Chapter 11 Study Guide

Fundamentals of the Nervous System and Nervous Tissue

Organization of the Nervous System

The nervous system is a complex network responsible for coordinating body activities and transmitting signals between different body parts.

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

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS; connects the CNS to limbs and organs.

• Functional Divisions: Sensory (afferent) division transmits information to the CNS; Motor (efferent) division carries commands from the CNS to effectors (muscles, glands).

• Autonomic Nervous System (ANS): Controls involuntary functions; subdivided into sympathetic and parasympathetic divisions.

Neuroglia (Glial Cells)

Neuroglia are supporting cells in the nervous system, each with specialized functions and locations.

FOUMD IN CNS:

• Astrocytes: Found in the CNS; maintain the blood-brain barrier, provide structural support, and regulate ion balance.

• Oligodendrocytes: CNS cells that form myelin sheaths around axons.

• Microglia: CNS immune cells; act as phagocytes.

• Ependymal Cells: Line ventricles of the brain and central canal of the spinal cord; produce and circulate cerebrospinal fluid.

  FOUND IN PSN:

• Schwann Cells: PNS cells that form myelin sheaths. (Similar to oligodendroctyes)

• Satellite Cells: Surround neuron cell bodies in the PNS; regulate environment. (Similar to Astrocytes)

General Features of a Neuron

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

• Dendrites: Receive incoming signals and convey to cell body

• Axon: Conducts electrical impulses away from the cell body

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

Characteridtics of neuron

• Conduct impulses -> electrical signals

• Extreme longevity -> live as long as people

• Amitotic -> they dont have centrioles -> cannot divide

• High metabolic rate -> will die if they dont have don't glucose or oxygen

Structural and Functional Classifications of Neurons

• Structural:

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

  • Bipolar: One dendrite, one axon (found in special senses like retina in eye).

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

• Functional:

  • Sensory (Afferent): Transmit impulses toward the CNS.

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

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

Ohm's Law and Its Application to the Body

Ohm's Law describes the relationship between voltage, current, and resistance.

• Formula: $V = IR$

• V (Voltage): Electrical potential difference across the membrane.

• I (Current): Flow of ions across the membrane.

• R (Resistance): Hindrance to current flow (e.g., membrane properties).

• Application: In neurons, ion channels affect resistance and thus influence current and voltage changes during signaling.

Ion Channels in Neuron Membranes

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

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

• Gated Channels:

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

  • Chemically (Ligand)-Gated: Open/close when a specific chemical binds.

  • Mechanically Gated: Open/close in response to mechanical deformation.

Resting Membrane Potential

The resting membrane potential is the electrical charge difference across the plasma membrane of a resting neuron.

• Typical Value: About -70 mV (inside negative relative to outside).

• Generated by:

  • Differences in ion concentrations (Na+, K+).

  • Selective permeability of the membrane.

  • Na+/K+ ATPase pump (3 Na+ out, 2 K+ in).

Depolarization vs. Hyperpolarization

• Depolarization: Membrane potential becomes less negative (moves toward zero).

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

• Significance: Depolarization can trigger action potentials; hyperpolarization inhibits them.

Graded Potentials

Graded potentials are small, localized changes in membrane potential that vary in size and decay with distance.

• Characteristics: Can be depolarizing or hyperpolarizing; summate to influence action potential generation.

• Location: Typically occur in dendrites and cell bodies.

Action Potentials

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

• Initiation: Triggered when threshold is reached at the axon hillock.

• Phases: Depolarization (Na+ influx), repolarization (K+ efflux), hyperpolarization.

• Propagation: Moves unidirectionally down the axon due to refractory periods.

• Saltatory Conduction: In myelinated axons, action potentials jump between nodes of Ranvier, increasing speed.

Factors Affecting Conduction Velocity

• Axon Diameter: Larger diameter = faster conduction.

• Myelination: Myelinated fibers conduct faster due to saltatory conduction.

• Temperature: Higher temperature increases conduction speed (within physiological limits).

Classification of Nerve Fibers

Nerve fibers are classified based on diameter, myelination, and conduction speed.

Type	Diameter	Myelination	Conduction Speed
A fibers	Large	Myelinated	Fastest
B fibers	Medium	Lightly myelinated	Intermediate
C fibers	Small	Unmyelinated	Slowest

Synapses and Synaptic Transmission

Synapses are junctions where neurons communicate with other neurons or effectors.

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

• Electrical Synapses: Direct cytoplasmic connections via gap junctions; allow rapid signal transmission.

• Postsynaptic Potentials:

  • Excitatory (EPSP): Depolarize the postsynaptic membrane, increasing likelihood of action potential.

  • Inhibitory (IPSP): Hyperpolarize the postsynaptic membrane, decreasing likelihood of action potential.

Neurotransmitter Receptors

• Ionotropic Receptors: Ligand-gated ion channels; fast, direct effects.

• Metabotropic Receptors: G-protein-coupled receptors; slower, indirect effects via second messengers.

Functional Classifications of Neurotransmitters

• Excitatory: Promote depolarization (e.g., glutamate).

• Inhibitory: Promote hyperpolarization (e.g., GABA, glycine).

• Modulatory: Influence the effects of other neurotransmitters (e.g., dopamine, serotonin).

Neural Integration

Neural integration refers to the processing of incoming signals by neurons to produce appropriate responses.

• Summation: EPSPs and IPSPs combine temporally and spatially to determine if threshold is reached.

• Diverging and Converging Circuits: Patterns of synaptic connections that allow for complex processing.

Example:

When you touch a hot object, sensory neurons rapidly transmit signals to the CNS, where interneurons process the information and motor neurons trigger muscle contraction to withdraw your hand.