A5 - Motor Unit

• Muscle contacts in response to stimulation from the nervous system
• The central nervous system includes the brain and spinal cord and is responsible for stimulating the nervous system to carry nervous impulses to the required muscle in order to produce movement
• The nerve that connects with the muscle fibre is called a motor neurone
• The motor neurone only stimulates a part of that muscle, which is called the motor unit
• The number of muscle fibres that a motor neurone stimulates varies between different muscles
• Different muscle fibres and different numbers of muscle fibres will be stimulated at different intensities
• When the muscles are working at a high intensity, a larger number of motor units are stimulated to produce these large amounts of force
• When a person is working at a lower intensity, less motor units are recruited
• The muscles need to be able to report back to the central nervous system (CNS) what they are doing and how they are responding to the stimulation that they are receiving. They are able to do this through two forms or proprioceptors: muscle spindles and Golgi tendon organs
• Proprioception is the body's ability to sense where its body are without having to look at them
• A muscle spindle is an organ placed within the muscle which communicates with the CNS
• The purpose of the muscle spindle is to detect when the muscle is being stretched to prevent over-stretching, which could damage the muscle
• The muscle spindles signal to the CNS if the muscle is being stretched too far to stop stretching. This is also known as the stretch reflex
• Golgi tendon organs (GTOs) are placed in the muscle tendons that attach the muscle to the bone. These organs detect tension within the muscle
• When a muscle is contracted, it changes the tension on the GTOs; this is relayed to the CNS which deals with this information accordingly - by either increasing the contraction of the muscle or relaxing the muscle
• Other receptors are located within the body to report back to the CNS what is happening so that the CNS can respond accordingly
• Chemoreceptors - these receptors detect chemical changes in the body. Usually these changes take place in the blood stream. They are located in the medulla in the CNS and in the periphery they are mainly located in the carotid body which is located close to the carotid artery
• Thermoreceptors - these receptors detect a change in temperature of the body and report back to the CNS if the body it too hot or too cold. They are located in the hypothalamus in the CNS and also close to the skin's surface and other areas such as the bladder and eye
• Baroreceptors - these receptors detect a change in blood pressure and are located in the main blood vessels, including the aorta and carotid artery