Muscle Tissue and Electrophysiology of Neurons: Study Guide

Muscle Tissue

Microscopic Anatomy of Skeletal Muscle Fiber

Skeletal muscle fibers are composed of repeating units called sarcomeres, which are the basic contractile units of muscle. Understanding the structure of the sarcomere is essential for comprehending muscle contraction.

• Sarcomere: The segment between two Z discs; the functional unit of muscle contraction.

• Key Components:

  • Z disc: Defines the boundaries of a sarcomere; anchors thin (actin) filaments.

  • M line: Middle of the sarcomere; holds thick (myosin) filaments together.

  • H zone: Central region containing only thick filaments.

  • A band: Dark band; length of thick filaments, includes overlapping thin filaments.

  • I band: Light band; region with only thin filaments.

Example: During contraction, the I band and H zone decrease in width, while the A band remains the same length.

Muscle Fiber Types and Blood Flow

Muscle fibers are classified based on their metabolic properties and contraction speed.

• Slow oxidative fibers: Rely on aerobic metabolism; fatigue-resistant; require continuous blood flow for oxygen supply.

• Fast glycolytic fibers: Rely on anaerobic metabolism; fatigue quickly; less dependent on blood flow.

• Fast oxidative fibers: Intermediate properties.

Key Point: Reduction in blood flow most affects slow oxidative fibers due to their reliance on oxygen.

Sarcoplasmic Reticulum (SR)

The sarcoplasmic reticulum is a specialized endoplasmic reticulum in muscle cells.

• Function: Stores and releases calcium ions (Ca2+) necessary for muscle contraction.

• Not involved in: Synthesizing ATP or releasing acetylcholine (ACh).

Muscle Contraction Types

• Isotonic Concentric Contraction: The muscle shortens while tension remains constant.

• Isometric Contraction: Muscle tenses but does not change length.

• Eccentric Contraction: Muscle lengthens while maintaining tension.

Sliding Filament Model of Contraction

Muscle contraction occurs as thin (actin) filaments slide past thick (myosin) filaments, shortening the sarcomere.

• Key Steps:

  • Myosin heads bind to actin, forming cross-bridges.

  • ATP is hydrolyzed to provide energy for the power stroke.

  • Filaments slide, Z discs move closer together.

Example: If thick and thin filaments no longer overlap, no muscle tension can be generated.

Electrophysiology of Neurons

Properties of Neurons

Neurons are specialized cells for communication via electrical and chemical signals.

• Excitability: Ability to respond to stimuli.

• Conductivity: Ability to transmit electrical signals over distances.

• Secretion: Release of neurotransmitters at synapses.

Functional Classes of Neurons

• Sensory (afferent) neurons: Transmit information from receptors to the CNS.

• Interneurons: Integrate information within the CNS.

• Motor (efferent) neurons: Send signals from the CNS to effectors (muscles/glands).

Structure of a Neuron

• Soma (cell body): Contains nucleus and organelles.

• Dendrites: Receive incoming signals.

• Axon: Conducts action potentials away from the soma.

• Axon terminals: Release neurotransmitters.

Structural Classification of Neurons

n Type	Structure	Location/Function
Multipolar	One axon, multiple dendrites	Most CNS neurons
Bipolar	One axon, one dendrite	Retina, olfactory cells
Unipolar	Single process from soma	Sensory neurons
Anaxonic	Many dendrites, no axon	Brain, retina

Axonal Transport

• Anterograde transport: Movement from soma to axon terminal.

• Retrograde transport: Movement from axon terminal to soma.

• Fast transport: 200–400 mm/day (organelles, vesicles).

• Slow transport: 0.2–0.5 mm/day (enzymes, cytoskeletal components).

Neuroglia (Glial Cells)

Supportive cells in the nervous system that protect and assist neurons.

• CNS Glia:

  • Oligodendrocytes: Form myelin sheaths in CNS.

  • Ependymal cells: Line ventricles, secrete cerebrospinal fluid (CSF).

  • Microglia: Phagocytic, remove debris and pathogens.

  • Astrocytes: Most abundant; support, regulate environment, form blood-brain barrier.

• PNS Glia:

  • Schwann cells: Form myelin in PNS, aid in axon regeneration.

  • Satellite cells: Surround neuron cell bodies in ganglia, regulate environment.

Myelin Sheath

• Definition: Spiral layers of insulation around axons, increasing conduction speed.

• Formed by: Schwann cells (PNS) and oligodendrocytes (CNS).

• Nodes of Ranvier: Gaps between myelinated segments; sites of action potential generation.

• Myelination: Begins in fetal development, rapid in infancy, complete by adolescence.

Resting Membrane Potential (RMP)

The RMP is the electrical charge difference across the plasma membrane of a neuron at rest, typically about -70 mV.

• Maintained by:

  • Unequal distribution of ions (Na+, K+) across the membrane.

  • Sodium-potassium pump ($3 \text{Na}^+$ out, $2 \text{K}^+$ in per ATP).

• Equation:

  • $\text{RMP} \approx -70\ \text{mV}$

Action Potentials

Action potentials are rapid, all-or-none electrical impulses that travel along axons.

• Phases:

  • Depolarization: Na+ channels open, Na+ enters, membrane potential becomes positive.

  • Repolarization: K+ channels open, K+ exits, membrane potential returns to negative.

  • Hyperpolarization: Membrane potential becomes more negative than RMP.

• All-or-none law: If threshold is reached, an action potential fires at full amplitude.

• Refractory period: Time during which a neuron cannot fire another action potential (absolute and relative phases).

Graded Potentials

• Definition: Local changes in membrane potential that vary in size and decay with distance.

• Characteristics: Can be depolarizing or hyperpolarizing; not all-or-none.

Signal Conduction in Nerve Fibers

• Unmyelinated axons: Conduct signals via continuous conduction; slower.

• Myelinated axons: Conduct signals via saltatory conduction (jumping from node to node); faster.

Synapses and Neurotransmitters

• Synapse: Junction where a neuron communicates with another cell.

• Neurotransmitters: Chemical messengers (e.g., acetylcholine, GABA, glutamate, dopamine).

• Synaptic transmission: Action potential triggers neurotransmitter release, which binds to receptors on the postsynaptic cell.

• Termination: Neurotransmitter is degraded, reabsorbed, or diffuses away.

Table: Major Types of Neuroglia

Type	Location	Function
Astrocytes	CNS	Support, regulate environment, form blood-brain barrier
Oligodendrocytes	CNS	Form myelin sheaths
Microglia	CNS	Phagocytosis of debris/pathogens
Ependymal cells	CNS	Produce and circulate CSF
Schwann cells	PNS	Form myelin, aid regeneration
Satellite cells	PNS	Insulate and regulate neuron cell bodies

Example: Action Potential Sequence

1. Stimulus depolarizes membrane to threshold.

2. Voltage-gated Na+ channels open; Na+ influx causes rapid depolarization.

3. Na+ channels inactivate; K+ channels open; K+ efflux repolarizes membrane.

4. Hyperpolarization occurs as K+ channels remain open briefly.

5. Resting membrane potential is restored.

Additional info: This guide integrates both muscle tissue and nervous system electrophysiology, as both are essential for understanding neuromuscular function in Anatomy & Physiology.