Muscles & Nerves

Muscles & Nerves

Dr. Shahida Shahana

Learning Outcomes

  • Understand the organisation of muscle fibres within skeletal muscles.

  • Understand the role of connective tissue in binding muscle fibres together, forming tendons and influencing the line of pull of muscles.

  • Understand the basic concept of muscle action.

  • Appreciate that muscle fibres within a named muscle may have different orientations and the effects of this on lines of pull of muscles.

  • Understand that skeletal muscles are often grouped together in fascial compartments (e.g., as flexors or extensors) and that the individual muscles within these groups commonly have the same nerve supply.

  • Describe the anatomical differences between the main subdivisions of the nervous system.

  • Describe the basic anatomy of a motor unit.

Lecture Overview

  • Spinal cord

  • Muscle

    • Muscle types (Histology)

    • Muscle morphology

    • Tendons vs. Aponeurosis

    • Muscle functions

    • Innervation of muscles

  • The nervous system divisions

  • Peripheral nerve structure

Muscle Functions

  • Movement: Enables locomotion and changes in position.

  • Skeleton: Facilitates joint movement and support, allowing skeletal integrity.

  • Intestines: Assists in the movement of food through the digestive system.

  • Heart & blood vessels: Contributes to circulation through involuntary muscle contractions.

  • Static Support: Provides stability to the body structure.

  • Heat Production: When muscles contract, they generate heat, helping regulate body temperature.

Types of Muscle: Based on Histology

  • Skeletal Muscle

    • Attached to skeleton

    • Responsible for locomotion

    • Voluntary Control: Under conscious control

  • Cardiac Muscle

    • Found in the heart

    • Responsible for blood circulation

    • Involuntary Control: Not under conscious control

  • Smooth Muscle

    • Located in walls of blood vessels and organs

    • Moves substances and restricts flow

    • Involuntary Control: Also not under conscious control

Skeletal Muscle Structure

  • Components:

    • Bone: Attachment point for muscle.

    • Perimysium: Connective tissue surrounding fascicles.

    • Blood vessels: Supply oxygen and nutrients to the muscle.

    • Muscle fiber: Basic unit of muscle tissue.

    • Fascicle: Bundles of muscle fibers.

    • Tendon: Connective tissue that attaches muscle to bone.

    • Epimysium: Outer layer covering the entire muscle.

    • Endomysium: Connective tissue surrounding individual muscle fibers.

Types of Skeletal Muscle: Based on Morphology

  • Aponeurosis: Type of flattened tendon.

  • Tendon: Connective tissue that attaches muscle to bone.

  • Based on shape & architecture:

    • Parallel muscles: Fibers run parallel to the length of the muscle.

    • Pennate muscles: Fibers are arranged at an angle to the tendon, providing greater force at the cost of range of motion.

Tendons Vs Aponeuroses

  • Tendons

    • Composed of dense fibrous connective tissue.

    • Do not shorten when muscles contract.

    • Can alter the direction of force generated by muscles.

    • Some muscles share a common tendon insertion.

  • Aponeurosis

    • A broad, flat sheet-like tendon that may support muscle tissues, connecting muscles to bones or to other muscles.

Muscles in Action

  • Muscles acting over a joint will affect that joint.

  • Some muscles traverse multiple joints and can act on each.

    • Example: Triceps acting on the elbow joint.

Functions of Specific Muscles

  • Erector spinae: Stabilizes posture and opposes gravitational force.

  • Deltoid: Has multiple actions due to muscle fibers running in different directions:

    • Posterior fibers: Extend the shoulder.

    • Anterior fibers: Flex the shoulder.

    • Middle fibers: Abduct the shoulder.

Muscles Working Together: Collaborative Function

  • Agonist-Antagonist pairs:

    • Agonists: Primary muscles responsible for producing a movement.

    • Antagonists: Oppose the action of agonists for balance.

    • Example: In elbow flexion, the Biceps brachii (agonist) contracts while the Triceps brachii (antagonist) relaxes.

  • Synergists: Additional muscles that assist the agonists.

    • Example: Brachialis aids in elbow flexion by enhancing the action of the Biceps brachii.

  • Stabilisers: Muscles that stabilize joints during movement.

    • Example: The shoulder joint stability is maintained by the rotator cuff muscles.

Specific Muscle Actions

  • Hand Muscles: Complex interactions between intrinsics for precision grip versus forearm muscles that provide powerful grip.

Muscle Compartments

  • Arm:

    • Divided into flexor and extensor compartments.

  • Forearm:

    • Divided into flexor-pronator and extensor-supinator compartments.

    • Posterior: Extensors

    • Anterior: Flexors

Nerve Supply of Muscles

  • Muscles within the same compartment are generally supplied by the same nerve.

    • Example: The Radial nerve innervates all the extensor muscles in the upper limb.

  • Brachial plexus: Network of nerves supplying the arm, including the radial nerve which is critical for extensor compartments.

Nervous System Divisions

  • Structural divisions:

    • Central Nervous System (CNS): Comprises the brain and spinal cord.

    • Peripheral Nervous System (PNS): Includes 12 cranial nerves, 31 spinal nerves, and peripheral ganglia.

  • Functional divisions:

    • Somatic (voluntary): Controls voluntary actions.

    • Visceral (Autonomic, involuntary): Regulates involuntary functions.

    • Motor: Controls the autonomic functions.

      • Sympathetic: Mediates 'fight or flight' responses.

      • Parasympathetic: Manages 'rest and digest' functions.

    • Visceral Sensory: Carries sensory signals from visceral organs.

    • Enteric: Pertains to the gastrointestinal system.

Nerve Structure

  • Components of a Nerve:

    • Epineurium: Outermost layer surrounding an entire nerve.

    • Perineurium: Middle layer that encapsulates fascicles.

    • Endoneurium: Innermost layer that encloses individual nerve fibers.

    • Fascicle: A bundle of nerve fibers.

    • Nerve fibre (myelinated): Actual nerve cells covered with a myelin sheath for efficient signal transmission.

Spinal Nerves

  • 31 pairs of spinal nerves: Differentiated into five regions corresponding to the vertebral column.

    • Each pair named after associated vertebrae, e.g., C1 through C8, T1 to T12.

Motor Units: Innervation of Muscle

  • Neuromuscular junction: The synapse or junction where communication occurs between a motor neuron and a muscle fiber.

  • Motor Unit: Defined as one motor neuron that may innervate a varying number of muscle fibers, affecting the control of muscular contractions and precision in muscle movements.

Acknowledgment

  • Thank you!

  • Slides adaptation from Dr. Groening.