Introduction

1. MBS 221Lecture 10: The muscle spindle and the spinocerebellar pathway Dr R. McBrideDepartment of Medical Biosciences, University of the Western CapeOffice number B5.1 (021) 959-2333rmcbride@uwc.ac.za Consultation hrs: Wed 12h00 – 13h00Thurs 12h00 – 13h00

2. Introduction • proprioception, like touch/pressure, temperature and pain, is one of the somatosensory modalities and refers to thesense of position and movement of one’s own limbs and body without using vision • there are two submodalities: • sense of stationary limb position (limb-position sense) • sense of speed and direction of limb movement (kinesthesia) • there are three types of muscle and skeletal mechanoreceptors that detect these submodalities: • muscle spindle receptors • Golgi tendon organs • joint kinesthetic receptors

3. Introduction 1. muscle spindle • located within fleshy part of muscle • detects: • rate of change at which the muscle fibers are stretched • changes in length of muscle fibers • aids in coordination and efficiency of muscle contraction

4. Introduction 2. Golgi tendon organ • located at junction of tendon and muscle • detects: • force of muscle contraction • tension applied to tendon • protects tendon and muscle from excessive tension

5. Introduction 3. joint kinesthetic receptors • located in synovial joint capsules • detect: • flexion/extension of the joint • acceleration and deceleration of joint movement • pressure in joint • excessive joint strain • stretch-sensitive receptors in skin (Ruffini endings and Merkel’s disks) also detect postural changes

6. Receptor: muscle spindle • consists of intrafusal muscle fibers = 2-10 specialized muscle fibers enclosed in a connective tissue capsule receptor (extrafusal muscle fibers = voluntary contractile muscle fibers of skeletal muscle) • two types of intrafusal muscle fibers: nuclear bag and chain fibers 1. nuclear bag fibers • approx. 1-3 fibers per spindle • so-named because nuclei concentrated in a “bag type” central part of the fiber • ends of fibers are striated (contain actin and myosin filaments), contractile and are attached to the extrafusal fibers • sensitive to rate of change in muscle length 2. nuclear chain fibers • approx. 3-7 fibers per spindle • so-named because nuclei spread in a chain-like fashion in the center of the fiber • ends of fibers are striated (contain actin and myosin filaments), contractile and attached to ends of the  nuclear bag muscle fibers • sensitive to absolute length of muscle

7. Receptor: muscle spindle

8. Receptor: muscle spindle • the dendritic endings of two types of first order sensory neurons receive stimuli from the nuclear bag and nuclear chain fibers: • primary endings • type Ia fibers • 17 microns (m) in diameter • conduct impulses at 100m/s • respond to rate of change in muscle fiber length 2. secondary endings • type II fibers • 8 m in diameter • conduct impulses more slowly • respond to overall length of muscle fiber • these first order sensory neurons conduct impulses away from the muscle spindle

9. Receptor: muscle spindle

10. Receptor: muscle spindle Type II secondary fibers Type 1a primary fibers

11. Receptor: muscle spindle • is a typical receptor for the general senses i.e. it is the modified dendritic ending of a first order sensory (pseudounipolar) neuron • how does signal transduction occur? • the dendritic endings within the muscle spindle are sensitive to stretch • when skeletal muscle stretches (which usually happens during extension), mechanically-gated ion channels are opened by the pulling of spectrin strands, and there is an influx of Na+/Ca2+ into the dendritic endings • the subsequent depolarization causes a receptor potential to develop in the dendritic endings (receptor region) of the type 1a and II fibers • if sufficiently large, the receptor potential will cause an action potential in the spike-generating region (first node of Ranvier) that will propagate down the length of the type 1a and II fibers (conducting region) by saltatory conduction

12. Receptor: muscle spindle • recall that the receptor potential is a local or non-propagated potential that is graded: • as the stimulus strength (amount of stretch) increases, so the amplitude (size) of the receptor potential increases – why? • because as the amount of stretch increases, so the time duration that ion channels are open increases, allowing more Na+/Ca2+ to enter the dendritic endings, causing a larger depolarization

13. Sensory pathway: spinocerebellar • what is the major difference between the pathways carrying touch/pressure sensation (dorsal column and anterolateral pathway) and those carrying proprioceptive information? • we aren’t consciously aware of the proprioceptive information that reaches our cerebellum • somatosensory pathways carry proprioceptive information about the position of skeletal muscles, tendons and joints in the spinocerebellar pathway,which consists of two ascending tracts: 1. the posterior or dorsalspinocerebellar tracts 2. the anterior or ventralspinocerebellar tracts

14. Sensory pathway: spinocerebellar • the posterior/dorsal spinocerebellar tracts • carries the axons of second order sensory neurons that don’t cross over to the opposite side of the spinal cord and enter the cerebellum via the inferior cerebellar peduncle • the anterior/ventral spinocerebellar tracts - carries the axons of crossed and uncrossed second order sensory neurons that enter the cerebellum via the superior cerebellar peduncle • many of the crossed over axons cross over again in the cerebellum i.e. they synapse on the ipsilateral (same) side as the original stimulus

16. Sensory pathway: spinocerebellar • the axons of first order sensory neurons (type Ia and II fibers) run through the dorsal roots of spinal nerves and divide into several collateral branches: • one collateral branch synapses with the cell bodies of second order sensory neurons (interneurons) which ascend to the cerebellum in the anterior and posterior spinocerebellar tracts, ensuring that the cerebellumis aware of the tone of muscles at all times • other collateral branches synapse with somatic motor neurons and GABA-secreting inhibitory interneurons, respectively but they will be discussed later in the context of reflexes

17. Sensory pathway: spinocerebellar • the axons of the second order sensory neurons ascending in the anterior and posterior spinocerebellar tracts synapse with third order sensory neurons in one of several precerebellar nuclei in the reticular formation of the brain stem, either the • lateral reticular nucleus • reticularis tegmenti or • paramedian reticular nucleus

18. Sensory processing region in the brain: cerebellum • the axons of third order sensory neurons in the posterior and anterior spinocerebellar tracts give rise to direct mossy fiber pathways that carry proprioceptive information from the precerebellar nuclei in the reticular formation of the brain stem to the spinocerebellum (vermis and intermediate hemisphere) - the mossy fibers synapse on granule cells which in turn synapse with Purkinje cells Direct pathway from precerebellar nuclei

19. Sensory processing region in the brain: cerebellum • recall that the spinocerebellum (vermis and intermediate hemisphere) is the functional region of the cerebellum that receives proprioceptive inputs from the spinocerebellar pathway - other somatosensory and special sense inputs arrive via other pathways spinocerebellum

20. Sensory processing region in the brain: cerebellum • proprioceptive information from each part of the body is relayed to a specific portion of the spinocerebellum • the spinocerbellum contains two inverted somatotopic maps of the body: • the head, neck and trunk are represented in the vermis • the arms and legs are represented in the intermediate hemisphere • compared to the somatotopic map in the primary somatosensory cortex, the same body parts are have multiple representations in different locations • this is because info. from a local area of skin, for example, diverges to multiple patches of granule cells • this is known as fractured somatotopy

21. Possible test/exam questions from this lecture… unless otherwise indicated: • all scanned images are from Kandel ER, Schwartz JH and Jessell TM. Principles of Neural Science. 4th International Edition. McGraw-Hill Companies, Inc. • Define/describe the following terms/concepts (2-5 marks): • proprioception, muscle spindle, golgi tendon organ, joint kinesthetic receptor, intrafusal muscle fiber (nuclear bag and chain fibers), extrafusal muscle fiber, primary endings, secondary endings, type Ia and II fibers, spinocerebellar pathway, posterior/dorsal spinocerebellar tract, anterior/ventral spinocerebellar tract, precerebellar nuclei, reticular formation, direct mossy fiber pathway, spinocerebellum, fractured somatotopy

22. Possible test/exam questions from this lecture… • Answer the following questions (5-10 marks): • briefly describe the two submodalities of proprioception • describe the structure of a muscle spindle • briefly describe the generation of receptor and action potentials in a muscle spindle • briefly explain why the receptor potential in a muscle spindle is graded • briefly explain the major difference between the dorsal column and anterolateral pathway versus the spinocerebellar pathway • compare the posterior and anterior spinocerebellar tracts • describe the spinocerebellar pathway (your answer should include where in the CNS the first, second and third order sensory neurons project to and synapse) • compare the somatotopic map of the primary somatosensory cortex with that of the spinocerebellum