The purpose of this thesis is to study aspects of the function of the primary sensory endings of muscle spindles using morphometric and immunohistochemical techniques. For morphometry the right tenuissimus muscles of three adult cats, whose right hind limbs were fixed with the hip and knee flexed at various acute angles, have been compareed with the corresponding muscles from the fully extended left hind limbs of the same animals. The aim of this project is a better understanding of morphometric changes in the muscle spindle including intrafusal fibre diameter, sarcomere length, and (in particular) changes in the primary nerve endings in response to passive stretch. Immunohistochemical studies of the calcium-binding protein, calretinin, and immunogold labelling for glutamate were also carried out on the primary nerve endings as detailed below. All the work reported in this thesis was carried out in the School of Biological and Biomedical Sciences, at the University of Durham. This thesis is divided into five chapters. Chapter one is an introduction to the structure and innervation of the muscle spindle andtendon organ, discussing briefly the presence of calretinin and glutamate in muscle spindle nerve endings. This provides general introduction and background to the work. Chapter two describes the deformation of the nerve endings in the muscle spindle when the cat tenuissimus muscle is subjected to different degrees of flexion of the hip and knee on the experimental side, the control side being extended so that the angles between the hip and the knee were approximately equal in the each case. For rapid and optimum fixation, both sides were perfused with fixative solution. The results show morphometric changes in the experimental muscles compared with the controls in tem1s of diameter, sarcomere length, and sensory nerve terminal deformation. Muscle fibres in the experimental (flexed) side have a greater diameter, and shorter sarcomeres, with nerve endings more indented into the intrafusal fibres, compared with those on the extended, control side. Nuclei of the equatorial region in both the experimental and control muscles differed in arrangements of nuclei and their outline. Chapter three discusses the calcium-binding proteins, with the emphasis on calretinin and its distribution in the nerve endings of rat muscle spindles and tendon organs as shown by histochemical examination of rat soleus muscle. The calretinin distribution was evident in the primary endings of both bag 1 and bag2 fibres and also in the primary endings of the chain fibres, although no secondary endings or tendon organs were observed.Chapter four describes quantitative immunological study carried out on the muscle spindle of the cat tenuissimus muscle to assess the comparative level of glutamate -like immunoreactivity in the equatorial region of the primary nerve endings, and their associated intrafusal fibres, were all found to show less glutamate -LI compared with the secondary nerve endings and their associated intrafusal fibre, but not the intrafusal fibrenuclei. In chapter five the results are summarised, and possible further studies outlined.