Download Skeletal Muscle: General Properties and reflexes

MCB 32 FALL 2000
SKELETAL MUSCLE: GENERAL PROPERTIES AND REFLEXES
Reading: Chapter 6
I. Muscle types and energetics
striated, cardiac and smooth
Chemical reactions to generate contraction and resynthesis of ATP
ATP --> ADP + Pi + contraction + heat; then ADP + Pi --> ATP
Resynthesis by mitochondria and oxidative phosphorylation processes is slow but efficient.
Generate 32 ATP from each glucose.
Resynthesis by glycolysis is fast but inefficient. This will occur when oxygen supplies are
limited or when energy requirements outstrip rates of mitochondrial production. 2 ATP per glucose, but
64 ATP can be generated by glycolysis in same time as 32 ATP from 1 glucose by mitochondria. Byproduct of glycolysis is lactic acid, which may contribute to fatigue by interfering with uptake and
release of Ca from SR and also may affect actin-myosin interactions.
Resynthesis by creatine-Pi + ADP --> ATP. Very fast, but limited supply of creatine-Pi.
Skeletal muscle: fast vs slow and oxidative vs glycolytic.
Three general types:
(i) fast glycolytic (weight lifter): few mitochondria and capillaries, little myoglobin (Mb, which
serves as a store for O2 in tissue) [in a turkey this would be the dark meat]
(ii) fast aerobic (leg muscles of long distance runner): more mitochondria and capillaries and
some Mb
(iii) slow aerobic (posture muscles in back), many mitochondria and capillaries and less Mb [this
would be the light meat]
All human muscles are mixtures of these different types. Particular types of conditioning will
cause the relevant type of muscle fibers to become larger. Thus, a weight lifter will have larger biceps
composed primarily of fast, glycolytic muscles. His or her biceps muscles will be large because there
has been a hypertrophy (increase in size of the cells already present) of the muscle fibers that are
available. Muscle does not undergo hyperplasia (increase in number of cells). When a weight lifter
stops lifting weights, the muscles get smaller because the proteins of the muscle cells are degraded, but
the number of muscle cells does not decrease.
II.
Muscles and movement: Figs 6.1-6.8
Muscles generate movement and perform work by contracting, i.e., by shortening. The
contraction becomes manifest to the world through the connections of the muscles to bones through
tendons. Isotonic (“constant tone”) contractions occur when you pick up a weight, e.g., a weight lifter
performing a “curl” maneuver with a bar bell. Isometric (constant length) occur when the muscle
contracts and generates a force, but the overall muscle length does not change. In both cases the muscle
is shortening internally and generating a force. In general, muscles pull on the bones like levers, e.g.,
biceps contracts and causes the forearm to move up (and also lengthens the triceps, which is relaxed).
Then the triceps contracts and pulls the forearm back down to the original position (pulling the biceps
back to its original position).
III.
Cell and tissue level control: nerves and muscles
Muscle cell = muscle fiber. Muscle cells are multinucleated because cells fuse during
development. Muscle cells in general do not divide after this fusion has occurred. There can be small
amount of repair to damaged muscles from "sattelite cells" (small number of cells which surround the
muscle cell, and are located in the connective tissue covering the muscle. Fig. 6.12
Nerves and muscles act together as motor units (nerve plus all the muscles innervated). Each
muscle cell is innervated by one and only one motor nerve, though one motor nerve often branches and
innervates more than one muscle cell (fiber). Contraction is initiated when the motor nerve generates an
action potential, which spreads along the axon and all its branches into the neuromuscular junctions
(regions where the nerves connect to the muscle). The action potential triggers release of small
membrane bound vesicles containing the neurotransmitter acetylcholine (several hundred molecules in
each packet), which then diffuses across to the muscle and binds to the acetylcholine receptor. When
the receptor is activated by bound acetylcholine, the muscle cell membrane voltage depolarizes
(becomes less negative) inside, which triggers opening of Na channels in the plasma membrane of the
muscle cell. The resulting action potential spreads along the muscle fiber from one end to the other, and
contraction of the muscle occurs. One action potential triggers one small contraction (“twitch”). When
the action potential has terminated, the muscle relaxes.
Motor units come in different sizes, depending on the particular muscle. Small motor units
(small number of branches of the motor nerves) have good control but low strength. Large motor units
(large number of brancher of motor nerves) have poor control but great strength. Examples: ocular
muscles in eye and posture muscles of back and leg. Strength of contraction by this mechanism largely
controlled by the number of motor units stimulated to contract. Large contractions involve more motor
units. Regulation and coordination through complicated feedback between nerves and muscles. Fig.
6.18
Second and continuing stimulations of the nerves lead to repeated contractions, which generate
larger and larger strengths. The different types of contractions are called twitch, summation and tetanus.
Depending on rate of stimulation of muscle, strength of contraction increases. Twitch (1 AP) leads to
one weak contraction. Summation leads to multiple intermediate contractions. Tetanus leads to
sustained maximal contraction. Depends on stretching of series elastic elements in muscle. Fig 6.17
Muscle strength is also a function of how long it is. This is called the length-tension relation.
Muscle generates different tensions depending on length, and this characteristic relation is due to the
special relation between actin and myosin, the overlap of the molecules determining the amount of
contractile strength. Fig. 6.19
Overall, strength of contraction is controlled by: number and size of motor units, length of the
muscle and rate of stimulation from the nerve. Under normal physiological conditions, most muscle
contractions are tetanic, one chooses a muscle length near its resting length (where strength is largest)
and the overall strength is controlled by the number of motor units activated.
IV.
Bone, ligaments, tendons and cartilage
These are all examples of connective tissue. Figs. 6.3-6.6
Ligaments and tendons are composed primarily of collagen and other fibrous connective tissue.
Tendons connect muscles to bone. Ligaments attach bone to bone. Both of these types of connective
tissue are secreted by particular types of fibroblasts, which secrete.
Cartilage (produced by chondrocytes) forms the cushioning at joints.
Bones are composed of both collagen and hydroxyapatite (mostly calcium phosphate). Thus, it
has strength provided by the calcium phosphate and flexibility provided by the collagen, similar to the
concrete structures on the freeways with their internal iron bars. Bone is produced by osteoblasts and
broken down by osteoclasts. Bone is constantly being renewed by the combined actions of these two
types of cells. This allows bone to change its shape and structure according to “demand.”