Chapter 12: Nervous Tissue - Comprehensive Notes

The Nervous System

• Includes the brain, spinal cord, receptors of sense organs (eyes, ears, etc.), and nerves connecting to other systems.
• Nervous tissue contains:
  • Neurons: For intercellular communication.
  • Neuroglia (glial cells): Essential for neuron survival and function, preserving nervous tissue structure.

Divisions of the Nervous System

Anatomical Divisions

• Central Nervous System (CNS)
  • Brain and spinal cord
  • Composed of nervous tissue, connective tissue, and blood vessels
  • Functions:
    • Processes and coordinates sensory data (internal and external)
    • Motor commands control peripheral organ activities (e.g., skeletal muscles)
    • Higher functions: intelligence, memory, learning, emotion
• Peripheral Nervous System (PNS)
  • All nervous tissue outside the CNS and ENS
  • Functions:
    • Delivers sensory information to the CNS
    • Carries motor commands to peripheral tissues
  • Nerves (peripheral nerves):
    • Bundles of axons with connective tissues and blood vessels
    • Carry sensory information and motor commands
    • Cranial nerves: Connect to the brain
    • Spinal nerves: Attach to the spinal cord

Functional Divisions of the PNS

• Afferent Division
  • Carries sensory information from receptors in peripheral tissues and organs to the CNS.
• Efferent Division
  • Carries motor commands from the CNS to muscles, glands, and adipose tissue.
• Receptors
  • Detect changes or respond to stimuli.
  • May be neurons or specialized cells.
  • May be single cells or complex sensory organs (e.g., eyes, ears).
• Effectors
  • Target organs that respond to motor commands.
• Efferent Division of PNS
  • Somatic Nervous System (SNS)
    • Controls skeletal muscle contractions (voluntary and involuntary/reflexes).
  • Autonomic Nervous System (ANS)
    • Controls subconscious actions: smooth/cardiac muscle contractions, glandular secretions.
      • Sympathetic division: Stimulating effect.
      • Parasympathetic division: Relaxing effect.
• Enteric Nervous System (ENS)
  • Approximately 100 million neurons in the walls of the digestive tract (as many or more than in the spinal cord).
  • Uses the same neurotransmitters as the brain.
  • Initiates and coordinates visceral reflexes locally without CNS instructions.
  • Can be influenced by the ANS.

Neurons

• Basic functional units of the nervous system.
• Send and receive signals for communication, information processing, and control.
• Cell Body (Soma)
  • Large nucleus and nucleolus.
  • Perikaryon (cytoplasm).
  • Mitochondria (produce energy).
  • RER and ribosomes (synthesize proteins).
• Cytoskeleton of Perikaryon
  • Neurofilaments and neurotubules (similar to intermediate filaments and microtubules).
  • Neurofibrils: Bundles of neurofilaments supporting dendrites and axon.
• Nissl Bodies
  • Dense areas of RER and ribosomes in perikaryon, making nervous tissue appear gray (gray matter).
• Dendrites
  • Short, highly branched processes extending from the cell body.
  • Dendritic Spines
    • Fine processes on dendrites that receive information from other neurons.
    • Account for 80–90% of the neuron's surface area.
• Axon
  • Single, long cytoplasmic process that propagates electrical signals (action potentials).
  • Axoplasm: Cytoplasm of the axon, containing neurofibrils, neurotubules, enzymes, and organelles.
• Structures of the Axon
  • Axolemma: Plasma membrane of the axon, covering the axoplasm.
  • Initial Segment: Base of the axon.
  • Axon Hillock: Thick region attaching the initial segment to the cell body.
  • Collaterals: Branches of the axon.
  • Telodendria: Fine extensions of the distal axon.
  • Axon Terminals (Synaptic Terminals): Tips of telodendria.
• Axonal (Axoplasmic) Transport
  • Movement of materials between the cell body and axon terminals along neurotubules, powered by mitochondria, kinesin, and dynein.

Structural Classification of Neurons

• Anaxonic Neurons
  • Small, with all cell processes looking similar.
  • Found in the brain and special sense organs.
• Bipolar Neurons
  • Small and rare, with one dendrite and one axon.
  • Found in special sense organs (sight, smell, hearing).
• Unipolar Neurons (Pseudounipolar Neurons)
  • Axon and dendrites are fused, with the cell body to one side.
  • Most sensory neurons of the PNS.
• Multipolar Neurons
  • Have one long axon and two or more dendrites.
  • Common in the CNS and all motor neurons that control skeletal muscles.

Functional Classifications of Neurons

• Sensory Neurons
• Motor Neurons
• Interneurons

Sensory Neurons (Afferent Neurons)

• Unipolar, with cell bodies grouped in sensory ganglia.
• Processes (afferent fibers) extend from sensory receptors to the CNS.
  • Somatic sensory neurons: Monitor the external environment.
  • Visceral sensory neurons: Monitor the internal environment.
• Types of Sensory Receptors
  • Interoceptors: Monitor internal systems (e.g., digestive, urinary) and internal senses (stretch, deep pressure, pain).
  • Exteroceptors: Monitor the external environment (e.g., temperature) and complex senses (e.g., sight, smell, hearing).
  • Proprioceptors: Monitor the position and movement of skeletal muscles and joints.

Motor Neurons (Efferent Neurons)

• Carry instructions from the CNS to peripheral effectors via efferent fibers (axons).
  • Somatic motor neurons (SNS): Innervate skeletal muscles.
  • Visceral motor neurons (ANS): Innervate all other peripheral effectors, including smooth/cardiac muscle, glands, and adipose tissue.
• Signals from the CNS to visceral effectors cross autonomic ganglia that divide axons into preganglionic and postganglionic fibers.

Interneurons

• Most are located in the brain and spinal cord, with some in autonomic ganglia.
• Located between sensory and motor neurons.
• Responsible for the distribution of sensory information and coordination of motor activity.
• Involved in higher functions like memory, planning, and learning.

Neuroglia

• Support and protect neurons; make up half the volume of the nervous system.

Types of Neuroglia in the CNS

• Astrocytes
  • Large cell bodies with many processes that maintain the blood-brain barrier (BBB), create a three-dimensional framework for the CNS, repair damaged nervous tissue, guide neuron development, and control the interstitial environment.
• Ependymal Cells
  • Form an epithelium lining the central canal of the spinal cord and ventricles of the brain; produce and monitor cerebrospinal fluid (CSF); have cilia to circulate CSF.
• Oligodendrocytes
  • Small cell bodies with few processes that cooperate to form myelin sheath, which insulates myelinated axons, increases the speed of action potentials, and makes nerves appear white.
    • Internodes: myelinated segments of axon.
    • Nodes (Nodes of Ranvier): lie between internodes, where axons may branch.
  • White Matter: Regions of the CNS with many myelinated axons.
  • Gray Matter: Regions of the CNS containing unmyelinated axons, neuron cell bodies, and dendrites.
• Microglia
  • Smallest and least numerous neuroglia with many fine-branched processes; migrate through nervous tissue, cleaning up cellular debris, wastes, and pathogens.

Neuroglia of the PNS

• Insulate neuronal cell bodies and most axons.
  • Satellite Cells: Surround ganglia (clusters of neuronal cell bodies) and regulate interstitial fluid around neurons.
  • Schwann Cells (Neurolemmocytes): Form myelin sheath or indented folds of plasma membrane around axons; the neurolemma is the outer surface of the Schwann cell. A myelinating Schwann cell sheaths only one axon, and many Schwann cells sheath an entire axon.

Neural Responses to Injuries

• Wallerian Degeneration: Axon distal to injury degenerates.
• Schwann Cells: Form a path for new growth and wrap around the new axon.
• Nerve Regeneration in the CNS: Limited by astrocytes, which produce scar tissue and release chemicals that block regrowth.

Membrane Potential

• All plasma (cell) membranes produce electrical signals via ion movements; membrane potential is particularly important to neurons.

Resting Membrane Potential

• The membrane potential of a resting cell.

Graded Potential

• Temporary, localized change in resting potential caused by a stimulus.

Action Potential

• An electrical impulse produced by a graded potential that propagates along the surface of the axon to the synapse.
• Three Important Concepts
  • The extracellular fluid (ECF) and intracellular fluid (cytosol) differ greatly in ionic composition.
    • Extracellular fluid contains high concentrations of Na^+ and Cl^-.
    • Cytosol contains high concentrations of K^+ and negatively charged proteins.
  • Cells have selectively permeable membranes.
  • Membrane permeability varies by ion.

Passive Processes Across Cell Membrane

• Current: Movement of charges to eliminate a potential difference.
• Resistance: How much the membrane restricts ion movement; high resistance means small current.

Chemical Gradients

• Concentration gradients of ions.

Electrical Gradients

• Charges are separated by the cell membrane; cytosol is negative relative to extracellular fluid.

Electrochemical Gradient

• Sum of chemical and electrical forces acting on an ion across the membrane (a form of potential energy).

Equilibrium Potential

• Membrane potential at which there is no net movement of a particular ion across the cell membrane.
  • Plasma membrane is highly permeable to K^+.
    • Explains the similarity of the equilibrium potential for K^+ and the resting membrane potential.
  • Resting membrane’s permeability to Na^+ is very low, so Na^+ has a small effect on the resting potential.

Active Processes across the Membrane

• Sodium-Potassium Exchange Pump
  • Powered by ATP, ejects three Na^+ for every two K^+ brought in, balances passive forces of diffusion, and stabilizes the resting membrane potential at approximately -70 mV when the ratio of Na^+ entry to K^+ loss through passive channels is 3:2.

Membrane Potential Existence

• Cytosol differs from extracellular fluid in chemical and ionic composition.
• Plasma membrane is selectively permeable.
• Membrane potential changes in response to temporary changes in membrane permeability resulting from the opening or closing of specific membrane channels in response to stimuli.
• Na^+ and K^+ are the primary determinants of membrane potential, and their channels are either passive or active.

Passive Ion Channels (Leak Channels)

• Are always open, and their permeability changes with conditions.

Active Ion Channels (Gated Ion Channels)

• Open and close in response to stimuli; at resting membrane potential, most are closed.

Types of Active Channels

• Chemically Gated Ion Channels (Ligand-Gated Ion Channels)
  • Open when they bind specific chemicals (e.g., ACh).
  • Found on the cell body and dendrites of neurons.
• Voltage-Gated Ion Channels
  • Respond to changes in membrane potential.
  • Found in the axons of neurons and sarcolemma of skeletal and cardiac muscle cells.
  • The activation gate opens when stimulated, and the inactivation gate closes to stop ion movement.
  • Three Possible States
    • Closed but capable of opening.
    • Open (activated).
    • Closed and incapable of opening (inactivated).
• Mechanically Gated Ion Channels
  • Respond to membrane distortion.
  • Found in sensory receptors responding to touch, pressure, or vibration.

Graded Potentials (Local Potentials)

• Changes in membrane potential that cannot spread far from the site of stimulation.
• Produced by any stimulus that opens gated channels.
  • Chemically gated Na^+ channels open, Na^+ ions enter the cell, and the membrane potential rises (depolarization).
• Na^+ ions move parallel to the plasma membrane, producing a local current that depolarizes nearby regions (graded potential), with the change proportional to the stimulus.
• Repolarization
  • When the stimulus is removed, the membrane potential returns to normal.
• Hyperpolarization
  • Results from opening potassium ion channels, increasing the negativity of the resting potential.
  • Opposite effect of opening sodium ion channels.
• Characteristics of Graded Potentials
  • Membrane potential is most changed at the site of stimulation; the effect decreases with distance.
  • The effect spreads passively due to local currents.
  • Graded change may involve depolarization or hyperpolarization.
  • Stronger stimuli produce greater changes in membrane potential and affect a larger area.
• Graded potentials often trigger specific cell functions, such as exocytosis of glandular secretions, or the graded potential caused by ACh at the motor end plate at the neuromuscular junction.

Action Potential (Nerve Impulses)

• Propagated changes in membrane potential affecting the entire excitable membrane, beginning at the initial segment of the axon.
• Do not diminish as they move away from the source.
• Stimulated by a graded potential that depolarizes the axolemma to threshold (-60 to -55 mV).

All-or-None Principle

• Any stimulus that changes the membrane potential to threshold will cause an action potential.
• All action potentials are the same, no matter how large the stimulus; an action potential is either triggered or not.

Generation of Action Potentials

• Step 1: Depolarization to threshold.
• Step 2: Activation of voltage-gated Na^+ channels, causing Na^+ to rush into the cytosol, changing the inner membrane surface from negative to positive, resulting in rapid depolarization.
• Step 3: Inactivation of Na^+ channels and activation of K^+ channels, which occurs at +30 mV. Inactivation gates of voltage-gated Na^+ channels close, voltage-gated K^+ channels open, and K^+ moves out of the cytosol, beginning repolarization.
• Step 4: Return to resting membrane potential. Voltage-gated K^+ channels begin to close as the membrane reaches the normal resting potential. K^+ continues to leave the cell, briefly hyperpolarizing the membrane to -90 mV. After all voltage-gated K^+ channels finish closing, the resting membrane potential is restored, and the action potential is over.

Refractory Period

• From the beginning of the action potential to the return to the resting state, during which the membrane will not respond normally to additional stimuli.

Absolute Refractory Period

• All voltage-gated Na^+ channels are already open or inactivated, and the membrane cannot respond to further stimulation.

Relative Refractory Period

• Begins when Na^+ channels regain their resting condition and continues until the membrane potential stabilizes; only a strong stimulus can initiate another action potential.

Ion Involvement

• Depolarization results from the influx of Na^+.
• Repolarization involves the loss of K^+.
• Sodium-Potassium Exchange Pump
  • Returns concentrations to prestimulation levels, maintaining concentration gradients of Na^+ and K^+ over time and uses one ATP for each exchange of two extracellular K^+ for three intracellular Na^+.
• Propagation: moves an action potential along an axon in a series of steps.

Types of Propagation

• Continuous Propagation
• Saltatory Propagation

Continuous Propagation

• Occurs in unmyelinated axons, affecting one segment at a time.
  • Step 1: Action potential develops at the initial segment, depolarizing the membrane to +30 mV.
  • Step 2: Local current develops, depolarizing the second segment to the threshold.
  • Step 3: An action potential occurs in the second segment while the initial segment begins repolarization.
  • Step 4: Local current depolarizes the next segment, cycling repeatedly, and the action potential travels in one direction (1 m/sec).

Saltatory Propagation

• Occurs in myelinated axons, is faster than continuous propagation and requires less energy.
• Myelin prevents continuous propagation.
• Local current “jumps” from node to node.
• Depolarization occurs only at the nodes.

Axon Diameter Effect

• The larger the diameter, the lower the resistance, and the faster the speed.

Types of Axons

• Type A Fibers
• Type B Fibers
• Type C Fibers

Type A Fibers

• Myelinated, large diameter, and transmit information to and from the CNS rapidly (120 m/sec).
  • Sensory information such as position and balance.
  • Motor impulses to skeletal muscles.

Type B Fibers

• Myelinated. Medium diameter, transmitting information at intermediate speeds (18 m/sec).

Type C Fibers

• Unmyelinated, small diameter, transmitting information slowly (1 m/sec).
  • Most sensory information.
• Messages carried by nerves are routed according to priority; critical information is transmitted through Type A fibers.
  • Sensory information about things that threaten survival.
  • Motor commands that prevent injury.

Synapses

• Specialized site where a neuron communicates with another cell.
  • Presynaptic Neuron: Sends the message.
  • Postsynaptic Neuron: Receives the message.

Types of Synapses

• Electrical Synapses
  • Direct physical contact between cells.
• Chemical Synapses
  • Signal transmitted across a gap by neurotransmitters.

Electrical Synapses

• Presynaptic and postsynaptic membranes are locked together at gap junctions, ions pass between cells through pores, local current affects both cells, and action potentials are propagated quickly.
• Found in some areas of the brain, the eye, and ciliary ganglia.

Chemical Synapses

• Most common type of synapse between neurons and the only type of synapse between neurons and other cells.
• Cells are separated by the synaptic cleft.
  • Presynaptic Cell: Sends the message.
  • Postsynaptic Cell: Receives the message.

Types of Chemical Synapses

• Neuromuscular Junction
  • Synapse between a neuron and skeletal muscle cell.
• Neuroglandular Junction
  • Synapse between a neuron and gland cell.
• Neurotransmitters: Chemical messengers contained within synaptic vesicles in the axon terminal of the presynaptic cell, released into the synaptic cleft, and affect receptors of the postsynaptic membrane.
  • Broken down by enzymes or reabsorbed and reassembled by the axon terminal.

Function of Chemical Synapses

• The axon terminal releases neurotransmitters that bind to the postsynaptic plasma membrane, producing localized changes in permeability and graded potentials.
• An action potential may or may not be generated in the postsynaptic cell, depending on the amount of neurotransmitter released and the sensitivity of the postsynaptic cell.

Cholinergic Synapses

• Release acetylcholine (ACh) at all neuromuscular junctions involving skeletal muscle fibers, many synapses in the CNS, all neuron-to-neuron synapses in the PNS, and all neuromuscular and neuroglandular junctions in the parasympathetic division of the ANS.
• Events at a Cholinergic Synapse
  • The action potential arrives at the axon terminal and depolarizes the membrane.
  • Extracellular calcium ions enter the axon terminal and trigger exocytosis of ACh.
  • ACh binds to receptors on the postsynaptic membrane and depolarizes it.
  • ACh is removed from the synaptic cleft by acetylcholinesterase (AChE), which breaks ACh into acetate and choline.
• Synaptic Delay
  • A synaptic delay of 0.2–0.5 msec occurs between the arrival of the action potential at the axon terminal and its effect on the postsynaptic membrane, mostly due to the time required for calcium ion influx and neurotransmitter release.
  • Fewer synapses lead to faster responses; some reflexes involve only one synapse.
• Synaptic Fatigue
  • Occurs when neurotransmitter cannot be recycled fast enough to meet the demands of intense stimuli, and the response of the synapse weakens until ACh is replenished.

Neurotransmitters and Neuromodulators

Classes of Neurotransmitters

• Excitatory Neurotransmitters
  • Cause depolarization of postsynaptic membranes and promote action potentials.
• Inhibitory Neurotransmitters
  • Cause hyperpolarization of postsynaptic membranes and suppress action potentials.
• The effect of a neurotransmitter on the postsynaptic membrane depends on the properties of the receptor, not on the nature of the neurotransmitter.

Major Classes of Neurotransmitters

• Biogenic Amines
• Amino Acids
• Neuropeptides
• Dissolved Gases

Biogenic Amines

• Norepinephrine (NE)
  • Released by adrenergic synapses, has an excitatory and depolarizing effect, and is widely distributed in the brain and portions of the ANS.
• Dopamine
  • A CNS neurotransmitter that may be excitatory or inhibitory and is involved in Parkinson’s disease and cocaine use.
• Serotonin
  • A CNS neurotransmitter that affects attention and emotional states.
• Gamma-aminobutyric Acid (GABA)
  • Has an inhibitory effect and its functions in the CNS are not well understood.
• Neuromodulators: Chemicals released by axon terminals that alter the rate of neurotransmitter release or response by the postsynaptic cell with long-term, slow-to-appear effects.
  • Responses involve multiple steps and intermediary compounds and affect the presynaptic membrane, postsynaptic membrane, or both.
  • Released alone or with a neurotransmitter.

Neuromodulators

• Neuropeptides
  • Small peptide chains synthesized and released by axon terminals.
• Opioids
  • Bind to the same receptors as opium and morphine.
    • Classes of Opioids in the CNS
      • Enkephalins
      • Endorphins
      • Dynorphins
• Dissolved Gases: Important neurotransmitters like nitric oxide (NO) and carbon monoxide (CO).
• Neurotransmitters and neuromodulators may have a direct effect on membrane potential by opening or closing chemically gated ion channels (e.g., ACh, glutamate, aspartate). An indirect effect through G proteins (e.g., E, NE, dopamine, serotonin, histamine, GABA), or an indirect effect via intracellular enzymes (e.g., lipid-soluble gases NO, CO).

Indirect Effects

• G Protein Links
  • The first messenger (neurotransmitter) and second messengers (ions or molecules in the cell).
• G Proteins Include
  • An enzyme that is activated when an extracellular compound binds (e.g., adenylate cyclase) producing the second messenger cyclic-AMP (cAMP).
• Indirect Effects by Intracellular Enzymes
  • Lipid-soluble gases (NO, CO) diffuse through lipid membranes and bind to enzymes inside brain cells.

Information Processing

• The response of the postsynaptic cell (integration of stimuli), where many dendrites receive neurotransmitter messages simultaneously (some excitatory, some inhibitory), and the net effect on the axon hillock determines if an action potential is produced.
• Postsynaptic Potentials: Graded potentials developed in a postsynaptic cell in response to neurotransmitters.

Types of Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP)
  • Graded depolarization of the postsynaptic membrane.
• Inhibitory Postsynaptic Potential (IPSP)
  • Graded hyperpolarization of the postsynaptic membrane.

Postsynaptic Potential Effects

• A neuron that receives many IPSPs is inhibited from producing an action potential because the stimulation needed to reach the threshold is increased.
• To trigger an action potential, one EPSP is not enough.
• EPSPs (and IPSPs) combine through summation (temporal and spatial).
• Temporal Summation: Rapid, repeated stimuli at a single synapse.
• Spatial Summation: Simultaneous stimuli arriving at multiple synapses.
• A neuron becomes facilitated as EPSPs accumulate and raise the membrane potential closer to the threshold, until a small stimulus can trigger an action potential.
• Summation of EPSPs and IPSPs Neuromodulators and hormones: Can change membrane sensitivity to neurotransmitters and shifting the balance between EPSPs and IPSPs.
• Axoaxonic Synapses: Synapses between axons of two neurons.
  • Presynaptic Inhibition: Decreases the rate of neurotransmitter release at the presynaptic membrane.
  • Presynaptic Facilitation: Increases the rate of neurotransmitter release at the presynaptic membrane.
• Information may be conveyed simply by the frequency of action potentials received, depending on the degree of depolarization above the threshold.
• Holding the membrane potential above the threshold has the same effect as a second, large stimulus.
• The maximum rate of action potentials is reached when the relative refractory period is eliminated.

Summary

• Information is relayed in the form of action potentials.
• Neurotransmitters released at a synapse may have excitatory or inhibitory effects.
• Neuromodulators can alter the rate of neurotransmitter release or the response of a postsynaptic neuron.
• Neurons may be facilitated or inhibited by chemicals other than neurotransmitters or neuromodulators.
• The response of a postsynaptic neuron can be altered by neuromodulators or other chemicals that cause facilitation or inhibition, activity under way at other synapses, or modification of the rate of neurotransmitter release through facilitation or inhibition.