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

Summary Table: Types of Neuroglia

Key Equations

• Resting Membrane Potential: Nernst Equation (for equilibrium potential):

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.