Somatic nervous System And Skeletal Muscular Contractions

Question 1

What do sensory neurones do?

Answer 1

• Sense touch, stretch, pain etc.
• Enter spine
• Relay information to spinal cord and brain via ascending tracts
• To somatosensory cortex
• Alternatively reflex arc

Question 2

What does the ascending track do?

Answer 2

relay information from the spinal cord to the sensory cortex

Question 3

What does the descending track do?

Answer 3

relay information from the motor cortex to the spinal cord

Question 4

What is the difference between upper and lower motor neurones?

Answer 4

• 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

Question 5

What are some properties of motor neurones?

Answer 5

*only one alpha motor neurone
* myelinated
* exit the spine

Question 6

What makes up a neuromuscular junction?

Answer 6

Synapse somatic motor neurone and a muscle fibre

Question 7

What happens at the neuromuscular junction?

Answer 7

• 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)

Question 8

types of muscle (picture)

Answer 8

look in folder

Question 9

go through skeletal muscle structure

Answer 9

.

Question 10

go through structure of muscle fibre

Answer 10

.

Question 11

What are the key proteins involved in contractions (in myofibril)

Answer 11

• Myosin
• Actin
• Troponin (binds calcium)
• Tropomyosin (supports actin)

Question 12

What does the H zone consist of?

Answer 12

myosin only

Question 13

What does the I zone consist of?

Answer 13

actin only

Question 14

What does the A band consist of?

Answer 14

entire length of myosin

Question 15

What is step 1of cross bridge cycling?

Answer 15

ATP bonds to myosin → myosin releases actin

Question 16

What is step 2 of cross bridge cycling?

Answer 16

Myosin hydrolyzes ATP.
Energy from ATP rotates the
myosin head to the cocked
position. Myosin binds weakly
to actin

Question 17

What is step 3 of cross bridge cycling?

Answer 17

Power stroke (myosin pushes actin along) begins when tropomyosin moves off the binding site.

Ca²⁺ binds to tropomyosin

Question 18

What is step 4 of cross bridge cycling?

Answer 18

Myosin releases ADP at the end of the power stroke

Question 19

What happens during excitation contraction coupling?

Answer 19

• Action Potential arrives at NMJ, MEP depolarized,
• Ca2+ release in muscle fibre causes contraction – Key link

Question 20

What are the four events at the NM?J

Answer 20

1: resting state
2: AP arrival = release of Ach + depolarisation of pre synapse
3: depolarisation of MEP + more
4: contraction

Question 21

What happens during stage 3 of NMJ (details)?

Answer 21

*Wave of depolarisation passes down fibre
*pre-synapse repolarises
* N2 channels activated

Question 22

What happens during stage 4 of NMJ (details)?

Answer 22

muscle fibre depolarises
MEP repolarises

Question 23

What happens during the latent phase of a twitch after single nerve activation?

Answer 23

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

Question 24

What happens during contraction phase (twitch)?

Answer 24

Myosin binds to actin, moves (powerstroke, ADP ejected),
releases (new ATP binds) and reforms many times causing
sarcomeres to shorten

Question 25

What happens during relaxation phase (twitch)?

Answer 25

Ca2+ actively transported back into SR Troponin-tropomyosin complex blocks myosin binding Muscle fibre lengthens passively (relaxation)

Question 26

What does a motor unit consist of?

Answer 26

Motor unit = 1 motor neuron & its muscle fibres

Question 27

What does the motor unit do?

Answer 27

* 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

Question 28

what are some details on cardiac muscle?

Answer 28

* 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

Question 29

how does cardiac muscle function in contractions?

Answer 29

* 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

Question 30

Describe details of smooth muscle?

Answer 30

* 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).

Question 31

describe the structure of smooth muscle fibres

Answer 31

* 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

Question 32

What are the similarities between smooth and skeletal muscle?

Answer 32

* Force - actin - myosin crossbridge / sliding filaments. * Contraction (cross bridge movements) initiated by an increase in free cytosolic Ca2+

Question 33

Describe the 6 stages of smooth muscle contraction

Answer 33

1. External Ca2+ ions enters cell (opened calcium channels in the sarcolemma released from SR) 2. Bind to calmodulin 3. Ca2+ / calmodulin complex then activates an enzyme called myosin (light chain) kinase (MLCK) 4. MLCK in turn, activates the myosin heads by phosphorylating them (converting ATP to ADP and Pi, with the Pi attaching to the head) 5. The mylosin heads can then attach to actin – they slide along thin (actin) filaments. Causes fibre to contract 6. 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

Question 33

What are the differences between smooth and skeletal muscle?

Answer 33

* 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

Question 34

Describe structures of smooth muscle and what they do

Answer 34

* 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

Question 35

What triggers a smooth muscle response?

Answer 35

* Neural stimulation by the ANS, * hormones * local factors eg stretch receptors * May have own pacesetter / pacemaker cells