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:

  • Receives information from internal and external stimuli

  • Processes information

  • Initiates 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 nervous system (CNS) and peripheral nervous system (PNS).

• 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 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, adipose tissue, and glands (involuntary)

  • Enteric Nervous System (ENS): Neurons in the digestive tract, coordinates local reflexes

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), mitochondria, RER, ribosomes, and Nissl bodies

• Dendrites: Highly branched processes that receive information

• Axon: Single, long process that propagates electrical signals (action potentials)

  • Axoplasm: Cytoplasm of the axon

  • Axolemma: Plasma membrane of the axon

  • Initial segment: First segment of axon

  • Axon hillock: Attaches initial segment to cell body

  • Collaterals: Branches of the axon

  • Telodendria: Fine branches of distal axon

  • Axon terminals: Expanded tips of telodendria

Axonal Transport

• Movement of materials between cell body and axon terminals along neurotubules

• Requires ATP

• Anterograde: Cell body to axon terminals (kinesin)

• Retrograde: Axon terminals to cell body (dynein)

Structural Classification of Neurons

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

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

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

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

Functional Classification of Neurons

• Sensory neurons (afferent): Carry information from receptors to CNS

  • Somatic sensory neurons: Monitor external environment and position

  • Visceral sensory neurons: Monitor internal environment

  • Types of sensory receptors:

    • Interoceptors: Monitor internal systems

    • Exteroceptors: Monitor external environment

    • Proprioceptors: Monitor position and movement of muscles and joints

• Motor neurons (efferent): Carry instructions from CNS to effectors

  • Somatic motor neurons: Innervate skeletal muscles

  • Visceral motor neurons: Innervate smooth/cardiac muscle, glands, adipose tissue

• Interneurons: Integrate sensory information and coordinate motor commands; involved in higher functions (memory, planning, learning)

Neuroglia

Types and Functions

Neuroglia are supportive cells that protect and maintain neurons, making up half the volume of the nervous system.

• Central Nervous System:

  • 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 cells, remove debris and pathogens

• Peripheral Nervous System:

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

  • Schwann cells: Form myelin sheath around axons, assist in repair after injury

Myelination and Repair

• Myelin: Lipid insulation that increases speed of action potential propagation

• Internodes: Myelinated segments of axon

• Nodes of Ranvier: Gaps in myelin sheath

• White matter: Regions with many myelinated axons

• Gray matter: Regions with unmyelinated axons, cell bodies, dendrites

Neural Responses to Injury

• PNS:

  • Wallerian degeneration: Axon distal to injury degenerates

  • Schwann cells proliferate and form a path for new axonal growth

  • Regrowth may restore function

• CNS:

  • Regeneration limited by astrocytes (scar tissue, chemical inhibition)

Membrane Potential

Resting Membrane Potential

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

• ECF: High Na+ and Cl-

• Cytosol: High K+ and negatively charged proteins

• Selective permeability allows passive movement through leak channels and active transport via sodium-potassium pump

• Inner membrane surface is more negative at rest

Electrochemical Gradient

• Current: Movement of charges to eliminate potential difference

• Resistance: Restriction of ion movement by membrane

• Chemical gradient: Movement driven by concentration differences

• Electrical gradient: Movement driven by charge differences

• Electrochemical gradient: Sum of chemical and electrical forces acting on an ion

Active Transport and Membrane Channels

• Sodium–potassium exchange pump: Powered by ATP, moves 3 Na+ out and 2 K+ in per ATP

• Keeps resting membrane potential stable by opposing passive ion leaks

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

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 outward and decrease with distance

• Stronger stimuli produce stronger graded potentials

• Can cause depolarization (shift toward less negative potential) or hyperpolarization (shift toward more negative potential)

• Repolarization returns membrane to resting potential

• Often trigger specific cell functions (e.g., exocytosis, muscle contraction)

Action Potential

Generation and Propagation

An action potential is a large depolarization that, once initiated, affects the entire excitable membrane and does not diminish as it moves away from the point of stimulation.

• All-or-none principle: Action potential is triggered only if threshold is reached; all action potentials are identical

• Steps:

  1. Depolarization to threshold

  2. Rapid depolarization (Na+ influx)

  3. Repolarization (K+ efflux)

  4. Hyperpolarization (brief, due to slow closing of K+ channels)

• Refractory period:

  • Absolute: No response possible

  • Relative: Larger stimulus required

Propagation of Action Potentials

• Continuous propagation: Unmyelinated axons; action potential moves segment by segment

• Saltatory propagation: Myelinated axons; action potential "jumps" from node to node, faster and more energy-efficient

• 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

A synapse is a specialized site where a neuron communicates with another cell. There are electrical and chemical synapses.

• Electrical synapses: Direct physical contact, rapid transmission

• Chemical synapses: Use neurotransmitters to transmit signals across a synaptic cleft

• Types of chemical synapses: Axoaxonic, axosomatic, axodendritic, neuromuscular, neuroglandular

Cholinergic Synapses

• Release acetylcholine (ACh)

• Events:

  1. Action potential arrives and depolarizes axon terminal

  2. Ca2+ influx triggers ACh exocytosis

  3. ACh binds to postsynaptic receptors, causing depolarization

  4. ACh is broken down by acetylcholinesterase

Neurotransmitters and Neuromodulators

Classes and Effects

• Excitatory: Cause depolarization, promote action potentials

• Inhibitory: Cause hyperpolarization, suppress action potentials

• Effect depends on receptor properties

• Major classes:

  • Acetylcholine

  • Biogenic amines (norepinephrine, dopamine, serotonin)

  • Amino acids (glutamate, glycine, GABA)

  • Neuropeptides (substance P, opioids)

• Mechanisms:

  • Ionotropic: Direct effect via ion channels

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

  • Direct intracellular effects (e.g., lipid-soluble gases)

Information Processing

Postsynaptic Potentials and Summation

• Postsynaptic potentials: Graded potentials in response to neurotransmitters

  • Excitatory (EPSP): Depolarization, increases likelihood of action potential

  • Inhibitory (IPSP): Hyperpolarization, decreases likelihood of action potential

• Summation: Integration of all incoming graded potentials

  • 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 various chemicals

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