Nervous Tissue

Overview of Neurons and Glial Cells

Course Structure and Worksheets

  • Models of Neurons: Use provided models for practical learning.

  • Worksheets: Additional worksheets available focusing on muscles and neurons.

  • Upcoming Topics: Systemic side of the neural system will be covered next week (CNS vs. PNS).

Topics to be Covered

  • Structure and basic physiology of the neuron.

  • Muscle cells and their relation to systemic phenomena (e.g., muscle twitch vs. overall muscle function).

  • Discussion of key concepts like myelin, nodes of Ranvier, white matter vs. gray matter.

Neuron Structure and Function

Basic Structure of Neurons

  • Dendrites: Receive signals from other neurons.

  • Cell Body: Contains nucleus and organelles.

  • Axon Hillock: Critical junction for decision-making on signal transmission (threshold evaluation).

  • Axon: Conducts impulses away from the cell body.

Types of Neurons

  • Multipolar Neurons: Most common, have many dendrites and one axon (e.g., motor neurons).

  • Bipolar Neurons: Have one axon and one dendrite (e.g., sensory neurons).

  • Unipolar Neurons: Single process extending from the cell body, often associated with sensory input; also known as pseudo-unipolar.

Neuronal Processes

  • Resting Membrane Potential: Average of -70 mV; maintained by sodium-potassium pump and leak channels.

  • Graded Potentials: Fluctuations that vary in strength; can depolarize or hyperpolarize the neuron based on stimuli.

  • Action Potentials: Occur when graded potentials reach a threshold of approximately -55 mV, leading to an all-or-nothing response.

Cellular Components of the Nervous System

Types of Glial Cells

  • Astrocytes: Star-shaped cells providing structural support, contributing to the blood-brain barrier, and supplying nutrients to neurons (ATP production).

    • Functions:

    • Support and brace neurons.

    • Regulate blood flow and nutrient exchange.

    • Metabolic support facilitating ATP production.

  • Microglia: Act as immune cells of the CNS; involved in phagocytosis to clear debris and pathogens.

    • Illustration: Resemble "Roombas" as they traverse neural tissue to clean up.

  • Ependymal Cells: Line cavities of the CNS and are responsible for producing and circulating cerebrospinal fluid (CSF).

    • Function: Their microvilli help circulate CSF in the brain ventricles; can indicate issues (e.g., CSF leakage).

  • Oligodendrocytes: Create myelin sheaths around axons in the CNS.

    • Function: Myelin serves as an insulator to facilitate faster signal transmission.

  • Schwann Cells: Function similarly to oligodendrocytes but in the peripheral nervous system (PNS).

    • Role: Wrap around individual axons to form myelin; play a crucial role in axonal repair post-injury.

  • Satellite Cells: Provide structural and metabolic support in the PNS, surrounding cell bodies of neurons.

Myelination and Signal Transmission

Myelin Sheaths and Nodes of Ranvier

  • Myelin: Hydrophobic protein that serves as an insulator, promoting faster impulse propagation along the axon.

  • Nodes of Ranvier: Gaps in the myelin sheath where action potentials can occur; enable saltatory conduction for rapid signaling.

Signal Propagation

  • Saltatory Conduction: Faster than continuous conduction due to myelin, allowing action potentials to jump between nodes of Ranvier up to speeds of 100 m/s.

  • Continuous Conduction: Occurs in unmyelinated fibers at approximately 2 m/s.

Action Potentials

Phases of Action Potentials

  1. Threshold Triggering: When the membrane depolarization reaches -55 mV.

  2. Rapid Depolarization: Sodium channels open, allowing Na+ influx.

  3. Repolarization: Sodium channels close, and potassium channels open, allowing K+ efflux.

  4. Hyperpolarization: Membrane potential becomes more negative than -70 mV before returning to resting potential.

Refractory Periods

  • Absolute Refractory Period: Time during which no new action potentials can be initiated (approx 1-2 ms after initiating an action potential).

  • Relative Refractory Period: Neurons can fire again, but require a stronger than normal stimulus due to residual hyperpolarization.

Neurotransmission

Action Potential to Neurotransmitter Release

  1. Calcium Influx: Action potential triggers calcium channels to open, allowing Ca2+ to enter the presynaptic terminal.

  2. Exocytosis: Increased intracellular calcium triggers the release of neurotransmitters into the synaptic cleft.

  3. Receptor Binding: Neurotransmitters bind to receptors on the postsynaptic cell, leading to either excitatory or inhibitory responses.

Types of Neurotransmitters

  • Examples include acetylcholine, epinephrine, norepinephrine, and serotonin.

  • Excitatory vs. Inhibitory: Depolarizing graded potentials (excitatory) vs. hyperpolarizing graded potentials (inhibitory).

Practical Implications and Exam Preparation

  • Review structures, functions, and pathways of neurons and glial cells thoroughly ahead of practical exams.

  • Understand physiological processes as they relate to neural communication and pathology (e.g., demyelinating diseases like multiple sclerosis).

  • Prepare for exam on March 5 or 6, including practical components using models.