7 Muscle, ~8 Nerve

• Muscle Cells (Myocytes, Myofibers - implies the cells are very long)
• Stem (Satellite) Cells - for regeneration + renewal
• Nerve Supply
• somatic motor nerves for voluntary muscles
• visceral motor nerves for involuntary muscles (can be sympathetic or parasympathetic)
• sensory (protection + proprioception: so you know where in space your extremities are)
• al muscle cells have excitable plasma membranes (means they’re capable of conveying APs, important in contraction)
• connective tissues
• blood vessels (large tissues so they have their own blood supply)
• lymphatics

1. Myoepithelial Cells: epithelial cells that contain contractile proteins; are abundant in glands & help them contract & release their product

2. Myofibroblasts: fibroblasts differentiate into myofibroblasts (can contract) during wound healing to help close wounds

• 1. Skeletal (Striated): voluntary muscles associated with skeletal movement (biceps, trapezius, deltoid - generally attached to bones)
• 2. Visceral (Striated): voluntary visceral muscles (upper esophagus, pharynx, tongue - don’t attach to bones)
• 3. Cardiac (Striated): under only autonomic (symp/parasymp) control (no voluntary control)
• 4. Smooth (NON-Striated): under autonomic control; wide distribution (gut, bronchioles, BVs, ureter, gall bladder, etc. - any place where there’s a tube that constricts)

• Skeletal: skeletal movement; cross & activate JOINTS
• Visceral: DON'T connect bone to bone
• in both, striations run perpendicular to cell line

• it's referring to muscle
• myofiber = myocyte = muscle cell
• mysium means flesh

term used to describe a muscle cell’s plasma membrane & it’s external lamina (equivalent to the basal lamina)

cytoplasm of a muscle cell

• smooth ER of a muscle cell
• important for calcium storage

• run the full length of the muscle
• are unbranched, striated & multinucleated (needed because of how long they are)
• controlled voluntarily
• has a continuous external lamina; because of that there are no places where the plasma membranes of different cells meet ~ therefore there are no cell-cell gap junctions
• UNLIKE cardiac/smooth muscle, the whole skeletal muscle cell is insulated from it's surroundings

• Lower Pharynx & Diaphragm are made up of skeletal muscle but are involuntary
• Diaphragm contracts on its own via the autonomic nervous system

• 1. EPImysium surrounds the entire muscle
• 2. PERImysium surrounds each fascicle (group of cells)
• 3. ENDOmysium surrounds an individual muscle fiber/cell
• *ALL THESE CT ELEMENTS COME TOGETHER AT THE TENDON (which attach muscle to bone)*

• a way to describe the multinucleated muscle fibers
• a syncytium is a multinucleated cell that results from multiple cell fusions of uninuclear cells - this is what happens during muscle cell development

• because skeletal muscle is so jam-packed with contractile proteins, nuclei get PUSHED out to the periphery
• spaces between the cells correspond to ENDOmesium (connective tissue)
• can see muscle cell nuclei (& maybe fibroblast nuclei) near the endomesium because they’re pushed to the fiber periphery
• cytoplasm of 1 muscle cell has “stippled” appearance because it’s packed with myofibrils

• visually, are the dark & light regions that identify striated muscle as 'striated'
• myofibrils themselves contain contractile proteins called myofilaments
• myofilaments consist of repeating sarcomeres that are in sync with adjacent sarcomeres

the muscle fiber cytoplasm (sarcoplasm), which itself contains cell organelles (sarcoplasmic recticulum, mitochondria, lysosomes, etc.)

• sarcomere exists from the Z (dark) line in one thin actin filament to the Z line in the next
• Z line: serves as an anchoring point for thin actin filaments (Z disk)
• I band: thin actin filament bisected by the Z line; light staining
• A band: thick myosin filaments; dark staining
• M line: line in the middle of the A (thick myosin) band that anchors the myosin
• H band: slightly lighter area just around the M line in the A (myosin) band where you only have only myosin (think) filaments, no actin (thin) filaments
• the bubbly/dotty stuff between myofibril striation = cell organelles

• when muscles contract the SARCOMERE shortens
• occurs by a sliding filament mechanism
• the Z-lines MOVE closer together
• neither the I nor A bands shorten (Z within the I just moves)
• H band (surrounding M line) disappears - is filled in with thin filaments that move toward the M line
• 
• I (actin thin) & H bands narrow; A (myosin thick) band width doesn’t change

• protein located on the surface of actin (thin, I filament)
• in the absence of calcium, it - via the troponin complex - BLOCKS the active sites of actin
• no calcium → actin can’t bind to myosin heads
• when calcium is present, it changes tropomyosin's conformation → actin & myosin can now interact (+ ATP) & provide muscle contraction
• control of muscle contraction has a lot to do with calcium

• neuron meets muscle at motor endoplate, aka neuromuscular junction
• action potential impulse can be transferred to surface of muscle cell, aka the sarcolemma

• *T tubule: indentation of plasma membrane that runs DEEP into the muscle (surrounds myofibrils)
• cisterns of smooth ER also surround T tubules & they're filled with CALCIUM (structure is overall called a triad)
• action potential → T tubule → sarcoplasmic reticulum → voltage gated calcium channels open → calcium floods sarcoplasm (relieving inhibition of myosin/actin interaction via Tropomyosin & the troponin complex)

• connects actin cytoskeleton (thin filaments) to cell's plasma membrane which is itself linked to the external lamina (CT) by another dystrophin-associated complex
• it links cell contractile elements to the surrounding connective tissue to mobilize muscles connected to tendon
• 
• located around cell periphery (brown staining)

• LOSE the ability to transfer mechanical force to move muscles (the cytoskeletal contractile elements are NOT connected to the external lamina CT)
• hypercalcemia also results, leading to increased osmosis → mitochondrial rupture
• 

• motor & sensory components
• 1. muscle fiber types
• 2. motor unit density
• 3. sensory fibers (propioception)

1. RED (slow twitch): many mitochondria & oxidative enzymes (AERobic); don't contract fast but CAN contract for long periods without fatiguing; eg. back muscles

2. WHITE (fast twitch): bigger muscle cells, contract quickly; fewer mitochondria & oxidative enzymes (ANaerobic); quicker to fatigue; eg. quadraceps

Coarse Control: a single neuron controls many muscle fibers (neuron can split ~1,000 times) → LESS control

Fine Control: a single neuron exerts control on only a few muscle fibers (splits maybe 3-4 times) → MORE control

• provide proprioceptive information because they're made up of muscle cells AND nerves
• eg. muscle spindle & golgi tendon organs

• a combination of muscle fibers & afferent (sensory) nerves that provide proprioceptive information (the extent to which muscles are extended or relaxed) & mediate the stretch reflex
• if a muscle stretches really quickly the muscle spindle can stimulate a contractile reflex to shorten the muscle (what you do when you test reflexes)
• 

• 
• located in the TENDON & provide sensory information about how much a muscle is being stretched
• involved in both proprioception AND inhibition reflex - protects muscle from OVER-stretching

reflex activated by golgi tendon organ that causes a overstretched muscle about to tear from the tendon to relax

• involuntary striated muscle only found in the heart controlled autonomically
• sympathetic speeds up heart beat
• parasympathetic slows heart down
• consists of relatively short cells connected by end-to-end junctions
• has a DISCONTINUOUS external lamina - there are gap junctions between cells
• cells are BInucleate
• CT = endocardium & epicardium
• 

• specialized end-to-end junctions between individual cardiac muscle cell
•  
• little dark lines in between the ends of cells
• can see BRANCHING
• can see centered nuclei (unlike skeletal, whose nuclei are pushed to the periphery)

A: Intercalated Disc (~Fascia Adherens), which is analagous to the zonula adherens; exists perpendicular to long line of cell & provides STRENGTH

B: on the lateral surface that’s parallel to the long line of the cell is where most cell-cell communication (gap junctions) occur; this is where the external lamina is DISCONTINUOUS for COMMUNICATION

desmosomes exist in BOTH places

the parallel gap junctions

• called a diad (sarcoplasmic reticulum + T tubule because there's less smooth ER surrounding T tubules (plasma membrane invaginations)
• LESS efficient than skeletal muscle but that's okay because cardiac muscle has GAP JUNCTIONS for communication
• it can also spontaneously contract

• 
• specialized myocardial (muscle) cells that can contract & secrete ANF (Atrial Natriuretic Factor) to surrounding cardiac cells (small black dots = granules)

hormone released by endocrine cells in cardiac muscle that acts as a vasoDilator in response to ↑ BP, ↑ Na+, & Angiotensin II (a vasoconstrictor)

• 
• specialized myocardial cells that can contract & conduct impulses/signals (contract so they do contain myofibrils)
• exist just deep of the endocardial surface near lumen of ventricles where blood is
• are rich in glycogen (cells look filled with space)

• 
• non-striated
• involuntary (under autonomic control)
• discontinuous external lamina
• fewer CT investments (CT = epimysium & endomysium only)
• many gap junctions
• found throughout hollow organs of body (eg. GI tract, blood vessels, bladder, & arrector pili in skin)
• often see both longitudinal and cross-sectional view in the same section
• lacks true sarcomeres

• dense body
• smooth muscle cells have similar contractile units as skeletal/cardiac striated
• except instead of being anchored on a Z line, smooth muscle contractile elements are anchored on spots known as dense bodies

• 1. electrically: via gated Ca2+ channels
• used by single (common BVs, GI tract) & multiunit (restricted trachea, large elastic arteries, iris) smooth muscle
• -atypical neuromuscular junction: neurotransmitters released at 10- 200μ from muscle = slow excitation

2. mechanically: receptors in smooth muscle that facilitate stretching; stimulates mechano-sensitive ion channels ("myogenic" response) - NOT a nerve response

3. hormonally: form a synapse-like structure (eg. oxytocin stimulates uterine contractions

• indentation in smooth muscle cells which allow propagation of nerve impulses and Ca 2+ activated channels throughout a smooth muscle
• analogous to T tubules in striated muscle

benign tumors of smooth muscle

satellite (stem) cells reside in external lamina; activated in damage or injury, proliferate, and replace damaged muscle cells

satellite cells are stimulated to proliferate in response to disease (eg. muscular dystrophy) or damage (eg. small tears from excessive weight training)

• weight lifting causes hypertrophy, not hyperplasia
• you're increasing cells' SIZE, not their number

• satellite cell replacement as well as limited mitosis
• most repair is fibrotic - fibrous scar that can hinder how the heart beats
• limited though: most you start with is what you end with

actively proliferates in throughout life in response to damage or physiological needs (e.g. uterine expansion during pregnancy)

• afferent: TO the cell body (dendrites)
• efferent: AWAY from the cell body (axons

• Motor Nerve: single neuron, whose cell body lies in the CNS
• Sensory Nerve: single neuron, whose cell body lies in the dorsal root ganglion
• Autonomic Nerve: two neurons; 1st neuron’s cell body is in the CNS, it synapses with a second neuron cell body in the autonomic (motor) ganglion, that 2nd neuron goes on to innervate muscle
• any nerve in the body are MIXED
• can distinguish Motor & Sensory nerves from Autonomic ones because they’re myelinated while Autonomic nerves are not

axon → myelin → endoneurium / Schwann cell cytoplasm (neurilemma)

• multiple axons will NOT be myelinated by a single Schwann cell
• however because neuron axons are so long, it takes multiple Schwann cells to myelinate the entire length of one axon

• microtubules
• intermediate filaments (neurofilaments)
• mitochondria
• [NO mention of microfilaments]

• residual protein components after the lipid parts have leeched out
• when you look at preps you don’t necessarily only see only an empty space where the myelin has been leeched out, but you also see wispy material that represents leftover protein from the neurilemma (Schwann cell cytoplasm)
• 
• wispy material surrounding the axon where the myelin used to be

• junction between 2 myelinating Schwann cells
• only place where there’s depolarization (saltatory conduction)
• have myelin on Sensory & Somatic Motor neurons because you want those pathways to be as FAST as possible
• 

• gap that exists in the myelin where the Schwann cell neurilemma (plasma membrane) & axon are connected (communicate)
• Schwann cells don’t JUST myelinate the axon, they’re also a source of support for it
• communication between the Schwann cell & axon are called Schmidt-Lanterman Incisures
• 
• appear at oblique angle
• there’s a continuous cytoplasmic pathways from the Schwann cell all the way to the axon
• can also see it as a “gap” between circles of myelin around an axon
• 

clusters of neuron cell bodies in the peripheral nervous system

• dorsal root ganglion will contain only a cell body, NO synapsing
• pseudounipolar neurons
• will have pretty complete ring of satellite cells surrounding neuron cell bodies - complete because sensory neurons don’t have any dendrites that can extend out & disrupt that ring of satellite cells
• central nucleus
• 

• enteric looks markedly different from symp/parasymp
• will contain synapsing
• multipolar neurons (have a ton of processes that STICK OUT)
• have many dendrites present, which disrupts the ring of satellite cells
• eccentric nucleus
• 

• see neurons between 2 layers of smooth muscle cells running up/down & into/out of the plane
• this arrangement of smooth muscle is characteristic of peristalsis
•