Nurs 1002-Obj #10

Muscle System Overview

The muscular system is essential for producing movement, maintaining posture, and supporting bodily functions. This chapter explores the relationship between muscle structure and function, the characteristics of different muscle types, and the major muscles of the human body.

Learning Objectives

• Describe the relationship between body movement and muscle action.

• Define the terms origin and insertion as they relate to muscle contraction.

• Identify major muscles, their points of attachment, and their function.

The Relationship Between Body Movement and Muscle Action

Effects of Exercise on Muscles

• Aerobic exercise (e.g., jogging, swimming, biking):

  • Increases the number of capillaries and mitochondria in muscle fibers.

  • Enhances endurance, strength, and resistance to fatigue.

• Anaerobic exercise (e.g., weight lifting, isometric exercises):

  • Leads to hypertrophy (increase in muscle fiber size).

  • Increases mitochondria, myofilaments, glycogen stores, and connective tissue.

  • Results in greater muscle strength and size.

• Muscle atrophy (degeneration and loss of mass):

  • Occurs due to immobilization or loss of neural stimulation.

  • Can begin almost immediately; muscle strength can decline 5% per day.

Muscle Contractions and Force Generation

• The force of muscle contraction depends on the number of cross bridges attached, which is influenced by several factors:

• Number of muscle fibers stimulated (recruitment): More motor units activated = greater force.

• Relative size of fibers: Bulkier muscles can develop more tension. Muscle cells can increase in size (hypertrophy) with regular exercise.

• Frequency of stimulation: Higher frequency = greater force.

• Degree of muscle stretch: Muscle fibers with sarcomeres at 80–120% of their normal resting length generate more force.

Table: Factors Affecting Muscle Contraction

Factor	Duration	Force
Fiber type	Classified as slow or fast fibers
Load	Muscles contract fastest when no load is added. The greater the load, the shorter the duration of contraction and the slower the contraction.
Recruitment		The more motor units contracting, the faster and more forceful the contraction.
Number of muscle fibers stimulated		The more motor units activated, the greater the force.
Relative size of fibers		Bulkier muscles develop more tension. Muscle cells can increase in size (hypertrophy) with regular exercise.
Frequency of stimulation		Higher frequency = greater force.
Degree of muscle stretch		Muscle fibers with sarcomeres at 80–120% of normal resting length generate more force.

Characteristics of Smooth Muscle

Location and Structure

• Found in the walls of most hollow organs (e.g., respiratory, digestive, urinary, reproductive, circulatory systems—except the heart).

• Organs contain two layers of sheets with fibers oriented at right angles:

  • Longitudinal layer: Fibers run parallel to the long axis of the organ. Contraction shortens the organ.

  • Circular layer: Fibers run around the circumference of the organ. Contraction constricts the lumen.

• Alternating contractions and relaxations of these layers mix and propel substances through the lumen.

Microscopic Features

• Contains varicosities (bulbous swellings) of autonomic nerve fibers instead of neuromuscular junctions.

• Varicosities store and release neurotransmitters into a wide synaptic cleft, called a diffuse junction.

• Innervated by the autonomic nervous system.

• No striations and no sarcomeres, but contains overlapping thick and thin filaments.

Functional Properties

• Exhibits slow, synchronized contractions..

• Cells are electrically coupled by gap junctions (cell junctions that allow ions and electrical signals to pass between cells).

• Some smooth muscle cells are self-excitatory (can depolarize without external stimuli).

• Can contract and relax but maintain contraction for prolonged periods with little energy cost.

• Maintains moderate degree of contraction constantly without fatiguing.

Regulation of Contraction

• Contraction is controlled by nerves, hormones, or local chemical changes.

• Neurotransmitter binding can cause either graded (local) potentials or action potentials.

• The same neurotransmitter can have a stimulatory effect in one organ and an inhibitory effect in another.

Origin and Insertion in Muscle Contraction

Definitions

• Origin: The attachment point of a skeletal muscle that does not move during contraction (usually proximal).

• Insertion: The attachment point that moves during contraction (usually distal).

During muscle contraction, the insertion moves toward the origin, producing movement at the joint.

Major Muscles, Points of Attachment, and Function

Head and Neck

• Muscles of facial expression: Control facial movements and expressions.

• Muscles of mastication: Temporalis and masseter (jaw closure); pterygoids (side-to-side movements); buccinator (cheek compression).

• Tongue movement: Genioglossus, hyoglossus, styloglossus anchor and move the tongue.

Neck and Vertebral Column

• Muscles of swallowing: Push food back toward the pharynx.

• Muscles of the back: Iliocostalis, longissimus, spinalis extend the trunk and maintain posture.

Thorax and Abdomen

• Muscles of respiration: Diaphragm and external intercostals enlarge the rib cage for inhalation; internal intercostals and abdominal muscles assist in forced expiration.

• Abdominal wall: Rectus abdominis, external oblique, internal oblique, transversus abdominis support abdominal organs and flex/rotate the trunk.

• Pelvic diaphragm: Supports pelvic organs and helps control defecation.

Shoulder and Upper Limb

• Shoulder muscles: Stabilize and move the shoulder girdle, increasing the range of arm movement.

• Muscles of the arm: Biceps brachii (flexes and supinates forearm), brachialis (flexes forearm), triceps brachii (extends forearm).

Hip and Lower Limb

• Anterior, medial, and posterior thigh muscles:

  • Flex femur at hip, extend leg at knee (walking): Iliopsoas (prime mover of flexion), rectus femoris, vastus muscles.

  • Extend thigh, flex leg (backswing of walking): Hamstrings (biceps femoris, semitendinosus, semimembranosus).

  • Adduct thigh: Adductor magnus, adductor longus, adductor brevis, gracilis.

  • Medial thigh muscles press thighs together.

• Muscles of the leg:

  • Anterior compartment: Tibialis anterior, extensor digitorum longus, extensor hallucis longus (dorsiflex the ankle and extend toes).

  • Posterior compartment: Gastrocnemius, soleus (plantar flex the ankle).

Example: Muscle Leverage and Movement

• Muscles use bones as levers to move body parts. The origin is the fixed point, and the insertion moves toward it when the muscle contracts.

• Levers operate at different mechanical advantages depending on the position of the fulcrum (joint), effort (muscle contraction), and load (bone plus any added weight).

Additional info: For a more detailed study, refer to anatomical diagrams and models to visualize muscle locations, origins, insertions, and actions.