Somatic nervous System And Skeletal Muscular Contractions Flashcards
What do sensory neurones do?
- Sense touch, stretch, pain etc.
- Enter spine
- Relay information to spinal cord and brain via ascending tracts
- To somatosensory cortex
- Alternatively reflex arc
What does the ascending track do?
relay information from the spinal cord to the sensory cortex
What does the descending track do?
relay information from the motor cortex to the spinal cord
What is the difference between upper and lower motor neurones?
- Upper motor neurons take message to relevant area of
spinal cord - Lower motor neuron relays nerve impulses from the
spine to trigger contraction of skeletal muscle
What are some properties of motor neurones?
*only one alpha motor neurone
* myelinated
* exit the spine
What makes up a neuromuscular junction?
Synapse somatic motor neurone and a muscle fibre
What happens at the neuromuscular junction?
- Nerve impulse communicates with muscle
- Acetylcholine is the neurotransmitter at skeletal muscle NMJs
- Binds to and activates Nicotinic Acetylcholine Receptor
- Ionotropic (ligand gated ion channel)
types of muscle (picture)
look in folder
go through skeletal muscle structure
.
go through structure of muscle fibre
.
What are the key proteins involved in contractions (in myofibril)
- Myosin
- Actin
- Troponin (binds calcium)
- Tropomyosin (supports actin)
What does the H zone consist of?
myosin only
What does the I zone consist of?
actin only
What does the A band consist of?
entire length of myosin
What is step 1of cross bridge cycling?
ATP bonds to myosin → myosin releases actin
What is step 2 of cross bridge cycling?
Myosin hydrolyzes ATP.
Energy from ATP rotates the
myosin head to the cocked
position. Myosin binds weakly
to actin
What is step 3 of cross bridge cycling?
Power stroke (myosin pushes actin along) begins when tropomyosin moves off the binding site.
Ca²⁺ binds to tropomyosin
What is step 4 of cross bridge cycling?
Myosin releases ADP at the end of the power stroke
What happens during excitation contraction coupling?
- Action Potential arrives at NMJ, MEP depolarized,
- Ca2+ release in muscle fibre causes contraction – Key link
What are the four events at the NM?J
1: resting state
2: AP arrival = release of Ach + depolarisation of pre synapse
3: depolarisation of MEP + more
4: contraction
What happens during stage 3 of NMJ (details)?
*Wave of depolarisation passes down fibre
*pre-synapse repolarises
* N2 channels activated
What happens during stage 4 of NMJ (details)?
muscle fibre depolarises
MEP repolarises
What happens during the latent phase of a twitch after single nerve activation?
Motor end-plate depolarisation
Depolarisation (AP) transmitted down T tubules
Ca2+ channels open in SR
↑ [Ca2+] in the sarcoplasm
Ca2+ binds to troponin revealing myosin binding site on actin
What happens during contraction phase (twitch)?
Myosin binds to actin, moves (powerstroke, ADP ejected),
releases (new ATP binds) and reforms many times causing
sarcomeres to shorten
What happens during relaxation phase (twitch)?
Ca2+ actively transported back into SR
Troponin-tropomyosin complex blocks myosin binding
Muscle fibre lengthens passively (relaxation)
What does a motor unit consist of?
Motor unit = 1 motor neuron &
its muscle fibres
What does the motor unit do?
- 1 motor neuron branches
and contacts several muscle
fibres - The number of muscle fibres
depends on the muscle - Fine motor control requires a
smaller ratio of muscle fibres
to nerve fibres
what are some details on cardiac muscle?
- Only found in the heart
- Striated
- Organized into sarcomere with same banding organization
- Muscle fibres are shorter usually contain only one nucleus
- Connected by Gap junctions
how does cardiac muscle function in contractions?
- Gap Junctions: channels between adjacent cardiac muscle fibres
- allow depolarizing current to flow from one cardiac muscle cell to the next
- quick transmission of action potentials and the coordinated contraction of the entire
heart - contract in a wave-like pattern so that the heart can work as a pump
Describe details of smooth muscle?
- More variable in structure than skeletal
- Don’t have regular sarcomeres / banding pattern
- Located: includes the following
- Blood vessel walls
- Walls of GI tract / associated organs
- Urinary system (walls of bladder and ureters)
- Respiratory system (airway passages)
- Reproductive system (both females and males), and
- Ocular muscles (eye).
describe the structure of smooth muscle fibres
- Lack striations – tissue appears uniform/bright
- Small, spindle-shaped cells with a single nucleus
- Have actin and myosin contractile proteins, and
generate force through thick and thin filaments - Filaments occur in parallel with each other, but run
obliquely - therefore get contraction in different directions
What are the similarities between smooth and skeletal muscle?
- Force - actin - myosin crossbridge / sliding filaments.
- Contraction (cross bridge movements) initiated by an increase in free cytosolic Ca2+
Describe the 6 stages of smooth muscle contraction
- External Ca2+ ions enters cell (opened calcium
channels in the sarcolemma released from SR) - Bind to calmodulin
- Ca2+ / calmodulin complex then activates an
enzyme called myosin (light chain) kinase
(MLCK) - MLCK in turn, activates the myosin heads by
phosphorylating them (converting ATP to ADP
and Pi, with the Pi attaching to the head) - The mylosin heads can then attach to actin –
they slide along thin (actin) filaments. Causes
fibre to contract - Muscle contraction continues until ATP-
dependent calcium pumps actively transport
Ca2+ out of the cell.
* low concentration of calcium remains to maintain
muscle tone. Important around blood vessels
What are the differences between smooth and skeletal muscle?
- Layers of smooth muscle may run in several directions
- Contract and relax much more slowly
- Less energy to generate amount of force
- Some continuously/tonically contracted
- Controlled by the autonomic nervous system
- Most of calcium comes from outside cell
- No T-tubules
- No troponin in actin filaments – calcium binds to calmodulin
Describe structures of smooth muscle and what they do
- Receptors are found all over the cell
surface - Series of neurotransmitter-filled bulges
called varicosities on axon - Forms loose motor units
- Varicosity releases neurotransmitters
into the synaptic cleft. - Bind to receptors on smooth muscle
What triggers a smooth muscle response?
- Neural stimulation by the ANS,
- hormones
- local factors eg stretch receptors
- May have own pacesetter /
pacemaker cells
