Physiology Flashcards
State four functions of skeletal muscle
- maintenance of posture - purposeful movement - respiratory movement - heat production
Why is skeletal muscle striated?
Due to the thick bands of myosin and thin bands of actin
What is a motor unit?
All muscle fibres supplied by an alpha motor neuron
What does the number of fibres tell you about the function of the muscle?
fever fibres - more precise movements e.g. eye lots of fibres - stronger more generalised movement e.g. leg
What is the functional unit of a muscle fibre?
Sarcomere
Name the parts of the sarcomere
- sarcomere
- H Zone
- actin
- mysoin
- Z line
- A band
- I band
- M Line
What does gradation of skeletal msucle tension depend on?
- number of muscle fibres
- tension developed by each contracting muscle fibre
How can a stronger contraction be generated?
Summation of twitches through repeated stimulation
Define a twitch
singel contraction produced when sketal muscle is stimulated
How is tension developped?
By a second stimulation before the muscle has completely relaxed.
When is optimum length of skeletal muscle reached?
At rest - maximal tetanic tension can be achieved and there is optimum overlap
How is tension transmitted from muscle to bone?
By stretching and tightening of connective tissue and tendons
Describe isotonic contraction
- body movements and moving objects
- muscle tension remains constant as muscle length changes
Describe isometric contraction
- maintaining posture & holding objects
- muscle tension develops as length stays constant
What happens to the velocity of muscle shortening as the load increases?
Decreases
What are the main differences between different types of muscle?
- Enzymatic pathways for ATP synthesis
- Resistance to fatigue
- Activity of myosin ATPase
How can ATP be supplied?
- energy transfer from creatine phosphate to ADP
- oxidative phosphorylation
- anaerobic glycolysis
Describe type I fibres
prolonged low work activity, aerobic, oxidative phosphorylation & slow speed of contraction
Describe type IIa fibres
Both aerobic and anaerobic useful for prolonged moderate work with fast speed of contraction
Describe type IIb/x fibres
anaerobic metabolism used for short term high intensity activities with fast speed of contraction
Define a reflex
stereotyped response to specific stimuli, in order to maintain optimal muscle length
Describe a reflex pathway
sensory receptor –> muscle spindle –> increases afferent neuron firing –> synapse in spinal cord –> alpha motor neuron –> neuromuscular junction –> contraction
How do muscle spindles increase firing in afferent neurons?
nerve endings on spindle cells (annulospiral fibres) increase firing when there is stretch in the muscle
Where are muscle fibres found?
Intrasfusal fibres in belly of muscle
What neurons supply the muscle spindles?
Gamma motor neurons - adjust tension in order to keep them proportional and sensitive to change during muscle contraction
What neurons supply skeletal muscle?
alpha motor neurons
Where are the cell bodies of alpha motor neurons?
Spinal cord (trunk & limbs)
Brain stem (face)
Describe the structure of the nerve at the muscle fibre
Axon branches into unmyelinated fibres that innervate motor units. Presynaptic terminal (bouton) synapse at endplate region of skeletal muscle
What is the unit of ACh in a vesicle called?
Quantum
Describe vesicle formation
Arrival of action potential
- depolarisation
- opening of calcium voltage gated channels
- calcium enters terminal
- causes exocytosis of vesicle
- ACh diffuses into synaptic cleft to activate ACh receptors in the endplate region
How many molecules of ACh are needed to activate nictoinic ACh receptors?
Two
Describe the structure of a nictonic ACh receptor
five subunits surrounding a central pore - ligand gated channel which allows sodium and potassium to move through
What direction do sodium and potasssium move in the ACh receptor?
Sodium in and Potassium out
When the ions move throught the channel what happens?
depolarisation –> end plate potential –> activation of sodium voltage gated channels –> action potential
Describe the result of the generation of an action potential by ACh
- Action potential moves down transverse tubules of muscle cell
- This triggers calcium release from the lateral sacs of the SR
- Calcium binds to troponin C on actin to uncover cross bridge
- Myosin cross bridges attach and pull the actin towards the centre of the sarcomere by energy supplied by the breakdown of ATP - ADP + Pi
What happens when there is no longer an action potential in skeletal muscle?
Calcium is actively uptaken by the SR allowing actin to slide back into place and bind to troponin - this requires ATP
What happens after the ACh has been used to open the channels?
It unbinds and teh enzyme acetylcholinesterase hydrolyses ACh to acetate (diffuses out the cleft) and choline (back into the nerve fibre to generate more ACH)
Name three types of joint
- fibrous
- cartilaginous
- synovial
Describe fibrous joints
unite bones, do not allow movement e.g. skull
Describe cartilaginous joint
bones united by cartilage allow limited movement e.g intervertebral discs
Describe synovial joints
bones separated by a cavity containing synovial fluid and united by a fibrous capsule, articular surfaces are covered with cartilage
What is the purpose of the inner synovial membrane?
Contains;
- vascular connective tissue with capillary networks
- lymphatics
- synovial cells (fibroblasts) which produce synovial fluid
Explain the difference between a simple and compound joint
Simple - one pair of articular surfaces e.g. fingers
Compound - more than one pair of articular surfaces e.g. elbow
What is the purpose of synovial fluid?
- lubricates joint
- facilitates movement
- minimises wear & tear
- supplies chondrocytes with oxygen & removes carbon dioxide
Describet the viscocity of synovial fluid
Changes during movement usually high due to mucin. Rapid movement is associated with decreased viscocity and increased elasticity
What does synovial fluid look like?
clear & colourless (<200 WBC/mm3). WBC increases in inflammatory & septic arthritis. Turns red in traumatic synovial tap, haemorrhagic arthritis
What is the purpose of articular cartilage?
- low friction lubricated surface
- distributes contact pressure
Describe the structure of articular cartilage
superficial, middle, deep & calcified zones - differ in organisation of collagen & content of cartilage components
What are the three components of the extracellular matrix ?
- Water
- Collagen (type II)
- Proteoglycans
What is the function of water in the ECM?
Maintains resilience & contributes to nutrition & lubrication
What is the function of collagen in the ECM?
maintains architecture, tensile stiffness & strength
What is the main role of proteoglycans and where are they found?
Middle & Deep zones - mainly GAGs. Load bearing
How does the ECM receive nutrients?
Synovial fluid
How is the ECM made?
synthesised, organised, degraded by chondrocytes
What factors contribute to replacement of ECM?
Anabolic Factors
- TGF beta and IGF -I stimulate proteoglycan syntheis and counteract IL-1
What factors contribute to degradation of ECM?
Catabolic Factors
- TNF alpha & IL-1 stimulate proteolytic enzymes (metalloproteinases)
