Proprioception And
Reflexes
We are aware of the orientation of our
limbs with respect to one another,
we can perceive the movements of our joints and we can accurately assess the amount
of resistance (force) that opposes the movements we make. This ability is
called proprioception. The three qualities of this modality are position,
movement and force. The receptors or proprioceptors that mediate
this modality are principally found in the joint capsules (joint receptors),
muscles (muscle spindles) and tendons (Golgi tendon organs).
The joint capsule is compressed
or stretched when the joint moves, and mechanoreceptors within it signal
the position of the joint, as well as the direction and velocity of the movement.
Individual receptors respond to the position of the joint, as well as the direction
and the velocity of the movement, but not the force. The receptor types found in
the joint capsule are Ruffini-type (slowly adapting) stretch receptors (Chapter
55).
Each muscle contains a number of small
muscle fibres (intrafusal muscle fibres: 15–30 μm in diameter and 4–7 mm
in length) that are thinner and shorter than the ordinary muscle fibre (extrafusal
muscle fibres: 50–100 μm in diameter and varying in length from a few millimetres
to many centimetres). Several intrafusal fibres are grouped together and encased
in a connective tissue capsule, called the muscle spindle, a specialized
receptor that responds to the stretch of a muscle (Fig. 60a). Muscle spindles lie
in parallel to the extrafusal muscle fibres and are elongated when the muscle
is stretched. The primary sensory innervation of the muscle spindle consists of
afferent fibres which wind themselves around the centre of the intrafusal
muscle fibres (annulospiral ending). These are large myelinated fibres (group
Ia afferents). These endings are called primary sensory endings and, when
excited, they evoke a monosynaptic stretch reflex involving an excitation
of the homonymous α-motor neurones and reciprocal inhibition of the heteronymous
α-motor neurones (Fig. 60b).
