Neural Tissue and Neurophysiology: Structure, Function, and Communication

Neural Tissue

Functions of Neural Tissue

Neural tissue is specialized for control and communication within the body, playing a central role in maintaining homeostasis. It performs three major functions:

• Monitor: Detects changes in stimuli and gathers sensory input.

• Integration: Evaluates sensory input and determines appropriate responses.

• Motor Output: Responds to stimuli via effector organs such as muscles and glands.

Organization of the Nervous System

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord and is responsible for integrating sensory information and coordinating responses. Responses may be based on reflexes, experience, or current conditions.

Peripheral Nervous System (PNS)

The PNS includes spinal and cranial nerves and neural tissue outside the CNS. It is responsible for monitoring sensory input and executing motor output.

• Afferent System: Carries sensory information from receptors to the CNS.

• Efferent System: Transmits motor commands from the CNS to effector organs.

  • Somatic Nervous System (SNS): Controls voluntary actions in skin and skeletal muscles, including reflexes.

  • Autonomic Nervous System (ANS): Regulates involuntary functions in smooth muscle, heart, and glands.

    • Sympathetic Division: Activates responses to stress.

    • Parasympathetic Division: Maintains normal bodily functions.

Cells of the Nervous System

Neurons

Neurons are the structural and functional units of the nervous system. They exhibit extreme longevity, high metabolic rates, and limited cell division after early childhood. Some stem cells remain active in specific regions (e.g., olfactory area, hippocampus).

• Dendrites: Branch-like extensions that carry impulses toward the cell body; form receptors at their distal ends.

• Axon: Single, long process that carries impulses away from the cell body; ends in teliodendria at the synaptic terminal.

• Neurofibrils: Provide internal support and distribute nutrients and depolarization waves.

• Nissl Bodies: Specialized rough endoplasmic reticulum for protein synthesis; give gray color to gray matter.

• Myelin Sheath: Segmented, fatty covering that insulates axons and speeds conduction; formed by Schwann cells in PNS and oligodendrocytes in CNS.

• Nodes of Ranvier: Gaps in the myelin sheath where axon collaterals can emerge.

• Neurolemma: Nucleated membrane around the fiber or myelin sheath in PNS; essential for nerve fiber regeneration.

• Cell Body (Perikaryon, Soma): Biosynthetic center of the neuron.

• Golgi Apparatus: Well-developed for processing proteins.

• Plasma Membrane: Part of the receptive surface.

• Structural Components:

  • Receptive/Input Region: Dendrite

  • Conducting Component: Axon

  • Secretory/Output Component: Axonal terminal

• Nuclei: Clusters of cell bodies in CNS.

• Ganglia: Clusters of cell bodies in PNS.

Synapse

A synapse is the functional connection between a neuron and another cell, requiring neurotransmitters (e.g., acetylcholine, norepinephrine, serotonin, dopamine). Enzymes are needed to terminate neurotransmitter action and prevent continuous impulses.

• Presynaptic Neuron: Sends information toward the synapse.

• Postsynaptic Neuron: Receives information away from the synapse.

• Presynaptic Membrane: Releases neurotransmitters.

• Postsynaptic Membrane: Contains receptors for neurotransmitters.

• Neuromuscular Junction: Neuron-to-muscle connection.

• Neuroglandular Junction: Neuron-to-gland connection.

• Axoplasmic Transport: Movement of materials between synaptic knobs and cell body (anterograde and retrograde).

Classification of Neurons

Structural Classification

• Multipolar: One axon, several dendrites; most common in CNS and skeletal muscle.

• Bipolar: One axon, one dendrite; rare, found in retina, inner ear, olfactory area.

• Unipolar (Pseudounipolar): Axon and dendrite continuous, cell body to one side; sensory neurons in PNS.

• Anaxonic: No clear distinction between axons and dendrites; found in brain and special sense organs.

Functional Classification

• Sensory (Afferent): Carry information from receptors to CNS; unipolar, cell bodies in sensory ganglia.

• Motor (Efferent): Carry impulses from CNS to effector organs; multipolar, cell bodies in CNS.

• Association Neurons (Interneurons): Connect sensory and motor neurons; multipolar, found only in CNS, involved in memory, learning, and planning.

Receptor Categories

• Exteroceptors: Detect external environment.

• Proprioceptors: Sense body position and movement.

• Interoceptors: Monitor internal organ systems, taste, deep pressure, pain.

Neuroglia (Glial Cells)

Neuroglia are small, numerous cells that support, protect, and anchor neurons. They play roles in myelin sheath formation, defense, and maintaining the chemical environment.

Neuroglia of the CNS

• Astrocytes: Most abundant; provide structural support, repair injury, direct neuron development, maintain blood-brain barrier, regulate chemical environment.

• Microglia: Smallest, least numerous; migrate and perform phagocytosis, important for CNS defense.

• Oligodendrocytes: Form myelin sheath around CNS axons, support neurons.

• Ependymal Cells: Line ventricles and central canal, may be ciliated, contribute to CSF circulation.

Neuroglia of the PNS

• Schwann Cells: Form sheath around peripheral axons, aid in regeneration.

• Satellite Cells: Surround cell bodies in ganglia, function similar to astrocytes.

Neural Repair in the PNS

Peripheral nerves can regenerate if the cell body remains intact. The likelihood of regeneration decreases with greater distance between severed ends.

1. Wallerian Degeneration: Axon and myelin distal to injury disintegrate.

2. Macrophages: Phagocytize debris.

3. Neurolemma: Remains intact within endoneurium.

4. Cell Body Changes: Disperses genetic material to support regeneration.

5. Schwann Cells: Proliferate, release growth factors, form regeneration tube, remyelinate axon.

Neurophysiology

Membrane Potential and Neural Communication

Neurons communicate via changes in membrane potential, which are initiated by alterations in ion permeability or concentration. Two types of signals are produced:

• Graded Potentials: Localized, temporary changes; decrease in strength over distance.

• Action Potentials: Long-distance signals; brief reversal of membrane potential, generated only by axons.

Resting Membrane Potential

At rest, the neuron membrane is polarized: positive outside, negative inside. This is maintained by ion gradients and the sodium-potassium pump.

• Na+: Higher concentration outside the cell.

• K+: Higher concentration inside the cell.

• Sodium-Potassium Pump: Transports 3 Na+ out for every 2 K+ in; requires ATP.

Equation:

$\text{Na-K Pump:} \, 3\, \text{Na}^+ \rightarrow \text{out}, \, 2\, \text{K}^+ \rightarrow \text{in}$

Membrane Channels

• Passive (Leak) Channels: Always open, permeability can vary.

• Active (Gated) Channels: Open/close in response to stimuli.

Depolarization, Repolarization, and Hyperpolarization

• Depolarization: Membrane becomes less negative; influx of Na+.

• Repolarization: Restoration of resting potential; sodium-potassium pump restores original distribution.

• Hyperpolarization: Membrane becomes more negative; increased membrane potential.

Action Potentials

• Threshold Stimulus: Minimum stimulus required to initiate an impulse.

• Subthreshold Stimulus: Weaker than threshold; may be summed to reach threshold.

• All-or-None Law: Action potential is triggered only if threshold is reached.

• Absolute Refractory Period: Membrane cannot respond to further stimulus; ensures one-way propagation.

• Relative Refractory Period: Follows absolute period; Na+ gates closed, K+ gates open.

Propagation of Action Potentials

• Continuous Propagation: Occurs in unmyelinated fibers; action potential moves in small steps.

• Saltatory Propagation: Occurs in myelinated fibers; action potential jumps from node to node, faster and more energy-efficient.

Rate of Impulse Propagation

Fiber Type	Diameter	Myelination	Function	Speed
Type A	Largest	Myelinated	Sensory info (position, balance, fine touch), motor commands to skeletal muscle	300 mph
Type B	Mid-sized	Myelinated	Pain, temperature, general touch, motor commands to smooth/cardiac muscle, glands	40 mph
Type C	Smallest	Unmyelinated	Less urgent info, incapable of saltatory propagation	2 mph

Synaptic Activity

• Electrical Synapses: Rare; direct transmission via gap junctions.

• Chemical Synapses: Use neurotransmitters; separated by synaptic cleft.

• Excitatory Neurotransmitters: Cause depolarization and action potential propagation.

• Inhibitory Neurotransmitters: Cause hyperpolarization and suppress action potentials.

Major Neurotransmitters

• Acetylcholine (ACh): Cholinergic synapses; muscle contraction.

• Norepinephrine: Brain and ANS; usually excitatory.

• Dopamine: Brain; excitatory or inhibitory; motor function regulation.

• Serotonin: CNS; emotional behavior; low levels linked to depression.

• GABA: Inhibitory; may reduce anxiety.

• Neuromodulators: Regulate neurotransmitter release or response; e.g., endorphins.

Information Processing in the Nervous System

• Postsynaptic Potentials: Graded potentials in response to neurotransmitters.

• Excitatory Postsynaptic Potential (EPSP): Graded depolarization; may trigger action potential.

• Inhibitory Postsynaptic Potential (IPSP): Graded hyperpolarization; reduces action potential likelihood.

• Summation: Combining EPSPs and IPSPs to initiate action potential.

  • Temporal Summation: Rapid stimuli at a single synapse.

  • Spatial Summation: Simultaneous stimuli at multiple synapses.

• Facilitation: Lowering threshold for action potential initiation; e.g., caffeine.

Conditions Affecting Neural Tissue

• Demyelination: Destruction of myelin sheaths; loss of sensation and motor control.

• Rabies: Retrograde transport of virus; cell death.

• Tay-Sachs Disease: Genetic abnormality; buildup of by-products, neuron deterioration.

• Hyperkalemia: High extracellular K+ concentration.

• Tumors: CNS tumors from abnormal neuroglia division.

• Neurotoxins: Disrupt normal nerve function; e.g., tetrodotoxin, venoms.

Example: Action Potential Generation

When a neuron receives a strong enough stimulus, voltage-gated Na+ channels open, causing rapid depolarization. If the threshold is reached, an action potential is generated and propagated along the axon. The sodium-potassium pump restores the resting membrane potential after the impulse passes.

Equation:

$\text{Resting Membrane Potential:} \, V_m = -70\, \text{mV}$

$\text{Threshold Potential:} \, V_{th} \approx -55\, \text{mV}$

$\text{Action Potential:} \, V_m \rightarrow +30\, \text{mV}$

Additional info: Academic context was added to clarify mechanisms of neural repair, synaptic activity, and action potential propagation, as well as to provide examples and equations for membrane potentials.