NJN 1. Muscle Tissue: Characteristics and Functions

• There are four characteristics of muscle tissue that differentiate it from other tissues: excitability, contractility, extensibility, and elasticity.

Muscle Characteristics

• Excitability: Muscles can respond to stimuli, typically neural, by changing their membrane potential, leading to an action potential.
• Motor Neuron Stimulation: A motor neuron stimulates a muscle cell by releasing neurotransmitters like acetylcholine.
• Contractility: Muscles can shorten forcibly when adequately stimulated, due to the action potential.
• Extensibility: Muscles can be stretched beyond their normal resting length.
• Distensibility: Another word for extensibility is stretchability.
• Elasticity: Muscles resist being stretched and tend to recoil to their original size.
• When tissue is stetched, it will resist that desire to stretch, and recoil back to the smallest size possible.

Muscle Functions

• Muscles are responsible for producing locomotion by moving the skeleton.
• Muscles help maintain posture and stabilize body position against gravity.
• Muscles stabilize a lot of the body's joints.
• Muscles generate heat to maintain body temperature; shivering is an example of this.
• Cellular respiration produces heat.

Macroscopic Structure of Skeletal Muscle

• Epimysium: A dense fibrous irregular connective tissue covering the entire muscle belly.
• Epimysium can fuse with the periosteum of the bone, forming direct fleshy attachments.
• Fascicles: Bundles of muscle fibers within the muscle belly.
• Perimysium: A dense fibrous irregular connective tissue surrounding each fascicle.
• The parimysium just like the epimysium is continuous with each other.
• Muscle Fiber: An individual muscle cell within a fascicle. Muscle fiber and muscle cell are the same.
• Endomysium: An areolar connective tissue covering each muscle fiber or muscle cell.
• Sarcolemma: The plasma membrane (phospholipid bilayer) surrounding the muscle fiber, beneath the endomysium.

Tendons

• Tendons are rope-like connective tissues rich in collagen, connecting muscle to bone.
• When a muscle fiber contracts, it pulls on the endomysium, which in turn pulls on the perimysium and epimysium.
• This action pulls on the tendon, moving the bone.
• Muscle Contraction Flow Pattern
  • Muscle fiber contracts
  • Connective tissue sheaths (endomysium, epimysium, perimysium) are pulled
  • Tendons are pulled
  • Bone is pulled or moved, producing locomotion.

Insertion and Origin.

• Origin: The point where the tendon is connecting is fixed; the part of the bone that doesn't move during muscle contraction.
• Insertion: The part that moves.
• During muscle contraction, the insertion moves towards the origin.
• Aponeurosis: A sheet-like connective tissue connecting muscle to bone vs a tendon which is a cord
• Connective tissue sheaths contribute to elasticity.
• Blood vessels and nerve fibers run through these connective tissue sheaths.

Muscle to Bone Connections

• Direct Attachment: Less common, involves the epimysium fusing with the periosteum of the bone or the pericondrium.
  • Epimysium fuses with the periosteum or pericondrium.
• Indirect Attachment: More common, mediated through tendons and aponeuroses.
  • mediated through tendons and aponeuroses.
• Tendons are smaller, conserving space, and are resilient due to collagen, resisting abrasion and friction.
• Tendons can better resist abrasion than fleshy connections as bones rub up against each other

Muscle Fiber Components

• Myofibril: A structure inside the muscle cell.
• Muscle cells have thousands of myofibrils within each cells.
• Sarcoplasmic Reticulum: A structure around muscle cells which stores calcium.
• Muscle fibers are cylindrical, multinucleated, and striated.
• Striated: Muscle fibers take on a striped appearance
• Sarcomere: The functional and structural unit of the muscle cell.