Motor units, muscle spindle, golgi tendon organs and joint receptors

-skeletal muscles and the elements of the nervous system that control them -comprises upper motor neurones (in brain) and lower motor neurones (in soma in brain stem & ventral horn of the spinal cord) UMN- supply input into LMNs to modulate their activity LMNs- receive input from proprioceptors and interneurones. they command muscle contraction and form the “final common pathway” • muscle spindle, innervated by Gamma motor neurones • alpha motor neurones

• Act on joint to perform the same action i.e. are agonists • sets of these muscles create movement when one contracts and the other relaxes • control movements of the trunk • found in shoulder, elbow, pelvis and knees • move the hands, feet and digits

• they exist in the spinal cord in the ventral roots • ventral root and dorsal root join together, contains both sensory and motor fibres -They belong to a spinal segment (C 1-8, T 1-12, L 1-5 and S 1-5) three are a greater number in the cervical enlargement (C3-T1) fro arm and the lumber enlargement (L1-S3) for leg

an Alpha motor neurone and all of the skeletal muscle fibres it innervates , i.e. the smallest functional unit of the motor system • co-ordinated contraction of the motor units • the collection of alpha MN that innervate a single muscle -frequency of AP discharge from the alphaMN The recruitment of additional, synergistic motor units

-in the ventral horn -somatotopic distribution LMNs innervating axial muscles are medial to those innervating distal muscles LMNs innervating flexors dorsal to those supplying extensors -central terminals of dorsal root ganglion cells whose axons innervate the muscle spindles UMNs in the motor cortex and brain stem Spinal interneurones

Neuromuscular activation & force production by innervated muscle fibres -Neuromuscular activation the firing rate of the LMNs involved The no. of LMNs that innervate a muscle (i.e no. of motor units) the co-ordination of movement Force production by innervated muscle fibres Fibre size Fibre phenotype

-frequency summation of muscle fibre contraction A single action potential in an α-MN causes a muscle fibre to twitch. Summation of twitches causes a sustained contraction as the number of incoming action potentials increases

-small for fine movements (innervated by small Alpha MNs) large in large postural muscles (innervated by larger alpha MNs) the force of contraction that each motor unit produces is dependent upon its size • these contain only muscle fibres of a single type the AMNs innervating fast type tend to be larger and have faster conducting axons • each muscle fibre innervated by a single motor axon at endplate (NM junction) which is usually at the centre of the fibre

• Fast and slow • differ in how fast myosin ATPase splits ATP to provide energy, this is reflected in time to develop peak tension -Slow oxidative type 1 fibres ATP from oxidative phosphorylation slow contraction and relaxation fatigue resistant Red fibres as high myoglobin content Fast type II fibres: Type IIa ATP derived from oxidative Phosphorylation. Fast contraction and relaxation fatigue resistant Red and reasonably well vascularised TypeIIx ATP from glycolysis. Fast contraction but not fatigue resistant, Pale in colour and poorly vascularised -3 types: slow- most numerous with aMNS with a low threshold Fatigue resistant fast fatiguing, Mans have a high threshold

• the force increases until max reached • all or nothing -temporal summation in individual fibres/motor units recruitment of more motor units (occurs second) -muscle fibre types and the load on muscle

The susceptibility of an aMN to discharge APs is a function of it’s size

the fine and graded development of muscle force

• When a skeletal muscle is pulled, it pulls back • change in length is registered by muscle spindle -fibrous capsule intrafusal muscle fibres sensory afferents that innervate intrafusal fibres gamma MN efferents that innervate intrafusual fibres monosynaptic reflex etc e.g. patellar tendon stretch of muscle spindle -> activation of Ia afferent -> excitatory synaptic transmission in spinal cord -> activation of aMN -> contraction of homonymous muscle -to asses integrity of spinal cord at specific levels: Supinator: C5-C6 Biceps: C5-C6 Triceps: C7 Quadriceps: L3-L4 Gastrocnemius: S1

-Gamma MNs -non-contractile equatorial region innervated by the Ia sensory neurones contractile polar ends that receive efferent input from gMNS with cell bodies in the ventral horn of the spinal cord • of MNs causes the spindle to contact • alpha and gamma MNs are co-activated so the intrafusal muscle fibres contract in parallel with the extrafusal fibres. maintains the sensitivity of the spindle and the Ia response is maintained

-by morphology & function: Nuclear bag fibres Bag 1: dynamic, very sensitive to the rate of change of muscle length. innervated by dynamic gMNs Bag 2: states, more sensitive to to the absolute length of the muscles, innervated by static gMNs Chain fibres sensitiv to the absolute length of the muscle, innervated by static gMNs -Ia afferents form a primary annulospiral nerve ending winding around the centre of all intrafusal fibres II fibres more slowly conducting, form flower spray endings on all intrafusual fibres except the bag 1 dynamic type both respond to stretch, Ia- rate of change II- absolute length of intrafusal fibres

-rate of change of muscle length (dynamic) absolute length of muscle (static) • static: causes static response to increase dynamic: enhances dynamic response to stretch -static: muscle length changes slowly and predictably Dynamic: muscle length changes rapidly and unpredictably

• at the junction of muscle and tendon • monitor changes in muscle tension, they are in series with, rather than parallel to, extrafusal fibres • by group Ib sensory afferents • act to regulate muscle tension to protect muscle from overload and regulate tension to optimal range

enter the spinal cord and synapse upon inhibitory interneurones which then synapse upon the alpha motor neurone of the homonymous muscle forming the basis of the reveres myotatic reflex

• polysynapotic pathway in which an inhibitory neurone is interposed between the Ib afferent and aMN • for proper execution of fine motor acts

• connective tissue of joints • changes in angle, direction and velocity of movement of joint. prevent excessive flexion or extension -RA- rapidly adapting SA- slowly adapting HT- high threshold LT- low threshold free nerve endings, found in capsule and connective tissue they are most numerous and HT, SA with nociceptive function Golgi-type endings in ligaments HTm Sam protective role? Pacinifrom endings periosteum near articular lattachments & fibrous part of the joint capsule, LT, SA, acceleration detectors Ruffini endings Joint capsule, LT, SA, static position and speed of movements