Control of Muscular Control of Muscular ContractionContraction

Methods of ControlMethods of ControlThere are 3 main mechanisms within the body to ensure that smooth and safe movement occurs: -

1. Proprioceptors – Sense organs located in joints, tendons and muscles. They provide kinaesthetic feedback concerning the body’s movement.

2. Muscle Spindle Apparatus – Sensitive receptors that exist between skeletal muscle fibres. They relay information via Afferent neurones concerning the state of muscle contraction and the length or extension of the muscle.

3. Golgi Tendon Organs – Thin capsules of connective tissue which exist where muscle fibre and tendon meet. They cause a muscle to relax if high tensions within the muscle occur.

Muscle Spindle ApparatusMuscle Spindle ApparatusChanges in the length and rate of change of the muscles are sensed by receptors in the muscle fibre called Muscle Spindles. These consist of very specialized muscle fibres called Intrafusal Fibres. These are surrounded by the normal fibres known as Extrafusal Fibres.

If the muscle is stretched or shortened it is detected by the intrafusal fibres of the muscle.

Nervous SupplyNervous SupplyThere are different types of nerves that have different roles within the muscle spindle apparatus and muscle contraction itself.

Afferent Neurones – Carry messages away from muscle/sensory organ to the CNS.

Efferent Neurones – Carry messages from the CNS to the muscle/sensory organ.

Alpha Motor Neurones – Nerves that supply the ‘normal’ extrafusal muscle fibres.

Gamma Motor Neurones – Nerves that supply the Intrafusal muscle fibres.

Neural Input into the Muscle

Extrafusal fibers are input by alpha motor neurons

These neurons are large and fast.

Intrafusal Fibers are input by gamma motor neurons

These neurons are relatively small and slow.

They are involved in the control of muscle tone.

Biceps ExampleBiceps Example

Stretch ReflexStretch ReflexThe activation of the muscle spindle is seen in the stretch reflex. This is where a muscle is suddenly stretched, the muscle contracts to resist the stretch and prevent the muscle from tearing.

PRACTICAL

Test the reflexes in a partners knee by tapping there patellar tendon. What happens to their leg and what causes this?

Now look back at the picture on the previous slide and suggest what would happen if a weight was placed in the hand?

motor neuron

sensory neuron interneuron

This stretch is detected by the sensory neurons in the arm and transferred to the interneuron in the spinal chord.

motor neuron

sensory neuron interneuron

A command to further contract the muscle is sent out the alpha motor neuron.

motor neuron

sensory neuron interneuron

The arm is returned to its commanded position.