Lecture 9: Muscle Anatomy and Physiology Part 1

Question 0

Motor units

Answer 0

A motor unit consists of a single motor neuron and the muscle fibers it controls

A motor unit controls only a few muscle fibers in an entire muscle

Large muscles have more motor units than do smaller muscles

Each muscle fiber obeys the all or none principle. A muscle fiber contracts completely or not at all

When a motor unit is stimulated all of muscle fibers under its control will contract

Question 1

Properties of muscle tissue

Answer 1

Four unique characteristics of muscle tissue:

1. Excitability: outside stimuli can initiate electrical changes in the muscle fiber (cell) leading to contraction of that muscle fiber (cell)
2. Contractility: stimulation of muscle fiber can lead to contraction or shortening of the muscle fiber
3. Elasticity: a muscle fiber’s ability to return to its original length when the tension of contraction is released
4. Extensibility: the ability of a muscle fiber to be stretched beyond its relaxed state

Question 2

Properties of whole muscles

Answer 2

Not all muscle cells in a muscle contract at the same time (only the ones activated by the same neuron)

The number of motor units that are activated determines the strength of the contraction small number of units = weak contraction; large number of units at greater frequency = stronger contraction

Muscle tone is the continued steady low level of contraction that stabilizes joints and maintains general muscle health

Question 3

Dystonia

Answer 3

Disorder characterized by involuntary sustained muscle contractions resulting in twisting and repetitive movements or abnormal postures

Question 4

Components of the NMJ

Answer 4

Synaptic knob: expanded end of the neuron

Synaptic vesicles: membrane bound sacs filled acetylcholine

Motor end plate: region of sarcolemma across from the synaptic knob that has folds and indentations to increase the surface area in that region

Synaptic cleft: narrow space separating the synaptic knob from the motor end plate

ACh receptors: in the motor end plate that bind to ACh

Acetylcholinesterase (AChE): an enzyme in the synaptic cleft that rapidly breaks down ACh

Question 5

Steps of propagation

Answer 5

1. Motor neuron action potential
2. Ca2+ enters voltage gated channels
3. Acetylcholine release
4. ACh binding opens ion channels
5. Na+ entry
6. Local current between depolarized end plate and adjacent muscle plasma membrane
7. Muscle fiber action potential initiation
8. Propagated action potential in muscle plasma membrane
9. Acetylcholine degradation

Question 6

Muscle fiber terminology

Answer 6

Cytoplasm = sarcoplasam
Plasma membrane = sarcolemma
Smooth ER = sarcoplasamic reticulum

Question 7

Excitation contraction coupling

Answer 7

1. Somatic motor neuron releases ACh at NMJ
2. Net entry of Na+ through ACh receptor channel initiates a muscle potential
3. Action potential in t-tubules alters confirmation of DHP receptor
4. DHP receptor opens ryanodine receptor Ca2+ release channels in sacrcoplasmic reticulum and Ca2+ enters cytoplasm
5. Ca2+ binds to troponin allowing actin-myosin binding
6. Myosin heads execute power stroke
7. Actin filament slide toward center of sarcomere

Question 8

Multiminicore disease

Answer 8

Disorder that primarily affects muscles used for movement (skeletal muscle) this condition causes muscle weakness and related health problems that range form milf to life threatening

Mutation in the RYR1 and SEPN1 genes cause multiminicore disease

RYR1 mutations are also associated with an increased risk of malignant hyperthermia

Question 9

NMJ disorders

Answer 9

Myasthenia Gravis
Myasthenic syndrome (eaton lambert disease)
Muscular dystrophy (such be muscular dystrophy)
Mystonias
MS
Guilloan-Barre (acute idiopathic polyneuritis)
Amyotrophic lateral sclerosis (als)

Question 10

Myasthenia Gravis

Answer 10

Post junctional alteration
Auto immune response to ACh receptors 
Onset pharyngeal and ocular weakness 
Worsens with exercise 
Treatment: anticholinestrase

Question 11

Myasthenic syndrome/ eaton lambert syndrome

Answer 11

Pre-junctional alteration 
Decreased ACh release
Peripheral and pelvic muscles affected
Improves with exercise 
Do not improve with anticholinestrases

Question 12

Smooth muscle contraction

Answer 12

1. Intracellular Ca2+ concentrations increases when Ca2+ enters cell and is released from sarcoplasamic recticulum
2. Ca2+ binds to calmodulin (CaM)
3. Ca2+ calmodulin activates myosin light chain kinase (MLCK)
4. MLCK phosphorylates light chains in myosin heads and increases myosin ATPase activity
5. Active myosin crossbridges slide along actin and create tension

Question 13

Relaxation in Smooth Muscle

Answer 13

1. Free Ca2+ in cytosol decrease when Ca2+ is pumped out of the cell or back into the sarcoplasamic reticulum
2. Ca2+ unbinds from calmodulin
3. Myosin phosphatase removes phosphate from myosin which decreases myosin ATPase activity
4. Less myosin ATPase results in decreased muscle tension

Question 14

Myofibrils

Answer 14

The sarcoplasam of a a muscle fiber contains 100-1000 of cylindrical structures that extend the entire length of the cell- myofibrils

Myofibrils have the ability to shorten resulting in contraction of the muscle and the production of motion

Question 15

Myofilaments

Answer 15

Myofibrils are composed of short bundles of myofilaments

Myofilaments do not run the entire length of the muscle fiber but are organized into repetitive groupings

Myofilaments are of two types:

1. Thin filaments: actin and associated proteins
2. Thick filaments: myosin

Question 16

Thin filaments

Answer 16

5-6nm
Comprised of two strands ( f-actin or filamentous actin) of bead-shaped (G-actin or globular actin) molecules twisted around each other
Two regulatory proteins part of thin filaments:
1. Tropomyosin
2. Troponin

Question 17

Thick filaments

Answer 17

11nm in diameter (twice as thick as thin filaments)
Composed of bundled molecules of myosin
Myosin molecule has a head and elongated tail
The heads form crossbridges with thin filaments during contraction

Question 18

Organization of a sarcomere

Answer 18

The sarcomere is the functional contractile unit in a skeletal muscle fiber

Defined by the are between 2 adjacent Z discs

Myofibrils contain multiple and repeating sarcomeres

Each sarcomere shortens as the muscle fiber contracts

Question 19

Overview Muscle contraction

Answer 19

Muscle fibers shorten by the interaction between thin and thick filaments within each sarcomere

The mechanism for contraction is explained by the sliding filament theory

Question 20

Sliding filament theory

Answer 20

Thin filaments slide across thick filaments towards the center of the sarcomere resulting in shortening of the H zone and I zone. A band does not change. The z lines move closer together

Question 21

Molecular basis of contraction

Answer 21

1. Ca2+ levels rise in cytosol 
Actin binding 
2. Power stroke 
3. aDP release 
4. ATP binding and cross bridge detachment 
5. ATP hydrolysis

Question 22

Muscle contraction

Answer 22

Muscle tension: force created by muscle 
Load: weight that opposes contraction 
Contraction: creation of tension in muscle 
Relaxation: release of tension 
Steps leading to muscle contraction: 
Events at the NMJ
Excitation- contraction coupling
Contraction-relaxation cycle