CH 8 Study Guide

Organization of the Nervous System

Overview

The nervous system is a complex network responsible for coordinating body activities and responding to internal and external stimuli. It is organized into central and peripheral divisions, each with specialized functions.

• Central Nervous System (CNS): Consists of the brain and spinal cord; integrates and processes information.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS; transmits sensory and motor signals.

• Enteric Nervous System: Network of neurons in the digestive tract, functioning both autonomously and as part of the autonomic nervous system.

Peripheral neurons are subdivided into:

• Somatic motor division: Controls skeletal muscle.

• Autonomic division: Regulates smooth muscle, cardiac muscle, glands, and adipose tissue.

• Sympathetic and Parasympathetic: Subdivisions of the autonomic division with opposing effects on target organs.

Cell Types in the Nervous System

Neurons and Glial Cells

The nervous system is composed primarily of two cell types: neurons and glial cells.

• Neurons: Functional units that carry electrical impulses. Classified by shape and function (e.g., unipolar, bipolar, multipolar, pseudounipolar).

• Glial cells (neuroglia): Support neurons. Types include astrocytes, ependymal cells, oligodendrocytes, microglia, satellite cells, and Schwann cells.

Cell Type	Main Function	Location
Neurons	Transmit electrical signals	CNS & PNS
Astrocytes	Support, blood-brain barrier	CNS
Oligodendrocytes	Form myelin sheath	CNS
Schwann cells	Form myelin sheath	PNS
Microglia	Immune defense	CNS
Ependymal cells	Produce cerebrospinal fluid	CNS
Satellite cells	Support neurons	PNS

Structure of a Neuron

• Cell body (soma): Integrates incoming signals.

• Dendrites: Receive incoming signals.

• Axon: Carries outgoing signals.

Neurons are classified by shape and function, and by the direction of axonal transport (fast and slow).

Electrical Signaling in Neurons

Action Potentials

An action potential is a rapid, transient electrical signal that travels along the membrane of a neuron. It is essential for communication within the nervous system.

• Definition: A change in membrane potential due to the movement of ions across the membrane.

• Characteristics: All-or-none response, travels long distances, involves Na+ and K+ movement.

• Phases: Resting, rising (depolarization), falling (repolarization), after-hyperpolarization, refractory period.

Resting neurons: The inside of the membrane is negative relative to the outside due to ion distribution.

Sodium-Potassium ATPase Pump

The sodium-potassium pump maintains the resting membrane potential by actively transporting Na+ out and K+ into the cell.

• Equation:

$3\text{Na}^+_{\text{out}} + 2\text{K}^+_{\text{in}} + \text{ATP} \rightarrow 3\text{Na}^+_{\text{out}} + 2\text{K}^+_{\text{in}} + \text{ADP} + \text{P}_i$

Ion Movement and Electrical Signals

Movement of ions such as Na+, K+, and Cl- across the membrane creates electrical signals. Voltage, current, resistance, and conductivity are key concepts.

• Voltage (V): Electrical potential difference.

• Current (I): Flow of electric charge.

• Resistance (R): Opposition to current flow.

• Conductivity (G): Ability to conduct current.

Ohm's Law:

$V = I \times R$

Graded Potentials vs. Action Potentials

Comparison

Neurons use two types of electrical signals: graded potentials and action potentials.

Feature	Graded Potential	Action Potential
Trigger	Chemical/mechanical stimuli	Threshold depolarization
Distance	Short	Long
Signal Loss	Yes, signal decreases	No, all-or-none
Ions Involved	Various	Na+, K+
Propagation	Passive	Active

Graded potentials: Local changes in membrane potential, decrease with distance, triggered by stimuli.

Action potentials: Rapid, long-distance signals, involve sequential opening of voltage-gated Na+ and K+ channels ("domino effect").

Neuronal Communication: Electrical and Chemical Signaling

Synaptic Transmission

Neurons communicate via synapses, where electrical signals are converted to chemical signals through neurotransmitter release.

• Neurotransmitters: Chemical messengers released in response to electrical signals.

• Synaptic transmission steps:

  1. Action potential arrives at axon terminal.

  2. Voltage-gated Ca2+ channels open.

  3. Ca2+ influx triggers neurotransmitter release.

  4. Neurotransmitter binds to receptors on postsynaptic cell.

  5. Postsynaptic response generated.

Release of neurotransmitters can vary in response to weak or strong stimuli.

Additional Info

• Myelinated axons: Conduct action potentials faster via saltatory conduction; nodes of Ranvier are critical for signal propagation.

• Pre- and post-synaptic neurons: Refer to the direction of signal transmission at synapses.