Fundamentals of the Nervous System and Nervous Tissue: Study Guide

Chapter 11: Fundamentals of the Nervous System and Nervous Tissue

Overview

The nervous system is a complex network responsible for receiving, integrating, and responding to information. It is composed of specialized cells and supporting structures that allow for rapid communication throughout the body. This chapter covers the structure and function of neurons and neuroglia, the mechanisms of nerve signaling, and the integration of neural activity.

The Nervous System: Structure and Function

Main Functions of the Nervous System

• Sensory Input: Gathering information from sensory receptors about internal and external changes.

• Integration: Processing and interpreting sensory input to determine an appropriate response.

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

Example: Touching a hot surface activates sensory receptors in the skin, which send signals to the brain (integration), resulting in the rapid withdrawal of the hand (motor output).

Divisions of the Nervous System

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integration and command.

• Peripheral Nervous System (PNS): Consists of cranial and spinal nerves; connects the CNS to the rest of the body.

Organization of the Nervous System

Main Division	Subdivisions
CNS	Brain, Spinal Cord
PNS	Sensory (Afferent) Division, Motor (Efferent) Division
Motor Division	Somatic Nervous System (voluntary), Autonomic Nervous System (involuntary)
Autonomic Division	Sympathetic Division, Parasympathetic Division

Neuroglia: Support and Maintenance of Neurons

Types of Neuroglia and Their Functions

• Astrocytes: Support neurons, regulate the blood-brain barrier, and maintain the extracellular environment.

• Microglial Cells: Act as immune defense cells in the CNS.

• Ependymal Cells: Line cerebrospinal fluid-filled cavities and help circulate cerebrospinal fluid.

• Oligodendrocytes: Form myelin sheaths around CNS nerve fibers.

• Satellite Cells: Surround neuron cell bodies in the PNS.

• Schwann Cells: Form myelin sheaths around PNS nerve fibers.

Neurons: Structure and Function

Structural Components of Neurons

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

• Dendrites: Receive incoming signals from other neurons.

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

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

Classification of Neurons

• Multipolar: Many processes extend from the cell body; most common type in the CNS.

• Bipolar: Two processes (axon and dendrite); found in special sensory organs.

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

Functional Types of Neurons

• Sensory (Afferent) Neurons: Transmit impulses from receptors to the CNS.

• Motor (Efferent) Neurons: Carry impulses from the CNS to effectors (muscles/glands).

• Interneurons: Shuttle signals within the CNS; most abundant type.

Myelin Sheath

Importance and Formation

• Function: Increases the speed of nerve impulse transmission and insulates axons.

• Formation in CNS: By oligodendrocytes.

• Formation in PNS: By Schwann cells.

Myelin Sheath Gaps (Nodes of Ranvier): Allow for saltatory conduction, where action potentials jump from node to node, greatly increasing conduction velocity.

Membrane Potentials and Ion Channels

Current, Voltage, and Resistance

• Current (I): Flow of electrical charge ($I$)

• Voltage (V): Potential energy generated by separated charges ($V$)

• Resistance (R): Hindrance to charge flow ($R$)

Ohm's Law:

$I = \frac{V}{R}$

Types of Ion Channels

• Leakage Channels: Always open; allow ions to move along their gradients.

• Chemically Gated Channels: Open in response to binding of a neurotransmitter.

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

• Mechanically Gated Channels: Open in response to physical deformation of the membrane.

Resting Membrane Potential

• Typically about $-70$ mV in neurons.

• Maintained by differences in ion concentrations (mainly $K^+$ and $Na^+$) and selective permeability of the plasma membrane.

• Sodium-potassium pumps help stabilize the resting potential by moving $3Na^+$out and $2$ $K^+$ in per ATP hydrolyzed.

Graded and Action Potentials

Graded Potentials

• Short-distance, localized changes in membrane potential.

• Can be depolarizing or hyperpolarizing.

• Magnitude varies with stimulus strength.

Action Potentials

• Long-distance signals of axons.

• All-or-none phenomenon: once threshold is reached, the action potential always occurs with the same amplitude.

• Phases: depolarization, repolarization, hyperpolarization.

Characteristic	Graded Potentials	Action Potentials
Amplitude	Varies with stimulus	Always the same
Distance	Short	Long
Summation	Possible	Not possible
Initiation	By stimulus or neurotransmitter	By graded potential reaching threshold

Refractory Periods

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

• Relative Refractory Period: A stronger-than-usual stimulus is required to initiate another action potential.

Synapses and Neurotransmission

Types of Synapses

• Electrical Synapses: Direct flow of ions through gap junctions; rapid communication.

• Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft.

Description	Electrical Synapses	Chemical Synapses
Most common type		X
Direct cell-to-cell exchange	X
Neurotransmitter release		X
Channel-containing gap junctions	X
Transmission is unidirectional		X

Events at a Chemical Synapse

1. Action potential arrives at axon terminal.

2. Voltage-gated $Ca^{2+}$ channels open; $Ca^{2+}$ enters the terminal.

3. Neurotransmitter is released into the synaptic cleft.

4. Neurotransmitter binds to receptors on the postsynaptic membrane.

5. Ion channels open, causing graded potentials.

Postsynaptic Potentials

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

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

Neurotransmitters

Classification and Function

• Acetylcholine (ACh): Excitatory at neuromuscular junctions; can be inhibitory elsewhere.

• Amino Acids: Glutamate (excitatory), GABA (inhibitory), Glycine (inhibitory).

• Biogenic Amines: Dopamine, norepinephrine, serotonin; involved in mood and emotional behaviors.

• Neuropeptides: Substance P, endorphins; modulate pain and other functions.

Neurotransmitter Receptors

• Channel-Linked (Ionotropic) Receptors: Mediate fast synaptic transmission.

• G Protein-Coupled (Metabotropic) Receptors: Mediate slower, longer-lasting effects.

Description	Channel-Linked Receptors	G Protein-Coupled Receptors
Simple, immediate response	X
Complex, prolonged response		X

Neural Integration and Processing

Patterns of Neural Processing

• Serial Processing: Information travels along one pathway to a specific destination (e.g., reflex arc).

• Parallel Processing: Information is processed simultaneously along several pathways, allowing for complex responses.

Neuronal Circuits

• Diverging Circuit: One input, many outputs (e.g., muscle contraction).

• Converging Circuit: Many inputs, one output (e.g., sensory integration).

• Reverberating Circuit: Signal travels through a chain of neurons, each feeding back to previous neurons (e.g., rhythmic activities).

• Parallel After-Discharge Circuit: One input, many outputs at different times (e.g., complex mental processing).

Additional info: This study guide is based on structured questions and learning objectives from a college-level Anatomy & Physiology textbook, focusing on the nervous system and nervous tissue. It is suitable for exam preparation and review.