Topic 6

Functions of the Nervous System

  • Sensory Input:

    • Receptors gather information about internal and external changes.

  • Integration:

    • Interpretation of sensory input.

  • Motor Response:

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

Nervous System Organization

  • Central Nervous System (CNS):

    • Comprises the brain and spinal cord.

    • Functions as the integration and command center.

  • Peripheral Nervous System (PNS):

    • Consists of spinal and cranial nerves.

    • Responsible for conveying messages to and from the CNS.

Functional Divisions of PNS

  • Sensory (Afferent) Division:

    • Transmits impulses towards the CNS:

    • Somatic Afferent Fibers:

      • Carry impulses from skin, skeletal muscles, and joints.

    • Visceral Afferent Fibers:

      • Carry impulses from visceral organs.

  • Motor (Efferent) Division:

    • Transmits impulses away from the CNS to effector organs.

    • Further subdivided into:

    • Somatic (Voluntary) Nervous System:

      • Controls skeletal muscles consciously.

    • Autonomic Nervous System (ANS):

      • Involuntary regulation.

      • Regulates smooth muscle, cardiac muscle, and glands.

      • Further divided into:

      1. Sympathetic Division

      2. Parasympathetic Division

Nervous Tissue (Cell Types)

  • Neurons:

    • Excitable cells that transmit electrical signals.

  • Neuroglia (Glial Cells):

    • Supporting cells in the nervous system.

Neuron Structure

  • Three Primary Parts:

    1. Dendrites:

    • Receptive/input region that carries messages toward the cell body.

    1. Cell Body:

    • Functions as the metabolic center.

    1. Axon:

    • Carries impulses away from the cell body.

  • Important Notes:

    • Dendrites are often short and branching.

    • Motor neurons can have hundreds of dendrites clustered around their cell body.

    • A neuron typically has only one axon.

    • A "nerve fiber" refers to the axon of a neuron.

    • Neurons are amitotic but highly metabolic.

      • They don’t divide and use a lot of energy.

    • Axons lack rough endoplasmic reticulum (RER) and Golgi apparatus; they decay quickly if severed.

Structural Classification of Neurons

  • Multipolar Neurons:

    • Multiple processes extend from the cell body.

  • Bipolar Neurons:

    • Two processes extend from the cell body (one dendrite and one axon).

  • Unipolar Neurons:

    • One process extends from the cell body.

Functional Classification of Neurons

  1. Sensory (Afferent) Neurons:

    • Transmit signals from receptors in the skin or internal organs to the CNS.

    • Almost all are unipolar with cell bodies located within sensory ganglia (outside CNS).

      • ganglion: collection of neuron cell bodies in the PNS.

  2. Interneurons:

    • Located between sensory and motor neurons.

    • Carry signals through the CNS where integration occurs.

    • Comprise >99% of neurons, almost all are multipolar.

  3. Motor (Efferent) Neurons:

    • Transmit signals away from the CNS to effector organs.

    • Typically multipolar, most of their cell bodies reside in the CNS.

Overlap of Structural and Functional Classifications

  • Relationships:

    1. Most multipolar neurons are interneurons.

    2. Some multipolar neurons are motor neurons.

    3. Bipolar neurons are sensory neurons associated with special sense organs.

    4. Most unipolar neurons are sensory neurons.

Neuroglia (Types)

  1. Astrocytes:

    • Anchor neurons to capillaries.

    • Exchange materials between capillaries & neurons.

    • Control chemical environment surrounding neurons.

  2. Microglial Cells:

    • Act as macrophages that destroy invading microorganisms & remove cellular debris.

  3. Ependymal Cells:

    • Line ventricles of the brain & central canal of spinal cord.

    • Produce & circulate cerebrospinal fluid.

  4. Oligodendrocytes:

    • Form myelin sheaths around CNS axons.

  5. Satellite Cells:

    • Surround neuron cell bodies in the PNS and function similarly to astrocytes.

  6. Schwann Cells:

    • Form myelin sheaths around PNS axons by wrapping around the axon.

Myelin Physiology

  • Myelin Sheath Function:

    • Increases the speed of impulse conduction primarily through saltatory conduction.

Important Definitions

  • Nerve:

    • A bundle of axons in the PNS.

  • Tract:

    • A collection of axons in the CNS.

  • Ganglion:

    • A cluster of cell bodies located outside the CNS (in PNS).

  • Nucleus:

    • A cluster of cell bodies within the CNS.

  • White Matter (CNS):

    • Composed of myelinated fibers.

  • Gray Matter (CNS):

    • Composed primarily of cell bodies and their dendrites.

Applied Understanding of Nervous Tissue

  • Case Study - Matt's Headaches:

    • Matt experiences persistent headaches; ultimately diagnosed with bacterial meningitis.

    • Physician begins IV antibiotics and considers potential swelling/inflammation reasons for headaches.

    • Key Point: Targeting microglial cells (CNS macrophages) may be discussed for reducing inflammation.

Neuron Excitability

  • Neurons are characterized by high excitability in response to stimuli due to the role of electricity in their activity.

  • Key Concept:

    • Adequate stimulation leads to the generation of an electrical impulse known as an action potential or nerve impulse.

Resting Membrane Potential

Resting membrane potential -70 mV (milivolts)

  • Establishment Factors:

    1. Differences in sodium (Na+Na^+) and potassium (K+K^+) concentrations across the membrane.

    2. Differences in membrane permeability to these ions.

  • Concentration Differences:

    • Outside the neuron: High Na+Na^+ and Low K+K^+.

    • Inside the neuron: Low Na+Na^+ and High K+K^+.

  • Membrane Permeability:

    • There are numerous K+K^+ leak channels compared to Na+Na^+ leak channels.

    • The sodium-potassium pump maintains these concentration gradients.

Ion Channels and Impulse Generation

  • Types of Gated Ion Channels:

    1. **Ligand-gated Channels

    2. **Mechanically Gated Channels

    3. Voltage-gated Channels

Definitions Related to Membrane Potential Changes

  • Depolarization:

    • Decreasing membrane potential, where the inside becomes less negative.

  • Hyperpolarization:

    • Increasing membrane potential, where the inside becomes more negative.

Action Potential Overview

  • Definition:

    • A large transient depolarization event conducted along the membrane of muscle cells or nerve fibers.

  • Key Players:

    • Voltage-gated Na+Na^+ channels.

    • Voltage-gated K+K^+ channels.

  • States of Channels:

    • Channels can only be in closed or open states.

Stages of an Action Potential (AP)

  1. Resting State:

    • Voltage-gated Na+Na^+ and K+K^+ channels are closed.

  2. Depolarization:

    • An adequate stimulus opens voltage-gated Na+Na^+ channels.

  3. Repolarization:

    • Voltage-gated Na+Na^+ channels inactive; K+K^+ channels open.

  4. Hyperpolarization:

    • K+K^+ channels close slightly delayed; resets back to resting membrane potential (RMP).

Action Potential Characteristics

  • Cause:

    • Changes in membrane permeability due to ion flow during the action potential.

  • Propagation:

    • Action potentials are self-propagating; an influx of Na+Na^+ during one region of the membrane depolarizes adjacent regions.

Action Potentials and Stimulus Strength

  • All-or-none Events:

    • Once an action potential is initiated due to a threshold stimulus (-55 mV), subsequent action potentials cannot vary in intensity.

  • Perception of Stimuli:

    • Varying stimulus strengths are perceived by changes in action potential frequency; larger stimuli produce more frequent action potentials.

Refractory Periods

  • Absolute Refractory Period:

    • The neuron cannot respond to another stimulus until reset.

    • Ensures each action potential is all-or-none and enforces one-way transmission.

  • Relative Refractory Period:

    • Follows the absolute period; most sodium channels are reset, but some potassium channels remain open.

    • A stronger stimulus can still generate an action potential during this time.

Conduction Velocity Factors

  • Factors Influencing Speed:

    1. Axon Diameter:

    • Larger diameter results in faster conduction.

    1. Degree of Myelination:

    • Saltatory conduction (in myelinated axons) is approximately 30 times faster than continuous conduction (in non-myelinated axons).

Classification of Nerve Fibers Based on Diameter and Myelination

  • A Fibers:

    • Diameter: Large

    • Myelination: Heavy

    • AP Speed: >300 mph (approx. 134134 m/s)

  • B Fibers:

    • Diameter: Intermediate

    • Myelination: Light

    • AP Speed: hickapprox40hickapprox 40 mph (approx. 1818 m/s)

  • C Fibers:

    • Diameter: Small

    • Myelination: Non

    • AP Speed: hickapprox2hickapprox 2 mph (approx. 0.90.9 m/s)

Communication Between Neurons

  • Synapse:

    • A junction that mediates information transfer from one neuron to either another neuron or an effector cell.

Steps of Synaptic Transmission

  1. Action Potential (AP) arrives at axon terminal.

  2. Voltage-gated Ca2+Ca^{2+} channels open, allowing Ca2+Ca^{2+} to enter.

  3. The influx of Ca2+Ca^{2+} triggers neurotransmitter release by exocytosis.

  4. Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic membrane.

  5. Binding opens ion channels, leading to a response in the postsynaptic neuron.

  6. Signal termination can occur through neurotransmitter reuptake, degradation, or diffusion away from the synaptic cleft.

    • Change away from RMP in response to a stimulus that may trigger on AP.

Graded Potentials vs. Action Potentials

  • Graded Potential:

    • Produced by chemically/mechanically gated ion channels on dendrites or cell body.

    • Can be depolarizing or hyperpolarizing.

    • Proportional to stimulus strength (graded).

    • Reversible if stimulus stops before threshold is reached.

    • Localized; affects a small region.

  • Action Potential:

    • Generated by voltage-gated ion channels on the trigger zone and axon.

    • Only depolarizing.

    • All-or-none response.

    • Irreversible once initiated.

    • Self-propagating over a greater distance without losing strength.

Types of Graded Potentials

  • Excitatory Postsynaptic Potential (EPSP):

    • Local depolarization that brings the neuron closer to threshold.

    • Activated by neurotransmitters that open Na+Na^+ channels.

  • Inhibitory Postsynaptic Potential (IPSP):

    • Local hyperpolarization that drives the neuron away from threshold.

    • Activated by neurotransmitters that open K+K^+ or Cl−Cl^- channels.

Postsynaptic Potential Summation

  • Summation Types:

    • Temporal Summation:

      • Occurs when multiple APs arrive from one presynaptic neuron back-to-back.

    • Spatial Summation:

      • Occurs when APs arrive simultaneously from multiple presynaptic neurons.

  • Example Scenario:

    • If multiple signals from presynaptic neurons fail to produce enough postsynaptic potential for threshold, IPSPs must outnumber EPSPs to prevent an impulse transmission.