Chapter 11: Neural Tissue – Structured Study Notes

Neural Tissue

Nervous System Divisions

The nervous system is divided into two main anatomical divisions, each with distinct functions and components.

• Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for integrating, processing, and coordinating sensory and motor commands.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. Subdivided into sensory and motor divisions.

Subdivisions of the PNS

• Sensory Division: Brings information to the CNS from three types of receptors:

  • Somatic sensory receptors: Detect position, touch, pressure, pain, and temperature.

  • Special sensory receptors: Responsible for smell, taste, vision, balance, and hearing.

  • Visceral sensory receptors: Monitor internal organs.

• Motor Division: Carries motor commands from the CNS to effectors:

  • Somatic Nervous System (SNS): Controls skeletal muscles.

  • Autonomic Nervous System (ANS): Regulates smooth muscle, cardiac muscle, glands, and adipose tissue involuntarily.

General Functions of the Nervous System

• Receptors: Detect changes in internal or external environment.

• Sensory Division: Sends information to CNS.

• Information Processing: Occurs in CNS.

• Motor Division: Carries commands to effectors.

• Effectors: Respond by changing their activities.

Neurons

Neurons are the basic functional units of the nervous system, specialized for intercellular communication.

• Dendrites: Receive stimuli; highly branched with dendritic spines.

• Cell Body (Perikaryon): Contains nucleus and organelles; Nissl bodies (clusters of RER and ribosomes) are present.

• Cytoskeleton: Contains neurofilaments and neurofibrils.

• Axon: Carries information toward other cells.

  • Axon hillock: Origin of axon from cell body.

  • Initial segment: Site of action potential initiation.

  • Axolemma: Specialized plasma membrane.

  • Axoplasm: Cytoplasm of axon; contains neurofibrils, neurotubules, vesicles, lysosomes, mitochondria, enzymes.

  • Telodendria: Fine extensions at axon end.

  • Axon terminals: Synaptic terminals at telodendria ends.

Axoplasmic Transport

• Anterograde transport: Soma to axon.

• Retrograde flow: Axon to soma.

Synapse

• Presynaptic cell: Contains synaptic vesicles with neurotransmitters.

• Presynaptic membrane: Site of neurotransmitter release.

• Synaptic cleft: Narrow space between pre- and postsynaptic membranes.

• Postsynaptic cell: Receives signal.

• Postsynaptic membrane: Contains neurotransmitter receptors.

Types of Synapses

• Neuron-neuron synapse: Communication between neurons.

• Neuromuscular junction: Neuron to muscle communication.

Collateral Branches

• Allow single neuron to communicate with several cells.

Neuron Classification

Neurons are classified anatomically and functionally.

Anatomical Classes

• Anaxonic neurons: Found in brain and special sense organs.

• Bipolar neurons: Two distinct processes (one dendrite, one axon).

• Unipolar neurons: Dendrites and axons are continuous; cell body off to one side. Most sensory neurons in PNS.

• Multipolar neurons: Two or more dendrites, one axon. Most common in CNS; all motor neurons controlling skeletal muscles.

Functional Classes

• Sensory neurons: Detect stimuli via sensory receptors.

  • Interoceptors: Sensations of distension, deep pressure, pain.

  • Proprioceptors: Monitor body position and movement.

  • Exteroceptors: Monitor external environment.

• Interneurons: Located in CNS; receive information from PNS and CNS.

• Motor neurons: Cell bodies in CNS; axons extend in PNS.

  • Somatic motor neurons: Innervate skeletal muscles (conscious control).

  • Visceral motor neurons: Innervate effectors other than skeletal muscle.

Key Terms

• Afferent fibers: Carry sensory information to CNS.

• Efferent fibers: Carry information from CNS.

• Ganglion: Collection of neuron cell bodies in PNS.

• Sensory ganglia: Ganglia with unipolar sensory neuron cell bodies.

Neuroglia

Neuroglia (glial cells) support and protect neurons in both CNS and PNS.

Neuroglia of the CNS

• Ependymal cells: Form ependyma; line fluid-filled passageways (central canal, ventricles); produce cerebrospinal fluid (CSF).

• Microglia: Mobile phagocytic cells; remove debris, waste, pathogens.

• Astrocytes: Maintain blood–brain barrier (BBB).

• Oligodendrocytes: Stabilize axons; produce myelin in CNS.

  • Myelin: Coats axons, increases speed of impulse transmission.

  • Myelin sheath: Layers of myelin and plasma membrane.

  • Internodes: Myelin-wrapped areas.

  • Nodes (Nodes of Ranvier): Gaps between internodes.

  • Myelinated axons: Appear white due to lipid content.

White and Gray Matter

• White matter: Areas with many myelinated axons.

• Gray matter: Areas with neuron cell bodies, dendrites, unmyelinated axons.

Neuroglia of the PNS

• Schwann cells: Cover myelinated and unmyelinated peripheral axons; form myelin in PNS.

• Satellite cells: Surround peripheral cell bodies.

Axon Injury and Repair

• Wallerian degeneration: Repair process after axon/myelin degeneration; does not restore full function.

• CNS axon injury: Limited regeneration.

Membrane Potential

Membrane potential is the unequal charge distribution across a cell membrane, characteristic of all living cells.

• Inside: Slightly negative charge.

• Outside: Slightly positive charge.

Neuronal Activity

• Changes in membrane potential trigger muscle contraction, gland secretion, or information transfer.

• Resting membrane potential: Undisturbed cell; typically –70 mV.

• Graded potential: Temporary, localized change in resting potential.

• Action potential: Rapid rise and fall in membrane potential; spreads along axon.

Contributors to Resting Potential

• ECF: High Na+ and Cl–.

• Cytosol: High K+ and negatively charged proteins (Pr–).

• Passive forces: Diffusion of Na+ and K+ through leak channels.

• Active processes: Sodium–potassium exchange pump.

Leak Channels and Sodium–Potassium Pump

• K+ leak channels: Potassium diffuses out.

• Na+ leak channels: Sodium diffuses in.

• Sodium–potassium pump: Maintains stable resting potential.

Electrochemical Gradient

• Combination of chemical and electrical gradients.

• Chemical gradient: Ion concentration difference across membrane.

• Electrical gradient: Attraction/repulsion of charges.

Equilibrium Potential

• Membrane potential at which an ion's electrical and chemical gradients are equal and opposite.

• Potassium: –90 mV; Sodium: +66 mV.

Gated Channels

Gated channels open or close in response to specific stimuli, regulating ion flow across the membrane.

• Chemically gated channels: Open when binding specific chemicals (e.g., ACh at neuromuscular junction).

• Voltage-gated channels: Open/close in response to membrane potential changes (Na+, K+, Ca2+).

• Mechanically gated channels: Open/close in response to physical distortion.

Distribution of Gated Channels

• Chemically gated: Cell body and dendrites.

• Voltage-gated Na+ and K+: Along axon.

• Voltage-gated Ca2+: Axon terminals.

Graded Potentials

Graded potentials are local changes in membrane potential that do not spread far from the site of stimulation.

• Produced by opening gated sodium channels.

• Phases:

  • Depolarization: Shift toward more positive value.

  • Local current: Movement of positive charges along membrane.

  • Repolarization: Return toward resting potential.

  • Hyperpolarization: Shift to more negative than resting potential.

Action Potential Generation

Action potentials are propagated changes in membrane potential affecting the entire excitable membrane.

• Threshold: Membrane potential at which voltage-gated sodium channels open (about –60 mV).

• Refractory periods:

  • Absolute: Membrane cannot respond to further stimulation.

  • Relative: Membrane can respond only to stronger stimulus.

• All-or-none principle: Either a typical action potential is triggered or not at all.

Action Potential Propagation

Propagation is the repeated generation of action potentials along the axon.

• Continuous propagation: Occurs along unmyelinated axons.

• Saltatory propagation: Occurs along myelinated axons; ions cross membrane only at nodes of Ranvier.

Synapses

Synapses are sites of intercellular connection and information transfer between neurons or between neuron and effector cell.

• Chemical synapses: Most abundant; neurotransmitters released (e.g., cholinergic synapses release ACh).

• Synaptic fatigue: Temporary inability to function until neurotransmitter supply is replenished.

• Synaptic delay: Time lag between action potential arrival and postsynaptic effect.

• Electrical synapses: Rare; current flows directly between cells; more efficient but less versatile.

Information Processing in a Neuron

Postsynaptic potentials are graded potentials in response to neurotransmitters.

• Excitatory postsynaptic potential (EPSP): Graded depolarization; shifts membrane potential closer to threshold.

• Inhibitory postsynaptic potential (IPSP): Graded hyperpolarization; shifts membrane potential farther from threshold.

• Summation: Collective effects of EPSPs and IPSPs.

  • Temporal summation: Single synapse repeatedly active.

  • Spatial summation: Multiple synapses active simultaneously.

  • Degree of depolarization depends on number and location of active excitatory synapses.

Higher Levels of Information Processing

Regulatory neurons and multiple neurotransmitters modulate information processing in the nervous system.

• Regulatory neurons alter sensitivity of axon terminals.

• More than 100 neurotransmitters; effects can be direct (on ion channels) or indirect (via G proteins).

Summary Table: Types of Neurons

Anatomical Class	Location	Structure	Function
Anaxonic	Brain, special sense organs	No obvious axon	Unknown, likely integration
Bipolar	Special sense organs	One dendrite, one axon	Sensory
Unipolar	PNS (sensory neurons)	Dendrite and axon fused	Sensory
Multipolar	CNS, motor neurons	Multiple dendrites, one axon	Motor, integration

Summary Table: Types of Neuroglia

Type	Location	Main Function
Ependymal cells	CNS	Produce and circulate CSF
Microglia	CNS	Phagocytosis
Astrocytes	CNS	Maintain BBB, support neurons
Oligodendrocytes	CNS	Myelinate CNS axons
Schwann cells	PNS	Myelinate PNS axons
Satellite cells	PNS	Support PNS neuron cell bodies

Key Equations

• Resting Membrane Potential: $V_{m} = V_{K^{+}} + V_{Na^{+}} + V_{Cl^{-}}$ Nernst Equation (for equilibrium potential): $E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

Example: The sodium–potassium pump moves 3 Na+ out and 2 K+ in, maintaining the resting potential.

Additional info: The notes have been expanded with academic context and tables for clarity and completeness.