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 that connect to other body systems.

• Functions:

  • Receive information from internal and external stimuli

  • Process information

  • Initiate responses

• Cell Types:

  • Neurons: Specialized for intercellular communication

  • Neuroglia (glial cells): Support, protect, and preserve nervous tissue structure

Divisions of the Nervous System

The nervous system is divided anatomically and functionally into the central and peripheral nervous systems.

• Central Nervous System (CNS):

  • Includes the brain and spinal cord

  • Integrates, processes, and coordinates sensory information and motor commands

  • Responsible for higher functions: intelligence, memory, learning, emotion

• Peripheral Nervous System (PNS):

  • All nervous tissue outside the CNS

  • Delivers sensory information to the CNS and carries motor commands to peripheral tissues

  • Nerves: Bundles of axons with connective tissue and blood vessels

  • Cranial nerves: Connect to the brain

  • Spinal nerves: Connect to the spinal cord

• Functional Divisions of PNS:

  • Afferent division: Carries sensory information from receptors to CNS

  • Efferent division: Carries motor commands from 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, adipose tissue, and glands (involuntary; includes sympathetic and parasympathetic divisions)

  • Enteric Nervous System (ENS): Neurons in digestive tract walls, coordinates local reflexes independently of CNS

Neurons

Structure and Function

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

• Cell Body (Soma): Contains nucleus, nucleolus, perikaryon (cytoplasm), neurofilaments, neurotubules, neurofibrils, mitochondria, RER, ribosomes, and Nissl bodies

• Dendrites: Short, highly branched processes that receive information

• Axon: Single, long process that propagates electrical signals (action potentials); includes axoplasm, axolemma, initial segment, axon hillock, collaterals, telodendria, and axon terminals

Axonal Transport

Axonal (axoplasmic) transport moves materials between the cell body and axon terminals along neurotubules, requiring ATP.

• Anterograde: Cell body to axon terminals (kinesin)

• Retrograde: Axon terminals to cell body (dynein)

Structural Classification of Neurons

Neurons are classified by structure:

• Anaxonic: Many dendrites, no obvious axon (brain, special sense organs)

• Bipolar: One dendrite, one axon (rare, special sense organs)

• Unipolar: Axon and dendrites continuous, soma off to the side (most sensory neurons of PNS)

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

Functional Classification of Neurons

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

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

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

Neuroglia

Types and Functions

Neuroglia are supportive cells that 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 monitor cerebrospinal fluid

  • Microglia: Phagocytic, remove debris and pathogens

• PNS Neuroglia:

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

  • Schwann Cells: Form myelin sheath or folds around axons, aid in repair after injury

Neural Responses to Injury

• PNS: Wallerian degeneration, Schwann cells form path for regrowth

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

Membrane Potential

Resting Membrane Potential

The resting membrane potential is the voltage difference across the cell membrane of an unstimulated cell, resulting from differences in ion concentrations and selective permeability.

• ECF: High Na+ and Cl-

• Cytosol: High K+ and negatively charged proteins

• Leak Channels: Allow passive ion movement

• Sodium-Potassium Pump: Maintains gradient by moving 3 Na+ out and 2 K+ in per ATP

Electrochemical Gradient

• Chemical Gradient: Driven by concentration differences

• Electrical Gradient: Driven by charge differences

• Electrochemical Gradient: Sum of chemical and electrical forces

Membrane Channels

• Gated Ion Channels: Open/close in response to stimuli

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

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

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

Graded Potentials

Characteristics and Effects

Graded potentials are temporary, localized changes in the resting membrane potential, produced by stimuli that open gated channels.

• Spread passively and decrease with distance

• Stronger stimuli produce stronger graded potentials

• Can cause depolarization (less negative) or hyperpolarization (more negative)

• Repolarization restores resting potential

Action Potential

Generation and Propagation

Action potentials are large depolarizations that, once initiated, affect the entire excitable membrane and do not diminish as they move.

• Threshold: -60 to -55 mV

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

• 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: No response possible

  • Relative: Larger stimulus needed

Propagation

• Continuous Propagation: Unmyelinated axons, slower

• Saltatory Propagation: Myelinated axons, faster, local current jumps from node to node

• Propagation Speed: Increased by myelination and larger axon diameter

• Axon Types:

  • Type A: Myelinated, large diameter, fast (120 m/sec)

  • Type B: Myelinated, medium diameter, intermediate speed (18 m/sec)

  • Type C: Unmyelinated, small diameter, slow (1 m/sec)

Synapses

Types and Function

Synapses are specialized sites where neurons communicate with other cells.

• Electrical Synapses: Direct physical contact, rapid transmission

• Chemical Synapses: Use neurotransmitters across a synaptic cleft; most common

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

Cholinergic Synapses

• Release acetylcholine (ACh)

• Events:

  1. Action potential arrives, depolarizes terminal

  2. Ca2+ influx triggers ACh exocytosis

  3. ACh binds to postsynaptic receptors, depolarizes membrane

  4. ACh broken down by acetylcholinesterase

Neurotransmitters and Neuromodulators

Classes and Effects

• Excitatory: Cause depolarization, promote action potentials

• Inhibitory: Cause hyperpolarization, suppress action potentials

• Major Neurotransmitters:

  • Acetylcholine (ACh): Excitatory

  • Biogenic amines: Norepinephrine (NE), Dopamine, Serotonin

  • Amino acids: Glutamate (excitatory), Glycine (inhibitory), GABA (inhibitory)

  • Neuropeptides: Substance P, opioids (enkephalins, endorphins, dynorphins)

• Effects:

  • Ionotropic: Direct effect via ion channels

  • Metabotropic: Indirect effect via G protein-coupled receptors and second messengers

  • Intracellular: Lipid-soluble gases activate enzymes inside cell

Information Processing

Postsynaptic Potentials and Summation

• Excitatory Postsynaptic Potential (EPSP): Graded depolarization

• Inhibitory Postsynaptic Potential (IPSP): Graded hyperpolarization

• Summation:

  • Temporal: Rapid, repeated stimuli at one synapse

  • Spatial: Simultaneous stimuli at multiple synapses

• Facilitation: Brings membrane potential closer to threshold

• Presynaptic Regulation:

  • Presynaptic inhibition: Decreases neurotransmitter release

  • Presynaptic facilitation: Increases neurotransmitter release

• Rate of Action Potential Generation: Proportional to frequency and degree of stimulation

Summary

• 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 chemicals or activity at other synapses

Key Equations

• Sodium-Potassium Pump:

• Resting Membrane Potential: (Nernst equation for K+)

Additional info: Tables and equations were expanded for clarity and completeness.