Bob Full's talk on Invertebrate Locomotion What do Muscles do? They can develop force - look at a plot of force/bodyweight (BW) vs. body mass(BM) - can see that force depends on the activity (running, flying, jumping, swimming, etc.) - small animals can produce a lot of force relative to their BW e.g. 1mg BM => 100 BW force, 100kg BM => 0.1 BW force. (This relationship is linear on a log-log plot - negative slope) They can move - look a plot of speed (or speed/bodylength) vs. body mass - also depends on the mode of movement/activity - small things are generally quicker for their size (i.e. the raw speed is nearly constant, but speed/bodylength is much greater for small animals) - also can look at the cycle frequency of different modes of motion (highest frequency recorded is 1 kHz!). The cycle frequency again forms a linear relationship on a log-log plot, with the animals with the smallest BM having the highest frequency actions. (These plots are in Full's chapter in the "Handbook of Comparative Physiology) Muscle Structure Showed the structure of the muscle. The structure/function of the muscle is as follows: Each muscle is made up of a number of "bundles" of muscle "fibers". Each fiber (which runs longitudinally in the muscle) is made up of shorter units called "sarcomeres". The sarcomeres are where the motion (or contraction) of the muscle takes place. It functions similar to a pull solenoid (although graded motion is possible). The outer "housing" is the myosin fibers, while the inner "plunger" is the actin filament. This actin filament has little "grabbers" on it, which, in the presence of oxygen (required to produce ATP - the energy unit of biology) and calcium, can pull (like hand over hand pulling on a tug of war rope) on the myosin fiber and shorten the sarcomere. ->->->->->->- Actin fibril ^ "grabbers" ************* Myosin fiber Sarcomere: ************* ************** * ->->->->->->---<-<-<-<-<-<- * ************* ************** * ->->->->->->---<-<-<-<-<-<- * ************* ************** In mammalian muscle, the muscles are supplied oxygen (required for long term activity) by the circulatory system (the blood is oxygenated in the lungs). In insect muscle, the oxygen acquisition instead takes place through small holes and tunnels in the exoskeleton called "trachea". Another difference between mammalian muscle and roach muscle is that in a single mammalian muscle there may be multiple "motor units" (a motor unit is a part of a muscle activated by one nerve). In roach muscles, each muscle is one motor unit, and therefore can be controlled entirely through one nerve. There are 2 different types of muscle contractions - isometric: gives force, but there is no change in length - isotonic: gives a change in length, but no force - most muscle contractions (other then laboratory experiments) are somewhere in between In addition, muscles are controlled by pulse trains produced by the nerve. If these pulses occur at a very fast rate, the muscle will pull strongest - this state of pull with the highest force value is called "tetanus". If the pulses occur at a slower frequency, the muscle is pulling with less force and this state of pull is called "summation". Lastly, if single pulses come with slow frequency, the muscles are in a state of "twitch". In typical every day tasks, the muscles most often perform at the level of a "summation" pull. This points out a weak point of a lot of muscle research - many researchers "zap" the muscle with large potentials to create a pull - this type of energizing of the muscle puts it in a state of "tetanus" which is not biologically typical. However, because each roach muscle is innervated by one nerve, the research that Full's lab has done (by stimulating the nerve directly) is able to look at more physiologically reasonable muscle pulls. Modelling Showed an actuator model from Zajac, 1989. In this model, the muscle is modelled by a series of two elements, one of which is a compression element in parallel with a spring and the second is a spring. Can show the force vs. length relationship for different animals; you get lots of variation Some parameters they have looked at - Vmax (lengths/sec) (At zero load) - Fo (kN/m^2) (max force at zero velocity and best length) - Pmax (W/kg) (through many cycles of actuation) - T (degrees C) Structures inside muscles: sarcomere, myosin cause change in length Full's lab created a musco-skeletal model (same one we saw on the SGI in the lab). They measured the maximum force which was 0.1 to 0.5 N (?) and fiber length in mm. For the coackroach behavior of wedging, they push very hard with the hind legs. The leg force was measured. What joint angle do the roaches chose? Do they chose the right angle to create maximum moment? This was for the static case. It is more difficult to measure for the running case. You would want to measure a force/velocity curve. How to do this: do controlled stimulation. This is the "alien probe" setup we saw in the lab. You stimulate the motor neurons with a pattern, including magnitude, phase, and frequency. You can also servo an attachment to the muscles to test at different strain/length. The Workloop Technique (for measuring muscle work) - Change the strain sinusoidally - Measure the stress to get a stress vs. strain surve - From this you can measure the work done by the muscle - The stress vs. strain curve is different for different parts of the cycle: /| /| / | + = / | / | ____ /____| -> shorten lengthen loop - the shape of the workloop changes with frequency Invertebrate Muscle Data Parameters they look at: - Power (W/kg) - work per cycle (J/kg) - stress (kN/m^2) - strain (%) - strain rate (L/s) These change with frequency; bees are the maximum on most of these (?) They compared many parameters for low and high power output They choose inputs to the muscle; a series of impluses add up, you keep adding to get maximum power output Note that the workloop goes CW, now CCW. So can muscles only absorb energy? No, the workloop can also go the other direction for certain modes. For example, the muscles works as a shock absorber during running. Neuromechincal Integration - muscles can react different to the same neural signal. "Similar neural signals from the same anatomical group can lead to completely different behavior."