Chapter 11: Fundamentals of the Nervous System and Nervous Tissue - Study Guide

The Nervous System: Overview

Definition and Functions

The nervous system is a complex network of cells that receives, integrates, and responds to information from both internal and external environments. It is essential for maintaining homeostasis and coordinating bodily functions.

• Sensory Input: The nervous system detects changes (stimuli) through sensory receptors.

• Integration: Processes and interprets sensory input, deciding what action is needed.

• Motor Output: Responds by activating effector organs (muscles or glands).

• Example: Touching a hot surface triggers sensory input (pain), integration (processing in the brain), and motor output (moving the hand away).

Organization of the Nervous System

CNS and PNS Divisions

The nervous system is divided into two main parts:

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

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS. Connects the CNS to the rest of the body.

PNS Functional Divisions

• Sensory (Afferent) Division: Carries information to the CNS.

• Motor (Efferent) Division: Transmits commands from the CNS to effectors.

CNS Divisions

• Brain: Main control center.

• Spinal Cord: Pathway for signals between brain and body.

Voluntary vs. Involuntary Nervous Systems

• Voluntary (Somatic) Nervous System: Controls conscious movements (skeletal muscles).

• Involuntary (Autonomic) Nervous System: Regulates automatic functions (heart rate, digestion).

Neuroglia (Glial Cells)

Definition and Differences from Neurons

Neuroglia are supporting cells in nervous tissue, distinct from neurons. They do not transmit electrical impulses but provide structural and metabolic support.

• Neurons: Excitable cells that transmit signals.

• Neuroglia: Support, protect, and nourish neurons.

Main Neuroglia in CNS

• Astrocytes: Support and regulate the environment around neurons.

• Microglia: Immune defense cells.

• Ependymal Cells: Line ventricles, produce cerebrospinal fluid.

• Oligodendrocytes: Form myelin sheaths in CNS.

Main Neuroglia in PNS

• Satellite Cells: Surround neuron cell bodies in ganglia.

• Schwann Cells: Form myelin sheaths in PNS.

Neurons

Definition and Characteristics

A neuron is a specialized cell capable of transmitting electrical impulses. Key characteristics include:

• Long-lived

• Amitotic (do not divide after maturity)

• High metabolic rate

Neuron Cell Body

• Contains nucleus and organelles

• Site of metabolic activity

CNS vs. PNS Cell Bodies

• CNS: Cell bodies grouped in nuclei.

• PNS: Cell bodies grouped in ganglia.

Neuron Processes

• Dendrites: Receive signals.

• Axon: Transmits signals away from cell body.

Myelin Sheath

Definition and Types

The myelin sheath is a fatty layer that insulates axons, increasing the speed of impulse transmission.

• Myelinated Axons: Conduct impulses rapidly.

• Nonmyelinated Axons: Conduct impulses slowly.

Differences in CNS and PNS

• CNS: Myelin formed by oligodendrocytes.

• PNS: Myelin formed by Schwann cells.

Classification of Neurons

Structural Classes

Type	Structure	Location
Multipolar	Many dendrites, one axon	Most CNS neurons
Bipolar	One dendrite, one axon	Special senses (retina, olfactory)
Unipolar	Single process	Most sensory neurons in PNS

Functional Classes

• Sensory (Afferent) Neurons: Carry impulses to CNS.

• Motor (Efferent) Neurons: Carry impulses from CNS to effectors.

• Interneurons: Connect sensory and motor neurons; found in CNS.

Basic Principles of Electricity in Neurons

Role of Membrane Ion Channels

Neurons use ion channels to control the flow of ions across their membranes, which is essential for electrical signaling.

• Leak Channels: Always open; allow passive ion movement.

• Gated Channels: Open in response to specific stimuli.

• Types of Gated Channels:

  • Chemically Gated: Open in response to neurotransmitters.

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

  • Mechanically Gated: Open in response to physical deformation.

Concentration Gradient

A concentration gradient is the difference in ion concentration across a membrane, driving passive movement.

Resting Membrane Potential

Definition and Polarization

The resting membrane potential is the voltage difference across the membrane of a resting neuron, typically about -70 mV. The membrane is polarized because the inside is more negative than the outside.

• Voltmeter: Instrument used to measure membrane potential.

Changing the Resting Membrane Potential

Membrane Potential Changes

• Graded Potentials: Local changes in membrane potential; decrease with distance.

• Action Potentials: Large, rapid changes; propagate along axons.

Depolarization vs. Hyperpolarization

• Depolarization: Membrane potential becomes less negative.

• Hyperpolarization: Membrane potential becomes more negative.

Action Potentials

Definition and Steps

An action potential is a rapid, transient change in membrane potential that propagates along the axon.

• Resting: Membrane at -70 mV.

• Depolarization: Na+ channels open, membrane potential rises.

• Repolarization: K+ channels open, membrane potential falls.

• Hyperpolarization: Membrane potential temporarily becomes more negative than resting.

Equation for Resting Membrane Potential:

$V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

Additional info: This equation is a simplified version; the actual resting membrane potential depends on multiple ions.

Propagation of Action Potential

Mechanism and Differences

Propagation refers to the movement of the action potential along the axon, primarily due to Na+ influx.

• Nonmyelinated Axons: Action potential moves continuously along the axon.

• Myelinated Axons: Action potential jumps between nodes of Ranvier (saltatory conduction), increasing speed.

Clinical Homeostatic Imbalance: Multiple Sclerosis (MS)

Definition, Symptoms, and Treatment

Multiple Sclerosis (MS) is an autoimmune disease where the immune system attacks myelin in the CNS.

• Affected Population: Young adults, especially females.

• Symptoms: Muscle weakness, vision problems, impaired coordination.

• Treatment: Immunosuppressive drugs, physical therapy.

Additional info: MS leads to progressive loss of function due to impaired nerve conduction.