Chapter 7

Muscular System

refers ONLY to skeletal muscles

The functions of Muscles

1. Movement

• important in communication (speech,writing)

• breathing, propelling food through digestive tract, pumping blood, dilation, constriction of blood vessles

2. Stability

• Prevent unwanted movement (posture, holding bones in place)

3. Control of body openings & passages

• sphincter muscles: muscles around eyelids; control admission of light into the eye

• regulate waste elimination

4. Heat generation

• necessary for enzymes to function & regulate metabolism

• 30% of body heat is generated at rest

5. Glycemic control

• maintenance of blood glucose levels

• absorbs large shares of glucose

Skeletal Muscle

usually attached to 1 or more bones

1. Voluntary - subject to conscious control

2. Striated - alternating light & dark bands

   1. reflect overlapping arrangement of internal proteins

Why are Skeletal muscles called Muscle Fibers?

Due to their long, slender shape & multiple nuclei

Skeletal Muscles fibers functions

Generate force & produce voluntary body movement, & stabilize joints

Myofibrils

Thick bundles of contractile proteins

Packed b/w are:

• mitochondria, SER

  • Glycogen: carbohydrate

  • Myoglobin: red oxygen-binding pigment

Sarcolemma

Plasma Membrane

• Transverse (T) Tubules: infoldings of sarcolemma

  • penetrate muscle fiber

  • carry electrical current from surface to interior

Sarcoplasmic Reticulum (SR)

SER of muscle fiber

• forms web around myofibrils

• Dilated sacs around T tubules; terminal cisterns

• Releases calcium into cytosol when muscle fiber is stimulated

Myofilaments

Bundles of parallel protein microfilaments

2 main types:

1. Thick filaments: made of several hundred myosin proteins, each with a tail & double globular head

2. Thin filaments: made mostly of two intertwined strands of actin protein

• contains tropomyosin & troponin

Striations

Dark A bands alternating with I bands

I bands:

• bisect by thin dark line, Z disc - a protein providing anchorage for thin filaments

A bands:

Sarcomere: segment of myofibril from one Z disc to the next

• muscle shortening = sarcomere shortening, pulling Z disc closer to each other

I bands

• consists only of thin filaments

• bisect by thin dark line

A bands

• thick & thin filaments overlap

• middle part composed of myosin only

How does skeletal muscle contract?

if stimulated by a nerve

Motor Neurons

Nerve cells stimulate skeletal muscles

• axons of motor neurons; lead to the muscles

• branch to multiple muscle fibers, each muscle fiber recives only 1 nerve fiber

Motor Unit

One motor neuron and all muscle fibers it supplies

• contract at once

• Large motor neurons supply a larger number of muscle
  fibers than small motor neurons

  • need a stronger stimulus to excite them

Neuromuscular junction (NMJ)

Where a nerve fiber meets a muscle fiber

Synapse

Where a nerve fiber meets another cell

Axon Terminal

Bulbous swelling at end of nerve fiber
• Nestles onto muscle fiber
• Separated by tiny gap: synaptic cleft

Synaptic vesicles

Membrane-bounded sacs in synaptic knob
• Contain acetylcholine (ACh), a neurotransmitter
• Released by nerve fiber
• Diffuses across synaptic cleft and binds to ACh
receptors on muscle fiber; this stimulates muscle fibers to contract

Acetylcholinesterase (AChE)

Enzyme found on muscle fiber and synaptic cleft
• Breaks down acetylcholine to stop muscle stimulation

Muscle paralysis
• May be caused by interference with acetylcholine

Excitation

Process of converting an electrical nerve signal to
electrical signal in the muscle fiber

Steps of muscle excitation

1. Arriving nerve signal triggers ACh release

2. ACh binds to receptors on sarcolemma, opens ion channel, & Na+ & K+ ions exchange places

3. Action potentials spread from synapse down muscle fiber

4. Action potentials spread down T tubules; sarcoplasmic reticulum releases calcium

5. Calcium ions bind to troponin

6. Tropomyosin shifts position & exposes active sites

Contraction

The muscle develops tension and may shorten
• The thick and thin filaments slide past each other
• The mechanism of contraction: sliding
filament model

• The link between myosin and actin: cross-
  bridge

• Myosin tugs the thin filament: power stroke

• Recovery stroke – when the myosin head recocks, ready
  to bind actin again

Contracted State

The sarcomere is shorter
• Thick and thin filaments are the same length as at rest
• Filaments overlap more extensively

Relaxation

No more ACh is released when the nerve signal ceases
- ACh already in synapse is broken down by
acetylcholinesterase
• Cessation of muscle fiber stimulation

Antagonistic muscle

Muscles return to their original length by the action of gravity
or by the pull of the muscle

Muscle tone tonus

The nervous system maintains a low level of stimulation
that keeps muscles partially contracted

Muscle Twitch

The minimum contraction exhibited by a muscle; a single
cycle of contraction and relaxation
• Very brief and weak
• Must be added together to do useful work; this is called
summation

• 7 miliseconds

Incomplete Tetanus

a smooth muscle contraction; motor units take turns, work in shifts, to avoid fatigue

Complete Tetanus

a spasmodic muscle contraction where the muscle never relaxes; achieved only in a lab

Isometric Contraction

Contraction of a muscle WITHOUT a change in length
• Muscle produces tension but doesn’t shorten
• A phase of normal muscular action
• Important in stabilizing joints and in maintaining posture

Isotonic Contraction

Muscle shortens with tension

Concentric contraction
• Muscle shortens as it maintains tension
• EX: during ‘bicep curls’

Eccentric contraction
• Muscle lengthens as it maintains tension
• More likely to cause injury

All muscle contraction depends on what?

ATP

2 mechanisms to approve ATP

1. Anaerobic fermentation

Glucose is converted to lactate
• Yields 2 ATP per glucose
• Does not use oxygen; produces ATP when demand for it is
high but oxygen is not available

2. Aerobic fermentation

Intermediate product of glucose metabolism, pyruvate, is
oxidized into carbon dioxide and water
• Yields 30 ATP per glucose
• Requires oxygen, so can’t be used during intense exercise
when the body can’t deliver enough oxygen

phosphagen system

When ATP is used, ADP is produced; cells convert this
ADP back into ATP by adding a phosphate borrowed from
creatine phosphate or other ADP’s

Muscle Fatigue

Progressive weakness from prolonged muscle use

Numerous causes:
• Depletion of ATP and acetylcholine
• Leakage of calcium from sarcoplasmic reticulum
• Accumulation of potassium in extracellular fluid, which reduces
muscle excitability

Endurance

Tolerance of prolonged exercise

Depends on several factors:
• Muscle’s supply of myoglobin and glycogen
• Density of blood capillaries
• Supply of organic nutrients
• Number of mitochondria
• Maximal rate of oxygen uptake, which depends on physical
condition
• Psychological factors

Recovery Period

Elevated rate of increased respiration and heart rate
immediately after exercise

• due to an increased demand for oxygen, called
  excess post-exercise oxygen consumption (EPOC),
  also known as oxygen debt

Slow-twitch Fibers

Long, slow twitches, so are not good for quick responses
• Well adapted for aerobic respiration; do not fatigue easily

• Abundant mitochondria, myoglobin, and capillaries

  • Give tissue red color, so muscles with a lot of them arecalled red muscles

Fast-twitch Fibers

Fast twitches, so well adapted for a quick response
• Fatigue easily
• Have many enzymes for aerobic fermentation &
regenerating ATP from creatine phosphate
• More glycogen but less myoglobin, fewer mitochondria
and capillaries
• Relatively pale in color, so are called white muscles

What are the two types of exercise that increase muscle performance?

Resistance - weightlifting or chin-ups

&

Endurance (aerobic) exercise - running and swimming

Muscle fibers are arranged in bundles called?

Fascicles

In order to cause electrical excitation of the muscle fiber, ACh must do what?

Bind to troponin

Recruiting more and larger motor units for contraction will _______ the strength of muscle contraction.

Increase

Consider the following scenario. The biceps brachii tendon in the anterior elbow region is hit by a reflex hammer and subsequently contracts. This reflex is ______

Monosynaptic

Reflex arcs that utilize an interneuron in the integrating center are always ______

Polysynaptic