Medical brain notes 24

  Sensory Receptors Preliminary remarks about receptors and their classification          The peripheral terminations of afferent fibres are responsible for receiving stimuli and are, therefore, referred to as receptors. Receptors can be classified in various ways. 1.                From a functional point of view receptors can be classified on the basis of the kind of information they provide. They may be of the following types. a.                Cutaneous receptors  are concerned with touch, pain, temperature and pressure. These are also called  exteroceptive receptors  or exteroceptors . b.                Proprioceptive receptors  (or  proprioceptors ) provide information about the state of contraction of mus...

  Exteroceptive Receptors Free Nerve Endings         When the terminals of sensory nerves do not show any particular specialisation of structure they are called free nerve endings. Such endings are widely distributed in the body. They are found in connective tissue. They are also seen in relation to the epithelial lining of the skin, cornea, alimentary canal, and respiratory system.         Free nerve endings are particularly numerous in relation to hair follicles. They respond mainly to deformation of hair i.e., they are fast adapting mechanoreceptors. The abundance of free nerve endings in relation to hair follicles is to be correlated with the fact that hair increase the sensitivity of skin to touch. Free nerve endings may also be thermoreceptors and nociceptors.         Some free nerve endings present in relation to hair follicles are described as  lanceolate endings . ...

  Proprioceptive Receptors Golgi Tendon Organs        They are also called the neurotendinous organs of Golgi. These organs are located at the junction of muscle and tendon. Each organ is about 500 μm long and about 100 μm in diameter. It consists of a capsule made up of concentric sheets of cytoplasm (Fig. 4.2). Inside the capsule there are small bundles of tendon fibres. The organ is innervated by one or more myelinated nerve fibres that divide to form several branches (spray-like arrangement). These receptors are stimulated by pull upon the tendon during active contraction of the muscle, and to a lesser degree by passive stretching.   In the past Golgi tendon organs have been considered to be involved in myotactic reflexes that prevent the development of excessive tension in muscle. However, it is now believed that their role is mainly in providing proprioceptive information; and that they are slow adapting receptors. Similar endings are also p...

  Neuromuscular Junctions          We have seen that skeletal muscle fibres are supplied by ramifications of somatic efferent neurons. We have also seen that axonal branches arising from one neuron may innervate a variable number of muscle fibres (that constitute a motor unit).          Each skeletal muscle fibre receives its own direct innervation. The site where the nerve ending comes into intimate contact with the muscle fibre is a neuromuscular (or myoneural) junction. Details of these junctions vary in different skeletal muscle fibres as follows. 1.              Motor end plates or ‘en plaque’ endings: In most neuromuscular junctions the nerve terminal comes in contact with a specialised area near the middle of the muscle fibre. This area is roughly oval or circular, and is referred to as the  soleplate . The sole plate plus the axont...

  STRUCTURE OF A TYPICAL MOTOR END PLATE:        In the region of the motor end plate axon terminals are lodged in grooves in the sarcolemma covering the sole plate. Between the axolemma (over the axon) and the sarcolemma (over the muscle fibre) there is a narrow gap (about 40 nm) occupied by various proteins that form a basal lamina. It follows that there is no continuity between axoplasm and sarcoplasm.         Axon terminals are lodged in grooves in the sarcolemma covering the sole plate. In section (Fig. 4.5) this groove is seen as a semicircular depression. This depression is the  primary cleft . The sarcolemma in the floor of the primary cleft is thrown into numerous small folds resulting in the formation of  secondary  (or subneural)  clefts .         In the region of the sole plate the sarcoplasm of the muscle fibre is granular. It contains a number of nuc...

  SOME FACTS ABOUT MUSCLE ACTION 1.                We have seen that a muscle fibre requires a stimulus for contraction to occur. Stimuli below a threshold level do not cause contraction. When a stimulus above the threshold strength is applied the muscle contracts to its full extent. This is the  all or none law . 2.                It is very important to understand that the all or none law applies only to individual muscle fibres and not to the muscle as a whole. The force exerted by a muscle depends on the number of muscle fibres that are in contraction at a particular moment. This allows for a graded strength of muscle action depending upon need. A corollary of this is that large muscles with greater number of fibres can exert more power than smaller muscles. The concept of motor units has been already discussed. Large muscles that are ...