Somatic nervous system and muscle contractions Flashcards
What are the different types of muscle?
Cardiac Muscle
-striated and involuntary muscle in the heart
Skeletal Muscle
- striated and voluntary muscle
- connected to bone and helps in movement
Smooth Muscle
-non-striated and involuntary muscle found in lining of hollow visceral organs (e.g. digestive tract, respiratory system, blood vessels, bladder) except the heart
Describe the Overall Muscle Structure.
Muscle Belly Cross-Section
-numerous facsiculi surrounded by perimycium membrane
- each fasciculi contains a bundle of muscle fibres (150 muscle fibres) surrounded by endomycium membrane
- each muscle fibre in the fasciculi is made of 100-1000s of myofibrils which contain sarcomeres, surrounded by a sarcolemma membrane
What is the Sarcolemma?
membrane of myofibrils of skeletal muscle, containing a sarcoplasm surrounding the sarcomeres within the myofibrils
the sarcoplasm contains fat, enzymes, mitochondria, glycogen necessary for muscle function
intertwined between these myofibrils is the sarcoplasmic reticulum where calcium is stored
How do Myofibrils look under a microscope?
Contain sarcomeres (contractile units) with areas of dark signal and light signal:
Anisotropic Band (A-band) >high density; contain thick myosin filaments
Isotropic Band (I-band) -light density; contain thin actin filaments
Describe the Sliding Filament Theory.
actin filaments slide in between thick myosin filaments to shorten the sarcomere and provide a contraction
ATP facilitates the sliding action
Outline the process of Calcium Stimulated Muscle Contraction.
1) Action potential reaches sarcoplasmic reticulum, and calcium released into the sarcoplasm
2) Calcium binds troponin, causing a conformational change in the tropomyosin and exposes the myosin binding sites on the actin
3) Myosin head binds ATP which is hydrolysed by ATPase into ADP and Pi to prime myosin head into a high energy state for binding to myosin binding site on actin filament
4) Using a power stroke, it moves the actin filament towards the centre of the sarcomere for contraction
5) ATP then breaks the bond between actin and myosin head and cycle starts again
What is Rogor Mortis?
The pumps which pump out calcium fail and there is a massive flood of calcium into the cell, causing muscle fibres to contract, and because there is no ATP being formed, the myosin head remains attached to the actin (sustained contraction) which can last for days
-eventually the muscle relaxes as the cells break down
What is the Somatic Nervous System (SNS)?
the part of the peripheral nervous system that controls voluntary movement of skeletal muscles
upper motor neurones from the brain, brainstem and spinal cord synapse with lower motor neurones using glutamate as a neurotransmitter
upper motor neurones from the spinal cord also synapse with individual muscle cells using ACh as the neurotransmitter
What is a motor unit?
A motor neurone and all of the muscle fibres it innervates
How can you regulate strength of muscle contraction?
by changing the amount of motor units activated
What is the Neuromuscular Junction (NMJ)?
point of contact between a lower motor neurone and a skeletal muscle cell:
1) Lower motor neurone synapsed to skeletal muscle and releases acetylcholine (ACh)
2) Action potential moves through plasma membrane into t tubules
3) Stored calcium released from sarcoplasmic reticulum into sarcoplasm
4) Calcium binds troponin, causing tropomyosin conformational change revealing the myosin binding sites
5) Myosin head combines with ATP which is hydrolysed to ADP and Pi, myosin head is cocked/primed and binds myosin binding sites on actin filament, and moves the actin filament towards centre of sarcomere for contraction using power stroke
6) Calcium is then pumped out of the cytosol and back into the sarcoplasmic reticulum
7) ATP then breaks the bond between actin and myosin head and cycle starts again
What are T-tubules (transverse tubules)?
invaginations of the sarcolemma which extend excitation deep into the cell where it communicates and interacts with the sarcoplasmic reticulum (calcium store)
What are the 2 types of neuronal receptors?
Muscarinic Receptors
- primarily in CNS
- G-protein coupled receptors
- slow
Nicotinic Receptors
- neuronal/NMJs
- fast
Describe the Propagating action potential via nicotinic receptor.
2 molecules of Ach bind, resulting in a conformational change in the receptor
ion channel (pore) opens
rapid increase in Na+/Ca2+ and membrane is depolarised leading to muscle contraction
A single action potential will produce a…
single twitch in muscle (lasts 100ms)
-not enough to confer tension to the elastic tendons and bone
Define summation.
when a second action potential arrives right after the first and before the muscle relaxes, there is summation of the two, and there is greater tension applied
What is tetanus?
a continuous contraction that shows no evidence of relaxation
What is Duchenne Muscular Dystrophy (DMD)?
- X-linked degenerative disease
- mutations in gene coding for protein dystrophin
- degradation of structural proteins
- affects both skeletal and cardiac muscle
- unable to walk by 10-12 years; death by 20s due to heart failure
What is the Role of dystrophin protein?
connects actin filaments to the sarcolemma/extracellular matrix for mechanical stability and fibre integrity
What does a lack of dystrophin in DMD cause?
dysfunction of sarcolemma stretch, opening ion pores and resulting in an abnormal increase in intracellular Ca2+
What is the Indication of DMD?
Due to degradation of structural proteins, creatine kinase (required for ATP) is lost from damaged muscle cells into blood, indicating muscle damage
What is Motor Neurones Disease?
group of disorders that selectively affect motor neurones, the neurones that control voluntary muscle activity including speaking, walking and swallowing
What is Amyotrophic Lateral Sclerosis (ALS)?
- Neurodegenerative disease affecting various motor neurones
- Death usually occurs from respiratory failure within 5 years of diagnosis
- Sporadic probably caused by a combination of environmental and genetic factors- largely unknown
What is Myasthenia Gravis?
chronic autoimmune neuromuscular disease caused by antibodies attacking ACh receptors at neuromuscular junctions
antibodies block ACh receptor and increase degradation of receptor, causing impaired signal transduction as motor neurones unable to innervate muscle fibres, leading to muscle weakness
affects younger women (20-30) and older men (50-60)
-affects women more 3:2 (F:M)
