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Summary Human physiology
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Test Bank Human Physiology Silverthorn, 64 questions with anwsers, new 2024
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Look at notes/PowerPoint while reading this summary for corresponding figures.LECTRURE 1
11.2 The Somatic Motor Division
▪ Somatic motor pathways, controls skeletal muscles
▪ Somatic motor pathways have a single neuron that originates in the CNS and projects its
axon to the target tissue, which is always a skeletal muscle.
▪ Somatic pathways are always excitatory
A Somatic Motor Pathway Consists of One Neuron
▪ Somatic motor neurons branch close to their targets.
▪ Each branch divides into a cluster of enlarged axon terminals that lie on the surface of
the skeletal muscle fiber
▪ The synapse of a somatic motor neuron on a muscle fiber → the neuromuscular
junction (NMJ)
▪ the NMJ has three components:
1. the motor neuron’s presynaptic axon terminal filled with synaptic vesicles and
mitochondria
2. the synaptic cleft
3. the postsynaptic membrane of the skeletal muscle fiber.
▪ On the postsynaptic side of the neuromuscular junction, the muscle cell membrane that
lies opposite the axon terminal is modified into a motor end plate→ a series of folds
that look like shallow gutters
▪ Between the axon and the muscle, the synaptic cleft is filled with a fibrous matrix whose
collagen fibers hold the axon terminal and the motor end plate in the proper alignment.
▪ The matrix also contains acetylcholinesterase (AChE) → the enzyme that rapidly
deactivates ACh by degrading it into acetyl and choline
The Neuromuscular Junction Contains Nicotinic Receptors
▪ As in all neurons, action potentials arriving at the axon terminal open voltage-gated Ca2+
channels in the membrane. Calcium diffuses into the cell down its electrochemical
gradient, triggering the release of ACh-containing synaptic vesicles. Acetylcholine
diffuses across the synaptic cleft and combines with nAChRs on the skeletal muscle
membrane
▪ The nAChR channels of skeletal muscle, classified as NM subtype, are similar but not
identical to the nicotinic NN ACh receptors found on neurons.
▪ Toxin @bungarotoxin binds to nicotinic skeletal muscle receptors but not to those in
autonomic ganglia.
▪ skeletal muscle NM receptors have a, b, d, and e subunit isoforms, while neuronal nAChR
has only the a and b isoforms. The a and b isoforms of nAChR can become desensitized
and their channels closed with extended exposure to ACh or other agonists.
, ▪ Nicotinic cholinergic receptors are chemically gated ion channels with two binding sites
for ACh
▪ Acetylcholine acting on a skeletal muscle’s motor end plate is always excitatory and
creates muscle contraction.
▪ relaxation occurs when the somatic motor neurons are inhibited in the CNS, preventing
ACh release
CHAPTER 12
▪ Our muscles have two common functions: to generate motion and to generate force.
▪ Our skeletal muscles also generate heat and contribute significantly to the homeostasis
of body temperature (shivering to create heat).
▪ The human body has three types of muscle tissue: skeletal muscle, cardiac muscle, and
smooth muscle.
▪ skeletal muscles are attached to the bones of the skeleton, enabling these muscles to
control body movement. Voluntary.
▪ Cardiac muscle {kardia, heart} is found only in the heart and moves blood through the
circulatory system. Involuntary
▪ Skeletal and cardiac muscles are classified as striated muscles {stria, groove} because
of their alternating light and dark bands seen under the light microscope
▪ Smooth muscle is the primary muscle of internal organs and tubes, such as the stomach,
urinary bladder, and blood vessels → primary function is to influence the movement of
material into, out of, and within the body.
12.1 Skeletal Muscle
▪ Skeletal muscles are usually attached to bones by tendons made of collagen
▪ When the bones attached to a muscle are connected by a flexible joint, contraction of
the muscle moves the skeleton. The muscle is called a flexor if the centers of the
connected bones are brought closer together when the muscle contracts, and the
movement is called flexion.
▪ The muscle is called an extensor if the bones move away from each other when the
muscle contracts, and the movement is called extension.
▪ Most joints in the body have both flexor and extensor muscles because a contracting
muscle can pull a bone in one direction but cannot push it back.
▪ Flexor-extensor pairs are called antagonistic muscle groups because they exert opposite
effects.
Skeletal Muscles Are Composed of Muscle Fibers
▪ A skeletal muscle is a collection of muscle cells, or muscle fibers,
▪ Skeletal muscle fibers are the largest cells in the body, created by the fusion of many
individual embryonic muscle cells.
▪ Committed stem cells called satellite cells lie just outside the muscle fiber membrane.
Satellite cells become active and differentiate into muscle when needed for muscle
growth and repair.
, ▪ Each skeletal muscle fiber is sheathed in connective tissue, with groups of adjacent
muscle fibers bundled together into units called fascicles.
Muscle Fiber Anatomy
▪ The cell membrane of a muscle fiber is called the sarcolemma {sarkos, flesh + lemma,
shell}
▪ the cytoplasm is called the sarcoplasm.
▪ The main intracellular structures in striated muscles are myofibrils {myo-, muscle},
highly organized bundles of contractile and elastic proteins that carry out the work of
contraction.
▪ Skeletal muscle fibers also contain extensive sarcoplasmic reticulum (SR), a form of
modified endoplasmic reticulum that wraps around each myofibril like a piece of lace –
- SR consists of longitudinal tubules with enlarged end regions called the terminal
cisternae {cisterna, a reservoir}.
- SR concentrates and sequesters Ca2+ {sequestrare, to put in the hands of a trustee}
with the help of a Ca2+ATPase in the SR membrane.
- Calcium release from the SR creates calcium signals that play a key role in
contraction in all types of muscle.
▪ The terminal cisternae are adjacent to and closely associated with a branching network
of transverse tubules/ t-tubules.
- One t-tubule and its two flanking terminal cisternae are called a triad.
- The membranes of t-tubules are a continuation of the muscle fiber membrane,
which makes the lumen of t-tubules continuous with the extracellular fluid.
- T-tubules allow action potentials to move rapidly from the cell surface into the
interior of the fiber so that they reach the terminal cisternae nearly simultaneously.
- Without t-tubules, the action potential would reach the center of the fiber only by
conduction of the action potential through the cytosol, a slower and less direct
process that would delay the response time of the muscle fiber.
Myofibrils Are Muscle Fiber Contractile Structures
▪ Each myofibril is composed of several types of proteins organized into repeating
contractile structures called sarcomeres.
▪ Myofibril proteins include the motor protein myosin, which forms thick filaments.
▪ the microfilament actin, which creates thin filaments; the regulatory proteins
tropomyosin and troponin; and two giant accessory proteins, titin and nebulin.
▪ Myosin {myo-, muscle} is a motor protein with the ability to create movement
- The heavy chains of the myosin heads form the motor domain that uses energy from
the high-energy phosphate bond of ATP to create movement.
- Because myosin acts as an enzyme to hydrolyze ATP, the motor domain is
considered a myosin ATPase.
- The heavy chains of the myosin heads also contain the binding sites for actin.
▪ In skeletal muscle, about 250 myosin molecules join to create a thick filament.
, - Each thick filament is arranged so that the myosin heads are clustered at each end of
the filament, and the central region of the filament is a bundle of myosin tails
▪ Actin {actum, to do} is a protein that makes up the thin filaments of the muscle fiber.
- One actin molecule is a globular protein (G-actin)
- multiple G-actin molecules polymerize to form long chains or filaments, called F-
actin.
- In skeletal muscle, two F-actin polymers twist together like a double strand of beads,
creating the thin filaments of the myofibril.
▪ Most of the time, the parallel thick and thin filaments of the myofibril are connected by
myosin crossbridges that span the space between the filaments.
- Crossbridges form when the myosin heads of thick filaments bind to actin in the thin
filaments
- Crossbridges have two states: low-force (relaxed muscles) and high-force
(contracting muscles).
▪ Z disks.
- One sarcomere is composed of two Z disks and the filaments found between them.
- Z disks are zigzag protein structures that serve as the attachment site for thin
filaments.
- abbreviation Z comes from zwischen, the German word for “between.”
▪ I bands.
- These are the lightest color bands of the sarcomere and represent a region occupied
only by thin filaments.
- abbreviation I comes from isotropic, a description from early microscopists meaning
that this region reflects light uniformly under a polarizing microscope.
- A Z disk runs through the middle of every I band, so each half of an I band belongs to
a different sarcomer
▪ A band.
- This is the darkest of the sarcomere’s bands and encompasses the entire length of a
thick filament.
- At the outer edges of the A band, the thick and thin filaments overlap.
- The center of the A band is occupied by thick filaments only.
- The abbreviation A comes from anisotropic {an-, not}, meaning that the protein
fibers in this region scatter light unevenly.
▪ H zone.
- This central region of the A band is lighter than the outer edges of the A band
because the H zone is occupied by thick filaments only.
- The H comes from helles, the German word for “clear.”
▪ M line.
- This band represents proteins that form the attachment site for thick filaments,
equivalent to the Z disk for the thin filaments.
- Each M line divides an A band in half. \M is the abbreviation for mittel, the German
word for “middle
▪ The proper alignment of filaments within a sarcomere is ensured by two proteins: titin
and nebulin
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