Nervous System and Nervous Tissue: Study Guide (CH 11)

Nervous System and Nervous Tissue

Basic Functions of the Nervous System

The nervous system is responsible for controlling and coordinating the activities of the body. It enables rapid communication between different body parts and responds to internal and external stimuli.

• 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 produce a response.

Structural and Functional Divisions of the Nervous System

The nervous system is divided into structural and functional components for organization and specialization.

• Structural Divisions:

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

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

• Functional Divisions:

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

  • Motor (Efferent) Division: Carries commands from the CNS to effectors. Subdivided into:

    • Somatic Nervous System: Controls voluntary movements of skeletal muscles.

    • Autonomic Nervous System: Regulates involuntary functions (e.g., heart rate, digestion).

Structural Components of a Neuron and Their Functional Roles

Neurons are specialized cells for transmitting electrical signals. Their structure is closely related to their function.

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

• Dendrites: Receive incoming signals and convey them toward the cell body.

• Axon: Conducts electrical impulses away from the cell body to other neurons or effectors.

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

• Myelin Sheath: Insulates axons and increases the speed of impulse transmission.

Classification of Neurons by Structure and Function

Neurons can be classified based on their shape and their role in the nervous system.

• Structural Classification:

  • Multipolar: Many dendrites, one axon (most common in CNS).

  • Bipolar: One dendrite, one axon (found in sensory organs).

  • Unipolar: Single process that splits into two branches (sensory neurons in PNS).

• Functional Classification:

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

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

  • Interneurons: Connect sensory and motor neurons within the CNS.

Functions of Neuroglia

Neuroglia (glial cells) support and protect neurons. They are essential for maintaining the environment of the nervous system.

• Astrocytes: Support neurons, regulate the blood-brain barrier, and maintain ion balance.

• Microglia: Act as immune cells, removing debris and pathogens.

• Ependymal Cells: Line ventricles and produce cerebrospinal fluid.

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Schwann Cells: Form myelin sheaths in the PNS.

• Satellite Cells: Support neurons in the PNS.

Structure and Function of the Myelin Sheath

The myelin sheath is a fatty layer that wraps around axons, providing insulation and increasing the speed of electrical transmission.

• CNS: Myelin is produced by oligodendrocytes.

• PNS: Myelin is produced by Schwann cells.

• Function: Allows for rapid conduction of action potentials via saltatory conduction.

Nucleus vs. Ganglion; Nerve vs. Tract

• Nucleus: Cluster of neuron cell bodies within the CNS.

• Ganglion: Cluster of neuron cell bodies in the PNS.

• Nerve: Bundle of axons in the PNS.

• Tract: Bundle of axons in the CNS.

Types of Membrane Ion Channels

Ion channels are proteins that allow ions to pass through the cell membrane, crucial for electrical signaling.

• Leak Channels: Always open; maintain resting membrane potential.

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

• Ligand-Gated Channels: Open in response to binding of a chemical messenger.

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

Resting Membrane Potential and Its Electrochemical Basis

The resting membrane potential is the voltage difference across the cell membrane when the neuron is not transmitting signals.

• Typical Value: About -70 mV in neurons.

• Basis: Created by differences in ion concentrations and selective permeability of the membrane.

• Key Equation: $V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$ (Nernst equation for potassium)

Graded Potentials: Description and Examples

Graded potentials are local changes in membrane potential that vary in size and decrease with distance.

• Examples:

  • Postsynaptic potentials: Excitatory (EPSP) or inhibitory (IPSP) changes in postsynaptic neurons.

  • Receptor potentials: Generated by sensory receptors.

Comparison: Graded Potentials vs. Action Potentials

Feature	Graded Potentials	Action Potentials
Amplitude	Variable	All-or-none
Propagation	Decreases with distance	Self-propagating
Location	Dendrites/cell body	Axon
Function	Initiate action potentials	Transmit signals long distances

Generation and Propagation of Action Potentials

Action potentials are rapid, all-or-none electrical impulses that travel along axons.

• Generation: Triggered when membrane potential reaches threshold, opening voltage-gated Na+ channels.

• Propagation: Depolarization spreads, opening adjacent channels and moving the impulse along the axon.

• Key Equation: $I = g(V_m - E_{ion})$ (Ohm's law for ion flow)

Absolute and Relative Refractory Periods

• Absolute Refractory Period: Time during which a neuron cannot fire another action potential, regardless of stimulus strength.

• Relative Refractory Period: Time during which a stronger-than-normal stimulus is required to initiate another action potential.

Saltatory vs. Continuous Conduction

Saltatory conduction occurs in myelinated axons, where the action potential jumps between nodes of Ranvier, increasing speed. Continuous conduction occurs in unmyelinated axons, where the impulse travels along the entire membrane.

• Saltatory Conduction: Fast, efficient; only at nodes of Ranvier.

• Continuous Conduction: Slower; occurs along unmyelinated fibers.