W6 Somatic nervous system and skeletal muscular contraction Flashcards
What is the Somatic nervous system?
- Part of the peripheral nervous system
- Controls movement of skeletal muscles (voluntary muscles)
What are Ascending tracts?
They relay information from the spinal cord to the sensory cortex
What are Descending tracts?
They relay information from the motor cortex to the spinal cord
What do Sensory neurones detect?
Where do they relay info to?
Where do they enter the spine?
Where is their cell body located?
- Sense touch, stretch, pain etc.
- Relay information to spinal cord and brain via ascending tracts to the somatosensory cortex (reflex arc)
- Enter spine at the dorsal horn, via dorsal root
- Unipolar neurones – cell body is at dorsal root ganglion
- Myelinated
What are the function of Motor Neurones in the somatic NS?
How do they exit the spine?
Features?
- Relay nerve impulses from the spine to trigger contraction of skeletal muscle
- Exit spine via ventral root
- ONE Alpha motor neurone in somatic NS
- Multipolar and myelinated
What is an alpha motor neurone?
-Neurone that extends from the spinal cord all the way to the muscle (only ONE in somatic NS)
-It is myelinated
Differences between SNS and ANS
SNS has one neurone which stretches from spinal cord to muscle and is myelinated (alpha)
Whereas ANS has 2 neurones and are non-myelinated
Neuromuscular Junction
What is the neurotransmitter here and what does it bind to?
- Synapse where somatic motor neurone and a muscle fibre meet (NMJ)
- Nerve impulse communicates with muscle
- Acetylcholine is the neurotransmitter at skeletal muscle NMJs
- Binds to and activates Nicotinic Acetylcholine Receptor
Which is ionotropic (ligand-gated channel) - Post synaptic membrane termed Motor End Plate (MEP)
Structure of a NMJ in SNS
Pre synaptic cell= Alpha motor neurone
Synaptic cleft
Post-synaptic membrane= Motor End Plate (MEP) where cholinergic receptors found called NiAch Receptors
Post-synaptic cell= Muscle
Schwann cell surrounds synapse
What is always the neurotransmitter at a neuromuscular junction
Acetycholine (Ach)
What does a Schwann cell do?
Generate myelin
What is the Motor end plate?
Where cholinergic receptors are found (on the post-synaptic membrane at a NMJ)
e.g. Nicotinic Ach Receptors
Ach Binds to and activates Nicotinic Acetylcholine Receptors, what happens next?
When Ach binds to them, they will open and allow (sodium) ions to flow into the post-synaptic membrane
What is Skeletal muscle? (striated) (3)
- Part of the SNS
- Enables movement of limbs and other parts of the skeleton
- Connected to bone via tendons (origin) or via tendons (insertion)
-Stripy
Contain myofibrils, sarcolemma, sarcoplasm, T-Tubule etc
What is the structute of a muscle fibre? (5)
Myofibril:
Sarcolemma
T-tubule
Sarcoplasmic reticulum
What is Cardiac muscle? (striated)
- The pump in the circulation (heart)
- Functional syncytium
- Intrinsic pacemaker activity
What is Smooth muscle?
- Functional syncytium
- Around many hollow internal organs
What are the names of the plasma membrane and the cytoplasm and the endoplasmic reticulum of a muscle cell called?
Sarcolemma= Plasma membrane
Sarcoplasm= Cytoplasm
Sarcoplasmic reticulum= Smooth Endoplasmic reticulum
What are the 4 key proteins involved in contraction?
Thick filament
1. Myosin
Thin filament:
2. Actin
3. Troponin
4. Tropomyosin
Tropomyosin supports actin
Troponin binds calcium
What is a Sarcomere?
Smallest unit of a muscle cell
Structure of a sarcomere:
H zone- Myosin only
A band- Myosin only
I band-
Z disk- holds actin together
What happens in the sliding filament theory?
H zone gets smaller when a muscle contracts, two regions of actin meet and there is crossover
Actin and myosin slide across each other.
(Z lines get closer to each other)
What happens during Cross bridge cycling (sliding filament theory- 4 steps)?
Myosin is bound to actin (at the start)
1. ATP binds to myosin (myosin releases actin)
2. Myosin hydrolyses ATP (into ADP and Pi). Energy from ATP rotates the myosin head to the cocked position. Myosin binds weakly to actin.
3. Power stroke is initiated/stimulated by Ca2+ signal which is released from sarcoplasmic retic and Ca2+ binds to troponin
Power stroke begins when tropomyosin moves off the binding site.
4. Myosin releases ADP at the end of the power stroke.
‘Myosin walks along actin’
What is Excitation-Contraction Coupling?
What is needed?
What occurs?
How depolarisation leads to contraction.
Needs AP and Calcium
AP triggers calcium release
AP arrives at NMJ
Ach released and binds to NiAch receptors on MEP
MEP is depolarised
T tubules depolarise and open Ca2+ channels on SR
Sarcoplasmic Ca2+ inc
Muscle fibre contracts (sliding filament theory)
Ca2+ is pumped back into SR
Muscle fibre relaxes
What binds to the Myosin head?
2 binding sites
ATP and Actin
Activity at the neuromuscular junction is acetylcholinergic
ACh formed in the synaptic terminal
* Generated by ChAT
* Packaged into vesicles
* Released into the synaptic cleft
* Broken down by Ach-
esterase (into acetic acid and choline)
* Choline taken back up into pre-synaptic cell
Which ions flood in first into a neurone?
Ca2+ ions into pre-synaptic cell and then Na+ ions into post-synaptic cell
What is Botox?
Paralysing agent
Stops muscles from contracting
Developed to stop twitching in eye
Medical uses of Botox
- Severe muscular spasms - Blepharospasm
- Children with cerebral palsy, motor neuron disease
- Speech deficits following throat cancer
- Botulinum toxicity still occurs from preserved foods
bacterial exotoxin - How many poisons work eg snake venom - bungarotoxin
