BIO 201 Chapter 11

Nervous System and Nervous Tissue

Functions of the Nervous System

  • Sensory Input:

    • Information gathered by sensory receptors about internal and external changes.

  • Integration:

    • Interpretation of sensory input.

  • Motor Output:

    • Activation of effector organs (muscles and glands) produces a response.

Divisions of the Nervous System

  • Central Nervous System (CNS):

    • Consists of the brain and spinal cord.
    • Serves as the integration and command center.

  • Peripheral Nervous System (PNS):

    • Comprised of paired spinal and cranial nerves that carry messages to and from the CNS.

Peripheral Nervous System (PNS)

Functional Divisions

  1. Sensory (Afferent) Division:

    • Somatic Afferent Fibers: Convey impulses from skin, skeletal muscles, and joints.
    • Visceral Afferent Fibers: Convey impulses from visceral organs.

  2. Motor (Efferent) Division:

    • Transmits impulses from the CNS to effector organs.

Motor Division of PNS

  • Somatic (Voluntary) Nervous System:

    • Controls conscious control of skeletal muscles.

  • Autonomic (Involuntary) Nervous System (ANS):

    • Comprises visceral motor nerve fibers.
    • Regulates smooth muscle, cardiac muscle, and glands.
    • Two Functional Subdivisions:
    • Sympathetic: Mobilizes body systems during activity.
    • Parasympathetic: Conserves energy and promotes housekeeping functions during rest.

Histology of Nervous Tissue

Principal Cell Types

  1. Neurons:

    • Excitable cells that transmit electrical signals.

  2. Neuroglia (Glial Cells):

    • Supporting cells, include:
    • Astrocytes (CNS):
      • Most abundant and versatile glial cells that support and brace neurons, help determine capillary permeability, guide the migration of young neurons, control the chemical environment, and participate in information processing in the brain.
    • Microglia (CNS):
      • Serve as defensive cells by migrating toward injured neurons and phagocytizing microorganisms and neuronal debris.
    • Ependymal Cells (CNS):
      • Line the central cavities of the brain and spinal column.
    • Oligodendrocytes (CNS):
      • Branched cells that wrap CNS nerve fibers, forming insulating myelin sheaths.
    • Satellite Cells (PNS):
      • Surround neuron cell bodies in the PNS similar to astrocytes.
    • Schwann Cells (PNS):
      • Surround peripheral nerve fibers and form myelin sheaths essential for the regeneration of damaged peripheral nerve fibers.

Neuron Structure and Function

Neurons Characteristics

  • Longevity: Neurons can live for over 100 years.
  • Amitotic: Neurons generally lose their ability to divide, with few exceptions.
  • High Metabolic Rate: They require continuous supply of oxygen and glucose.
  • Electrical Signaling: Plasma membranes function in electrical signaling.

Neuron Components

  • Cell Body (Perikaryon or Soma):
    • Contains a spherical nucleus with nucleolus.
    • Rough ER called Nissl bodies (chromatophilic substance).
    • Network of neurofibrils (neurofilaments) important for maintaining cell shape.
    • Axon Hillock: Cone-shaped area from which the axon arises.
    • Clusters of cell bodies are referred to as nuclei in the CNS and ganglia in the PNS.

Processes of Neurons

  • Dendrites:

    • Short, tapering, diffusely branched.
    • Receptive region of a neuron that conveys electrical signals toward the cell body as graded potentials.

  • Axon:

    • One axon per cell arising from the axon hillock.
    • Long axons referred to as nerve fibers.
    • Numerous terminal branches (telodendria) with knoblike axon terminals (synaptic knobs or boutons) that release neurotransmitters.
    • Conducting region of a neuron that generates and transmits nerve impulses (action potentials) away from the cell body.

Myelin Sheath

  • Functions to protect and electrically insulate the axon and increase the speed of nerve impulse transmission.

  • Myelin Sheaths in the PNS:

    • Formed by Schwann cells wrapping many times around the axon.
    • Nodes of Ranvier: Gaps between adjacent Schwann cells.

  • Myelin Sheaths in the CNS:

    • Formed by processes of oligodendrocytes; do not have nephilemma and have no neurilemma.
    • Thinnest fibers are unmyelinated.

White Matter and Gray Matter

  • White Matter: Dense collections of myelinated fibers.
  • Gray Matter: Mostly neuron cell bodies and unmyelinated fibers.

Structural Classification of Neurons

Types of Neurons

  1. Multipolar Neurons:

    • One axon and several dendrites. Most abundant (99% of all neurons).

  2. Bipolar Neurons:

    • One axon and one dendrite. Rare, e.g., retinal neurons.

  3. Unipolar Neurons (Pseudounipolar):

    • Single short process that has two branches:
    • Peripheral Process: More distal branch, often associated with a sensory receptor.
    • Central Process: Branch entering the CNS. Example: Dorsal root ganglion.

Table 11.1 - Comparison of Structural Classes of Neurons

  • Multipolar Neurons:

    • Many processes extend from the cell body, all are dendrites except for a single axon.
    • Most abundant neuron type in the CNS.

  • Bipolar Neurons:

    • Two processes extend from the cell body: one is a fused dendrite and the other is an axon. Found in special sensory organs.

  • Unipolar Neurons:

    • One process extends from the cell body forming central and peripheral processes combined as an axon. Found mainly in the PNS, commonly in dorsal root ganglia.

Functional Classification of Neurons

Types of Neurons

  1. Sensory (Afferent) Neurons:

    • Transmit impulses from sensory receptors toward the CNS.

  2. Motor (Efferent) Neurons:

    • Carry impulses from the CNS to effectors.

  3. Interneurons (Association Neurons):

    • Shuttle signals through CNS pathways; most are entirely within the CNS.

Neuron Function

  • Neurons are highly irritable and respond to an adequate stimulus by generating an action potential (nerve impulse).
  • The impulse is always the same regardless of the stimulus.

Principles of Electricity

  • Opposite charges attract each other.
  • Energy is required to separate opposite charges across a membrane.

Role of Membrane Ion Channels

  • Membrane proteins that serve as ion channels.
  • Two types of ion channels:
    • Leakage (Nongated) Channels: Always open.
    • Gated Channels: Include:
    • Chemically Gated (Ligand-Gated): Open upon binding of a specific neurotransmitter.
    • Voltage-Gated: Open and close in response to changes in membrane potential.
    • Mechanically Gated: Open and close due to physical deformation of receptors.

Resting Membrane Potential (Vr)

  • The potential difference across the membrane of a resting cell, approximately –70 mV in neurons (cytoplasmic side is negatively charged relative to exterior).
  • Differences in ionic makeup:
    • Intracellular: Lower Na+ and Cl- than Extracellular fluid (ECF).
    • Intracellular: Higher K+ and negatively charged proteins than ECF.
  • The negative interior is caused by higher diffusion of K+ out of the cell than Na+ into the cell.
  • The Sodium-Potassium pump maintains concentration gradients for Na+ and K+ that stabilize the resting membrane potential.

Changes in Membrane Potential

  • Depolarization: A reduction in membrane potential (toward zero); inside membrane becomes less negative, increasing the probability of producing a nerve impulse.
  • Hyperpolarization: An increase in membrane potential (away from zero); inside membrane becomes more negative, reducing the probability of producing a nerve impulse.

Graded Potentials

  • Short-lived, localized changes in membrane potential that decay with distance.
  • Occur when a stimulus causes gated ion channels to open.
  • Magnitude varies directly with stimulus strength.

Action Potential (AP)

  • Brief membrane potential reversal with an amplitude of approximately 100 mV (-70 mV to +30 mV).
  • Action potential does not decrease in strength over distance (all or none principle).
  • Principal means of long-distance neural communication.

Stages of Action Potential

  1. Resting State: Na+ and K+ channels are closed.
  2. Depolarization Phase: Depolarizing local currents open voltage-gated Na+ channels; Na+ influx leads to further depolarization.
  3. Repolarization Phase: Na+ channels close; K+ channels open and K+ exits the cell to restore internal negativity.
  4. Hyperpolarization: K+ gates remain open causing excessive K+ efflux, leading to hyperpolarization (undershoot).

Role of Sodium-Potassium Pump

  • Restores resting electrical conditions but does not restore ionic conditions.
  • Ionic redistribution back to resting conditions is achieved by the thousands of sodium-potassium pumps.

Threshold

  • At threshold (-55 to -50 mV):
    • Membrane is depolarized by 15 to 20 mV, allowing Na+ influx, leading to the positive feedback cycle.
  • Subthreshold stimulus: Weak local depolarization that does not reach threshold.
  • Threshold stimulus: Strong enough to push membrane potential beyond threshold.

Coding for Stimulus Intensity

  • Strong stimuli generate action potentials more frequently than weaker stimuli.
  • The CNS determines stimulus intensity based on the frequency of impulses.

Absolute Refractory Period

  • Time from opening of the Na+ channels until reset of the channels; ensures that each AP is an all-or-none event and enforces one-way transmission of nerve impulses.

Relative Refractory Period

  • Follows the absolute refractory period; most Na+ channels have returned to resting state, but some K+ channels remain open.
  • A strong stimulus may generate an AP during this phase.

Conduction Velocity

  • Effect of Axon Diameter: Larger diameter fibers have less resistance to local current flow and faster impulse conduction.
  • Effect of Myelination: Continuous conduction in unmyelinated axons is slower than saltatory conduction in myelinated axons.
  • Saltatory conduction in myelinated axons is approximately 30 times faster due to voltage-gated Na+ channels located at nodes.

Multiple Sclerosis (MS)

  • An autoimmune disease affecting young adults with symptoms including visual disturbances, weakness, loss of muscular control, speech disturbances, and urinary incontinence.
  • Myelin sheaths in the CNS become nonfunctional scleroses, leading to shunting and short-circuiting of nerve impulses, slowing conduction.

Multiple Sclerosis: Treatment

  • Lifestyle changes: Avoid smoking, X-rays, and toxic substances.
  • Nutritional support: High potency multivitamins, fish oil may help reduce symptoms.
  • Immune system-modifying drugs: Including interferons and Copaxone to manage symptoms.

The Synapse

  • A junction mediating information transfer between:
    • One neuron to another neuron.
    • A neuron to an effector cell.
  • Presynaptic Neuron: Conducts impulses toward the synapse.
  • Postsynaptic Neuron: Transmits impulses away from the synapse.

Types of Synapses

  • Axodendritic: Between axon of one neuron and dendrite of another.
  • Axosomatic: Between axon of one neuron and soma of another.
  • Axoaxonic: Between axon and axon.
  • Dendrodendritic: Between dendrites of neurons.
  • Dendrosomatic: Between a dendrite and soma.

Synaptic Cleft

  • Fluid-filled space separating presynaptic and postsynaptic neurons, preventing direct nerve impulse transfer.
  • Transmission across the synaptic cleft is a chemical event involving release, diffusion, and binding of neurotransmitters to receptors, ensuring unidirectional communication.

Termination of Neurotransmitter Effects

  • Occurs in milliseconds by:
    • Degradation by enzymes.
    • Reuptake by astrocytes or axon terminal.
    • Diffusion away from the synaptic cleft.

Synaptic Delay

  • Time needed for neurotransmitter release, diffusion across, and binding to receptors (0.3-5.0 ms). This delay is the rate-limiting step of neural transmission.

Postsynaptic Potentials

  • EPSP (Excitatory Postsynaptic Potential): Depolarization that spreads to axon hillock; favors generation of AP.
  • IPSP (Inhibitory Postsynaptic Potential): Hyperpolarization that spreads to axon hillock; reduces probability of AP generation.

Comparison of Action Potentials with Graded Potentials

  • Action Potential (AP):
    • Long distance, occurring at axon hillock. All-or-nothing phenomenon.
  • Graded Potential (GP):
    • Short distance, occurs at dendrites and cell body, varies in amplitude with stimulus and decreases with distance.

Amplifying Conditions for APs and GPs

  • Only a strong enough EPSP can invoke an AP.
  • Multiple EPSPs can summate to reach threshold for AP generation (spatial and temporal summation).

Chemical Classes of Neurotransmitters

  • Over 50 neurotransmitters identified used for neuronal communication.
  • Acetylcholine (ACh): 1st identified neurotransmitter, involved in neuromuscular junctions.
  • Biogenic Amines: Include catecholamines (dopamine, norepinephrine, epinephrine) and indolamines (serotonin, histamine).
  • Amino Acids: Such as GABA, glycine, aspartate, glutamate.
  • Peptides: Such as substance P (pain mediator), endorphins (natural pain relievers), and gut-brain peptides.
  • Purines: ATP acts in both CNS and PNS, involved in fast/slow responses.
  • Gases and Lipids: Such as nitric oxide (NO) and carbon monoxide (CO), involved in various signaling processes.

Functional Classification of Neurotransmitters

  • Effects may be excitatory (depolarizing) and/or inhibitory (hyperpolarizing), determined by receptor type.
  • GABA and glycine are usually inhibitory, whereas glutamate is excitatory.

Neural Integration: Neuronal Pools

  • Functional groups of neurons that integrate incoming information and forward processed information to other destinations.

Types of Circuits in Neuronal Pools

  1. Diverging Circuit: One incoming fiber stimulates a growing number of fibers, amplifying signals.
  2. Converging Circuit: Opposite of diverging, leads to strong stimulation or inhibition.
  3. Reverberating Circuit: Contains collateral synapses which allow signal persistence.
  4. Parallel After-Discharge Circuit: One fiber stimulates several neurons in parallel to stimulate a common output cell.

Patterns of Neural Processing

  • Serial Processing: Input travels along one pathway to a specific destination, e.g., reflexes.
  • Parallel Processing: Input travels along several pathways; one stimulus can provoke several responses, important for higher-level mental functioning.