Nervous Tissue

LECTURE 11: NERVOUS TISSUE

Chapter 12: Dr. Dolan, BIOLOGY 220 - A&P I

Introduction to the Nervous System

  • Components of the nervous system:

    • Brain and spinal cord.

    • Receptors of sense organs (eyes, ears, etc.).

    • Nerves connecting to other systems.

  • Functions of the nervous system:

    • Receive information from internal and external stimuli.

    • Process the information.

    • Initiate responses.

  • Nervous tissue consists of two types of cells:

    • Neurons: specialized for intercellular communication.

    • Neuroglia (glial cells): support and protect neurons; preserve the structure of nervous tissue.

Divisions of the Nervous System
1. Central Nervous System (CNS)

  • Comprises the brain and spinal cord.

  • Contains nervous tissue, connective tissue, and blood vessels.

  • Functions of the CNS:

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

    • Sensory data includes input from both the internal and external environment.

    • Motor commands control peripheral organs, including skeletal muscles and various glands.

    • Higher brain functions include intelligence, memory, learning, and emotion.

2. Peripheral Nervous System (PNS)

  • Encompasses all nervous tissue outside the CNS.

  • Functions of the PNS:

    • Delivers sensory information to the CNS.

    • Carries motor commands from the CNS to peripheral tissues.

  • Nerves (peripheral nerves):

    • Bundles of axons with associated connective tissues 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 in peripheral tissues and organs to the CNS.

  • Receptors: structures detecting stimuli.

    • Can be single cells or complex sensory organs (like eyes and ears).

    • Types of receptors include:

    • Visceral: monitors internal organs.

    • Somatic: monitors muscles, joints, and skin.

    • Special: associated with special senses (smell, vision, etc).

Efferent Division

  • Carries motor commands from the CNS to effectors.

  • Effectors: musculature, glands, and adipose tissue responding to motor commands.

Efferent Division of the PNS
1. Somatic Nervous System (SNS)

  • Controls skeletal muscle contractions, involving both voluntary movements and involuntary reflexes.

2. Autonomic Nervous System (ANS)

  • Controls smooth muscle, cardiac muscle, adipose tissue, and glands.

  • Also referred to as the visceral motor system.

  • Functions involuntarily.

  • Subdivisions with opposing effects:

    • Sympathetic Division: typically stimulates the "fight or flight" response.

    • Parasympathetic Division: typically promotes the "rest and digest" response.

3. Enteric Nervous System (ENS)

  • Composed of neurons in the walls of the digestive tract, with approximately 100 to 600 million neurons.

  • Initiates and coordinates local visceral reflexes of the digestive tract without needing instructions from the CNS (local control).

  • Functionally influenced by sympathetic and parasympathetic divisions of the PNS.

Neurons

1. Definition and Function

  • Neuron (Nerve Cell):

    • Basic functional unit of the nervous system.

    • Possesses an excitable plasma membrane and can send and receive signals.

    • Functions in communication, information processing, and control.

2. Neuron Structure

  • Cell Body (Soma):

    • Contains a large round nucleus with a prominent nucleolus.

    • Perikaryon: cytoplasm within the cell body.

    • Cytoskeleton: contains neurofilaments and neurotubules, which are analogous to intermediate filaments and microtubules, respectively.

    • Neurofibrils: Bundles of neurofilaments supporting dendrites and axons.

  • Unique Features:

    • Lacks centrioles: No cell division occurs.

    • Rich in mitochondria: Provides energy.

    • Endoplasmic Reticulum (RER) and ribosomes are present.

    • Nissl Bodies: Dense areas of RER and ribosomes in the perikaryon, appearing gray in color.

3. Dendrites

  • Function: Short and highly branched processes extending from the cell body.

  • Purpose: Receive information from other neurons.

  • Dendritic Spines: Fine processes on dendrites that receive info; account for 80-90% of the surface area of a neuron.

4. Axon

  • Structure: One long cytoplasmic process.

  • Function: Propagates electrical signals (action potentials).

  • Axon Components:

    • Axoplasm: cytoplasm of the axon.

    • Axolemma: plasma membrane of the axon.

    • Initial Segment: the starting segment of the axon.

    • Axon Hillock: thick region connecting the initial segment to the cell body.

    • Collaterals: branches of the axon.

    • Telodendria: fine branches at the distal end of the axon or collaterals.

    • Axon Terminals (Synaptic Terminals): expanded tips of telodendria.

5. Axonal Transport

  • Definition: Movement of materials between the cell body and axon terminals.

  • Transport Mechanism: Materials move along neurotubules, facilitated by proteins (kinesin for anterograde and dynein for retrograde transport).

  • Energy Requirement: The process requires ATP.

  • Anterograde Transport: From cell body to axon terminals (carried out by kinesin).

  • Retrograde Transport: From axon terminals back to the cell body (executed by dynein).

  • Notable Instances: Infection with rabies virus utilizes retrograde transport; heavy metals can also hitch a retrograde ride.

6. Structural Classification of Neurons

  • Types of Neurons:

    • Anaxonic Neurons: Small with many dendrites; no obvious axon, found in brain and special sense organs.

    • Bipolar Neurons: One dendrite and one axon; rare, exclusively in special sense organs.

    • Unipolar (Pseudounipolar) Neurons: Continuous axon and dendrites with off-centered soma; predominantly sensory neurons of the PNS.

    • Multipolar Neurons: One long axon and multiple dendrites; prevalent in CNS and all PNS motor neurons.

7. Important Definitions

  • Nerve: Bundles of axons with connective tissue and blood vessels in the PNS (distinct from 'nerve cell' or neuron).

  • Tract: Bundle of axons in the CNS.

  • Ganglion: Collection of neuron cell bodies in the PNS.

  • Nucleus: Collection of neuron cells in the CNS.

8. Functional Classifications of Neurons
Sensory Neurons (Afferent Neurons)

  • Carry information from receptors to the CNS.

  • Cell Bodies: Grouped in peripheral sensory ganglia.

  • Processes (Afferent Fibers): Extend from sensory receptors to CNS.

  • Types:

    • Somatic Sensory Neurons: Monitor external environment and body position.

    • Visceral Sensory Neurons: Monitor internal environment.

  • Types of Sensory Receptors:

    • Interoceptors: Monitor internal systems (detect stretch, deep pressure, and pain).

    • Exteroceptors: Monitor external environment (detect touch, temperature, pressure, complex senses).

    • Proprioceptors: Monitor position and movement of skeletal muscles and joints.

Motor Neurons (Efferent Neurons)

  • Carry instructions from the CNS to peripheral effectors.

  • Somatic Motor Neurons: Innervate skeletal muscles.

  • Visceral Motor Neurons: Innervate all other peripheral effectors (smooth muscle, cardiac muscle, glands, adipose tissue).

  • Preganglionic Neurons: Visceral motor neurons with cell bodies in the CNS; innervate second set of neurons (postganglionic) in autonomic ganglia.

Interneurons (Association Neurons)

  • Neurons between sensory and motor neurons integrating sensory information and coordinating motor commands.

  • Found in the brain, spinal cord, and some autonomic ganglia.

  • Involved in higher functions (memory, planning, learning).

  • Integration involves more interneurons for complex responses.

Neuroglia

1. Definition

  • Neuroglia: Cells that support and protect neurons, comprising half the volume of the nervous system.

2. Types of Neuroglia in the CNS

  • Astrocytes:

    • Most numerous; large cell bodies with many processes.

    • Functions:

    • Maintain the blood-brain barrier (BBB).

    • Provide structural support.

    • Regulate ion and nutrient concentrations.

    • Absorb and recycle neurotransmitters.

    • Form scar tissue after injury.

  • Ependymal Cells:

    • Line ventricles (brain) and central canal (spinal cord).

    • Produce and monitor cerebrospinal fluid (CSF).

    • Possess cilia aiding in circulation of CSF.

  • Oligodendrocytes:

    • Smaller, prominent cytoplasmic extensions wrapping around axons.

    • Form myelin sheath along CNS axons.

    • Myelin: Lipid insulation increasing action potential propagation speed.

    • Differentiation between:

    • Internodes: myelinated segments of the axon.

    • Nodes (Nodes of Ranvier): Gaps in the myelin sheath.

    • White Matter: CNS regions rich in myelinated axons.

    • Gray Matter: CNS regions with unmyelinated axons, neuron cell bodies, and dendrites.

  • Microglia:

    • Smallest, least numerous neuroglia with fine-branched processes.

    • Migrate through nervous tissue to clear cellular debris and pathogens via phagocytosis.

3. Neuroglia in the PNS

  • Satellite Cells: Surround neuronal cell bodies in ganglia, regulating O2, CO2, nutrient, and neurotransmitter levels.

  • Schwann Cells (Neurolemmocytes): Encase all axons in the PNS, responsible for axon myelination, and contribute to repair processes post-injury.

4. Demyelination

  • Definition: Progressive destruction of myelin sheaths in CNS or PNS.

  • Causes of Demyelination:

    • Chronic exposure to heavy metals (like lead)

    • Diphtheria: bacterial disease damaging Schwann cells, causing demyelination in PNS and paralysis.

    • Multiple Sclerosis (MS): An autoimmune disorder destroying myelin in the CNS; consequences include speech, vision, balance, motor coordination, and bladder control issues.

5. Neural Response to Injuries

  • In the PNS:

    • Wallerian Degeneration: Degeneration of the axon distal to injury.

    • Schwann cells proliferate, forming a path for new axonal growth (regrowth variable).

  • In the CNS:

    • Regeneration is limited by astrocytes producing scar tissue and anti-regrowth chemicals.

Membrane Potential

1. Resting Membrane Potential

  • Definition: The membrane potential of an unstimulated, resting cell.

  • All neural activity begins with a change in the resting membrane potential.

2. Ionic Composition of Fluids

  • Extracellular Fluid (ECF): High concentrations of Na+ and Ca2+.

  • Intracellular Fluid (Cytosol): High concentrations of K+ and negatively charged proteins.

3. Selectively Permeable Membranes

  • At rest, ions move passively through leak channels and via active transport (sodium-potassium exchange pump).

  • Permeability and Ion Movement:

    • More potassium ions leak than sodium ions.

    • Negatively charged proteins inside the cell are too large to cross the membrane, making the inner membrane surface more negative at resting potential.

4. Electrochemical Gradient

  • Definitions:

    • Current: Movement of charges to erase a potential difference.

    • Resistance: Measure of membrane restriction to ion movement.

    • High resistance = small current (and vice versa).

  • Passive Processes (Gradients):

    • Chemical Gradient: Passive ion movement due to concentration differences.

    • Electrical Gradient: Passive ion movement due to charge differences.

    • Electrochemical Gradient: Sum of chemical and electrical forces acting on an ion across the membrane.

5. Active Transport across the Membrane

  • Sodium-Potassium Exchange Pump:

    • Powered by ATP, moves 3 Na+ out and 2 K+ into the cell.

    • Stabilizes resting membrane potential by opposing passive leaking of Na+ and K+.

6. Gated Ion Channels

  • Changes in resting membrane potential are due to the opening or closing of specific membrane channels in response to stimuli.

  • Types of Gated Ion Channels:

    • Chemically gated (ligand-gated) channels: Open when specific chemicals bind.

    • Voltage-gated channels: Respond to changes in membrane potential.

    • Mechanically gated channels: Open in response to physical distortion of the membrane.

7. Graded Potentials

  • Definition: Temporary, localized changes in resting membrane potential due to stimuli opening gated channels.

  • Propagation: Spread passively from the stimulation site and decrease with distance.

  • Stronger stimuli yield stronger graded potentials.

  • Can lead to:

    • Depolarization: Shift towards less negative potentials due to influx of Na+.

    • Hyperpolarization: Shift towards more negative potentials due to efflux of K+.

8. Action Potentials

  • Definition: Large depolarization that impacts the entire excitable membrane once initiated, without diminishing.

  • Threshold: Membrane potential for action initiation typically between -60 mV and -50 mV.

9. All-or-None Principle

  • An action potential is either triggered or not, based on whether the threshold is reached.

  • All initiated action potentials are uniform, irrespective of the initial stimulus size.

10. Action Potential Generation Steps

  • Step 1: Depolarization to Threshold: Graded potential depolarizes plasma membrane to threshold.

  • Step 2: Rapid Depolarization: Voltage-gated sodium channels open, Na+ floods in.

  • Step 3: Repolarization: At +30 mV, sodium channels close and potassium channels open; K+ exits.

  • Step 4: Hyperpolarization: Potassium channels close slowly, additional K+ leaves, returning potential to resting.

11. Refractory Periods

  • Absolute Refractory Period: Membrane cannot respond to further stimulation as sodium channels remain closed or inactive.

  • Relative Refractory Period: Membrane can respond to stronger stimuli only; sodium channels are in resting state but membrane is hyperpolarized.

12. Propagation of Action Potentials

  • Repetition of depolarization and repolarization across the axon.

  • Types:

    • Continuous Propagation: In unmyelinated axons.

    • Saltatory Propagation: In myelinated axons; faster and energy-efficient as local currents jump nodes (of Ranvier).

13. Axon Fiber Types

  • Type A Fibers: Myelinated, large diameter, rapidly transmitting sensory and motor signals (e.g., position, balance).

  • Type B Fibers: Myelinated, medium diameter, transmitting signals faster than C fibers but slower than A.

  • Type C Fibers: Unmyelinated, small diameter, transmitting signals slowly (sensory from skin and motor to smooth muscle).

Synapses

1. Definition

  • Synapse: Specialized site where a neuron communicates with another cell.

  • Presynaptic Cell: Neuron sending the message.

  • Postsynaptic Neuron: Neuron or cell receiving the message.

2. Types of Synapses

  • Electrical Synapses: Direct contact via gap junctions; fast action potential propagation.

  • Chemical Synapses: Signaling across synaptic cleft via neurotransmitters. Types include:

    • Axoaxonic: Between two axons.

    • Axosomatic: Between an axon and a cell body.

    • Axodendritic: Between an axon and a dendrite.

    • Neuromuscular Junction: Between an axon and a muscle cell.

    • Neuroglandular Junction: Between an axon and a secretory cell.

3. Function of Chemical Synapses

  • Neurotransmitters, held in synaptic vesicles, are released upon action potential arrival.

  • Neurotransmitter binds to receptors on the postsynaptic membrane, creating localized changes in permeability and potential.

  • Action potentials may or may not form in the postsynaptic neuron, depending on neurotransmitter quantity and postsynaptic sensitivity.

4. Cholinergic Synapses

  • Release acetylcholine (ACh) at:

    • All neuromuscular junctions with skeletal muscle fibers.

    • Numerous synapses in CNS.

    • All neuron-to-neuron synapses in PNS.

    • Neuromuscular and neuroglandular junctions within parasympathetic division of ANS.

5. Events at a Cholinergic Synapse

  • Action potential arrives, depolarizing the axon terminal.

  • Ca2+ enters axon terminal, triggering ACh release via exocytosis.

  • ACh binds to receptors on postsynaptic membrane, causing depolarization and graded potential.

  • Acetylcholinesterase (AChE) breaks ACh down into acetate and choline, these byproducts are cleared from the synaptic cleft.

6. Synaptic Delay

  • Delay between action potential arrival and its effect on the postsynaptic membrane, primarily due to Ca2+ influx and neurotransmitter release; fewer synapses yield faster responses.

7. Synaptic Fatigue

  • Weakens synaptic response due to insufficient neurotransmitter replenishment in high-demand scenarios.

Neurotransmitters and Neuromodulators

1. Classes of Neurotransmitters Based on Effect

  • Excitatory Neurotransmitters: Cause postsynaptic membrane depolarization, promoting action potential generation.

  • Inhibitory Neurotransmitters: Cause hyperpolarization, hindering action potential generation.

  • Neurotransmitter effect on postsynaptic membranes is contingent on receptor properties, rather than neurotransmitter nature.

2. Neurotransmitter Functions

  • Neurotransmitters: Rapid effects (milliseconds to seconds), transmit signals across synapse, binding to receptors.

  • Neuromodulators: Broader, slower effects, altering neuronal activity across a larger area (minutes to hours). Some neurotransmitters can also act as neuromodulators.

3. Classes of Neurotransmitters

  • Acetylcholine (ACh): Excitatory, found in CNS and PNS at neuromuscular junction.

  • Biogenic Amines:

    • Norepinephrine (NE): Adrenergic synapses, excitatory, present widely in CNS and portions of ANS.

    • Dopamine: Potentially excitatory or inhibitory; crucial in various brain regions; implicated in Parkinson’s disease and cocaine effects.

    • Serotonin: CNS neurotransmitter where deficiency is tied to depression; SSRIs inhibit serotonin reuptake, increasing cleft concentration.

  • Amino Acids:

    • Glutamate: Major excitatory neurotransmitter in the brain, vital for memory and learning.

    • Glycine: Inhibitory neurotransmitter; strychnine blocks its receptors.

    • Gamma-Aminobutyric Acid (GABA): Inhibitory in CNS; an example of antianxiety drugs enhancing GABA effects.

  • Neuropeptides: Small peptidic chains that can serve as neurotransmitters (e.g., Substance P for pain transmission).

  • Dissolved Gases:

    • Nitric Oxide (NO): Engaged at smooth muscle synapses and brain functions.

    • Carbon Monoxide (CO): Present in the CNS.

4. Receptor Interactions of Neurotransmitters and Neuromodulators

  • Ionotropic Effects: Direct impact on membrane potential through gate channels (e.g., ACh).

  • Metabotropic Effects: Indirect effects through G proteins activating enzymes like adenylate cyclase, leading to second messengers (e.g., NO, CO).

Information Processing

1. Postsynaptic Integration

  • Postsynaptic cell can receive numerous incoming signals, integrating them for overall response (net effect).

2. Postsynaptic Potentials

  • Types:

    • Excitatory Postsynaptic Potential (EPSP): Graded depolarization, increases action potential likelihood.

    • Inhibitory Postsynaptic Potential (IPSP): Graded hyperpolarization; reduces action potential likelihood.

3. Summation

  • Integrates effects of incoming graded potentials:

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

    • Spatial Summation: Simultaneous stimuli arriving at multiple synapses.

4. Facilitation and Inhibition

  • Facilitation: Increases membrane potential closeness to threshold induced by incoming EPSPs.

  • Inhibition: Function of EPSPs and IPSPs interactions that dictate initial segment's membrane potential and resultant action potential likelihood.

5. Presynaptic Regulation

  • At axoaxonic synapses, regulation occurs:

    • Presynaptic Inhibition: Reduces neurotransmitter release rate.

    • Presynaptic Facilitation: Enhances neurotransmitter release rate.

6. Action Potential Rate of Generation

  • Degree and frequency of stimulation correlate with action potential generation rate.

Summary

  • Information relayed through action potentials.

  • Neurotransmitters at synapses exhibit excitatory or inhibitory properties.

  • Neuromodulators may impact neurotransmitter release rates or postsynaptic responses.