Extrafusal muscle fiber Extrafusal muscle fiber Extrafusal muscle fibers are the skeletal standard muscle fibers that are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement. They make up the large mass of skeletal muscle tissue and are attached to bone by fibrous tissue extensions (tendons). Each alpha motor neuron and the extrafusal muscle fibers innervated by it make up a motor unit. The connection between the alpha motor neuron and the extrafusal muscle fiber is a neuromuscular junction, where the neuron's signal, the action potential, is transduced to the muscle fiber by the neurotransmitter acetylcholine. Extrafusal
Coffin bone Coffin bone The coffin bone, also known as the pedal bone (U.S.), is the bottommost bone in the equine leg and is encased by the hoof capsule. Also known as the distal phalanx, third phalanx, or "P3". The coffin bone meets the short pastern bone or second phalanx at the coffin joint. The coffin bone is connected to the inner wall of the horse hoof by a structure called the laminar layer. The insensitive laminae coming in from the hoof wall connects to the sensitive laminae layer, containing the blood supply and nerves, which is attached to the coffin bone.