BIOL 161 Exam 4

Muscular Structure

Types of Muscle Tissue

• Skeletal

  • Voluntary

  • Striated

  • Multinucleate

  • Attached to bone or fascia (connective tissue)

• Cardiac

  • Involuntary

  • Striated

  • Mononucleate

  • Autorhythmic (cells can self-regulate contraction)

• Smooth

  • Involuntary

  • Non-striated

  • Mononucleate

Properties of Muscle Tissue

1. Excitability

   1. Respond to chemicals released from motor neurons

2. Conductivity

   1. Ability to propagate electrical signals over membrane

3. Contractability

   1. Ability to shorten and generate force

4. Extensibility

   1. Ability to be stretched without damaging the tissue

5. Elasticity

   1. Ability to return to original shape after being stretched

Anatomy of skeletal muscle

• Muscle: formed by a group of fascicles

• Fascicle: formed by a group of muscle fibers

• Muscle fibers: formed by a group of myofibrils

• Myofibrils: contain bundles if protein filaments, organized into sarcomeres

Connective tissue

• Protects muscle cells

• Reduces friction

• Creates space for extracellular fluid

1. Epimysium: covers entire muscle

2. Perimysium: covers fascicles

3. Endomysium: covers muscle fibers

Muscle Connections: Muscle to Bone

• Indirect

  • Collagen fibers of epimysium form strong fibrous tendon (dense regular connective tissue) that merges into periosteum (outer covering of bone)

  • Most common connection

  • Ex. biceps brachii, calf muscle

• Direct (fleshy)

  • Collagen fibers of epimysium are directly continuous with periosteum

  • Looks like muscle emerges directly from bone

  • Ex. intercostal muscles (ribs)

Muscle Connections: Muscle to Fascia

• Muscles insert to broad sheet of connective tissue called an aponeurosis: similar in structure as a tendon, but broad and flat

  • Ex. abdominal aponeurosis

Muscle Shape

• Muscle shape is based on the organization of fascicles

• The shape of the muscle affects its properties

• Muscles with more muscle cells can generate more force, while muscles with longer muscle cells can contract further

Muscle Histology

Sarcolemma → muscle cell membrane

Sarcoplasm → cytoplasm of a muscle cell

• Inside sarcoplasm:

  • Myofibrils → long bundles of proteins

  • Myoglobin → stores oxygen for use

  • Glycogen → energy storage

Sarcolemma has tube-like structures which penetrate the interior of the cell → transverse tubules (T-tubules)

• Filled with extracellular fluid

Sarcoplasmic reticulum (SR) → muscle cell ER

• Terminal cisternae → sacs of the SR, closely associate with T-tubules

• Stores calcium

Each muscle cell contains many myofibrils

• Myofibrils are bundles of three types of proteins

  • Thick filaments (myosin)

  • Thin filaments (actin, troponin, tropomyosin)

  • Elastic filaments (titin)

• Filaments are responsible for contraction

• These filaments are organized into functional units called sarcomeres

Thick filaments consist of a polymer of myosin proteins

• A single myosin is shaped like a golf club (the head is mobile and can extend and flex)

• One thick filament consists of hundreds of myosin proteins

• Myosin can bind and hydrolyze ATP

  • The release of energy allows the head to change shape (flex) and ultimately generate force

Thin filaments consist of a polymer of actin with troponin and tropomyosin accessory proteins

• Each actin monomer (yellow ball) has an active site that can bind to the head of a myosin protein (myosin-binding site)

• Tropomyosin (brown stripe) blocks the active site of myosin in relaxed muscles

• Troponin (blue) binds to and regulates tropomyosin 

• In the presence of calcium, troponin moves tropomyosin off active sites of actin

Sarcomere

• Thick and thin filaments overlap each other in a pattern that creates striations

• I band: light region, only thin filaments

• A band: dark region, thick filaments

• Z disks: boundary between sarcomeres

• M line: midline of sarcomere

• H zone: contains thick filaments, or thin filaments

Interaction between thin and thick filaments (with ATP hydrolysis) causes sarcomeres to slide closer to each other. This is the sliding filament model.

• Thin filaments are pulled along thick filaments toward the M-line (middle) of the sarcomere

• Contraction of many sarcomeres, in many myofibrils, in many cells causes the muscle to contract as a whole

The neuromuscular junction (NMJ) is a synapse between a motor neuron and a muscle cell

• The motor endplate is the sarcolemma region associated with the NMJ

• Contains high density of chemically-gated ion channels (ACh receptors)

• ACh=acetylcholine (neurotransmitter)

Muscle contraction consists of four major stages

1. Excitation

   1. Communication between the neuron and muscle cell

   2. Leads to excitation of muscle cell (action potential)

2. Excitation-contraction coupling

   1. Conversion of action potential in muscle cell to activation of proteins in the sarcomere

3. Contraction

   1. Muscles develop tension and may shorten

   2. Sliding filament theory

4. Relaxation

   1. Return of muscle cells to resting length

Excitation

• Action potential from motor neurons reaches end of axon

• Voltage-gated calcium channels open

  • Calcium enters the neuron and causes synaptic vesicles to release ACh into the synapse

• Ach binds to chemically-gated channel in the motor end plate

  • Properly called a cholinergic receptor

  • Non-specific → allows diffusion of both sodium and potassium cations

• Sodium rushes into the cell, sine potassium exits cell

  • More sodium enters the potassium leaves, end result is depolarization

• This is called an end-plate potential (EPP)

• Depolarization from EPPs causes muscles cell to reach threshold leading to action potential

• The action potential involves same voltage-gated channels as neurons

Excitation-Contraction Coupling

• Action potential triggers voltage-gated Ca+ channels in T tubules to open, which are physically connected to mechanically-gated Ca+ channels in SR

• Calcium binds to troponin and causes it to move tropomyosin off the actin myosin-binding sites

Contraction: Step 1

• Myosin hydrolyzes an ATP molecule

  • Produces ADP + P

• Activates the myosin head in an extended position

Contraction: Step 2

• Myosin binds to the actin active site

  • Forms a cross-bridge between actin and myosin

  • ADP + P are still bound to myosin

Contraction: Step 3

• Interaction of cross-bridge causes release of ADP + P 

  • Causes myosin head to flex

  • Thin filament slides past the thick

Contraction: Step 4

• Another ATP molecule binds to myosin, breaking cross-bridge

Sliding Filament Theory

• Cycle of contraction will continue as long as there is enough Ca+ and ATP

• Thin and thick filaments do not become shorter, they just slide past each other

Relaxation

• Action potentials in axon cease 

  • No more Ach released from neuron

• AchE breaks down remaining Ach

  • Breakdown products are transported back to neuron and recycled

• No EPP (or action potentials) are produced in the muscle membrane

• Active transport needed to pump calcium back into SR

  • Calcium pumps use ATP to move calcium into SR

Each muscle cell must be excited by a branch of a motor neuron (muscle cells DO NOT excite each other)

• A motor unit is all the muscle fibers that ONE motor neuron controls

Size of motor units depends on muscle functions

• Muscles that move the eye have many small units, for fine control. One motor unit would contain ~12 muscle cells.

• Deltoid muscle has fewer larger motor units, for powerful but less precise movements. One motor unit would contain hundreds to thousands of muscle cells.

Not all motor units are activated at the same time. Progressive activation allows varying strengths of contraction.

• Contraction of one motor unit: weak contraction of entire muscle

• Contraction of subsequent motor units will increase strength of muscle contraction (recruitment)

Somatic sensory neurons (proprioceptors) and interneurons in the cerebrum inform the cerebellum via action potentials if contraction is strong enough. If not, more muscle cells are recruited.

Task: pick up a glass of milk

Goal: don't throw it over your shoulder

Step 1: plan to pick up glass of milk

• Interneurons from brain initiate action potentials

  • Planning in the motor cortex of the forebrain

  • Special senses (sight, balance)

Step 2: pick up glass of milk

• Proprioceptors (somatic sensory neurons) tell your body where your limbs are (i.e. position)

  • Also responsible for sense for force and heaviness

Step 3: Did you pick up the glass of milk? Do you need more force?

• Proprioceptors tell your body if you have moved the glass of milk

• Information from the somatic sensory proprioceptors will integrate with information from the forebrain in the cerebellum (hindbrain)=coordination

Skeletal muscle contraction (exercise) requires ATP

• Reminder: ATP hydrolysis is required to prepare myosin (extended conformation) to interact with actin

• ATP supply is limited. The body must have a way to regenerate ATP from ADP.

The body’s supply of ATP is limited. How does it regenerate ATP from ADP to sustain contractions?

• Three mechanisms:

  • Aerobic respiration

  • Anaerobic respiration

  • Phosphorylation using creatine phosphate (CP)

• Mechanism depends on type of exercise (muscle contraction)

  • The mechanism the cell uses depends on duration and intensity of the exercise

    • Ex. sprinting (quickly, high force) vs. marathon (slower, less force)