topic 12

Three muscle types

Skeletal, cardiac, smooth

Skeletal muscles:

Where:

Function:

Type of control:

Response:

– Striated muscle attached to bones of skeleton

– Control body movement

– Voluntary control

▪ Responds to somatic innervation, modulated by endocrine control

Cardiac muscle

Where:

Function:

Type of control:

Response:

– Striated muscle found only in heart

– Moves blood through circulatory system

– Involuntary control

▪ Responds to autonomic innervation, spontaneous contraction,

modulated by the endocrine system

Smooth muscle

Where:

Function:

Type of control:

Response:

– Primary muscle of the internal organs and tubes

– Influence movement of material into, out of and within body

– Involuntary control

▪ Responds to autonomic innervation, spontaneous contraction,

modulated by the endocrine system

Organization of muscle tissues from largest to smallest

Muscle → fascicle → muscle fiber → myofibril → sarcomere

Organization of a sarcomere: composed of __ and delineated _.

▪ Composed of overlapping thick and thin filaments

▪ Delineated at both ends by z-discs

Sarcomeres =

Myofilaments arranged in repeating units

Thin and thick filaments made up of

Alternating I-bands and A-bands

I bands

▪ Light-appearing regions that contain only thin filaments

▪ Bisected by Z disc

▪ Get smaller when muscle contracts

A band

▪ Dark-appearing region that contains thick filaments and

overlapping thin filaments

▪ Contains H zone and M line

▪ Makes up central region of sarcomere

Identify

Identify

H zone: central portion of A band that:

▪ Only thick filaments present; no thin filament overlap

▪ Disappears with maximal muscle contraction

Identify

Identify

Identify

Identify

M line:

middle of H zone

▪ Protein meshwork structure

▪ Attachment site for thick filaments

Parts of muscle cell (fiber):

Sarcoplasm And has a typical organelles plus contractile proteins

Sarcolemma (plasma membrane):

▪ Has T-tubules (transverse tubules) that extend deep into cell Containing voltage-sensitive calcium channels

Myofibrils are

(hundreds to thousands per cell)

▪ Bundles of myofilaments enclosed in sarcoplasmic reticulum

Sarcoplasmic reticulum

• Terminal cisternae: blind sacs of sarcoplasmic reticulum

–Serve as reservoirs for calcium ions

–Two cisternae with T-tubule in between = triad

Sarcoplasmic reticulum Contains

calcium pumps that import calcium

Sarcoplasmic reticulum Contains (extended)

▪ Contains calcium release channels

–Triggered by electrical signal traveling down T-tubule

–Calcium released into sarcoplasm

Myofibrils =

Muscle Fiber Contractile Structures

Myofilamemts

Contractile proteins within myofibrils

Thick filaments consist

Consist of bundles of many myosin protein molecules

Thin filaments

Twisted strands of actin with Myosin binding site where myosin heads attach

Thin filaments Regulatory proteins aren

Tropomyosin and troponin

Accessory proteins are

Titian and nebulin

Titin

– Elastic protein

– Stabilizes the position of the contractile elements

– Elasticity returns stretched muscles to their resting length

• Nebulin

– Inelastic protein

– Aligns actin filaments

Organization of myofibrils

Understand

Neuromuscular junction location and parts

• Location where motor neuron innervates muscle

• Has the following parts:

  –Synaptic knob

  –Synaptic cleft

  –Motor end plate

Synaptic knob

– Expanded tip of motor neuron axon

Synaptic knob Houses

synaptic vesicles

▪ Small sacs filled with neurotransmitter Ach

Innervation of skeletal fibers Has

Ca2+ pumps in plasma membrane

▪ Establish calcium gradient (more outside the neuron)

Innervation of skeletal fibers Has

Has voltage-gated Ca2+ channels in membrane

▪ Ca2+ flows into cell if channels open (down concern. Gradient)

Motor end plate

Specialized region of sarcolemma with numerous folds

Motor end plate has many

Has many ACh receptors

Motor end plate ACh receptors are

Plasma membrane protein channels

– Opened by binding of ACh

-Allows influx of Na+ (and efflux of K+)

Synaptic cleft

– Narrow fluid-filled space

– Separates synaptic knob from

motor end plate

Synaptic cleft

Acetylcholinesterase here

▪ Enzyme breaks down ACh molecules

Skeletal muscle fibers at rest

Muscle fibers exhibit resting membrane potential (RMP)

– RMP of muscle cell = ~ −90 mV

▪ Established by leak channels and Na+/K+ pumps

(voltage-gated channels are closed

In the Neuromuscular Junction, calcium enters at

The synaptic knob

When Calcium enters at the synaptic knob the nerve signal

travels down axon and opens voltage-gated Ca2+ channels

When voltage-gated Ca2+ channels open

–Ca2+ diffuses into knob

–Ca2+ binds to synaptic vesicles

Neuromuscular junction: excitation of skel. musc. fibers release

• Release of ACh from synaptic knob

–Exocytosis of ACh at

synaptic knob

Neuromuscular junction: excitation of skel. musc. fibers Bind

• Binding of ACh at motor end plate

–Diffuses across cleft

–Binds to receptors

–Excites fiber

Excitation-Contraction Coupling is stimulation

Stimulation of fiber coupled with sliding of filaments

• Synaptic cleft is a

– Narrow fluid-filled space

– Separates synaptic knob from motor end plate

Acetylcholinesterase is housed in _ and is an _.

Housed in synaptic cleft. Enzyme breaks down ACh molecules

Excitation-Contraction Coupling

–Na+ diffuses into cell through voltage-gated channels

▪ Cell depolarizes:

–Becomes less negative (eventually becomes +30 mV)

–Just after Na+ channels open, they close:

▪Voltage-gated K+ channels open

–K+ diffuses out of cell

–Cell repolarizes:

returns to -90mV

AP travels down T-tubules causing

Causes voltage-sensitive calcium channels in T-

tubule membrane to trigger opening of calcium release

channels in SR terminal cisternae

Release of Ca2+ from sarcoplasmic reticulum

–Ca2+ interacts with myofilaments triggering contraction

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Slide 44

Slide 45

Sarcomere: Crossbridge Cycling

–Cycling continues as long as Ca2+ and ATP present

–Results in sarcomere shortening as Z discs move closer

together

Sarcomere: Crossbridge Cycling

–Narrowing (or disappearance) of H zone and I band

–Thick and thin filaments remain same length

▪ But slide past each other

Events in Skeletal Muscle Relaxation

1. Termination of nerve signal and ACh release from

motor neuron

2. Closure of Ca2+ channels in sarcoplasmic reticulum

▪ Ca2+ returned to sarcoplasmic reticulum by pumps

3. Troponin returns to original shape

▪ Tropomyosin blockade returns

4. Return of muscle to original position due to its elasticity