BIO 273 UNIT 3 NOTES
3 types of muscle
No spaces between fibers
Only contract in response to somatic motor neurons
40% of body weight
Inside muscle, there are fascicles. Inside fascicle there are fibers. Inside fascicle there are
Membrane of muscle fiber is called sarcolemma
Sarcoplasmic reticulum contain terminal cisternae, which release ca2+ ions
Terminal cisternae are adjacent to t-tubules.
1 t-tubule and the 2 terminal cisternaes on each side is called a triad
Lumen of t-tubules contain extracellular fluid. (t-tubule = transverse tubule)
t-tubules move action potentials from outside of cell to inside of cell
Contain intercalated disks
Spaces between fibers
Autonomic and spontaneous contraction
No space between fibers
Autonomic and spontaneous contraction
Myofibrils are the structures that contract in muscle.
Myosin is the motor protein that creates movement in the muscle. 250 myosin molecules join to create the
Actin creates the thick filament. 1 actin molecule is a globular protein.
Under the light microscope, repetitions of thick and thin filaments create the repeating pattern of light and
dark bands. One repeat forms the sarcomere.
Z discs- 2 of them in 1 sarcomere. They are attachments for actin.
I band- lightest colour bands of the sarcomere, and represent a region that consists of only thin filaments.
1 z disc runs through every middle of 1 I band.
A band- darkest band of the sarcomere, and includes the entire length of the thick filaments. Edges
contain both thick and thin filaments.
H zone- middle part of A band, is lighter than a band because it contains only thick filaments.
M line- divides a band in half. Contains thick filaments with accessory pigments.
Titin is the largest protein. Contains more than 25000 amino acids, and a single molecule extends from
the z disc to the m line.
Nebulin helps align.
Titan and nebulin are gigantic accessory pigments.
Calcium binds to troponin which rotates tropomyosin, exposing the actin filaments to the myosin heads.
Actin filaments move towards the m line. Sliding filament theory process
1. Acetylcholine is released by the somatic motor neuron at the neuromuscular junction
2. Ach causes Na to enter the motor end plate, which sends an AP down the t-tubules.
3. AP changes the DHP receptor
4. DHP receptor opens Ca2+ release channels (aka RyR) in sarcoplasmic reticulum to allow the
entry of Ca2+ into the cytoplasm.
5. Ca2+ binds to troponin
6. Tropomyosin rotates, exposing actin filaments.
7. Myosin binds and pulls actin filaments towards the M line, causing muscle contraction
8. ATP binds to myosin head to lower it after muscle movement is complete.
slow twitch oxidative red muscle fibers
Small in diameter
Dark coloured due to presence of myoglobin
Lots of blood supply
Fast twitch oxidative glycolytic red muscle fibers
Large in diameter
Fast twitch glycolytic white muscle fibers
Force of contraction increases as there are more muscle twitches (page 425)
Motor unit- 1 somatic motor neuron and muscle fibres it innervates
Autonomic responses are also known as visceral responses because they often involve internal organs.
Urination and defecation are autonomic spinal reflexes that are modulate excitatory and inhibitory signals
from the brain. Other autonomic reflexes can be triggered by the hypothalamus, thalamus, and brain stem.
Brain stem- vomiting, salivating, sneezing, coughing, swallowing, gagging.
Limbic system (sometimes called the “visceral brain”): sex, fear, rage, aggression, hunger.
All autonomic responses are polysynaptic, with one synapse in the CNS between the sensory neuron and
the preganglionic autonomic neuron, and another synapse in the ganglion between the pre and
Autonomic reflexes (visceral reflexes)
Modulated by excitatory and inhibitory signals from the brain. Parts of CNS involved: hypothalamus,
thalamus, and brain stem. These maintain homeostasis. Brain stem controls salivating, coughing,
sneezing, vomiting, swallowing, and gagging.
Limbic system (Visceral brain, emotional stimuli): sex fear, rage, aggression, hunger
Sensory receptors (proprioceptors): located in skeletal muscle, ligaments, and joint capsules.
Sensory neurons: carry input signal from proprioceptors to CNS.
CNS: integrates input signal using pathways of excitatory and inhibitory interneurons.
Somatic motor neurons: carry output signals to muscle (alpha motor neurons).
Extrafusal muscle fibers: synapse onto most prominent surface of muscle.
3 types of proprioceptors are muscle spindles, Golgi tendon organs, and joint receptors.
Muscle spindles, respond to muscle stretch. Is intrafusal. Innervates with gamma motor neurons. At
muscle resting length, the muscle spindle is stretched to activate sensory fibers. Stretching of muscle
causes the spindles to fire, contracting the muscle.
Gamma: intrafusal Golgi tendon organs respond to tension at the location between the tendon and the muscle. If the force is
excessive on the muscle, the GTO relaxes the muscle to drop the load and protect it.
Monosynaptic stretch reflex: tap tendon to extend muscle. This causes the muscle spindle to fire and
contract the muscle that the tendon belonged to.
Cross extensor reflex: step on nail pain stimulates nociceptor sensory neuron sends signal to spine
extensor of injured leg inhibited, while flexor is excited. At the same time the extensor of the other leg
is excited and the flexor is inhibited.
Duration of muscle contraction in 3 types of muscle
Duchenne muscular dystrophy: dystrophin which links proteins to actin is absent. This creates tears in the
cell membrane allow extracellular to enter, which activates the intracellular enzymes which results in the
breakdown of fiber components. Patients die before the age of 30.
McArdle’s disease: myophosphorylase deficiency, is a condition in which the enzyme that converts
glycogen to glucose 6-phosphate is absent. This limits exercise tolerance, due to no energy supply to the
Smooth muscle: generate max tension when 25-30 percent of all cross bridges activated, and has low
oxygen consumption. Sphincters are examples of these contracted muscles. They close off openings made
by hollow organs, and relax when material is to enter or leave the organ.
Features of smooth muscle:
Lots of variety: 6 groups include vascular, gastrointestinal, urinary, respiratory, reproductive, and
More oblique than parallel.
Controlled by hormones, paracrines, and neurotransmitters: unlike skeletal muscle which is only
controlled by acetylcholine, smooth muscle can be controlled by acetylcholine, norepinephrine
and other neurotransmitters, hormones, and paracrines.
They can depolarize without action potentials.
Hormones can both stimulate and contract it. Blood vessels for example receive two signals: one
to excite and one to relax.
Smooth muscle is much smaller than skeletal muscle fibers.
Most smooth muscle is called single unit smooth muscle or visceral smooth muscle because the
individual muscle cells contract as one unit. All the fibers are electrically attached to each other.
There is also multi-unit smooth muscle, which consists of cells that are not electrically attached to
Smooth muscle has larger actin and myosin filaments.
Smooth muscle is not arranged in sarcomeres.