Neurons and Neuroglia

NEURONS AND NEUROGLIA

Course Information

  • An_Sci 3253/3254: Physiology of Domestic Animals

  • Section 4 - Lecture 2

  • University of Missouri

REVIEW: THE NERVOUS SYSTEM

Central Nervous System (CNS)

  • Comprises the brain and spinal cord.

  • Functions:

    • Processes sensory input.

    • Coordinates motor output.

  • Key Structures:

    • Cerebrum: Involved in decision-making.

    • Cerebellum: Responsible for balance and coordination.

    • Brainstem: Regulates vital functions.

Peripheral Nervous System (PNS)

  • Encompasses all nerves outside the CNS.

  • Connects the CNS to the rest of the body.

  • Divisions:

    • Sensory Division: Carries signals to the CNS (input).

    • Motor Division: Sends commands to muscles and glands.

Autonomic Nervous System (ANS)

  • A subdivision of the PNS controlling involuntary functions.

  • Divided into two branches:

    • Sympathetic Nervous System: Responds to stress ('Fight or Flight').

    • Example: Increases heart rate.

    • Parasympathetic Nervous System: Promotes relaxation ('Rest and Digest').

    • Example: Contributes to energy conservation.

NEURONS

  • The basic functional units of the nervous system.

  • Specialized for transmitting electrical signals.

Key Parts:

  • Dendrites: Receives signals.

  • Soma (Cell Body): Control center.

  • Axon: Transmits signals away from the cell body.

  • Nucleus: Contains genetic materials.

  • Node of Ranvier: Gaps in myelin sheath allowing for faster signal transmission.

  • Myelin Sheath: Insulating layer around axon, speeds signal conduction.

  • Synaptic Knobs: Ends of axon where neurotransmitters are released.

  • Trigger Zone: Location (axon hillock) where electrical signals initiate.

NEURON PROPERTIES

  • Excitability: Ability to respond to environmental changes or stimuli.

  • Conductivity: Ability to conduct electrical signals to other cells quickly.

  • Secretion: Release of neurotransmitters at the end of nerve fibers upon signal arrival.

NEURON CLASSES

  • Sensory (afferent) Neurons:

    • Function: Detect stimuli and transmit information toward the CNS.

  • Interneurons:

    • Function: Integrate signals from multiple neurons, making decisions on responses.

    • Location: Exclusively located within the CNS (about 90% of all neurons).

  • Motor (efferent) Neurons:

    • Function: Send signals to muscles and gland cells ('effectors').

SOMA (CELL BODY)

  • Acts as the control center of the neuron.

  • Contains:

    • Cytoplasm filled with organelles (e.g., nucleus, mitochondria).

  • Notable Points:

    • No centrioles: Hence, no mitosis or replication of neurons occurs, meaning new neurons cannot be generated.

DENDRITES

  • Branch-like structures extending from the soma.

  • Primarily responsible for receiving signals from other neurons.

  • More dendrites lead to increased information processing capacity.

AXON

  • A cylindrical nerve fiber originating from the soma.

  • Axon Hillock: The point where the axon originates.

  • Features:

    • Axon Collaterals: Branching structures that can occur at the distal end.

    • Axoplasm: Cytoplasm of the axon.

    • Axolemma: The plasma membrane surrounding the axon.

NEUROGLIA (SUPPORTIVE CELLS)

  • There are approximately 1 trillion neurons in the nervous system alongside 10 trillion neuroglia (10-to-1 ratio).

  • Functions of Neuroglia:

    • Protect neurons and assist in their various functions.

    • Bind neurons together and form the supportive framework for nervous tissue.

    • Aid in guiding migrating neurons to reach their destinations during fetal development.

Types of Neuroglia in the CNS

  • Oligodendrocytes:

    • Function: Form myelin sheaths in the CNS to speed up signal transmission.

    • Characteristic: Arm-like processes wrap around axons.

  • Ependymal Cells:

    • Function: Line the internal cavities of the brain, secreting and circulating cerebrospinal fluid.

    • Structure: Composed of cuboidal epithelium with cilia on the apical surface.

  • Microglia:

    • Function: Wander through the CNS to search for debris and damage.

    • Origin: Derived from white blood cells and typically concentrated in damaged areas.

  • Astrocytes:

    • Description: The most abundant type of neuroglia.

    • Functions:

    • Provide a supportive framework for neurons.

    • Extensions (perivascular feet) form a blood–brain barrier by sealing contact with blood capillaries.

    • Convert glucose to lactate, supplying it to neurons.

    • Secrete nerve growth factors.

    • Communicate electrically with neurons.

    • Absorb excess neurotransmitters and ions.

    • Regulate the chemical composition of the tissue fluid.

    • Respond to damage to neurons by forming scar tissue (this process is called astrocytosis or sclerosis).

Neuroglia in the PNS

  • Satellite Cells:

    • Function: Surround the soma of neurons in the PNS providing electrical insulation.

    • Role: Regulate the chemical environment of the neurons.

  • Schwann Cells:

    • Description: Envelop nerve fibers in the PNS.

    • Function: Produce a myelin sheath similar to that of oligodendrocytes in the CNS, assisting in the regeneration of damaged fibers.

MYELIN SHEATH

  • Definition: A protective insulating layer surrounding the axon.

  • Composition: 80% lipids and 20% proteins.

  • Functions:

    • Speeds up signal conduction.

    • Insulates to prevent signal dissipation.

    • Protects nerve fibers from damage.

MYELIN FORMATION

  • CNS:

    • Myelin produced by oligodendrocytes.

  • PNS:

    • Myelin produced by Schwann cells.

  • Myelination Process:

    • Begins at around week 14 of fetal development.

    • Rapidly proceeds during infancy.

    • Completed by late adolescence.

    • Importance: Dietary fat is crucial for CNS development.

Myelin Structure

  • The myelin sheath is segmented.

  • Nodes of Ranvier: Gaps between segments of the myelin sheath.

  • Internodes: Myelin-covered segments from one gap to the next.

MYELINATED AXONS

  • Initial Segment:

    • The short section of nerve fiber between the axon hillock and the first glial cell.

  • Trigger Zone:

    • Combination of the axon hillock and initial segment, significant in initiating a nerve signal.

AXON DISTAL ENDS

  • Terminal Arborization:

    • Composed of a complex arrangement of fine branches.

  • Synaptic Knobs (Terminal Buttons):

    • Bulbous endings that form synapses with target cells.

    • Houses synaptic vesicles filled with neurotransmitters for neuron-to-neuron communication.

SIGNAL TRANSMISSION

  • Involves four specific tasks:

    1. Signal Reception: Handled by dendrites and soma.

    • Incoming signals are converted to changes in membrane potential.

    1. Signal Integration: Occurs in the axon hillock, where incoming signals are assessed for strength.

    2. Signal Conduction: Involves the axon, which specializes in sending signals through varying distances.

    3. Signal Transmission: Via the axon terminal, resulting in neurotransmitter release to target cells (e.g., muscle cells).

CONDUCTION SPEED

  • Influential Factors:

    • Fiber Diameter: Larger fibers conduct signals more rapidly due to increased surface area.

    • Presence of Myelin: Myelin enhances speed of signal conduction.

Speed Characterization

  • Small unmyelinated fibers: 0.5 to 2.0 m/s.

  • Small myelinated fibers: 3.0 to 15.0 m/s.

  • Large myelinated fibers: Up to 120 m/s.

Application of Conduction Speed

  • Slower signals are fine for gastrointestinal (GI) tract function but faster signals are crucial for skeletal muscle control to enhance balance and coordinated movement.

REVIEW QUESTIONS

  1. What are the three main properties of neurons that allow them to function effectively in the nervous system?

  2. How do oligodendrocytes and Schwann cells differ in their function and location?

  3. What factors influence the speed of signal conduction in neurons, and how do they affect transmission?