WEEK 3 Flashcards
What is an exteroreceptor?
A receptor that transduces information we receive from the world around us
What is an interoceptor?
A receptor that transduces information we receive from the world inside us (eg. baroreceptors)
What is the result of a stronger stimulus?
Larger depolarisation, more APs (response is graded by stimulus strength)
Do sensory receptors respond to all stimuli or are they selective?
They are selective transducers that convert energy from specific stimuli to another form of energy
What is sensory modality?
The type of stimulus (eg. touch, taste, smell, sight, balance) which is carried on dedicated pathways called labelled lines-only one type of nerve will take one type of sensation. Can have sub-modalities (sight-colour, distance etc)
What is adequate stimulus?
The type of energy to which a receptor is most sensitive
What are the classifications of sensory receptors?
Mechanoreceptors (mechanical energy) Photoreceptor (electromagnetic, light) Chemoreceptor (chemical) Thermoreceptor (temperature) Nociceptor (noxious-chemical/thermal/mechanical)
How are stimuli detected by receptors?
Region of sensory receptor surface is stimulated which causes a change in the membrane potential of that neurone
What is the role of proprioceptors?
Sense of position
What is receptor adaptation?
The lessoning of receptor potential with maintained stimuli (can be rapidly/slowly adapting)
What are the mechanoreceptors of the somatic sensory system?
Merkel’s receptors (slowly adapting free nerve endings embedded between epithelial cells) and Meissner’s corpuscles (rapidly adapting nerve endings that are coupled to surrounding epithelium by strands of connective tissue)
How do Pacinian corpuscles work?
Pressure applied, concentric rings of connective tissue surrounding neurone are deformed and stretch, activating stretch-mediated Na+ ion channels, generator potential, AP, if stimulus is maintained, concentric rings slide over one another, effectively dampening stimulus energy and activity abates
What is a motor unit?
A somatic efferent (motor neurone) plus all the muscle fibres it supplies (minimal functional unit of motor system)
What are the three types of muscle?
Skeletal (striated, voluntary/neurogenic), Cardiac (striated, involuntary/myogenic) and Smooth (non-striated, involuntary/myogenic) muscle
What does neurogenic mean?
Stimulation by nerves
What does myogenic mean?
Spontaneously discharges electrical potentials causing depolarisation and muscle contractions
Functions of skeletal muscles
Movement
Stability of joints
Posture (muscle tone)
Heat generation (maintenance of body temp)
What is a myocyte?
A single cell of muscle (muscle fibre), formed by myoblast
What is a fasciculus/fascicle?
A collection of myocytes
What is a muscle?
A collection of fasciculi
Connective tissue enveloping of skeletal muscles, sequential ordering
Endomysium-contains myocyte
Perimysium-contains fascicle
Epimysium-contains muscle and neurovascular bundle running parallel to muscle fascicles
What are tendons?
Organised tough bands of fibrous connective tissue mass that forms a point of confluence of contraction (or pull) by single myocytes of a muscle
What makes up myocytes?
Myofibrils
What causes striations?
Dark (A bands-myosin) and light (I bands-actin) bands of tissue of myofibrils
What are the different aspects of a myofibril?
A band-myosin present I band-actin present H band-only myosin present M line-centre of A band Z line-centre of I band
What is the motor neurone composed of?
Upper and lower motor neurone
Where are the cell bodies of upper motor neurones located?
Cerebral cortex
Where are the cell bodies of lower motor neurones (alpha-motor neurones) located?
Ventral horn of spinal cord
Characteristics of the motor unit
Each muscle cell receives motor supply from only 1 motor neurone
Muscle cells randomly distributed throughout muscle thickness (allow contraction generation throughout fullness of the muscle-smooth and powerful movement)
Characteristics of muscle fibres of a motor unit
Same physiological profile (contraction speed and susceptibility to failure), same histochemical profile (myosin fibre typing)
Basis of classification of muscle fibres
How quickly force develops, how long until fatigue/relaxation
What are the 3 classes of muscle fibres?
Fast twitch (type FF)-develop force and fatigue quickly Intermediate twitch (type FR)-develop force and fatigue moderately quickly Slow twitch (type S)-develop force and fatigue slowly
What are tetanic contractions?
Prolonged, precise, smooth contraction involving fusion of many muscle fibre twitchings at high frequency
What factor determines motor unit properties?
Motor neurone (classification of this determines muscle fibre activity)
What is the neuromuscular junction?
A specialised synapse between an alpha-motor neurone and muscle cell
Role of Ca2+ in excitation-contraction coupling
AP in Pre-S terminal, depolarisation opens V-gated Ca2+ channels, Ca2+ diffuses into Pre-S terminal, binds to synaptotagmin causing conformational change-transport and fusion of vesicles with Pre-S membrane, ACh released via exocytosis
What is the transmitter that binds to the motor end-plate?
Acetylcholine (ACh)-binds to nicotinic ACh (ionotropic) receptors with 5 sub-units (2ACh binding sites)
What is a quantal release of neurotransmitter?
One packaged vesicle’s worth of neurotransmitter
Specialisations of NMJ
Pre-S: multiple quanta release
Post-S: junctional folding of motor end-plate, high density of nAChRs and V-gated Na+ channels
What is the resting membrane potential of a muscle cell?
-90mV
Process on end-plate
Em=-90mV, ACh binds to nAChRs, ligand-gated channel opens (permeable to Na+ and K+), Na+ influx»K+ efflux as Ena much further from Em, end-plate potential (EPP)=-20mV
What is the result of the end-plate potential?
AP due to opening of V-gated channels, which decays as it moves away from end-plate as nAChRs absent away from synapse
Why is EPP very large?
Due to many ACh vesicles released/high density of nAChRs, so threshold for AP generation easily passed
What is the process of post-synaptic action potential?
AP travels through T-tubules, depolarisation of sarcolemma, activation of L-type V-gated Ca2+ channels called DHP receptors, causes Ca2+ release channels called Ryanodine receptors in sarcoplasmic reticulum to open and release Ca2+ into sarcoplasm, moves into fluid surrounding myofibril initiating muscle contraction process by binding to troponin in actin filaments, exposing myosin binding sites by moving tropomyosin which normally blocks these sites
Describe the muscle contraction process
active site on actin exposed by Ca2+ binding with troponin, myosin head forms cross-bridge with actin (ATP hydrolysed), power stroke caused by myosin head bending (ADP and P released), new ATP molecule binds to myosin head, causing cross-bridge to detach
What is the fate of ACh?
hydrolysed by acetylcholinesterase (AChE) into acetate and choline, choline reuptaken, used to remake ACh via choline acetyl transferase enzyme
What is Myesthenia Gravis?
autoimmune disease of nAChR, reducing number at NMJ, causing muscle weakness during sustained activity (Ptosis occurs-drooping of eyelid(s))
What is the treatment for Myesthenia Gravis?
AChE inhibitors (AChEIs)-neostigmine
