L14 - Muscles Flashcards
Skeletal muscle overview
- Skeletal Muscles are under voluntary control
- The PRIMARY job is to develop FORCE
- This force is mainly used to move and maintain
postural control - Muscles develop force in only 1 direction
- they can “pull” (develop force by shortening,
or by resisting lengthening), not “push”.
Other (secondary) jobs: - Support and protect soft internal organs
- Convert energy (in part) to heat which is used to
maintain core temperature (eg shivering) - Provides a major “store” for energy and protein
overview skeletal muscle anatomy
Muscle fibres are gathered into bundles called
fascicles.
Fascilcles are gathered into bundles called
muscles.
Fibres, fascicles, and muscles are each
ensheathed in connective tissue.
Connective tissue collects together to form
tendons.
Tendons connect muscles to bones.
Myofibres are each a collection of bundles of myofibrils.
Myofibrils are bundles of contractile proteins running the length of the fibre, made up of
myofilaments.
The sarcomeric structure of myofilaments
arises due to the arrangement of the
contractile proteins actin and myosin.
Myosin is the thicker of these filaments and is comprised of heavy and light chains.
Actin is the thinner filament and is wrapped with regulatory proteins.
The flexing of the myosin head causes the thick
and thin filaments to slide past one another
* shortening the distance between sarcomeres
* developing force through contraction
describe muscle activation - neuromuscular junction
Each muscle fibre receives contact from one
myelinated motor neuron axon at one site
to form an excitatory synapse.
This special synapse is the neuromuscular
junction (NMJ).
Each motor neuron axon branches within
the muscle to make contact with many
muscle fibres (dozens to hundreds).
A motor neuron and all the muscle fibres it
controls is called a motor unit. Motor units
can be:
* Small (precise little movements, eg face and hands) – innervating few muscle fibres = small force generation
* Large (big muscles like quads for big movements/lots of force) – innervating many muscle fibres = large force generation
describe the 9 actual events that happen at the neuromuscular junction in order for an AP to be sent (look at diagram in lecture notes)
- Action Potential (AP) arrives at motor axon terminal
- Voltage gated Ca2+ channels open
- Vesicles containing ACh fuse with the axon terminal (botox acts here - prevents release of ACh)
and ACh is released into the NMJ - ACh binds nicotinic-ACh receptors, which are ligand-
gated ion channels - Na+ enters the post-synaptic cell
- Na+ depolarises the muscle cell
- Voltage gated Na+ channels open
- AP propagates across sarcolemma
- ACh is degraded by acetylcholinesterase (Novickoc acts here - nerve agent that blocks the activity of AChesterases)
describe excitation-contraction coupling (by describing the two important structures involved and what they do)
The cell has a system of structures organized to
regulate the activity of the force-producing elements.
Transverse tubules (T-tubules)
* invaginations of the surface membrane
* run from the cell surface deep into the fibre in
a regular array
* conduct electrical signals (action potentials)
deep into the core of the fibre.
Sarcoplasmic reticulum (SR)
* membranous tubular network associated
precisely with the T tubules.
* take up and store Ca2+
* release Ca2+ when AP is conducted along the
associated T-tubules.
describe excitation-contraction coupling (specific steps that lead to cross bridge formation)
Coupling of excitation from the motor neuron
with contraction of muscle
0. AP from the motor neuron causes synaptic
transmission at NMJ to trigger an AP in muscle fibre
1. AP propagates into t-tubules
2. Depolarisation releases Ca2+ from SR
3. Ca2+ binds to troponin to remove blocking of
tropomyosin
4. Myosin heads bind to actin and generate force
5. SERCA pumps Ca2+ back into SR
6. Ca2+ dissociates from troponin, which blocks tropomyosin
describe the proteins involved with cross-bridge formation and what events they result in)
Chains of tropomyosin molecules are arranged along actin filaments
* Partially cover myosin-binding sites on actin
* Tropomyosin is held in place by troponin
Troponin interacts with actin and tropomyosin
Troponin has 3 subunits:
* I = inhibitory
inhibits actin-myosin interaction and connects the troponin subunits
* T = tropomyosin-binding
connects the troponin to tropomyosin
* C = Ca2+ -binding
Ca2+ binding to troponin-C pulls tropomyosin out of the way allowing myosin to bind to actin
what are the stages involved in cross-bridge cycling?
Resting state:
* actin binding sites are blocked by
tropomyosin
* myosin head is energized
1. Myosin head binds to actin
2. Pi detaches and power stroke
occurs
3. ATP binds to myosin head and
myosin detaches from actin
4. ATP is hydrolised to ADP + Pi and
myosin head is energized/cocked
No ATP –> rigor mortis (since ATP needed to unbind myosin head and not present when dead)
describe the length-tension relationship
Optimal force generation requires:
1. Cross-bridge formation
2. Room to move
Too stretched = insufficient cross-
bridge formation.
Too overlapped = no room to move.
Consider resting mandible position
* neither fully occluded nor fully
relaxed.
describe frequency modulation and summation
- The amount of force a fibre can produce
is proportional to the frequency of its
stimulation (up to fibre’s limit) - One stimulus (AP) = short duration
contraction (twitch) - Multiple stimuli in quick succession =
increasing force generation
(summation) - High frequency stimulation results in
fused tetanus (no muscle relaxation) - More Ca2+ availability = more cross-
bridge formation = maximum force
Can end up generating a lot of force if high Ca2+ levels and letting the muscle have a slight rest but not full relaxation before sending another signal
describe recruitment
The amount of force a whole muscle can produce is a function of the force produced by each muscle fibre, AND of the number
of muscle fibres activated
Motor unit size varies from muscle to muscle.
Finer control of muscle tension is possible in muscles with small motor units:
* eye and hand muscles contain small motor units
* quadriceps muscle contains large motor units
NB, muscles with smaller motor units tend to occupy a larger
portion of the primary motor cortex)
The number of fibres activated is regulated by how many neurons are active at one time.
A small number of active neurons tends to produce low force from the muscle, with the amount of force generally increasing as more motor units are activated.
This process of activating more motor units to make more force is called recruitment
Increasing number of motor units activated = increased force generation.
describe muscle fibre types
Muscle fibres can be broadly categorized into types, generally as a function of their relative speed of contraction and of their resistance to fatigue
Muscles of head and jaw may not easily fit into these categories
* Human jaw-closing muscles express myosin isoforms typical
of immature muscle and cardiac muscle
* Many fibres are myosin hybrids (they express more than 1 myosin isoform)
* Type II fibres are often smaller diameter than type I
* Shortening velocities of jaw muscles are often different to limb muscles with similar myosins, probably due to different
myosin light chain isoforms
Types are:
- slow-oxidative fibres (small amounts of force, postural muscles)
- fast-oxidative glycolytic fibres (still uses oxygen to make fuel, but produces more force than the slow-ox fibres)
- fast-glycolytic fibres (for fast, powerful movements, not sustainable, produced lactic acid)
describe how jaw muscles have special features and how this relates to their movements
For such a small number of muscles it is unusual that they can create such a range of movements
- Complex anatomy and fiber layout of muscles allows each muscle to contribute to several functions, depending on which part of the muscle is activated
- Pennate arrangement increases force potential over short distance of contraction due to many fibres packed into smaller cross-sectional area, and allows
for variation in direction of force
