Functional Organization of Nervous Tissue: Chapter 11 Study Notes

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Functional Organization of Nervous Tissue

Introduction to the Nervous System

The nervous system is a complex communication network that consists of the central nervous system (CNS)—comprising the brain and spinal cord—and the peripheral nervous system (PNS), which includes nerves, ganglia, and receptors. Nervous tissue contains two main cell types: neurons and glial cells.

Nerve: Bundle of axons outside the brain and spinal cord.
Cranial nerves: Originate from the brain; 12 pairs.
Spinal nerves: Originate from the spinal cord; 31 pairs.
Ganglion: Collection of neuron cell bodies outside the CNS.
Plexus: Network of axons and sometimes neuron cell bodies outside the CNS.
Glial cells: Supportive cells with various functions.

11.1 Functions of the Nervous System

The nervous system performs several essential functions:

Maintaining homeostasis: Regulates and coordinates activities to maintain internal balance.
Receiving sensory input: Monitors internal and external stimuli.
Integrating information: Processes sensory input and initiates responses.
Controlling muscles and glands: Directs movement and secretion.
Establishing and maintaining mental activity: Responsible for consciousness, thinking, memory, and emotion.

11.2 Divisions of the Nervous System

Central nervous system (CNS): Brain and spinal cord.
Peripheral nervous system (PNS): Sensory receptors and nerves.

Branch of Nervous System	Components	Division	Direction of Signal	Type of Control	Subdivisions	Effectors	Response at Effector
CNS	Brain and spinal cord	-	-	-	-	-	-
PNS	Receptors, nerves, ganglia, plexuses	Sensory	Afferent	-	-	-	-
PNS	Receptors, nerves, ganglia, plexuses	Motor	Efferent	Somatic (Voluntary)	-	Skeletal muscle	Stimulates contraction
PNS	Receptors, nerves, ganglia, plexuses	Motor	Efferent	Autonomic (Involuntary)	Sympathetic	Cardiac and smooth muscle; glands	Stimulates physical activity
PNS	Receptors, nerves, ganglia, plexuses	Motor	Efferent	Autonomic (Involuntary)	Parasympathetic	Cardiac and smooth muscle; glands	Regulates resting functions

Divisions of the PNS

Sensory (afferent): Transmits action potentials from receptors toward the CNS.
Sensory receptors: Neuron endings or specialized cells that detect stimuli and send input to the CNS.
Motor (efferent): Transmits action potentials from CNS to effectors (muscles, glands).

Motor Division of PNS

Somatic nervous system: Controls skeletal muscles; voluntary; single neuron system.
Autonomic nervous system (ANS): Controls smooth muscle, cardiac muscle, and glands; involuntary; two-neuron system (CNS to ganglion, ganglion to effector).
- Sympathetic: Prepares body for physical activity.
- Parasympathetic: Regulates resting functions (e.g., digestion, bladder emptying).
- Enteric: Plexuses within the digestive tract wall.

11.3 Cells of the Nervous System

Neurons are electrically excitable cells with three major parts:

Neuron cell body (soma): Contains Nissl bodies (rough ER); responsible for protein synthesis and housekeeping.
Dendrites: Extensions that receive information; often highly branched with dendritic spines; conduct currents toward the cell body.
Axons: Arise from axon hillock; initial segment is the trigger zone for action potentials; ends at presynaptic terminal with synaptic vesicles full of neurotransmitter.

Axonic Transport Mechanisms

Anterograde transport: Moves materials from cell body to axon terminals (growth, repair, renewal).
Retrograde transport: Moves materials toward cell body (damaged organelles, recycled membrane, substances from endocytosis).

Types of Neurons

Functional classification:
- Sensory (afferent): Toward CNS.
- Motor (efferent): Away from CNS.
- Interneurons: Within CNS, between neurons.
Structural classification:
- Multipolar: Most CNS neurons; motor neurons.
- Bipolar: Sensory in retina and nasal cavity.
- Pseudo-unipolar: Single process divides into two branches; peripheral branch has sensory receptors.
- Anaxonic: Only dendrites; found in brain and retina; communicate via graded potentials.

Glial Cells

Glial Cells of the CNS

Astrocytes: Star-shaped; regulate brain fluid, form blood-brain barrier, promote synapse development, limit inflammation.
Ependymal cells: Line ventricles and central canal; form choroid plexuses that secrete cerebrospinal fluid (CSF).
Microglia: CNS macrophages; phagocytize necrotic tissue, microorganisms, and foreign substances.
Oligodendrocytes: Form myelin sheaths around axons; one cell can myelinate several axons.

Glial Cells of the PNS

Schwann cells: Form myelin sheath around one axon; outer layer is neurilemma.
Satellite cells: Surround neuron cell bodies in ganglia; provide support, nutrients, and protection.

Myelinated and Unmyelinated Axons

Myelinated axons: Myelin insulates and speeds transmission; gaps called nodes of Ranvier; degeneration occurs in multiple sclerosis.
Unmyelinated axons: Rest in Schwann cell or oligodendrocyte invaginations; not wrapped; form gray matter.

Nervous Tissue Response to Injury

Cut nerves may heal or be permanently damaged.
Degeneration: Axon distal to injury breaks down; Schwann cells and macrophages clear debris; Schwann cells form a column to guide regrowth.
Regeneration in CNS is limited due to lack of guiding columns.

11.4 Organization of Nervous Tissue

Gray matter: Unmyelinated axons, cell bodies, dendrites; integrative functions; cortex of brain.
White matter: Myelinated axons; propagate action potentials.
CNS: Clusters of cell bodies are nuclei; bundles of axons are nerve tracts.
PNS: Clusters of cell bodies are ganglia; bundles of axons are nerves.

11.5 Electrical Signals

Cells produce action potentials—electrical signals for communication.
Membrane potential: Results from ionic concentration differences and membrane permeability.

Ionic Concentration Differences Across the Plasma Membrane

Maintained by Na+/K+ pump and membrane permeability.
High Na+ and Cl- outside; high K+ and proteins inside.
Steep gradients for Na+ and K+ in opposite directions.

Permeability Characteristics of the Plasma Membrane

Proteins: Large, negatively charged, synthesized inside cell.
Cl-: Repelled by proteins, exit via open channels.
Gated ion channels: Open/close in response to stimuli, altering permeability.

Sodium-Potassium Pump

Uses ATP to pump 3 Na+ out and 2 K+ in per ATP molecule.
Maintains uneven ion distribution.
Equation: per ATP

Leak Ion Channels

Always open; responsible for resting permeability.
More channels for K+ and Cl- than Na+.

Gated Ion Channels

Ligand-gated: Open/close in response to ligand (e.g., neurotransmitter).
Voltage-gated: Open/close in response to voltage changes.
Other: Touch and temperature receptors respond to mechanical or thermal stimuli.

Resting Membrane Potential

Charge difference across membrane (~ -70 to -90 mV).
Maintained by permeability and ion concentration differences.
More permeable to K+; K+ diffuses out, making inside negative.
Na+/K+ pump maintains gradients.

Changing the Resting Membrane Potential

Depolarization: Inside becomes more positive (e.g., Na+ entry).
Hyperpolarization: Inside becomes more negative (e.g., K+ exit, Cl- entry).

Graded Potentials

Small, localized changes in membrane potential.
Magnitude varies with stimulus strength; can summate to reach threshold.
Can be depolarizing or hyperpolarizing.

Action Potentials

All-or-none electrical signals for neuron communication.
Phases: Depolarization, repolarization, afterpotential, return to resting potential.
Generated when graded potentials reach threshold at trigger zone.

Action Potential Mechanism

Resting: Voltage-gated Na+ and K+ channels closed.
Depolarization: Na+ channels open, Na+ enters.
Repolarization: Na+ channels inactivate, K+ channels open, K+ exits.
Afterpotential: K+ channels remain open briefly, causing hyperpolarization.
Return to resting: Channels reset, Na+/K+ pump restores gradients.

Refractory Period

Absolute: No new action potential possible.
Relative: Stronger stimulus can initiate another action potential.

Action Potential Frequency

Frequency increases with stimulus strength, but magnitude remains constant.

Propagation of Action Potentials

Continuous conduction: Unmyelinated axons; action potential moves along entire membrane.
Saltatory conduction: Myelinated axons; action potential jumps between nodes of Ranvier.
Speed increases with myelination and axon diameter.

Nerve Fiber Types

Type	Diameter	Myelination	Speed	Function
A	Large	Myelinated	15-120 m/s	Motor/sensory
B	Medium	Lightly myelinated	3-15 m/s	ANS
C	Small	Unmyelinated	2 m/s or less	ANS

11.6 The Synapse

Junction between two cells; site of signal transmission.
Presynaptic cell: Sends signal.
Postsynaptic cell: Receives signal.
Electrical synapses: Gap junctions allow direct current flow; rapid, synchronized activity.
Chemical synapses: Neurotransmitter released into synaptic cleft; binds to postsynaptic receptors.

Neurotransmitter Removal

Enzymatic breakdown (e.g., acetylcholinesterase for ACh).
Reuptake into presynaptic terminal (e.g., norepinephrine).
Diffusion away from synapse.

Receptor Molecules in Synapses

Neurotransmitter binds only to specific receptors.
Can be excitatory or inhibitory depending on receptor type.

Neurotransmitters and Neuromodulators

Chemical messengers secreted by neurons; over 100 types.
Criteria: Synthesized by neuron, stored in vesicles, released by action potential, binds receptor, evokes response.
Classified by structure, effect, and mechanism.
Major classes: Acetylcholine, biogenic amines (serotonin, dopamine, norepinephrine), amino acids (GABA, glycine, glutamate), purines (adenosine, ATP), neuropeptides (Substance P, endorphins), gases/lipids (nitric oxide, endocannabinoids).

Responses at Postsynaptic Cells

Excitatory postsynaptic potential (EPSP): Depolarization; may reach threshold for action potential.
Inhibitory postsynaptic potential (IPSP): Hyperpolarization; decreases likelihood of action potential.

Neuromodulation

Neuromodulators influence action potential production in postsynaptic cell.
Presynaptic inhibition/facilitation modulates neurotransmitter release.

Summation

Spatial summation: Multiple graded potentials from different locations combine.
Temporal summation: Multiple graded potentials from same location in rapid succession combine.
Combined EPSPs and IPSPs determine if threshold is reached.

11.7 Neuronal Pathways and Circuits

Serial pathway: Input travels along one pathway.
Convergent pathway: Many inputs converge on fewer neurons (data synthesis).
Divergent pathway: Few neurons synapse with many (information distribution).
Reverberating circuit: Reciprocal activation (rhythmic activities).
Parallel after-discharge circuit: Parallel stimulation converges on one output (complex processing).

Summary: The nervous system is organized into central and peripheral divisions, composed of specialized cells and structures that enable rapid communication, integration, and response to stimuli. Understanding the functional organization of nervous tissue is essential for comprehending how the body maintains homeostasis, processes information, and coordinates complex behaviors.