Chapter 12: Nervous Tissue and Neural Signaling – Study Outline

Chapter 12: Nervous Tissue and Neural Signaling

An Introduction to the Nervous System and Nervous Tissue

The nervous system is a complex network responsible for communication, coordination, and control throughout the body. It is divided into anatomical and functional subdivisions, each with specialized roles.

• Nervous System Overview: Includes all nervous tissue; the basic functional unit is the neuron.

• Anatomical Divisions:

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

  • Peripheral Nervous System (PNS): All nervous tissue outside CNS and ENS.

  • Enteric Nervous System (ENS): Neurons and networks in the digestive tract walls.

• PNS Subdivisions: Somatic nervous system (controls voluntary actions) and autonomic nervous system (controls involuntary actions).

• Nerves: Bundles of axons (nerve fibers) in the PNS.

• Functional Divisions:

  • Afferent Division: Brings sensory information from receptors to CNS.

  • Efferent Division: Carries motor commands to effectors (muscles and glands).

• Efferent Division Subdivisions:

  • Somatic Nervous System (SNS): Controls skeletal muscle contractions.

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

Neurons: Structure and Classification

Neurons are specialized cells for intercellular communication, with distinct structural and functional classifications.

• Perikaryon: Cytoplasm surrounding the nucleus in a multipolar neuron; contains organelles such as neurofilaments, neurotubules, and neurofibrils.

• Axon Hillock: Connects the initial segment of the axon to the soma (cell body).

• Axoplasm: Cytoplasm within the axon, rich in organelles.

• Axon Branching: Collaterals branch from the axon; telodendria branch from the axon's tip.

• Structural Classification:

  • Anaxonic: No obvious axon; found in CNS.

  • Bipolar: One dendrite and one axon; found in sensory organs.

  • Unipolar: Single process splits into dendrite and axon; common in PNS sensory neurons.

  • Multipolar: Multiple dendrites, one axon; most common in CNS.

• Functional Classification:

  • Sensory Neurons: Form afferent division; deliver information from interoceptors, exteroceptors, and proprioceptors to CNS.

  • Motor Neurons: Form efferent division; stimulate or modify activity of peripheral tissues/organs.

  • Interneurons: Located in CNS; distribute sensory inputs and coordinate motor outputs.

CNS and PNS Neuroglia: Support and Protection

Neuroglia, or glial cells, provide structural and functional support to neurons in both CNS and PNS.

• CNS Neuroglia Types:

  • Astrocytes: Largest, most numerous; maintain blood-brain barrier, regulate environment.

  • Ependymal Cells: Line ventricles; produce and circulate cerebrospinal fluid (CSF).

  • Oligodendrocytes: Myelinate CNS axons.

  • Microglia: Phagocytic cells; remove debris and pathogens.

• PNS Neuroglia:

  • Ganglia: Clusters of neuron cell bodies.

  • Satellite Cells: Surround neuron cell bodies in ganglia.

  • Schwann Cells: Cover axons; myelinate one segment or enclose several unmyelinated axons.

• Axon Repair: PNS can undergo functional repair via Wallerian degeneration; CNS repair is limited.

Membrane Potential: Ion Concentrations and Permeability

Neural signaling depends on the plasma membrane's properties, including ion gradients and permeability.

• Electrochemical Gradient: Sum of chemical and electrical forces across the membrane.

• Sodium–Potassium Exchange Pump: Stabilizes resting membrane potential at approximately −70 mV. Equation: $\text{Na}^+/\text{K}^+ \text{ pump:} \, 3\, \text{Na}^+ \text{ out}, \, 2\, \text{K}^+ \text{ in}$

• Ion Channels:

  • Passive (Leak) Channels: Always open.

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

• Types of Gated Channels:

  • Chemically Gated (Ligand-Gated): Open in response to specific chemicals.

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

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

• Graded Potentials: Localized depolarization or hyperpolarization; decrease with distance.

Action Potentials: Long-Distance Communication

Action potentials are rapid, all-or-none electrical events used for communication over long distances in neurons.

• Initiation: Occurs when membrane depolarizes to threshold.

• Steps:

  1. Depolarization to threshold

  2. Activation of voltage-gated sodium channels; rapid depolarization

  3. Inactivation of sodium channels; activation of potassium channels

  4. Return to normal permeability

• All-or-None Principle: Action potential either occurs fully or not at all.

• Refractory Period: Time from action potential onset to return to resting potential.

• Propagation Types:

  • Continuous Propagation: Action potential spreads in small steps across unmyelinated membrane.

  • Saltatory Propagation: Action potential leaps from node to node in myelinated axons; much faster.

• Axon Classification:

  Type	Diameter	Myelination	Speed
  Type A	Large	Myelinated	Fastest
  Type B	Medium	Myelinated	Intermediate
  Type C	Small	Unmyelinated	Slowest

Synapses: Intercellular Communication

Synapses are specialized sites where neurons communicate with other neurons or cells, using electrical or chemical signals.

• Synapse Structure: Information passes from presynaptic to postsynaptic cell.

• Neurotransmitters: Released from axon terminals; affect postsynaptic cell.

• Types of Synapses:

  • Electrical Synapses: Direct physical contact; rare; gap junctions allow current flow.

  • Chemical Synapses: Use neurotransmitters; more common; involve synaptic cleft.

• Cholinergic Synapses: Release acetylcholine (ACh); synaptic delay due to calcium influx and neurotransmitter release.

• Synaptic Fatigue: Occurs if ACh stores are exhausted; choline is recycled.

Neurotransmitters and Neuromodulators: Effects and Mechanisms

The effects of neurotransmitters and neuromodulators depend on their receptors and can be excitatory or inhibitory.

• Excitatory Neurotransmitters: Cause depolarization; promote action potentials.

• Inhibitory Neurotransmitters: Cause hyperpolarization; suppress action potentials.

• Receptor Properties: Determine effect, not neurotransmitter type.

• Adrenergic Synapses: Release norepinephrine (NE); other neurotransmitters include dopamine, serotonin, GABA.

• Neuromodulators: Influence postsynaptic cell's response; can alter rate of neurotransmitter release or response.

• Mechanisms of Action:

  • Direct effect on membrane potential

  • Indirect effect via G proteins

  • Indirect effect via intracellular enzymes (for lipid-soluble gases)

Integration of Excitatory and Inhibitory Stimuli

Neurons process information by integrating excitatory and inhibitory signals, determining their output based on these interactions.

• Postsynaptic Potentials:

  • Excitatory Postsynaptic Potential (EPSP): Depolarization caused by neurotransmitter.

  • Inhibitory Postsynaptic Potential (IPSP): Hyperpolarization caused by neurotransmitter.

• Summation: EPSPs can combine via temporal (same synapse, rapid succession) or spatial (multiple synapses, different locations) summation.

• EPSP–IPSP Interactions: Determine neural activity; integration occurs at axon hillock.

• Presynaptic Inhibition: GABA release inhibits calcium channels, reducing neurotransmitter release.

• Presynaptic Facilitation: Increases neurotransmitter release, enhancing postsynaptic effects.

• Frequency of Action Potentials: Depends on degree of depolarization above threshold; at high frequencies, refractory period is eliminated.

Summary Table: Types of Neuroglia

Location	Type	Main Function
CNS	Astrocytes	Support, blood-brain barrier, regulate environment
CNS	Ependymal Cells	Produce/circulate CSF
CNS	Oligodendrocytes	Myelinate axons
CNS	Microglia	Phagocytosis
PNS	Satellite Cells	Support neuron cell bodies
PNS	Schwann Cells	Myelinate axons

Example: In saltatory propagation, a myelinated axon transmits an action potential much faster than an unmyelinated axon, allowing rapid reflexes and efficient communication in the nervous system.

Additional info: The notes expand on brief outline points to provide definitions, examples, and context for key concepts in nervous tissue and neural signaling, suitable for exam preparation.