UNIT 9: Synaptic Transmission and Muscle Physiology Flashcards
What is synapses?
These are the junctions between nerve cells that allow information to be transmitted
What is the target cell in the CNS?
a neuron
What is the target cell in the PNS?
Can be either a neuron or an effector cell (such as muscle fiber or gland)
What are the latter synapses?
these are usually called neuromuscular junctions
What do most synapses use?
Chemical transmissions which involve the release of chemical substances from the presynaptic axon terminal
Can some transmissions be electrical?
Yes
What are gap junctions?
specialized connections that allow direct passage of current from one cell to another
What are the types of synapses?
Electrical, chemical, and, neuron-neuron synapses
What are electrical synapses?
it is in smooth and cardiac muscle, between some neurons of the brain, and between glial cells. The cells are joined by gap junctions. Stimulation causes phosphorylation or dephosphorylation of connexin proteins to open or close the channels
What are chemical synapses?
it involves the release of a chemical called a neurotransmitter from the axon’s terminal boutons. The presynaptic and postsynaptic cells are held close together by cell adhesion molecules (CAMs)
What are neuron-neuron synapses?
this usually involves a connection between the axon of one neuron and the dendrites, cell body, or axon of a second neuron.
neuron-neuron synapses are respectively called:
axodendritic, axosomatic, and, axoaxonic synapses
What happens in almost all synapses?
transmission is in one direction only (from the axon of the first neuron [presynaptic] to the second neuron [postsynaptic].
How do synapses in the nervous system occur?
this occurs through the release of chemical neurotransmitters from presynaptic axon endings (“boutons”) that are separated from the postsynaptic cell by a synaptic cleft that is only ~10nm wide
Where are presynaptic boutons located?
small, membrane-enclosed synaptic vesicles
What happens when action potentials arrive at the end of an axon?
This triggers the release of neurotransmitters very quickly
What are voltage-regulated channels?
they open in response to depolarization
what are chemically regulated channels?
they open in response to the binding of postsynaptic receptor proteins to their neurotransmitter ligands
What happens when the opening of ion channels?
produces depolarization (inside the postsynaptic membrane becomes less negative)
What is this method of depolarization called?
excitatory postsynaptic potential (EPSP) [this is because the membrane potential moves toward the threshold required for action potentials.
What happens if hyperpolarization occurs?
the inside of the postsynaptic membrane becomes more negative
What is this method of hyperpolarization called?
this is an inhibitory postsynaptic potential (IPSP) [this is because the membrane potential moves farther away from the threshold depolarization required to produce action potentials
What are the 4 major classes of neurotransmitters?
acetylcholine, bioamines, amino acids, neuropeptide
Where are neurotransmitter receptors located?
the dendrites and the cell body
How is depolarization produced?
This is produced by activating these receptors spread decrementally (decreasing) to the axon hillock, where the first action potentials are produced. After depolarization stimulus (EPSP) causes the opening of voltage-gated channels in the axon hillock, action potentials are then initiated in the axon
What are skeletal muscles?
they are typically attached to bones including the muscles we are most accustomed to: the biceps, triceps, and quadriceps
What are skeletal muscles composed of?
they are composed of longitudinal muscle fibers that are grouped into fascicles
What holds the fascicles together?
A tough connective tissue (epimysium) holds the fascicles together and forms tendons where muscles connect to the bones
What are the fascicles composed of?
they are composed of muscle fibers and are held together by perimysium
What are muscle fibers?
they are cells that have a fused membrane to form a syncytium, or a fused mass of cells that shares continuous cytoplasm
What can be found within these cells?
longitudinally arranged myofibrils that contain the contractile fibers actin and myosin
What is the cytoplasm of muscle fibers known as?
Sarcoplasm
How are cells held together?
held together by a connective tissue called endomysium
What neurotransmitter is released when somatic motor neurons synapse?
they release the neurotransmitter acetylcholine
Where do nerve impulses in somatic motor neurons synapse?
they synapse directly on the sarcolemma (plasma membrane of muscle fibers)
What does acetylcholine do?
it diffuses across the synaptic cleft and binds to nicotinic acetylcholine receptors which will open up Na+ and K+ channels simultaneously and result in muscle fiber depolarization
What does depolarization of the sarcolemma lead to?
this leads to muscular contraction which is termed excitation-contraction coupling
What is a motor unit defined as?
a specific single motor neuron and all the muscle fibers it innervates (all muscle fibers in a motor unit contract at once)
What is the degree of innervation dependent on?
the amount of branching of the synaptic terminals (some only form a few synaptic connections which results in weak, precise movements)
What happens in motor units that supply powerful muscles?
they synapse on thousands of myofibers which will result in very strong contractions
What happens when a motor unit is activated?
depolarization occurs and will activate muscle fibers through an all or none mechanism
What is the neuromuscular junction?
this is the site where a motor neuron stimulates a muscle fiber, while the motor end plate is the area of the muscle fiber sarcolemma where a motor neuron stimulates it using the neurotransmitter acetylcholine
What is the sarcomere?
skeletal muscle that exhibits a striated morphology (striated muscle)
What causes the striated morphology?
the ordered arrangement of myofilaments (thick and thin and produces myofibrils)
What do I bands contain?
only thin filaments that are mainly made up of actin
What do A bands contain?
thick filaments with some thin filament overlap; the thick filament is composed of myosin
What do H bands contain?
they are the center of the A band and have no filament overlap
What do Z discs contain
they are found in the center of each I band
What is the sarcomere composed of
specific overlapping of thick filaments (myosin) and thin filaments (actin) [A and I bands]
What is actin bound to?
z-discs
What are z-discs?
they are a connective tissue anchor and form the z-line (the boundary of the sarcomere)
What is the middle of a sarcomere occupied with?
myosin filaments which are anchored to the z-disc by a microfilament (titin)
Where do I bands occur?
where the actin does not overlap myosin, overlapped areas produce A bands
What is the sliding filament theory of contraction?
the sliding of thick and thin filaments which results in the shortening of the sarcomere (resulting in shortening of the myofibril, muscle fiber, fascicle, and entire muscle)
What happens when a muscle contracts
sarcomeres shorten (A bands move closer together and I bands shorten but thin filaments do not)
What happens to thin filaments when a muscle contracts?
the thin filaments slide toward the H zone which shortens or disappears
What do myosin filaments contain?
globular heads that can change conformation (powered by ATP) and interact with actin filaments
How are myosin head groups connected?
connected to the rest of the myosin filament through a hinged region.
What does the hinge region do?
move the head of the myosin head groups in a certain orientation that will cause the actin filament to slide across the myosin filament to shorten the sarcomere
How many steps are there in the sliding filament theory?
6
What is this cycle also known as?
The cross-bridge cycle (ATP powers the cycle but does not control the contraction. Excitation-contraction coupling is what provides the control of muscle contraction)
What two microfilaments are associated with actin?
tropomyosin and troponin
What is tropomyosin?
filamentous protein that winds along the actin filament and covers up the myosin binding sites
What happens once tropomyosin is activated?
it will undergo a conformational change that will reveal the myosin binding sites due to the attraction between the myosin head and the actin (cross–bridge formation occurs instantly)
What controls tropomyosin?
troponin
What is troponin?
a globular protein that is attached to tropomyosin along its entire length. it contains a high-affinity binding site for Ca2+ (once-bound change in tropomyosin)
What is Ca2+ responsible for?
controlling muscular contraction
What is cardiac muscle?
makes up the myocardium (only found in the heart) it is similar to skeletal muscle and contains striations, composed of the same myofilaments and is controlled by Ca2+ to induce cross-bridge formation to shorten muscle fibers
What is the major difference?
myocardial cells are shorter, highly branched, and form large cellular syncytium through gap junctions
Where are these gap junctions located?
at the long axis of cells and form intercalated discs
Cardiac muscle is…….
entirely involuntary
How is control of contraction measured?
by the left atrium (pacemaker)
