Synapses & Neurotransmitters

Communication Between Neurons

Synapses are specialized junctions where neurons communicate with other neurons or effector cells using neurotransmitters.

• Presynaptic Neuron: Sends signals toward the synapse.

• Types of Synapses: Chemical synapses are most common; electrical synapses are less frequent.

• Synaptic Vesicles: Contain neurotransmitters, which are released into the synaptic cleft upon arrival of an action potential.

• Neurotransmitter Release: Triggered by $\text{Ca}^{2+}$ entry into the axon terminal.

• Postsynaptic Potentials: Excitatory postsynaptic potential (EPSP) makes the cell more likely to fire an action potential; inhibitory postsynaptic potential (IPSP) makes it less likely.

• Neurotransmitters: Acetylcholine is used at neuromuscular junctions; dopamine, norepinephrine, and serotonin are biogenic amines; endorphins act as natural opiates.

• Modulation: Caffeine blocks adenosine receptors; THC acts on endocannabinoid receptors.

Example: At the neuromuscular junction, acetylcholine is released to stimulate muscle contraction.

Table: Major Neuroglia Types and Functions

Cell Type	Location	Main Function
Astrocytes	CNS	Support neurons, regulate blood-brain barrier, maintain ion balance
Microglia	CNS	Phagocytosis of debris and pathogens
Ependymal Cells	CNS	Line ventricles, circulate cerebrospinal fluid
Oligodendrocytes	CNS	Form myelin sheaths
Schwann Cells	PNS	Form myelin sheaths, aid regeneration
Satellite Cells	PNS	Support neuron cell bodies

Additional info:

• All major topics and subtopics were expanded with academic context and definitions.

• Key terms and processes were explained for clarity and completeness.

• Table was inferred and constructed based on standard neuroglia classification.