Nervous Tissue

An Introduction to the Nervous System

The nervous system is a complex network responsible for receiving, processing, and responding to internal and external stimuli. It consists of the brain, spinal cord, sensory receptors, and nerves connecting to other systems.

• Functions: Receives information, processes it, and initiates responses.

• Cell Types: Neurons (specialized for communication) and Neuroglia (support and protect neurons).

Divisions of the Nervous System

The nervous system is divided anatomically and functionally into several components, each with distinct roles.

• Anatomical Divisions:

  • Central Nervous System (CNS): Brain and spinal cord; integrates, processes, and coordinates sensory information and motor commands. Responsible for higher functions such as intelligence, memory, learning, and emotion.

  • Peripheral Nervous System (PNS): All nervous tissue outside the CNS; delivers sensory information to the CNS and carries motor commands to peripheral tissues. Includes cranial and spinal nerves.

• 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).

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

  • Autonomic Nervous System (ANS): Controls smooth muscle, cardiac muscle, glands, and adipose tissue (involuntary). Subdivided into Sympathetic and Parasympathetic divisions.

  • Enteric Nervous System (ENS): Neurons in the digestive tract; coordinates local reflexes independently of the CNS.

Neurons

Structure and Function of Neurons

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

• Cell Body (Soma): Contains nucleus, perikaryon (cytoplasm), neurofilaments, neurotubules, and neurofibrils for structural support. No centrioles (no cell division).

• Dendrites: Highly branched processes that receive information from other neurons.

• Axon: Single, long process that propagates action potentials. Includes axoplasm, axolemma, initial segment, axon hillock, collaterals, telodendria, and axon terminals.

Axonal Transport

• Anterograde: Movement from cell body to axon terminals.

• Retrograde: Movement from axon terminals to cell body (e.g., rabies virus infection).

Classification of Neurons

Neurons are classified structurally and functionally.

• Structural Classification:

  • Anaxonic: Many dendrites, no obvious axon; found in brain and special sense organs.

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

  • Unipolar: Axon and dendrites are continuous; most sensory neurons in PNS.

  • Multipolar: One long axon, two or more dendrites; common in CNS and all PNS motor neurons.

• Functional Classification:

  • Sensory Neurons (Afferent): Carry information from receptors to CNS.

  • Motor Neurons (Efferent): Carry instructions from CNS to effectors.

  • Interneurons: Integrate sensory information and coordinate motor commands; involved in higher functions.

Neuroglia

Types and Functions of Neuroglia

Neuroglia are supportive cells that maintain the environment and structure of nervous tissue. They make up half the volume of the nervous system.

• CNS Neuroglia:

  • Astrocytes: Maintain blood-brain barrier, repair tissue, guide development, regulate environment.

  • Oligodendrocytes: Form myelin sheath, increase speed of action potentials.

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

  • Microglia: Phagocytic cells that clean up debris and pathogens.

• PNS Neuroglia:

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

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

Demyelination and Neural Injury

• Demyelination: Destruction of myelin sheaths (e.g., lead exposure, diphtheria, multiple sclerosis).

• Regeneration: Limited in CNS (astrocytes block regrowth); possible in PNS (Schwann cells form path for regrowth).

Membrane Potential

Resting Membrane Potential

The resting membrane potential is the voltage difference across the cell membrane of an unstimulated neuron, typically around -70 mV.

• Ion Distribution: Extracellular fluid has high Na+ and Cl-; cytosol has high K+ and negatively charged proteins.

• Selective Permeability: More K+ leaks out than Na+ leaks in; proteins cannot cross membrane.

Electrochemical Gradients

• Chemical Gradient: Driven by concentration differences.

• Electrical Gradient: Driven by charge differences.

• Electrochemical Gradient: Sum of chemical and electrical forces; determines ion movement.

Equilibrium Potential

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

Sodium–Potassium Exchange Pump

• Active Transport: Moves 3 Na+ out and 2 K+ in per ATP molecule.

• Maintains Resting Potential: Opposes passive ion leaks.

Membrane Channels

• Gated Ion Channels: Open/close in response to stimuli; types include chemically gated, voltage-gated, and mechanically gated channels.

Graded Potentials

• Definition: Temporary, localized changes in membrane potential.

• Depolarization: Shift toward less negative potential (Na+ influx).

• Hyperpolarization: Shift toward more negative potential (K+ efflux).

• Repolarization: Return to resting potential.

Action Potential

Generation and Propagation of Action Potentials

An action potential is a large, rapid depolarization that propagates along the axon without diminishing.

• Threshold: Minimum depolarization required to trigger an action potential.

• All-or-None Principle: Action potential is either triggered or not; all are identical in magnitude.

• Steps:

  1. Depolarization to threshold

  2. Rapid depolarization (Na+ influx)

  3. Repolarization (K+ efflux)

  4. Hyperpolarization (brief, due to slow K+ channel closure)

Refractory Period

• Absolute Refractory Period: No response possible; Na+ channels open/inactivated.

• Relative Refractory Period: Response possible with larger stimulus; membrane hyperpolarized.

Propagation Types

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

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

Axon Types and Propagation Speed

• Type A Fibers: Myelinated, large diameter, fast transmission (sensory/motor to skeletal muscle).

• Type B Fibers: Myelinated, medium diameter, intermediate speed.

• Type C Fibers: Unmyelinated, small diameter, slow transmission (sensory/motor to smooth/cardiac muscle, glands).

Synapses

Structure and Function of Synapses

A synapse is a specialized site where a neuron communicates with another cell. It can be electrical (direct contact) or chemical (neurotransmitter-mediated).

• Presynaptic Cell: Sends the message.

• Postsynaptic Cell: Receives the message.

• Types of Chemical Synapses: Axoaxonic, axosomatic, axodendritic, neuromuscular, neuroglandular.

Cholinergic Synapses

• Release Acetylcholine (ACh): At neuromuscular junctions, CNS synapses, PNS neuron-to-neuron synapses, and parasympathetic junctions.

• Events: Action potential arrives, Ca2+ influx triggers ACh release, ACh binds to postsynaptic receptors, ACh broken down by acetylcholinesterase.

Synaptic Delay and Fatigue

• Synaptic Delay: Time required for neurotransmitter release and effect (0.2–0.5 ms).

• Synaptic Fatigue: Occurs when neurotransmitter supply is depleted during intense stimulation.

Neurotransmitters and Neuromodulators

Types and Effects

• Excitatory: Cause depolarization and promote action potentials.

• Inhibitory: Cause hyperpolarization and suppress action potentials.

• Major Classes:

  • Acetylcholine: Excitatory, CNS and PNS.

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

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

  • Neuropeptides: Opioids (pain relief), enkephalins, endorphins, dynorphins.

  • Dissolved Gases: Nitric oxide, carbon monoxide.

Mechanisms of Action

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

• Metabotropic (Indirect): Bind to G protein-coupled receptors, activate second messengers (e.g., NE, dopamine, serotonin, GABA).

• Intracellular Enzyme Activation: Lipid-soluble gases enter cell and activate enzymes (e.g., NO, CO).

Information Processing

Postsynaptic Potentials and Summation

• Excitatory Postsynaptic Potential (EPSP): Graded depolarization.

• Inhibitory Postsynaptic Potential (IPSP): Graded hyperpolarization.

• Summation:

  • Temporal Summation: Rapid, repeated stimuli at a single synapse.

  • Spatial Summation: Simultaneous stimuli at multiple synapses.

Facilitation and Inhibition

• Facilitation: Brings membrane potential closer to threshold.

• Interactions: EPSPs and IPSPs determine likelihood of action potential.

• Neuromodulators/Hormones: Can alter membrane sensitivity.

Presynaptic Regulation

• Presynaptic Inhibition: Decreases neurotransmitter release.

• Presynaptic Facilitation: Increases neurotransmitter release.

Rate of Action Potential Generation

• Frequency: Strength of response is proportional to frequency of stimulation.

• Maximum Rate: Achieved when relative refractory period is eliminated by strong depolarization.

Summary of Information Processing

• Information is relayed as action potentials.

• Neurotransmitters may have excitatory or inhibitory effects.

• Neuromodulators can alter neurotransmitter release or postsynaptic response.

• Neurons may be facilitated or inhibited by various chemicals.

• Postsynaptic response can be altered by neuromodulators, activity at other synapses, or presynaptic regulation.

Learning Outcomes

• Describe anatomical and functional divisions of the nervous system.

• Sketch and label neuron structure, describe functions, and classify neurons.

• Describe locations and functions of neuroglia.

• Explain how resting membrane potential is established, maintained, and changed.

• Describe events in action potential generation and propagation, and factors affecting speed.

• Describe synapse structure and synaptic activity mechanisms.

• Describe major neurotransmitters and neuromodulators and their effects.

• Discuss interactions enabling information processing in nervous tissue.