Tissues That Respond
Two tissue types are excitable: they respond to stimulation by changing their membrane potential. One converts that into movement, the other into signalling.
Muscle comes in three types:
| Type | Control | Striated | Nuclei | Where |
|---|---|---|---|---|
| Skeletal | Voluntary | Yes | Many, peripheral | Attached to bone |
| Cardiac | Involuntary | Yes | One, central | Heart only |
| Smooth | Involuntary | No | One, central | Walls of hollow organs |
Skeletal and cardiac muscle are striated, and the stripes are the sarcomeres, the repeating units of overlapping actin and myosin filaments. Contraction happens by the sliding filament mechanism: myosin heads bind actin, pivot, and pull the filaments past one another. The filaments themselves never shorten; only the overlap changes.
This produces the length-tension relationship. Too stretched and there is too little overlap for many cross-bridges to form; too compressed and the filaments interfere. Maximum force comes at an intermediate length where overlap is optimal, which is why joint angle affects how much force you can exert.
Cardiac muscle adds intercalated discs that couple cells electrically, so the heart contracts as a coordinated unit rather than fibre by fibre.
Nervous tissue has two cell populations. Neurons carry signals; glia outnumber them and support, insulate and defend. A neuron has dendrites receiving input, a cell body integrating it, and an axon carrying the output.
Conduction speed is the design problem, and there are two solutions. Increasing axon diameter works but scales poorly, the squid giant axon reaches 1 mm across to achieve about 25 m/s. Myelination works far better: a fatty sheath with gaps at the nodes of Ranvier lets the signal jump node to node, reaching 100 m/s or more in a fibre a fraction the diameter. Vertebrates chose insulation over bulk, and the loss of myelin in multiple sclerosis shows how completely conduction depends on it.
Common pitfall: thinking muscle filaments shorten during contraction. They do not, actin and myosin keep their length throughout, and the sarcomere shortens because the filaments slide past one another. The name of the mechanism is the correction.