Physiology of nerve and muscle cells Flashcards
What are the 3 types of muscle
- Skeletal muscle
- Cardiac musce
- Smooth muscle
What is resting membrane potential in nerves?
-70mV
What type of channels maintain the membrane potential?
K+ channels - K+ channel at rest is greater than Na+ channel
How do changes in Na and K affect the action potential
Changes in external K affect the resting membrane potential
- If the extracellular concentration of K is increased (hyperkalaemia) the resting potential becomes closer to the threshold potential (less negative as less K flow out of cell) and the neuron becomes more excitable
- If the extracellular level of K is reduced, the membrane potential is reduced and the membrane is hyperpolarised
Changes in external Na will affect the strength of the action potential
- Decreasing the external Na+ reduces the size of the action potential but has little effect on the resting membrane potential (because permeability to Na+ is low
Describe the sequence of a nerve action potential
Depolarising stimulus > voltage gated Na+ channels open > Na+ enters the cell and there is a slow initial depolarisation (approx 15 mV) > reaches threshold potential > rate of depolarisation increases as more Na+ channels open > overshoots isopotential to +35 mV > Na+ influx slows and voltage gated K+ channels open > net movement of positive charge out of the cell > repolarisation > slow return of K+ channels to closed state > small but prolonged overshoot (afterhyperpolarisation)
What is the “all or none” principle
Once threshold intensity is reached, a full-fledged action potential is produced with constant amplitude and form regardless of the strength of the stimulus
If a weak stimulus is applied - it takes longer for the neuron to reach threshold level
If a strong stimulus is applied -there is short time to reach threshold
What is the absolute refractory period?
From moment firing level is reached until repolarisation is about 1/3 complete
What is relative refractory period?
From about 1/3 of repolarisation to the start of after depolarisation
Stronger than normal stimuli can cause excitation of the nerve during this period
What is saltatory conduction?
The way in which an electrical impulse skips from node to node down the length of an axon
What type of nerve fibres are most susceptible to local anaesthetic?
C
Local is important even if you CBA
What type of nerve fibres are most susceptible to hypoxia
B
Hypoxia needs oxygen BAC
Describe the conduction of an action potential
- Nerve cell membrane is polarised at rest
- +ve charges along the outside of the membrane
- -ve charges along the inside of the membrane
- During the action potential, polarity is abolished, + charges from the membrane ahead of and behind the action potential flow into the area of negativity represented by the action potential
- By drawing off positive charges, this decreases the polarity of the membrane ahead of the action potential
- This results in electrotonic depolarisation, which initiates a local response and when the firing level is reached a propogated reponse occurs.
What are type Aa nerve fibres?
Somatic motor and proprioception
Conduction velocity 70-120 m/s
What are type Ab nerve fibres?
Touch, pressure
Conduction velocity 30-70m/s
What are type Ag nerve fibres
Motor to muscle spindles (detect stretch in muscles)
Conduction velocity 15-30 m/s
What are type Ad nerve fibres?
Pain, temperature
Conduction velocity 12-30m/s
What are type B nerve fibres
Preganglionic autonomic
Conduction velocity 3-15 m/s
What are type C nerve fibres?
C-dorsal root - pain and temperature 0.5-2 m/s
C-sympathetic - post ganglionic sympathetic 0/7-2.3 m/s
What are the 3 differnt types of macroglia?
- Oligodendrocytes - emit multiple processes that form myelin on many neighbouring axons
- Schwann cells - form myelin on a single axon
- Astrocytes - found throughout the brain and are sibdivided into fibrous astrocytes (white matter) and protoplasmic astrocytes (gray matter). They produce substances that are tropic to neurons.
What are 4 key features of skeletal muscle?
- Has well developed cross-striations
- Contracts only as a consequence of nervous stimulation
- No functional connections between muscle fibres
- Under voluntary control
What are 3 key features of cardiac muscle?
- It has cross striations
- Does not require external innervation (has pacemaker cells)
- Is functionally syncytial
What are the key features of smooth muscle?
- Lacks cross-striations
- Can be further subdivided into 2 types
- Visceral - found in hollow viscera, contain pacemakers, functionally syncytial
- Multiunit - found in the eye, not spontaneously active
Describe the morphology of skeletal muscle
- Each fibre is a single cell that is multinucleated, long, cylindrical and surrounded by the sarcolemma (cell membrane)
- Muscle fibres are made up of myofibrils that are divisible in to individual filaments.
- Myofilaments contain several proteins that make up the contractile machinery of the cell
What proteins are involved in the contractile mechanism of skeletal muscle?
- Myosin 2
- Actin
- Tropomyosin
- Troponin
Describe the structure of thick filaments
- Made up of myosin 2
- Has 2 heads (made up of light chains) and a long tail (made up of heavy chains)
- The heads form cross-links to actin and contain an actin-binding site and a catalytic site that hydrolyses ATP
- They are arranged symmetrically on either side of the centre of the centre sarcomere
Describe the structure of the thin-filaments
- 2 chains of actin that form a long double helix
- Tropomyosin forms long filaments located in the groove between the 2 chains of actin
-
Troponin molecules are located at intervals along the tropomyosin
- Trop T - binds the other troponin components to tropomyosin
- Trop I - inhibits the interaction of myosin with actin
- Trop C - contains the binding sites for Ca2+ that initiates contraction
What is meant by the term sarcomere?
The area between two adjacent Z lines
Describe the lines and bands seen in a sarcomere
- A band - Centre of the sarcomere, entire length of thick filament, appears darker due to overlap of actin and myosin filaments.
- I band - The peripheries of the sarcomere which appear lighter as actin only is present
- H zone - Light central region in the A band where only myosin is present
- The light I band is divided by the dark Z line
- The dark A band has the light H band at its centre
- An M line is seen in the middle of the H band
(Way to remember: A band = All the myosin, I band - thin, thin filament only, H zone - thick, thick filament only, Z is at the end, M is in the middle)
What happens to the lengths of the different sarcomere bands when the muscle contracts?
- The H zone reduces with contract (myosin filament not overlapping)
- The A band never changes (myosin filament length)
- The I band reduces with contraction (actin filament not overlapping)
- Distance between Z lines and M lines also reduces with contraction
What is the function of actinin, titin and desmin?
Actinin - binds actin to the Z-lines
Titin - connects the Z lines to the M lines
Desmin - binds Z lines to plasma membrane
What is the sarcotubular system?
The sarcotubular system is made up of:
- The T system - Continuus with the sarcolemma, forms a grid perforated by individual muscle fibrils. Allows rapid transmission of the action potential from the cell to the plasma membrane
- The sarcoplasmic reticulum - Important store of Ca2+, forms an irregular curtain around each fibril. Has terminal cisterns in close contact with the T system.