Nervous System I: Basic Structure and Function

Chapter 10: Nervous System I - Basic Structure and Function

Overview of Nervous System Function

  • Master regulatory system: The nervous system regulates systemic responses and behaviors.
  • Sends and receives information: Communication exchange between CNS and body.
  • Sensory input: Detects changes in internal and external environments.
  • Integration and processing: Involves making decisions based on sensory input.
  • Motor output: Stimulates muscles and glands to respond accordingly.
  • Maintains homeostasis: Regulates internal balance despite external changes.
  • Acts as the center for thought, learning, and memory: Facilitates cognitive functions.

Main Cell Types of Nervous System

  1. Neurons (nerve cells):

    • Respond quickly to changes/stimuli.
    • Conduct electrical impulses via neurotransmitters.

  2. Neuroglia (glial cells):

    • Protect, support, insulate, and nourish neurons.
    • Do not conduct electrical impulses like neurons.

Information Flow in the Nervous System

  • Information flows from sensory receptors → brain/spinal cord (CNS) → effector organs.

Overview of Nervous System Structure

Central Nervous System (CNS)

  • Comprises the brain and spinal cord.

Peripheral Nervous System (PNS)

  • Connects CNS to other body parts.
  • Composed of cranial nerves and spinal nerves.
  • Divided into two subdivisions:
    1. Afferent (sensory):
    • Carries signals from sensory receptors to CNS.
    1. Efferent (motor):
    • Carries signals from CNS to muscles and glands.

Sensory (Afferent) and Motor (Efferent) Divisions

Sensory Division

  • Function: Detects changes through sensory receptors; converts information into neural impulses.
  • Impulses are conducted along peripheral nerves to CNS for integration.

Motor Division

  • Function: Transmits impulses from CNS to effectors (muscles and glands).
  • Subdivided into:
    1. Somatic: Controls voluntary muscle movements (skeletal muscles).
    2. Autonomic: Controls involuntary muscle movements (smooth and cardiac muscles, glands).

Clinical Application 10.1: Migraine

  • Prevalence: Affects about 12% of the US population.
  • Signs: Include pounding headaches, nausea, aura (shimmering images), and light/sound sensitivity.
  • Triggers: Bright lights, certain foods, stress, and hormonal changes in women, especially before menstruation.
  • Duration: Lasts from 4 to 72 hours.
  • Mechanism: Period of neuron excitation followed by unresponsiveness leads to pain sensations.
  • Management: Identify and avoid triggers to reduce attack frequency.

Clinical Application 10.1 (Continued) - Migraine Treatments

  • Triptans: Medications that may stop a migraine attack.
  • Transcranial Magnetic Stimulation: Effective for both types of migraines.
  • Other Treatments:
    • Botox injections.
    • Medications targeting neurotransmitter imbalances.

10.2 Nervous Tissue Cells: Neurons and Neuroglia

Neurons

  • Structural Variations: Vary in size and shape; differences noticed in axons and dendrites.
  • Key Features:
    • Cell body (soma/perikaryon): Houses nucleus, cytoplasm, organelles, neurofilaments, and Nissl bodies (chromatophilic substance).
    • Dendrites: Branch out and receive signals; a neuron can have many.
    • Axon: Transmits impulses; usually only one per neuron.

Axon Structure

  • Axon Hillock: Cone-shaped area from where axon arises.
  • Collaterals: Branches that may arise from the axon.
  • Axon terminal: The end part of axons where neurotransmitters are released.
  • Synaptic knob: Rounded ending of the axon terminal.
  • Schwann cells: Wrap around some axons in layers, creating the myelin sheath which insulates axons, facilitating faster impulse transmission.

Myelination of Axons

  • Myelinated Axons:
    • Have a myelin sheath produced by Schwann cells in the PNS or oligodendrocytes in the CNS.
    • Comprise white matter in the CNS and speed up impulse conduction.
  • Unmyelinated Axons:
    • Encased by Schwann cell cytoplasm in PNS but lack myelin.
    • Comprise gray matter in the CNS.

Clinical Application 10.2: Multiple Sclerosis (MS)

  • Description: Autoimmune condition leading to the destruction of myelin sheaths in CNS.
  • Symptoms: Muscle atrophy, fatigue, mood changes, and impaired vision.
  • Treatment: Immunosuppressive drugs.

Classification of Neurons

Structural Classification

  1. Multipolar Neurons:
    • Multiple processes extend from cell body (many dendrites, one axon).
    • Comprise 99% of neurons; primarily found in CNS.
  2. Bipolar Neurons:
    • Two processes extend from the cell body (one dendrite and one axon).
    • Less common; found in sensory organs.
  3. Unipolar (Pseudounipolar) Neurons:
    • One process extends, dividing into two branches functioning as one axon; primarily found in PNS.

Functional Classification

  1. Sensory (Afferent) Neurons:
  • Carry impulses towards the CNS; often unipolar or bipolar.
  1. Interneurons (Association Neurons):
  • Multipolar neurons linking neurons in the CNS; relay information internally.
  1. Motor (Efferent) Neurons:
  • Carry impulses away from CNS to effectors; primarily multipolar.

Classification of Neuroglia

General Functions of Neuroglia

  • Provide structural support and nourishment to neurons.
  • Facilitate may functions during embryonic development.
  • Aid in creating synapses and regulating the extracellular environment.

Neuroglia of CNS

  1. Astrocytes:
    • Connect neurons with blood vessels, forming the blood-brain barrier.
  2. Oligodendrocytes:
    • Myelinate CNS axons and provide support.
  3. Microglia:
    • Act as immune defenders; phagocytic support.
  4. Ependymal Cells:
    • Line brain cavities and regulate cerebrospinal fluid composition.

Neuroglia of PNS

  1. Schwann Cells:
    • Produce myelin sheath for peripheral axons.
  2. Satellite Cells:
    • Support and nourish neuron cell bodies within ganglia.

Neuroglia and Axonal Regeneration

  • CNS Neurons: Lack regenerative capability due to absence of guiding sheath and reduced oligodendrocyte proliferation following injury.
  • PNS Neurons: Can regenerate if injury occurs outside the cell body; Schwann cells can guide axon regrowth.

10.3 Cell Membrane Potential

  • Cell Membrane Polarization: The difference in electrical charge across the membrane defines the membrane potential.
  • Resting Membrane Potential: Approximately −70 mV; predominantly attributable to ion distribution, primarily sodium (Na+) and potassium (K+).
  • Ion Distribution:
    • K+ is primarily intracellular
    • Na+ is primarily extracellular
    • Negatively charged impermeant proteins contribute to the negative interior.

Mechanism of Membrane Potential

  • Gated channels regulate ion flow; numerous factors can lead to the opening/closing of these channels.
  • Sodium-Potassium Pump maintains resting conditions by transporting ions against gradients: 3 Na+ out, 2 K+ in, utilizing ATP.
  • Action Potential: Series of events altering membrane potential: depolarization (inward sodium influx) and repolarization (outward potassium efflux).

Illustrations of Ion Movement During Action Potentials

  • Sequence of Events:
    1. Local stimulus excitability triggers sodium influx, leading to depolarization.
    2. Threshold potential is achieved at approximately −55 mV, resulting in voltage-gated sodium channels opening, rapidly changing the interior to positive (~ +30 mV).
    3. Potassium efflux follows, returning to resting potential and occasionally overshooting (hyperpolarization).

Summation of Local Potentials

  • Local changes can grade in intensity, meaning larger stimuli can induce larger changes in potential.
  • Excitatory vs Inhibitory:
    • EPSP (Excitatory Postsynaptic Potential): Makes neuron more likely to generate action potentials by depolarizing.
    • IPSP (Inhibitory Postsynaptic Potential): Decreases likelihood of an action potential by hyperpolarizing the membrane.

Neurotransmitters and Synapses

  • Communication at Synapse: Involves transferring impulses via neurotransmitters across a synaptic cleft from a presynaptic to a postsynaptic neuron.
  • Neurotransmitter Effects: Can include opening/closing of ion channels, thus affecting synaptic potentials.
  • Major neurotransmitters include: Acetylcholine, Biogenic amines, Amino acids, and Neuropeptides.

Clinical Application 10.3: Factors Affecting Impulse Conduction

  • Various ion concentrations can significantly impact neuron excitability and impulse transmission; e.g., high extracellular potassium can provoke convulsions, while low levels can lead to paralysis.

Conclusion: Understanding Impulse Processing

  • The structure and function variability of neurons and their connections dictate how the nervous system processes incoming impulses to act appropriately, this involves mechanisms such as convergence and divergence allowing for complex integration of sensory information.