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, allowing organisms to interact with their environment. It is responsible for rapid communication and control throughout the body.

• Sensory Input: The nervous system detects internal and external stimuli via sensory receptors.

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

• Motor Output: Initiates a response by activating effector organs (muscles or glands).

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

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): 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 and spinal cord.

• Voluntary vs. Involuntary Nervous Systems:

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

  • Involuntary (Autonomic): Regulates unconscious functions (smooth muscle, cardiac muscle, glands).

Neuroglia and Neurons

Neuroglia (Glial Cells)

Neuroglia are supporting cells in nervous tissue, distinct from neurons. They provide structural and metabolic support, maintain homeostasis, and protect neurons.

• Differences from Neurons: Neuroglia do not transmit electrical impulses; neurons do.

• CNS Neuroglia:

  • Astrocytes: Support and regulate the environment around neurons.

  • Microglia: Immune defense cells; phagocytize debris.

  • Ependymal Cells: Line ventricles; produce and circulate cerebrospinal fluid.

  • Oligodendrocytes: Form myelin sheaths in CNS.

• PNS Neuroglia:

  • Satellite Cells: Surround neuron cell bodies in ganglia; regulate environment.

  • Schwann Cells: Form myelin sheaths in PNS.

Neurons

Neurons are excitable cells that transmit electrical signals. They are the functional units of the nervous system.

• Special Characteristics: Longevity, amitotic (do not divide), high metabolic rate.

• Neuron Cell Body: Contains nucleus, cytoplasm, organelles; site of biosynthetic activity.

• CNS vs. PNS Cell Bodies:

  • CNS: Cell bodies grouped in nuclei.

  • PNS: Cell bodies grouped in ganglia.

• Neuron Processes:

  • Dendrites: Receive signals; short, branched.

  • Axon: Conducts impulses away from cell body; long, singular.

Myelin Sheath

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

• Myelinated vs. Nonmyelinated Axons:

  • Myelinated: Rapid conduction; saltatory transmission.

  • Nonmyelinated: Slower conduction; continuous transmission.

• Differences in CNS and PNS:

  • CNS: Myelin formed by oligodendrocytes.

  • PNS: Myelin formed by Schwann cells.

Classification of Neurons

Structural Classification

Neurons are classified by the number and arrangement of their processes.

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

Functional Classification

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

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

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

Basic Principles of Electricity in Neurons

Membrane Ion Channels

Ion channels are proteins that allow ions to cross the neuronal membrane, crucial for electrical signaling.

• Types of Ion Channels:

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

  • Gated Channels: Open/close in response to 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: Difference in ion concentration across a membrane; drives passive movement.

Resting Membrane Potential

The resting membrane potential is the voltage difference across the membrane of a resting neuron, typically around -70 mV.

• Polarized: The membrane has a negative charge inside relative to outside.

• Voltmeter: Instrument used to measure membrane potential.

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

Changing the Resting Membrane Potential

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

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

• Depolarization: Membrane potential becomes less negative.

• Hyperpolarization: Membrane potential becomes more negative.

Action Potentials

Definition and Steps

An action potential is a brief electrical impulse that travels along the axon, allowing communication between neurons.

• Four Main Steps:

  1. Resting State: All gated channels closed; membrane at resting potential.

  2. Depolarization: Na+ channels open; influx of Na+ makes inside positive.

  3. Repolarization: K+ channels open; efflux of K+ restores negative charge.

  4. Hyperpolarization: K+ channels remain open; membrane becomes more negative than resting.

$\text{Action Potential:}$ $\text{Resting} \rightarrow \text{Depolarization} \rightarrow \text{Repolarization} \rightarrow \text{Hyperpolarization}$

Propagation of Action Potential

Propagation refers to the movement of the action potential along the axon.

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

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

• Role of Na+: Influx of sodium ions initiates depolarization.

Clinical Homeostatic Imbalance: Multiple Sclerosis (MS)

Overview

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

• Who it affects: Primarily young adults, more common in women.

• Symptoms: Visual disturbances, muscle weakness, loss of coordination, speech difficulties.

• Treatment: Immunosuppressive drugs, physical therapy, symptom management.

Example: In MS, demyelination slows or blocks nerve impulses, leading to neurological deficits.