What is cross nerve innervation?” the friend asked dad.
“If the nerve that innervates or controls the slow twitch muscle or Type I muscle is switched over and connected to the fast twitch muscle or Type II muscle, it is called as cross-reinnervation,” dad answered.
“What does the word twitch mean?” the friend enquired again.
“When we want to do a movement, the intention initiates signals in the brain. The signals are brought from the brain to the muscle through the nerves. The nerve cell that innervates and controls a certain number of muscle fibres is called as a motor unit, a term coined by Charles Scott Sherrington. The contraction of the muscle fibres that are under one motor unit responds to a nerve stimulus called as action
potential, also known as a twitch,” mom answered.
“When the muscle responds quickly, it is called as fast twitch and if it responds slowly, it is called as slow twitch. The motor unit is either fast twitch or slow twitch depending on the nerve that innervates. If a person wants to sprint, jump or throw, the motor unit recruited would be a fast twitch unit and the long distance runners would recruit the slow twitch unit,”
she added.
“What is the use of this cross-reinnervation?” the friend asked.
“It is mainly to understand the properties of the muscle fibre and the changes that happen within it depending on the nerve that controls it. If it is controlled by small nerve cells or neurons, the fibres responding to their recruitment would behave like endurance fibres. If the same fibres were to be innervated by large-size neurons by the cross-reinnervation technique, the properties of the muscle fibres alter to meet the demand of the nerve and behave as a
fast twitch fibre unit,” dad explained.
“What is the ‘size principle?,” the friend asked.
“It refers to one of the greatest contributions to neurobiology by Elwood Henneman in 1965. Motor neurons with a small body, hence capable of only a small spike, innervate motor units that produce less force. Those with large bodies are capable of large spikes and innervate motor units that generate
considerable force,” dad said.
“Large motor neurons innervate muscle fibres that are fast twitch and fatigue sensitive. Small motor units innervate slow twitch fibres that are fatigue resistant. It is also a rule that fast twitch fibres would develop properties of slow twitch but not vice versa,”
mom added.
“Why is that not possible?” the friend asked.
“The slow twitch muscle fibres are capable of tonic or sustained contractions and are important for the maintenance of posture. The changes induced by speed or strength training on them would be overridden by the sustained demand of gravity inducing postural changes, which is throughout the day. The fast twitch fibres cannot be involved with posture as their capacity is limited for sustained work,” mom explained.
“The interesting thing is that the slow twitch fibres would not receive enough blood supply if they are continuously contracted by postural demand of gravity. This is overcome by undertaking the job in rotation instead of involving all the fibres at one time. When a few motor units are done then the next group of motor units with higher threshold would be brought into action until all slow twitch muscles are exhausted, forcing the person to sit or even lie down,” dad said.
“The reflex action can never use slow twitch fibres. They always use fast twitch fibres,” mom said.
“The slow twitch postural fibres are highly innervated for optimal sensory input for posture maintenance and control. Interestingly, stressful conditions causing pain is usually through the involvement of the slow twitch fibres and not the fast twitch fibres,” dad concluded.
— sparrc@gmail.com