Muscle

Muscle cells like neurons, can be excited chemically, electrically, mechanically to produce an action potential along their cell membrane

Unlike Neurons they have a contractile mechanism activated by the action potential and mediated by the contractile proteins actin and myosin.

Muscle is divided into 3 types:
skeletal (as in somatic musculature)
cardiac (making up the myocardium)
smooth (as in viscera)

Skeletal Muscle is made up of individual muscle fibers, multinucleate, long and cylindric in shape

Muscle Fibers are arranged in parallel between the tendon ends so that the force of contraction of each single unit is additive

Each fiber is formed of Fibrils and these of filaments organized into Sarcomeres made up of proteins: Myosin and Actin (during contraction myosin and actin break and reform crosslinkages), Tropomyosin, and Troponin (these last 2 "relaxing proteins" form a complex which inhibits myosin and actin interaction)

Steps in Contraction

1) discharge of motor neuron at myo-neural plate (endplate)
2) release of the neurotransmitter acetylcholine at the endplate
3) binding of acetylcholine to its receptors
4) increased Na+ and K+ conductance in endplate membrane
5) generation of action potential at endplate
6) generation of action potential along muscle fibers
7) spread of potential along muscle fibers
8) release of Ca++ from sarcoplasmic reticulum (extracellular space surrounding muscle fibrils) and diffusion to thick and thin filaments
9) binding of Ca++ to troponin and thereby uncovering myosin binding sites on actin
10) formation of cross-linkages between actin and myosin and sliding of thin on thick filaments, producing muscle shortening

Steps in Relaxation

1) Ca++ pumped back in sarcoplasmic reticulum
2) release of Ca++ from troponin
3) cessation of interaction between actin and myosin

There are at least 2 fiber types in skeletal muscle:

TYPE 1, SLOW, RED MUSCLES (e.g. long muscles of back):
long latency
adapted for slow posture maintaining contractions and have moderate diameter
high oxidative capacity
large blood supply

TYPE IIB, FAST, WHITE MUSCLES (e.g. hand muscles):
short latency
adapted for fine, skilled movements and have large diameter
low oxidative capacity
less blood supply

Muscle differentiates from mesoderm
First muscular activity recorded in 8 week-embryo
Recognizable contractions observed after 16-18 fetal weeks when:
* nerve fibers to muscle are developed
* nerve fibers have achieved contact with muscle cells (myo-neural junction) and neurotransmission apparatus (acetylcholine/cholinesterase) has developed

Cross-innervation experiments:
* a specific substance is secreted at nerve endings, or
* a pattern of nerve impulses on muscle fibers acts to cetermine contraction velocity

Skeleto-muscular (M-S) development at adolescence

M-S development involves several systems:
Muscle: size, strength, metabolism, power
CNS: coordination of motor activity, voluntary, autonomic, motivation, fatigue
Respiration: provides O2, removes CO2
Circulation: circulates O2, nutrients
Temperature regulation: during exercise X20 heat production than at rest
Stress: corticoids
Hormones: insulin, GH, IGF-I, T3, Calcitonin, PTH, androgens, estrogens

Muscle fiber number is virtually fixed at birth
The increase in mass or Hypertrophy (sometimes as much as 50%) is due to increase in length and cross-sectional area of muscle fibers due to an increase in the number of myofibrils (from 75 to over 1000)
Capacity for plasticity and regeneration in response to neural, hormonal and nutritional influences

Development of muscles cells
Reader, pp287-293

With early development:
mesodermal origin:
*myoblasts (no distinguishable features)
*4th month, myotubules (myofibrils, some motor activity)
*increased myosin, actin, Ca++ channels

With further development:
*1-7 years, slow growth
*8-17 years, accelerated growth
*18-25 years, slow growth
*increased number of myofibrils
*increased number of nuclei
*hyperplasia, hypertrophy
*no built-in time limit to thickness/strength potential (increased physical activities, gymnastics)

At birth, all limb muscles have same contraction velocity
After birth, velocity differentiates in fast and slow muscles depending on innervation, ACh, AChE levels/activity, increased electrolytes, metabolic changes

Development of myoneural junction (MJ)
*MJ number increases with development
*increased acetylcholine (ACh) levels
*receptors

Before birth, Ach sensitivity is spread through length of muscle;
After birth, it is localized to MJ

Power output = rate of doing work
Work: moving force through a distance

Contraction may be:
isometric: muscle does not shorten but produces force
isotonic: force remains constant but muscle shortens