Nervous Tissue: Structure, Function, and Clinical Correlates

Chapter 12: Nervous Tissue

Introduction to the Nervous System

The nervous system, together with the endocrine system, is essential for maintaining homeostasis and coordinating body functions. It is responsible for behavior, memory, and movement. The study of the nervous system's structure and function is called neurophysiology.

• Homeostasis: Maintained by rapid communication and control.

• Major Functions: Sensory input, integration, and motor output.

Basic Functions of the Nervous System

• Sensory: Sensing changes with sensory receptors (internal and external environment).

• Integrative: Interpreting and remembering sensory input, making decisions.

• Motor: Reacting to stimuli by activating effectors (muscles and glands).

Major Structures of the Nervous System

• Central Nervous System (CNS): Brain and spinal cord.

• Peripheral Nervous System (PNS): Cranial nerves, spinal nerves, ganglia, sensory receptors.

Organization and Divisions of the Nervous System

Central Nervous System (CNS)

• Consists of the brain and spinal cord.

• Responsible for integration and command center functions.

Peripheral Nervous System (PNS)

• All nervous tissue outside the CNS.

• Includes cranial nerves, spinal nerves, ganglia, and sensory receptors.

Subdivisions of the PNS

• Somatic Nervous System (SNS): Voluntary control of skeletal muscles; sensory neurons from skin, muscles, joints to CNS.

• Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands; divided into sympathetic and parasympathetic divisions.

• Enteric Nervous System (ENS): "Brain of the gut"; controls GI tract independently but communicates with CNS and ANS.

Neuronal Structure and Function

Neurons

Neurons are the functional units of the nervous system, capable of producing action potentials (electrical signals).

• Cell Body (Soma): Contains nucleus and organelles; site of most protein synthesis.

• Dendrites: Receive input; typically short, branched, and unmyelinated.

• Axon: Conducts impulses away from the cell body; long, cylindrical process; may be myelinated or unmyelinated.

Axonal Transport

• Moves substances (e.g., proteins, organelles) between cell body and axon terminals.

• Fast and slow axonal transport mechanisms exist.

Structural Classification of Neurons

• Multipolar: Many dendrites, one axon (most common in CNS).

• Bipolar: One dendrite, one axon (e.g., retina, inner ear).

• Unipolar: Single process that splits into two branches (sensory neurons).

Functional Classification of Neurons

• Sensory (Afferent): Transmit impulses from receptors to CNS.

• Motor (Efferent): Transmit impulses from CNS to effectors (muscles/glands).

• Interneurons (Association): Connect sensory and motor neurons; most abundant type in CNS.

Neuroglial Cells

Neuroglia support, protect, and nourish neurons. They do not conduct impulses and are more numerous than neurons.

• Astrocytes: Maintain blood-brain barrier, regulate ion balance, provide structural support.

• Microglia: Phagocytic cells that remove debris and pathogens.

• Ependymal Cells: Line ventricles of brain and central canal of spinal cord; produce cerebrospinal fluid (CSF).

• Oligodendrocytes: Myelinate axons in CNS.

• Schwann Cells: Myelinate axons in PNS.

• Satellite Cells: Support neurons in PNS ganglia.

Myelination and Nerve Fibers

Myelin Sheath

• Multilayered lipid and protein covering produced by Schwann cells (PNS) and oligodendrocytes (CNS).

• Increases speed of nerve impulse conduction.

Axon Coverings in PNS

• Neurilemma: Outer cytoplasmic layer of Schwann cell; aids in axon regeneration.

• Nodes of Ranvier: Gaps in myelin sheath; facilitate saltatory conduction.

Gray and White Matter

• White Matter: Myelinated axons (white appearance).

• Gray Matter: Neuron cell bodies, dendrites, unmyelinated axons.

Electrical Signals in Neurons

Ion Channels

• Flow of ions through gated channels in the cell membrane generates electrical signals.

• Types of gated channels: voltage-gated, ligand-gated, mechanically-gated.

Resting Membrane Potential

• Inside of membrane is negative relative to outside (typically -70 mV).

• Maintained by unequal distribution of ions (Na+, K+, Cl-), selective permeability, and Na+/K+ ATPase pump.

Graded Potentials

• Small, localized changes in membrane potential.

• Vary in amplitude and duration; can summate to trigger action potentials.

Action Potentials

• Rapid, large changes in membrane potential that propagate along axons.

• All-or-none response; threshold must be reached to initiate.

• Phases: depolarization (Na+ influx), repolarization (K+ efflux), hyperpolarization.

Key Equation:

$V_m = V_{inside} - V_{outside}$

Refractory Periods

• Absolute Refractory Period: No new action potential can be generated.

• Relative Refractory Period: Stronger stimulus required for new action potential.

Propagation of Action Potentials

• Action potentials travel along axons as nerve impulses.

• Continuous Conduction: Unmyelinated fibers; stepwise depolarization.

• Saltatory Conduction: Myelinated fibers; impulse jumps from node to node, increasing speed.

Speed of Impulse Propagation

• Depends on fiber diameter (larger = faster), myelination (myelinated = faster), and temperature (warmer = faster).

Encoding Stimulus Intensity

• Stronger stimuli produce action potentials at higher frequency and may recruit more neurons.

Signal Transmission at Synapses

Types of Synapses

• Electrical Synapses: Direct current flow via gap junctions; rapid, synchronized activity.

• Chemical Synapses: Neurotransmitter release from presynaptic neuron to postsynaptic cell; one-way transmission.

Chemical Synapse Mechanism

• Action potential triggers Ca2+ influx, causing neurotransmitter release.

• Neurotransmitter binds to receptors, generating postsynaptic potential.

Excitatory and Inhibitory Potentials

• Excitatory Postsynaptic Potential (EPSP): Depolarizes membrane, increases likelihood of action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes membrane, decreases likelihood of action potential.

Removal of Neurotransmitter

• By diffusion, enzymatic degradation, or uptake by cells.

Comparison of Graded and Action Potentials

Feature	Graded Potential	Action Potential
Origin	Dendrites/cell body	Axon hillock/axon
Amplitude	Varies with stimulus	All-or-none
Channels	Ligand/mechanically-gated	Voltage-gated
Propagation	Localized	Propagated
Refractory Period	No	Yes

Summation

• Spatial Summation: Multiple presynaptic neurons release neurotransmitter at various locations.

• Temporal Summation: Single presynaptic neuron releases neurotransmitter rapidly in succession.

Neurotransmitters

• Include acetylcholine, amino acids (GABA, glutamate), biogenic amines (dopamine, serotonin), neuropeptides (substance P), ATP, and others.

• Can be excitatory or inhibitory depending on receptor and location.

Small-Molecule Neurotransmitters

• Acetylcholine (ACh): Excitatory at neuromuscular junctions, inhibitory in some CNS synapses.

• Amino Acids: Glutamate (excitatory), GABA (inhibitory in brain).

• Biogenic Amines: Dopamine, norepinephrine, serotonin; involved in mood, arousal, and autonomic functions.

• ATP and Purines: Act as neurotransmitters in both CNS and PNS.

Neuropeptides

• Short chains of amino acids; include substance P (pain perception), endorphins (pain relief).

Clinical Correlates

Strychnine Poisoning

• Blocks inhibitory neurotransmitter glycine in spinal cord, causing muscle contractions and possible respiratory failure.

Regeneration and Repair

• Plasticity maintained throughout life; limited repair in CNS, possible in PNS if neurilemma is intact.

Neurogenesis in the CNS

• Formation of new neurons from stem cells occurs in some brain regions (e.g., hippocampus).

Multiple Sclerosis (MS)

• Autoimmune destruction of CNS myelin; symptoms include muscle weakness, vision problems, and progressive loss of function.

Epilepsy

• Neurological disorder characterized by recurrent seizures due to abnormal electrical activity in the brain.

• Causes include brain injury, tumors, infections, and genetic factors.

Additional info: This summary expands on the provided slides with definitions, examples, and clinical context for clarity and completeness.