Muscle Tissue and Signaling Pathways

Signaling Pathways and Excitable Tissues

• Majority of signaling pathways are found in excitable tissues like nervous and muscle tissue.

• Ligand-gated channels are prevalent in these tissues.

• Calcium is crucial for bone development and regulation of excitable tissues (muscle, nerves).

• Calcium regulation occurs via:

  • Ligand-gated (chemical).

  • Voltage-gated (electrical).

  • Mechanical stimulation.

• Calcium acts as an intracellular and secondary messenger, initiating downstream enzymatic activity.

Short-Term and Long-Term Signaling Molecules

• Short-acting paracrine molecules (e.g., nitric oxide) play a significant role.

• Nitric oxide:

  • A potent vasodilator.

  • Used in nitroglycerin for heart attack patients (administered sublingually).

  • Mechanism: Opens blood vessels, improving blood flow to prevent ischemia and necrosis (cell death) during myocardial infarction (heart attack).

• Foods rich in nitric oxide (e.g., beets) can increase energy by:

  • Enhancing blood flow.

  • Increasing oxygen delivery.

  • Boosting ATP production.

Arachidonic Acid and Eicosanoids

• Arachidonic acid is derived from fats and is a precursor to eicosanoids.

• Eicosanoids:

  • Examples: Prostaglandins, thromboxanes, leukotrienes.

  • Prostaglandins: Pro-inflammatory, responsible for pain, and involved in creating the mucosal lining of the stomach (protecting it from stomach acid).

• COX-1 and COX-2 enzymes are responsible for the creation of prostaglandins, thromboxanes, and leukotrienes.

• Leukotrienes are specific cytokines involved in allergic or inflammatory responses.

NSAIDs and Prostaglandins

• NSAIDs (Non-Steroidal Anti-Inflammatory Drugs):

  • Examples: Ibuprofen, Advil.

  • Mechanism: Inhibit COX-1 and COX-2 enzymes, reducing prostaglandin production, thus decreasing inflammation and pain.

  • Side effects of prolonged use: Can inhibit prostaglandin production in the stomach, reducing the protective mucosal layer and leading to internal bleeding and reflux.

Agonists, Antagonists, and Synergists

• Agonist: A molecule that stimulates a response.

• Antagonist: A molecule that blocks a response.

• Synergist: Molecules that amplify a response when working together.

• Examples:

  • Glucagon and cortisol are synergistic hormones that increase blood sugar levels.

  • Epinephrine binds to different receptors (alpha receptors) in intestinal versus skeletal regions, causing either constriction or dilation, respectively.

Reflex Pathways and Speed of Responses

• Reflex: An involuntary response.

• Examples:

  • Patellar reflex (knee-jerk reflex).

  • Pupillary reflex (pupils dilating in response to darkness).

• Differences between nervous and endocrine systems:

  • Nervous system: Faster speed, shorter duration.

  • Endocrine system: Slower speed, longer duration.

• Neural coding for stimulus intensity: Increase in action potential frequencies.

Secondary Messengers and Signal Transduction

• Downstream signal transduction can be simple or complex, involving multiple downstream events to generate a single cellular response.

Muscle Tissue Overview

• Two key characteristics of muscle tissue:

  • Excitability: Ability to generate electrical impulses.

  • Contractibility: Ability to shorten or extend.

• Three types of muscle tissue: Skeletal, cardiac, and smooth.

Types of Muscle Tissue

• Skeletal Muscle:

  • Voluntary (conscious control).

  • Attached to the skeleton.

  • Striated (has lines when viewed under a microscope).

  • Multinucleated (poly nucleated).

  • Referred to as somatic.

• Smooth Muscle:

  • Involuntary (no conscious control).

  • Found in digestive, respiratory, and circulatory systems (visceral functions).

  • Non-striated (smooth appearance).

  • Uninucleated.

  • Referred to as autonomic or visceral.

• Cardiac Muscle:

  • Involuntary.

  • Found in the heart.

  • Striated.

  • Uninucleated.

  • Referred to as autonomic or visceral.

  • Highest energy demand due to constant activity.

Exceptions to the Rules

• Diaphragm: Contains both skeletal and smooth muscle.

• Cardiac Muscle:

  • Contains intercalated discs: Unique structures composed of gap junctions (permeable) and anchoring junctions (impermeable).

  • Functions as a functional syncytium, allowing rapid and coordinated contraction.

  • Has its own pacemaker and doesn't necessarily need neural input.

  • Connected via intercalated discs to ensure synchronized contraction.

  • If intercalated discs are blocked (theoretically by a drug), it can cause arrhythmias (irregular heartbeats like A-fib or V-fib).

Stimulants and Health

• Stimulants (e.g., cocaine) can lead to high heart rates and cardiac arrest with chronic use.

• Other stimulants: Coffee (in moderation, from natural sources).

• Amphetamines (e.g., Adderall, Ritalin):

  • Used for focus and weight loss.

  • Highly addictive; chemically similar to crystal meth (one methyl group away).

  • Side effects: High blood pressure, increased heart rate, anxiety.

  • Magnesium glycinate could reduce anxiety.

Origins and Insertions

• Bones have bone markings/landmarks (ridges, holes) for muscle attachment.

• Muscles attach to bones via tendons (made of dense regular connective tissue).

• Ligaments (made of dense regular connective tissue) connect bone to bone.

• Origins and insertions are attachment sites for muscles on bones, enabling motion.

• Origins: Attachment sites where the bone does not move (usually proximal).

• Insertions: Attachment sites where the bone moves (usually distal).

• Examples:

  • Bicep Brachii:

    • Action: Flexion (decreasing the angle) of the elbow.

    • Bones involved: Humerus (brachium), radius, and ulna (antibrachium).

    • Origin: Humerus (stationary).

    • Insertion: Radius and ulna (moving).

  • Hamstrings:

    • Action: Flexion of the knee.

    • Bones involved: Femur (femoral region), tibia, and fibula (lower leg).

    • Origin: Femur (stationary).

    • Insertion: Tibia and fibula (moving).