Microanatomy of Muscles

contraction produces

locomotion, propulsion, and pressure regulation

muscle originates from which layer?

mesoderm

sarcoplasm

cytoplasm containing glycogen and myoglobin

sarcolemma

plasma membrane of the muscle cell

sarcoplasmic reticulum

specialized smooth ER regulating calcium flow

- present in smooth and striated muscle

What kind of muscle is this?

skeletal muscle; striated

What kind of muscle is this?

cardiac muscle; striated

What kind of muscle is this?

smooth; non-striated

myoepithelial cells

contractile non-muscle cells

cardiac myocytes

- cells branch and connect to neighboring cardiomyocytes

- intercalated discs

- striated

- involuntary

smooth muscle myocytes

- fusiform spindle shaped cells

- single elongated central nucleus with blunt ends

- no striations; contain actin and myosin but do not have sarcomeres

- involuntary contraction

skeletal muscle myocytes

- contracts rapidly and is under somatic control

- most skeletal muscles insert on bones

- also forms voluntary sphincters and occurs in tongue and parts of esophagus

skeletal muscle development

- myoblasts align and fuse to form multinucleated myotubes

- myotubes differentiate and develop functional myofilaments

- nuclei become peripheral

- undifferentiated satellite cells persist for repair

skeletal muscle in cross section

fasciles are bounded by perimysium

individual fibers are separated by delicate endomysium

skeletal muscle in longitudinal section

parallel fibers and cross-striations can be identified

myofibrils

cylindrical bundles within the sarcoplasm

myofilaments

contractile and regulatory proteins

actin and myosin

contractile proteins

tropomyosin and troponin

regulate contraction

Myofilaments microscopically

myofilaments can be seen in EM only

Why is skeletal muscle striated?

- ordered actin and myosin create the striated appearance

- A band

- I band

A band

dark, region of thick and thin filament overlap, actin and mysoin

I band

pale, thin actin filaments only

Steps of contraction

1. calcium released 2. calcium binds to troponin

3. tropomyosin moves 4. actin-myosin interaction 5. ATP drives cross bridge cycling 6. sarcomere shortens

actin does what during contraction?

slides over myosin and the I band shortens

Cardiac muscle cell characteristics

- single cells with a central nucleus

- mitochondria may occupy up to 20% of cell volume, reflecting high oxygen demand

intercalated discs

- attach cardiac muscle cells to one another

- contain desmosomes and gap junctions

- provide mechanical strength and electrical coupling

- allow cardiac muscle to function as a synctium

mechanical coupling (intercalated discs)

desmosomes / adherens junctions

electrical coupling (intercalated discs)

gap junctions

SR in smooth muscle

poorly developed

smooth muscle nucleus

single central elongated nucleus

smooth muscle functions

- peristalsis in the GI tract

- vascular tone and regulation of blood flow and pressure (vasodilation and vasoconstriction)

- propulsion in urinary bladder and uterus

- minor secretory role

peristalsis

Involuntary waves of muscle contraction that keep food moving along in one direction through the digestive system.

tunica muscularis

has an inner circular layer and outer longitudinal layer

contractile non muscle cells

myoepithelial cells and myofibroblasts

myoepithelial cells

ectodermal, contain actin/myosin, basket like around exocrine glands

- contract and expel exocrine gland secretions

myofibroblasts

mesodermal and important in would contraction

muscle regeneration

- skeletal muscle has regeneration capacity through satellite cells

- cardiac muscle cells cannot regenerate

- smooth muscle regeneration is limited; scar tissue contributes to repair in all types

satellite cells

lie between basal lamina and sarcolemma and retain mitotic potential

stage of satellite cells from left to right

furthest left: uninjured

middle: repair in progress, proliferating satellite cells

furthest right: repaired

skeletal, cardiac, smooth: branching

skeletal: long cylindrical

cardiac: branching

smooth: fusiform

skeletal, cardiac, smooth: nuclei

skeletal: multiple, peripheral

cardiac: 1-2 central

smooth: 1 central

skeletal, cardiac, smooth: special cue

skeletal: peripheral nuclei

cardiac: intercalated discs

smooth: no striations

skeletal, cardiac, smooth: regeneration

skeletal: some

cardiac: extremely limited

smooth: greater/variable