Nervous Tissue: Structure, Function, and Information Processing

Nervous Tissue

An Introduction to the Nervous System

The nervous system is a complex network responsible for communication, coordination, and control throughout the body. It consists of the brain, spinal cord, sensory receptors, and nerves that connect to other systems. Nervous tissue is composed of two main cell types: neurons and neuroglia.

• Neurons: Specialized for intercellular communication, information processing, and control.

• Neuroglia (glial cells): Support, protect, and maintain the environment for neurons.

Divisions of the Nervous System

Anatomical Divisions

• Central Nervous System (CNS): Composed of the brain and spinal cord. Functions include processing and coordinating sensory data, issuing motor commands, and higher functions such as intelligence, memory, learning, and emotion.

• Peripheral Nervous System (PNS): Includes all nervous tissue outside the CNS and the enteric nervous system (ENS). Delivers sensory information to the CNS and carries motor commands to peripheral tissues.

Functional Divisions of the PNS

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

• Efferent Division: Carries motor commands from the CNS to effectors (muscles, glands, adipose tissue).

• Receptors: Detect changes or stimuli; may be neurons or specialized cells.

• Effectors: Target organs that respond to motor commands.

Subdivisions of the Efferent Division

• Somatic Nervous System (SNS): Controls voluntary and involuntary (reflex) skeletal muscle contractions.

• Autonomic Nervous System (ANS): Controls subconscious actions, including smooth and cardiac muscle contractions and glandular secretions. Divided into:

  • Sympathetic Division: Stimulating effect.

  • Parasympathetic Division: Relaxing effect.

• Enteric Nervous System (ENS): Network of neurons in the digestive tract, capable of local reflexes independent of CNS input but influenced by the ANS.

Neurons

Structure and Function

Neurons are the basic functional units of the nervous system, specialized for communication and information processing.

• Cell Body (Soma): Contains the nucleus, nucleolus, perikaryon (cytoplasm), mitochondria, rough endoplasmic reticulum (RER), and ribosomes.

• Cytoskeleton: Includes neurofilaments, neurotubules, and neurofibrils for structural support.

• Nissl Bodies: Dense areas of RER and ribosomes, giving gray matter its color.

• Dendrites: Short, branched processes that receive information; dendritic spines increase surface area for synaptic input.

• Axon: Long process that propagates action potentials. Key structures include:

  • Axolemma: Plasma membrane of the axon.

  • Initial Segment: Base of the axon.

  • Axon Hillock: Region connecting the soma to the axon.

  • Collaterals: Axon branches.

  • Telodendria: Fine extensions at the axon's end.

  • Axon Terminals: Synaptic terminals at the tips of telodendria.

• Axonal (Axoplasmic) Transport: Movement of materials between soma and axon terminals via neurotubules, powered by mitochondria, kinesin, and dynein.

Classification of Neurons

• Structural Classification:

  • Anaxonic: Small, processes indistinguishable; found in brain and special sense organs.

  • Bipolar: One dendrite, one axon; rare, found in special senses.

  • Unipolar (Pseudounipolar): Fused axon and dendrite; most sensory neurons in PNS.

  • Multipolar: One axon, multiple dendrites; most common in CNS and all motor neurons controlling skeletal muscle.

• Functional Classification:

  • Sensory (Afferent) Neurons: Unipolar, cell bodies in sensory ganglia, transmit sensory input to CNS.

  • Motor (Efferent) Neurons: Carry instructions from CNS to effectors; somatic motor neurons innervate skeletal muscle, visceral motor neurons innervate smooth/cardiac muscle, glands, adipose tissue.

  • Interneurons: Located in CNS and autonomic ganglia; integrate sensory input and coordinate motor output, involved in higher functions (memory, planning, learning).

Types of Sensory Receptors

• Interoceptors: Monitor internal systems and internal senses (e.g., stretch, pain).

• Exteroceptors: Monitor external environment and complex senses (e.g., sight, smell, hearing).

• Proprioceptors: Monitor position and movement of muscles and joints.

Neuroglia

Types and Functions

Neuroglia support and protect neurons, making up about half the volume of the nervous system. They differ in the CNS and PNS.

Neuroglia in the CNS

• Astrocytes: Maintain the blood-brain barrier, provide structural support, repair tissue, guide development, and control the interstitial environment.

• Ependymal Cells: Line ventricles and central canal, produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Form myelin sheaths around CNS axons, increasing action potential speed; myelinated regions are white matter, unmyelinated regions are gray matter.

• Microglia: Phagocytic cells that remove debris, waste, and pathogens.

Neuroglia in the PNS

• Satellite Cells: Surround neuron cell bodies in ganglia, regulate interstitial fluid.

• Schwann Cells (Neurolemmocytes): Form myelin sheaths around PNS axons; a single Schwann cell myelinates one axon segment, or can enclose multiple unmyelinated axons.

Neural Response to Injury

• Wallerian Degeneration: Distal axon degenerates after injury; Schwann cells guide regrowth in PNS.

• CNS Regeneration: Limited due to astrocyte scar tissue and inhibitory chemicals.

Membrane Potential

Establishment and Maintenance

Neurons generate electrical signals due to differences in ion concentrations across their plasma membranes.

• Resting Membrane Potential: The voltage across the membrane of a resting cell (typically −70 mV).

• Graded Potential: Local, temporary change in membrane potential due to a stimulus.

• Action Potential: Propagated electrical impulse along the axon, triggered by graded potentials reaching threshold.

Key Concepts

• ECF is high in Na+ and Cl−; cytosol is high in K+ and negatively charged proteins.

• Membranes are selectively permeable; permeability varies by ion.

• Current: Movement of charges to eliminate potential difference.

• Resistance: How much the membrane restricts ion movement.

• Chemical Gradient: Difference in ion concentration across the membrane.

• Electrical Gradient: Difference in charge across the membrane.

• Electrochemical Gradient: Combined chemical and electrical forces acting on an ion.

• Equilibrium Potential: Membrane potential at which there is no net movement of a particular ion.

Sodium–Potassium Exchange Pump

• Powered by ATP.

• Ejects 3 Na+ for every 2 K+ brought in.

• Maintains resting membrane potential and concentration gradients.

$3 \mathrm{Na}^+_{\text{out}} : 2 \mathrm{K}^+_{\text{in}}$

Ion Channels

• Passive (Leak) Channels: Always open; permeability varies.

• Active (Gated) Channels: Open/close in response to stimuli; most are closed at rest.

• Chemically Gated: Open when bound to specific chemicals (e.g., ACh).

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

• Mechanically Gated: Open in response to physical distortion.

Graded Potentials

• Local changes in membrane potential; effect decreases with distance from stimulus.

• Depolarization: Membrane potential becomes less negative (e.g., Na+ influx).

• Repolarization: Return to resting potential after stimulus removal.

• Hyperpolarization: Membrane potential becomes more negative (e.g., K+ efflux).

• Stronger stimuli produce greater changes and affect larger areas.

Action Potentials

• All-or-none events: Once threshold is reached (−60 to −55 mV), an action potential is triggered.

• Phases:

  1. Depolarization to threshold.

  2. Activation of voltage-gated Na+ channels; Na+ influx.

  3. Inactivation of Na+ channels, activation of K+ channels; K+ efflux (repolarization).

  4. Return to resting potential; brief hyperpolarization.

• Refractory Period: Time during which the neuron cannot (absolute) or is less likely to (relative) fire another action potential.

• Sodium–potassium pump restores ion gradients after action potential.

Propagation of Action Potentials

• Continuous Propagation: Occurs in unmyelinated axons; action potential moves segment by segment.

• Saltatory Propagation: Occurs in myelinated axons; action potential jumps from node to node (nodes of Ranvier), increasing speed and efficiency.

• Propagation speed increases with axon diameter and myelination.

Types of Axons

Type	Myelination	Diameter	Speed	Example
Type A	Myelinated	Large	120 m/s	Sensory info (position, balance), motor to skeletal muscle
Type B	Myelinated	Medium	18 m/s	Intermediate sensory/motor
Type C	Unmyelinated	Small	1 m/s	Slow sensory info

Synapses

Structure and Function

• Synapse: Site where a neuron communicates with another cell.

• Presynaptic Neuron: Sends the message.

• Postsynaptic Neuron: Receives the message.

Types of Synapses

• Electrical Synapses: Direct physical contact via gap junctions; rapid transmission.

• Chemical Synapses: Use neurotransmitters to cross a synaptic cleft; most common.

Chemical Synapse Events

1. Action potential arrives at axon terminal.

2. Ca2+ influx triggers neurotransmitter (e.g., acetylcholine, ACh) release.

3. Neurotransmitter binds to postsynaptic receptors, causing graded potential.

4. Neurotransmitter is removed by enzymatic breakdown (e.g., acetylcholinesterase) or reuptake.

Special Synapses

• Neuromuscular Junction: Neuron to skeletal muscle cell.

• Neuroglandular Junction: Neuron to gland cell.

Synaptic Delay and Fatigue

• Synaptic Delay: 0.2–0.5 ms delay due to neurotransmitter release; fewer synapses = faster response.

• Synaptic Fatigue: Occurs when neurotransmitter supply cannot keep up with demand during intense activity.

Neurotransmitters and Neuromodulators

Types and Effects

• Excitatory Neurotransmitters: Cause depolarization, promote action potentials.

• Inhibitory Neurotransmitters: Cause hyperpolarization, suppress action potentials.

• Effect depends on receptor properties, not just neurotransmitter type.

Major Classes

• Biogenic Amines: Norepinephrine (excitatory), dopamine (excitatory/inhibitory), serotonin (modulates mood), GABA (inhibitory).

• Neuropeptides: Small peptides (e.g., opioids like enkephalins, endorphins, dynorphins) modulate pain and other functions.

• Dissolved Gases: Nitric oxide (NO), carbon monoxide (CO) act as neurotransmitters.

Mechanisms of Action

• Direct: Open/close ion channels (e.g., ACh, glutamate).

• Indirect via G Proteins: Activate second messengers (e.g., cAMP) to modulate cell activity (e.g., norepinephrine, dopamine).

• Indirect via Intracellular Enzymes: Lipid-soluble gases diffuse and activate enzymes inside the cell.

Information Processing in Nervous Tissue

Integration and Summation

• Postsynaptic Potentials: Graded potentials in response to neurotransmitters.

• Excitatory Postsynaptic Potential (EPSP): Graded depolarization.

• Inhibitory Postsynaptic Potential (IPSP): Graded hyperpolarization.

• Summation of EPSPs and IPSPs determines if action potential is generated.

• Temporal Summation: Rapid, repeated stimuli at one synapse.

• Spatial Summation: Simultaneous stimuli at multiple synapses.

• Facilitation: Accumulation of EPSPs brings neuron closer to threshold.

• Neuromodulators and hormones can alter membrane sensitivity and shift the balance between EPSPs and IPSPs.

Presynaptic Modulation

• Axoaxonic Synapses: Synapses between axons can modulate neurotransmitter release.

• Presynaptic Inhibition: Decreases neurotransmitter release.

• Presynaptic Facilitation: Increases neurotransmitter release.

Frequency Coding

• Information is encoded by the frequency of action potentials.

• Maximum firing rate is limited by the refractory period.

Summary

• Neural information is relayed as action potentials.

• Neurotransmitters and neuromodulators can excite or inhibit postsynaptic neurons.

• Neurons are influenced by the sum of all synaptic inputs and modulatory chemicals.