Muscle Tissue

Types of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement, posture, and vital functions. There are three main types of muscle tissue, each with distinct characteristics:

• Skeletal Muscle: Striated, voluntary, multinucleated cells; attached to bones and responsible for body movement.

• Cardiac Muscle: Striated, involuntary, branched cells with intercalated discs; found only in the heart.

• Smooth Muscle: Non-striated, involuntary, spindle-shaped cells; located in walls of hollow organs (e.g., intestines, blood vessels).

Comparison Table:

Neuromuscular Signaling and Muscle Contraction

Muscle contraction is initiated by electrical signals from motor neurons. The process involves several key steps:

1. Motor neuron releases acetylcholine (ACh) at the neuromuscular junction.

2. ACh binds to receptors on the muscle fiber, generating a muscle action potential.

3. Action potential triggers Ca2+ release from the sarcoplasmic reticulum.

4. Calcium enables actin–myosin cross-bridge cycling, leading to contraction.

5. ATP is required for both contraction (cross-bridge movement) and relaxation (detachment of myosin from actin).

Sliding Filament Theory: Sarcomeres shorten as myosin heads pull actin filaments toward the center, resulting in muscle contraction.

Muscle Physiology: Recruitment, Stimulation, and Fatigue

• Recruitment: Increasing the number of active motor units to generate greater force.

• Stimulation Frequency: Higher frequency leads to summation and tetanus (sustained contraction).

• Muscle Fatigue: Decline in ability to generate force, often due to ATP depletion or lactic acid buildup.

• ATP Sources:

  • Brief activity: Creatine phosphate and anaerobic glycolysis.

  • Sustained activity: Aerobic respiration.

Equation for ATP hydrolysis:

Skeletal Muscles: Movement and Clinical Application

Muscle Attachments and Actions

• Origin: Fixed attachment point of the muscle.

• Insertion: Movable attachment point.

• Joint Action: Movement produced at a joint when the muscle contracts.

• Lever Mechanics: Bones act as levers, joints as fulcrums; muscles provide the force.

Fascicle Arrangement and Force Production

• Parallel Fascicles: Fibers run parallel to the long axis; allow greater range of motion but less force (e.g., sartorius).

• Pennate Fascicles: Fibers attach obliquely to a central tendon; produce more force but less range of motion (e.g., rectus femoris).

Muscle Roles in Movement

• Agonist (Prime Mover): Main muscle responsible for movement (e.g., biceps brachii in elbow flexion).

• Antagonist: Opposes the agonist (e.g., triceps brachii in elbow flexion).

• Synergist: Assists the agonist (e.g., brachialis in elbow flexion).

Clinical Application: Running Injury & Compartment Syndrome

• Compartment Syndrome: Increased pressure within a muscle compartment impairs blood flow and nerve function.

• Affected Tissue: Muscle and connective tissue within the compartment.

• Functional Changes: Weakness, pain, and possible tissue death if untreated.

• Danger of Pressure: High pressure can cause permanent damage to muscles and nerves.

Nervous System: Organization and Cells

Functional Organization

The nervous system coordinates sensory input, integration, and motor output to maintain homeostasis.

1. Sensory Input: Receptors detect changes and send signals to the CNS.

2. CNS Integration: Brain and spinal cord process information.

3. Motor Output: Commands sent to effectors (muscles/glands).

Divisions of the Nervous System

• CNS (Central Nervous System): Brain and spinal cord; integration and processing center.

• PNS (Peripheral Nervous System): Nerves outside the CNS; transmits signals to and from the CNS.

• Somatic Division: Voluntary control of skeletal muscles.

• Autonomic Division: Involuntary control; includes sympathetic ("fight or flight"), parasympathetic ("rest and digest"), and enteric (digestive tract) divisions.

Neurons and Glial Cells

• Neuron Parts:

  • Dendrites: Receive signals.

  • Soma (Cell Body): Contains nucleus and organelles.

  • Axon: Conducts impulses away from soma.

  • Myelin: Insulating sheath that speeds conduction.

  • Axon Terminals: Release neurotransmitters to communicate with other cells.

• Glial Cells:

  • Support: Physical and metabolic support for neurons.

  • Protection: Immune defense (e.g., microglia).

  • Myelination: Oligodendrocytes (CNS) and Schwann cells (PNS) form myelin sheaths.

  • Homeostasis: Maintain extracellular environment (e.g., astrocytes).

Spinal Cord and Spinal Nerves

Protection and Structure

• Vertebral Column: Bony protection for the spinal cord.

• Cerebrospinal Fluid (CSF): Cushions and nourishes the spinal cord.

• Meninges: Three protective layers—dura mater (outer), arachnoid mater (middle), pia mater (inner).

Spinal Cord Anatomy

• Gray Matter (Horns): Central region; contains neuron cell bodies.

• White Matter (Columns): Surrounds gray matter; contains myelinated axons.

Spinal Nerve Pathways

1. Dorsal Sensory Root: Carries sensory input; contains dorsal root ganglion (sensory neuron cell bodies).

2. Ventral Motor Root: Carries motor output.

3. Mixed Spinal Nerve: Formed by merging dorsal and ventral roots; branches into dorsal and ventral rami.

• Plexuses:

  • Cervical: Neck and diaphragm.

  • Brachial: Upper limb.

  • Lumbar: Lower limb (anterior).

  • Sacral: Lower limb (posterior).

Spinal Tracts and Injury

• Ascending (Sensory) Tracts: Carry sensory information to the brain (e.g., spinothalamic tract for pain/temperature).

• Descending (Motor) Tracts: Carry motor commands from the brain (e.g., corticospinal tract for voluntary movement).

• Injury Level: Higher injuries cause more extensive deficits; lower injuries affect fewer functions.

Reflexes: Rapid Responses Supporting Homeostasis

Reflex Arc Components

A reflex is an automatic, rapid response to a stimulus. The basic components are:

1. Receptor: Detects stimulus.

2. Sensory Neuron: Transmits signal to CNS.

3. Integration Center: Processes information (may be a single synapse or multiple).

4. Motor Neuron: Sends command to effector.

5. Effector: Muscle or gland that responds.

Types of Reflexes

• Monosynaptic Reflex: Single synapse between sensory and motor neuron (e.g., stretch reflex).

• Polysynaptic Reflex: Involves interneurons and multiple synapses (e.g., withdrawal reflex).

Examples of Reflexes

• Stretch (Patellar) Reflex: Maintains muscle tone and posture.

• Tendon Reflex: Prevents muscle damage from excessive tension.

• Withdrawal Reflex: Removes body part from painful stimulus.

• Crossed-Extensor Reflex: Maintains balance during withdrawal.

Reciprocal Inhibition: When one muscle contracts, its antagonist is inhibited to allow smooth movement.