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Lesson Overview

  • Lesson Title: Muscle Physiology and Neurons

  • Instructor: Dr. Robin Bairley

  • Unit 3 Schedule:

    • Week 7 (10/5/25): Muscular Tissue and Physiology (Chapter 10)

    • Week 7 (10/9/25): Muscular Physiology and Nervous Tissue (Chapters 10, 11)

    • Week 8 (10/14/25): Nervous Tissue (Chapter 11)

    • Week 8 (10/16/25): Skeletal and Smooth Muscle Pathophysiology (Chapter 10)

    • Week 9 (10/21/25): Central Nervous System (Chapter 12)

    • Week 9 (10/23/25): CNS

    • Week 10 (10/28/25): Review and Unit 3 Test (Due 10/8/25)

Lesson Objectives

  • Describe the general functions and components of the nervous system.

  • Determine the divisions of the nervous system and how they interact.

  • Assess the functional and structural characteristics of neurons.

  • Assess the requirements of an action potential for neurons.

  • Explain how and why an action potential stimulates a nerve impulse.

  • Relate the actions of motor neurons to muscle contraction and relaxation processes.

Main Components of the Nervous System

  • Brain: Contains the nervous tissue in the cranium.

  • Spinal Cord: Extension of nervous tissue within the vertebral column.

  • Neurons and Neuroglia: Cellular components of the nervous system.

Divisions of the Nervous System

  • Autonomic Nervous System (ANS): Handles involuntary motor responses.

Functions of the Nervous System

  1. Receive Information:

    • Detect stimuli from the environment (sensation).

    • Sensory functions are responsible for registering changes from homeostasis or environmental events.

  2. Generate Responses:

    • Produce a voluntary or involuntary response based on stimuli perceived by sensory structures.

    • This is referred to as the response function.

  3. Integrate Sensory Input:

    • The Central Nervous System (CNS) translates sensations from neural impulses into perceptions, termed the integration function.

    • Integrates sensory input with memories, emotional states, or learning processes.

Characteristics of Neurons

  • Basic Unit of the Nervous System:

    • Composed of a cell body filled with cytoplasm (referred to as axoplasm in neurons) containing organelles.

    • Dendrites: Axoplasm-filled extensions of the cell body that receive communications from other neurons.

    • Axon Hillock: Thickening of the cell body where the axon originates.

    • The plasma membrane of the axon is known as the axolemma.

    • Myelin Sheath: Encloses larger axons to increase the speed of nerve impulses.

    • Axon terminals (or synaptic knobs): Communicate with target cells including other neurons.

Resting Potential and Action Potential

  • Resting Potential:

    • The resting membrane potential of neurons is typically around −70extmV-70 ext{ mV} .

Stages of Action Potential

  • Action Potential Definition: Rapid depolarization and repolarization of the membrane potential of a cell.

  • Initiation:

    • Local potentials cause depolarization at the axolemma to reach a threshold of −55extmV-55 ext{ mV} (threshold potential).

  • Depolarization:

    • Activation gates for voltage-gated Na+ channels open.

  • Repolarization:

    • Inactivation gates for voltage-gated Na+ channels close while voltage-gated K+ channels open.

  • Hyperpolarization:

    • Voltage-gated K+ channels may release additional K+ before returning to resting stage.

Types of Ion Channels Important to Neurons

  1. Leak Channels: Always open; contribute to resting potential.

  2. Ligand-Gated Channels: Open upon binding of a ligand to a receptor.

  3. Voltage-Gated Channels: Open in response to voltage changes across the plasma membrane (axoplasm).

  4. Mechanically Gated Channels: Open when mechanical deformations occur (pressure, stretch, etc.).

Action Potential Transmission

  • Refractory Periods:

    • Absolute Refractory Period: No additional action potential can occur.

    • Relative Refractory Period: A stronger stimulus can possibly produce an additional action potential.

  • Propagated Action Potentials: These result in what is known as a nerve impulse.

Neuromuscular Junction and Muscle Contraction

  • Skeletal Muscle Fibers: Innervated by motor neurons controlling cell membrane potentials.

  • Neuromuscular Junction: The synapse between a motor neuron and a skeletal muscle fiber, separated by a synaptic cleft.

  • Neurotransmitter (e.g., Acetylcholine - ACh): Released into the synaptic cleft to activate ligand-gated channels in muscle fibers.

Phases of Muscle Contraction

  1. Phase 1 - Excitation:

    • Neural impulse reaches the axon terminal, leading to the release of ACh into the synaptic cleft.

    • Binding of ACh to ligand-gated sodium channels produces a motor end-plate potential.

  2. Phase 2 - Excitation-Contraction Coupling:

    • Action potential propagates to the T-tubules, leading to depolarization and activation of voltage-gated calcium channels opening.

    • Calcium enters the cytosol, exposing actin binding sites.

  3. Phase 3 - Contraction:

    • Crossbridges form between myosin and actin leading to ATP hydrolysis and myosin head movement (power stroke).

  4. Phase 4 - Relaxation:

    • Acetylcholinesterase degrades excess ACh; calcium is returned to the sarcoplasmic reticulum (SR), and actin binding sites are recovered by tropomyosin.

Smooth Muscle Contraction

  • Involuntary Control: Contraction is stimulated by various factors including stretch receptors, hormonal, and neural stimuli, and pacemaker cells.

  • Calcium Role:

    • Calcium ions enter from both the sarcoplasmic reticulum and extracellular fluid.

    • Calcium binds to calmodulin, activating myosin light-chain kinase (MLCK) that enables crossbridge formation.

Conclusion and Next Steps

  • Upcoming Assignments: Complete the Review Quiz on Chapters 10/11 by October 27th.

  • Focus of Next Lecture: Overview of additional nervous tissue anatomy and physiology including neuroglial cells, summation, and classes of neurotransmitters.