Motor Control of Tail Spine Rotation of the Horseshoe Crab, Limulus Polyphemus

1. Limulus polyphemus (L.), the horseshoe crab, rotates its tail spine in order to right tself and to keep itself balanced. 2. Eight muscles, discrete bundles of muscle fibres, move the tail spine. Fibres of the muscles contract in sequence and thereby pull consecutively on the several tendons of each muscle in order to rotate the tail spine in either a clockwise or counter-clockwise direction. 3. Motoneurones in nerves to different muscles, fibres within a muscle and units in a nerve to a single muscle fire in different sequences during clockwise and counter-clockwise rotation of the tail spine. 4. The firing pattern consists of a major burst of small to large neurons which fire in clusters, and a minor burst of small neurones which appear to fire randomly. Motoneurones in both bursts are excitatory. The major burst develops tension, the minor burst acts during extension of the muscle and presumably impedes relaxation in order to produce stable deflexion and smooth rotation of the tail spine. 5. Muscle fibres respond to motor output with small excitatory junctional potentials of < 5 mV. E.j.p.s sum and show facilitation, and in some cases develop spike-like potentials of 10-20 mV. Both spiking and the greatest increase in tension occur during the clustered firings in major bursts. 6. Muscle fibres have sarcomere lengths of 6·5±0·8 µm and diameters of 10·60 µm. Nerve fibres range from less than 3 to 32 µm in diameter in large nerve branches, which contain between 50 and 100 fibres. 7. These findings indicate that two different motor programs evoke contraction of muscle fibres in opposite sequences. Sequential contraction of fibres within a muscle means that muscle fibres which are activated together, rather than whole muscles, are the functional contractile entities.