Muscular Function Overview

B.1.3 Muscular Function

Overview - Understandings

• B.1.3.1 The body utilizes various types of muscular contractions that serve distinct roles in creating movement and stability.

  • Muscles are organized into functional groups known as motor units that contract based on the all-or-none principle.

  • Muscular contraction necessitates the metabolism of ATP (adenosine triphosphate) within muscle cells.

  • Motor units can be classified by fibre type and neuron diameter, specifically:

  • Type I: Slow-twitch fibers

  • Type IIa: Fast-twitch fibers, fatigue-resistant

  • Type IIx: Fast-twitch fibers, quick to fatigue

  • The recruitment patterns of these motor units are influenced by the specific physical activity being performed.

  • Hypertrophy (increase in muscle size) and atrophy (decrease in muscle size) can lead to changes in how motor unit recruitment occurs.

  • Muscle contractions can be categorized in four main types:

  • Isometric: Muscle tension without change in length.

  • Isotonic Concentric: Muscle shortens while contracting.

  • Isotonic Eccentric: Muscle lengthens while contracting.

  • Isokinetic: Movement occurs at a constant speed.

  • Muscles typically operate in pairs within a mechanism known as reciprocal inhibition, functioning as agonists (primary movers) and antagonists (opposing muscles).

Overview – Understandings AHL

• B.1.3.2 (HL) The sliding filament theory explains how myofilaments interact during muscle contraction at the sarcomere level.

  • Critical substances involved in this process include:

  • Calcium (Ca²⁺)

  • ATP

  • The proteins actin, myosin, troponin, and tropomyosin each serve specific functions in muscle contraction.

• Movement:

  • When skeletal muscles contract, they pull on tendons, hence moving bones at joints.

• Movement of Substances:

  • Smooth muscle facilitates food movement through the digestive system.

  • Cardiac muscle is responsible for blood circulation.

  • Skeletal muscle aids in returning venous blood to the heart.

• Posture and Stability:

  • Postural muscles help maintain body position even while inactive.

• Heat Production:

  • Muscle contraction generates heat, such as in shivering, contributing to approximately 85% of body heat.

Chapter Focus

• This chapter places emphasis on large skeletal muscles and their importance in joint movement.

Properties of Muscles

Key Properties

• Contractility:

  • The ability of muscle tissue to contract and generate force in response to nerve stimulation.

  • A distinctive property solely of muscle tissue.

• Extensibility:

  • The capacity for a muscle to stretch beyond its resting length.

• Elasticity:

  • Refers to the ability of a muscle to return to its original resting length after being stretched.

Example: Elbow Movement

• Upon nerve signal transmission:

  • The biceps brachii contracts (demonstrating contractility).

  • As a result, the elbow flexes; simultaneously, the triceps brachii is elongated (showing extensibility).

  • When the arm lowers, the triceps return to their resting length (demonstrating elasticity).

Muscle Fibre Capacity

• Muscles can shorten to approximately 50% of their resting length.

• Conversely, they can elongate to about 150% of their resting length.

Neuromuscular Function

Nervous System Overview

• Composed of millions of nerve fibres transmitting electrical signals.

• Divided into:

  • Central Nervous System (CNS): Encompasses the brain and spinal cord, playing roles in sensing and controlling activities.

  • Peripheral Nervous System: Nerves extend from the spinal cord to limbs and throughout the body.

Motor Neurons

• Responsible for conveying signals from the CNS to muscles, triggering contractions or relaxation.

• The human body has roughly 200,000 motor neurons, sometimes referred to collectively as the efferent system.

Muscle Activation

• Nerve cells transmit electrical impulses originating from the brain, facilitating coordinated muscle contractions.

Energy for Contraction

• ATP (adenosine triphosphate) serves as the principal energy currency in biological systems.

• PCr (phosphocreatine) aids in replenishing energy during muscle contractions.

  • ATP is integral, transferring chemical energy to power metabolic reactions.

  • Structurally, ATP is a molecule composed of adenosine and three phosphate groups.

Main Components of a Neuron

• Structures of a neuron include:

  • Soma (cell body): Located within the spinal cord or ganglia.

  • Dendrites: Facilitate connections between neurons, ensuring the flow of information between them.

  • Axon: Serves as the principal channel for nerve signal transmission, akin to an electrical wire.

  • Myelin Sheath: Acts as insulation for the axon, enhancing the speed of electrical signal conduction.

  • Nodes of Ranvier: Gaps in myelin which accelerate signal transmission.

Neuromuscular Junction

• The axon becomes unmyelinated at the muscle region, connecting to the muscle fibre at the motor endplate.

• The synapse represents the diminutive gap between the neuron and muscle, wherein the electrical signal initiates muscle stimulation.

Motor Units

What is a Motor Unit?

• A motor unit comprises one motor neuron coupled with all the muscle fibers it innervates.

Innervation Ratio

• A high innervation ratio (approximately 2,000 fibres) correlates with the capability to generate large forces but results in less precision (e.g., gluteus maximus).

• A low innervation ratio (around 10 fibres) yields small forces but allows for heightened precision (e.g., eye muscles).

Force vs. Precision

• An increase in fibres per neuron results in increased muscle force production.

• Conversely, fewer fibres per neuron yield greater control and precision.

All-or-None Principle

• When a motor neuron activates, all muscles fibers within that unit undergo contraction.

• The muscle fibers operate either fully contracted or fully relaxed.