Muscle spindles are stretch receptors present in the skeletal muscles. These are meant for proprioceptive mechanism and so are a type of proprioceptors. Each skeletal muscle
contains muscle spindles of variable number depending upon the task performed. Muscles involved in precision movements contain many more spindles than the muscles used to maintain posture. For example, hand muscles have approximately 80 spindles, which is 20% of the number of spindles contained in back muscles weighing 100 times as much.
Structure
Each muscle spindle consists of 3—10 small muscle fibres (called intrafusal muscle fibres), encapsulated in a thin connective tissue capsule containing fluid. The muscle spindles are present in between and parallel to the extrafusal fibres (large force-generating muscle fibres). Either end of the muscle spindle is attached to the endomysium of the extrafusal muscle fibres.
Intrafusal muscle fibres consist of a central non-contractile portion, which does not contain actin and myosin filaments and is thus devoid of striations. Portions on either side of the central part are contractile (as they contain actin and myosin filaments) and are called striated poles. The central part of each intrafusal fiber is sensory portion. Intrafusal fibres are of two types:
a. Nuclear bag fibres: Each spindle contains about 2—5 nuclear bag fibres, which are about 30 nm in diameter and 7 mm in length. In these fibres, many nuclei are congregated into an expanded bag in the central portion, hence the name.
b. Nuclear chain fibres: These are 15 microns in diameter and 4 mm in length. In these fibres nuclei are arranged in a single file in the central part in the form of a chain, hence the name. Approximately, 6—10 nuclear chain fibres exist in each typical spindle.
Nerve Supply of the Muscle Spindle
The muscle spindle is innervated by both sensory and motor nerve fibres. It is the only receptor in the body which has got motor nerve supply also.
(i) Sensory nerve supply:-
Central non-contractile portion of each intrafusal fibres is the receptor portion. Sensory fibres supply this area. There are two types of sensory fibres:
(A) Group la fibres, also known as primary sensory endings, supplying central receptor portions of both nuclear bag as well as nuclear chain fibres. Since these fibres spirally wind around the intrafusal fibres, these are also called annulospiral endings. They have diameter of about 17 microns and carry impulses at the rate of 70—120 m/s.
The primary endings supplying both the nuclear bag as well as the nuclear chain intrafusal fibres are stimulated when the muscle spindle is stretched. But the pattern of response is different:-
a) Dynamic response is shown by nerve endings supplying the nuclear bag fibres.
b) Static response is shown by the nerve endings supplying the nuclear chain fibres.
(B) Type II fibres, also known as secondary sensory endings, innervate the receptor portion of mainly nuclear chain fibres on one side of the primary endings. They are also known as flower spray endings. They have a diameter of about 8 microns. These nerve endings respond mainly to sustained stretch, therefore measure the muscle length.
(ii) Motor supply:-
The efferent fibres to the muscle spindle are called ɣ-fibres because their axons belong to the Aɣ group of fibres. There are two types of ɣ-fibres:
a) Dynamic ɣ-fibres primarily innervate the striated poles of nuclear bag fibres, where they end as motor end plate, hence also called plate endings. These fibres increase the
sensitivity of the la afferent fibres to stretch.
b) Static ɣ-fibres primarily innervate the striated poles of nuclear chain fibres where they end as a network of branches called trail endings. They increase the tonic activity in the la afferent fibres at any given muscle length.
Functions of Muscle Spindle
(i) Role in stretch reflex:-
Muscle spindle forms the receptor organ of stretch reflex and thus plays a key role in stretch reflex.
(ii) Role in maintaining muscle tone:-
Muscle spindle plays an important role in maintaining the muscle tone by controlling the discharge from the ɣ-motor neurons.
(iii) Role in maintaining skeletal muscle at a certain physiological length:-
Most important function of muscle spindle is to act as a comparator of the extrafusal fibre length. The muscle spindles, through the stretch reflex, act as a feedback device to maintain the skeletal muscle at a certain physiologically useful length. This action of muscle spindles (particularly in the antigravity muscles) is of fundamental importance in the maintenance of standing posture
(iv) Role as proprioceptor:
Muscle spindle plays the role of proprioceptor in Unconscious proprioceptive sensations and Conscious kinesthetic sensations.