Muscle Flashcards
Divisions of the nervous system
Brain labelled with medulla and hypothalamus
Spinal cord
- Afferent information (information coming in) is sensory, it comes in one of the nerve roots
- Sensory information goes in the dorsal side and goes into the CNS
- Motor commands come out through motor neurons (efferent) on the ventral side, and goes out into the skin, muscles, etc. (viscera of the body)
Peripheral nervous system summary
a) Afferent (sensory)
b) Efferent (motor)
1. Somatic
2. Autonomic (visceral motor)
- Sympathetic
- Parasympathetic
c) Enteric nervous system (gut)
Diagram showing the innervation of the viscera
- The viscera are smooth muscle, cardiac muscle, and glands (not skeletal muscle- that’s somatic motor)
- Parasympathetic on the left. The very end of the spinal chord has neurons that go out of the CNS, controls parasympathetic innervation of the gut
- The sympathetic output from the CNS comes from the middle portion of the spinal chord (in the thoracic (T) and lumbar (L) region
- Some neurons (look at the top of the diagram) come but don’t directly innervate the tissue - it synapses onto a second-order neuron that goes out and innervates the tissue, e.g. the heart and lungs
Ganglia
- Ganglia are the collection of cell bodies in the periphery
- The pre-ganglionic neuron has a cell body in the CNS and it brings an axon to the ganglia
- The post-ganglionic neuron has a cell body in the ganglia and it targets its synapses onto the tissue
- The sympathetic ganglia tend to be a little closer to the spinal cord and the parasympathetic tend to have ganglia that are a little closer to the target tissue
Sympathetic and parasympathetic innervation of tissue
- Some tissue (e.g. the heart) is innervated by both sympathetic and parasympathetic and the effect is antagonistic. Sympathetic innervation of the heart increases the heart rate; parasympathetic slows it down.
- However, some tissue only gets one type of innervation e.g. the smooth muscles in arterioles are only innervated by sympathetic fibers.
Diagram showing detail about effect of parasympathetic and sympathetic responses
Neurotransmitters and receptors for parasympathetic
- The parasympathetic neurons (postganglionic) release the neurotransmitter acetylcholine
- All parasympathetic responses are mediated by muscurinic acetylcholine receptors
Neurotransmitters and receptors for parasympathetic and sympathetic
- Postganglionic neurons in the parasympathetic nervous system release acetylcholine
- The sympathetic release norepinephrine, receptors are adrenergic (epinephrine released by the adrenal gland can also bind to these)
- Different types of adrenergic receptors: alpha, beta 1, beta 2
- The different receptors have a different effect on the tissue
Muscarinic receptor
- G-protein coupled receptor - effect on the cell depends on what protein the receptor interacts with
Comparison of somatic motor NS and autonomic (sympathetic and parasympathetic) NS
- All motor neurons, whether somatic or autonomic, that have a cell body in the CNS, release acetylcholine onto nicotinic acetylcholine receptors
- In the autonomic NS, the cell bodies of post-ganglionic neurons have acetylcholine receptors
- In the sympathetic division, the post-ganglionic neuron releases norepinephrine
- Also, in the sympathetic division, pre-ganglionic neurons release ACH onto glandular cells, which release epinephrine into the bloodstream (which will bind to adrenergic receptors)
- The nicotinic ACH receptors in the ganglia neurons are different than those in the skeletal muscle
More detail on autonomic pathways
What do different receptors respond to?
- They primarily respond to norepinephrine, released by post-ganglionic neurons
- Beta 1 receptors respond primarily to norepinephrine as well, but are also affected by epinephrine (has a slight binding affinity for it)
- Beta 2 receptors primarily respond to epinephrine from the blood
- This distinction is important because the effects mediated by the different receptors will be different, but also, there’s a timing difference. The information coming from the CNS and affecting the target tissue will come in a split second. Any response mediated by the adrenal gland and epinephrine in the blood will take at least a few minutes.
- So it’s a ‘two-pronged’ attack
Summary of information about somatic and autonomic NS
- Axon leaving spinal cord
(Somatic and Preganglionic Autonomic)
ACh and Nicotinic ACh receptors - Postganglionic
- Ach and Muscurinic Ach receptors
- Norepinephrine and Alpha, Beta NE receptors
Heart- parasympathetic and sympathetic responses + type of adrenergic receptor
- Parasympathetic: slows rate
- Sympathetic increases rate and force of contraction with B1 receptors
Arteries and veins- parasympathetic and sympathetic responses + type of adrenergic receptor
- Parasympathetic: ———-
- Sympathetic constricts using a receptors and dilates using B2 receptors
Lungs- parasympathetic and sympathetic responses + type of adrenergic receptor
- Parasympathetic: contracts!
- Sympathetic: relaxes using B2 receptors
GI tract- parasympathetic and sympathetic responses + type of adrenergic receptor
- Parasympathetic: increased motility and secretion
- Sympathetic: decreased motility and secretion using a and B2 receptors
Kidney- parasympathetic and sympathetic responses + type of adrenergic receptor
Sympathetic increases renin secretion with B1 receptors
Ventricles- parasympathetic and sympathetic responses + type of adrenergic receptor
- Parasympathetic: decreased contractility
- Sympathetic: increased contractility using B1 and B2 receptors
SA node- parasympathetic and sympathetic responses + type of adrenergic receptor
- Parasympathetic: decreased heart rate
- Sympathetic: Increased heart rate using B1 receptors
What are the three types of muscle?
What types of muscle are striated?
Skeletal and cardiac
What types of muscle are involuntary?
Cardiac and smooth
Movement of muscles
- Muscles only pull (not push) about joints
- Agonist and antagonist
A muscle as a lever
Agonists vs. antagonists
- Muscles that give you the same movement when you contact are called agonists
- Muscles that oppose that movement (on the other side of the joint) are called antagonists
Macro and micro diagram of muscle
Diagram of sarcomere relaxed and contracted
Diagram of cross-bridge movement
Close up of myosin heads and actin fibers
Cycle of muscle contraction
What is a muscle fiber?
- A muscle cell, but very long and multinucleated
- They run from tendon to tendon
Myofibrils
- Inside a single muscle fiber, there are bundles of contractile filaments called myofibrils
- Here you can see the striations
- Each myofibril has thousands of sarcomeres linked together in a chain
What is the Z-line?
The border between one sarcomere (contractile element) and a neighboring one
How is power of muscle measured?
Number of cross-bridges per second
Structure of myosin heads
- At the top are actin binding sites
- The myosin heads bind ATP (at ATP binding sites) to get energy
- THe light chains allow them to be flexible
Structure of actin filament
- The binding sites (grey circles) are covered by tropomyosin fibers to control when the muscle contracts and when it relaxes
- Troponin molecules control this: they have a binding site for calcium
- Calcium levels inside a cell go up, calcium binds to troponin, this initiates a confirmation change in troponin that twists the tropomyosin away from the binding sites, exposing them to the myosin head groups, so now the myosin heads can bind and power stroke to contract the muscle
- You can twist the tropomyosin away and get some cross-bridges, or all the way
What does the number of cross-bridges per second depend on?
- The amount of calcium available and the degree to which you can twist the tropomyosin away
- The more you stimulate muscle, the higher the concentration of calcium is, and the more cross-bridges are formed
Sarcoplasmic reticulum
- Endoplasmic reticulum of a muscle cell
- Stores calcium in a very high concentration
- Other muscle types can have calcium channels in the membrane that let calcium from the extracellular fluid come in
Where does most of the calcium in skeletal muscle come from?
Most of the calcium that comes into the cytosol comes from the sarcoplasmic reticulum
When does actin have a high affinity for myosin?
When ADP and P are bound, the actin-binding sites are desperate to find an actin to bind to (have a very high affinity)
2D and 3D representation of sarcomeres
How is calcium released inside the cell to trigger contraction? And then how do we control contractions?
- A neuron synapses onto the muscle cell
- This diagram shows skeletal muscle- it only responds when a motor neuron fires action potentials and releases ACH onto nicotinic ACH receptors
- Calcium is stored in the sarcoplasmic reticulum (blue tubes), which is throughout the whole length of the cell
- The SR comes into very close proximity of the T-tubule, which is an invagination of the cell membrane that pokes through the thickness of the muscle cell
T-tubules
- Inside have extracellular fluid, high in sodium chloride, very low in potassium
- There are voltage-gated sodium channels in the membrane of the T-tubules to carry an action potential on the surface of the cell and dived down into the T-tubules, depolarizing the membrane
- The depolarization triggers the release of calcium from the nearby sarcoplasmic reticulum
Mechanism of calcium release
- Depolarization goes down into the T-tubule membrane
- Protein molecules from the membrane of the T tubule and the membrane of the SR are so close to each other that they interact
- The protein inside the SR membrane has a calcium channel
- When the T tubule membrane depolarizes, it shifts the position of the receptor in the outward director, pulling open the gate on the calcium channel, and releasing calcium from inside
How is calcium returned to the SR?
- While calcium is in the cytosol, it can cause cross-bridges to occur, but then it’s transported back to the SR through primary active transport
- This is called sequestering the calcium
- This takes more time than releasing the calcium
What is the other way (other than through calcium concentration) that the nervous system can control the force of muscle contraction?
- By controlling the number of muscle fibers being used
Synapse- how does a motor neuron activate a muscle cell?
- The synapse releases ACH onto nicotinic ACH receptors, opening ion channels
- These channels are permeable to positively-charged ions, so sodium can come in, potassium can go out, but the driving force is the greatest for sodium to come in
- Nearby the receptors, there are voltage-gated sodium channels, and when you depolarize the membrane, they open, and only let sodium come in –> action potential
- The surface of the membrane and the T-tubules have voltage-gated sodium channels all along them
Diagram of relative tension vs. number of action potentials
- The y-axis is relative tension or force, but could also read ‘number of cross-bridges per second’ or ‘concentration of intracellular calcium’, both of which are proportional to the force
