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  • Alpha motor neuron (MN) receives excitatory and inhibitory inputs from thousands of neurons carrying information from various receptors and supraspinal centres (e.g. motor cortex, cerebellum, etc.). <br /> MNs remain silent when inhibitory inputs prevail. When excitatory inputs reach certain threshold (bring resting membrane potential from -65 mV up to -45mV) MN is excited and fires an action potential. <br /> Understanding of the regulation of muscle contraction at MN level is very relevant for sports and athletic activities. For example, metabolic changes resulting from muscle contraction are sensed by chemoreceptors and that increases inhibitory drive on MN. Whereas mechanism involved in stretch reflex can be used to facilitate activation of MN (more details about that in upcoming slides) <br />
  • Reflexes is a way for the body to react quickly to the signal that typically would push physiological systems of the organism away from their “comfort zone”, homeostasis, or would help to prepare for some events that were signalled to happen (think of a sour cherry melting in your mouth for the latter example; salivation that might just have happened is a way to prepare for the effective absorption of nutrient from that to be eaten cherry). <br /> The reflex arc works as follows: 1) receptors sense changes in what they are monitoring (e.g., length of the muscle) and send neural inputs (action potentials) via sensory nerves; 2) the signal then is passed to motor neurons and interneurons in the spinal cord (integration centre) for processing (meaning, something would have to be inhibited or activated in response to it); 3) “program to act” in a form of action potentials is sent to the effectors (skeletal muscle in this examples). <br /> Postural reflexes involving skeletal muscles, which operate at a subconscious level, is a quick and efficient way to adjust motor programs in response to perturbations that may lead to the loss of balance and falls, or move away from environmental stimuli that may be harmful (for instance, removal of the finger from an object which turns out to be hot). <br /> We will focus on a couple of reflexes which are also relevant for various athletic activities. <br />
  • Muscle spindle is a sensor organ for the length and the rate of change in length of the muscle. <br /> Golgi tendon organ is a sensor for tendon tension. <br /> Understanding of the role of afferent feedback is relevant in sports and exercise related activities. For example Plyometric Training method capitalizes on the stretch reflex. Spinal reflexes originating from muscle afferents also play important role in restricting range of motion in the joints (flexibility). <br />
  • Reciprocal inhibition assist coordination of antagonists in various motor tasks. <br />
  • Flexibility, characterized by range of motion which in turn is limited by mechanical resistance (of tissues surrounding joints) and stretch tolerance. Activation of stretched muscle groups (measured by EMG activity of the muscle) contributes to mechanical resistance. This activation may originate from the stretch of muscle spindles. <br />

11 spinal reflexes sr2002 2013 al 11 spinal reflexes sr2002 2013 al Presentation Transcript

  • Motor units in movements: Spinal reflexes SR2002 October 31, 2013 Dr. Arimantas Lionikas
  • Spinal reflexes Plan • • • • • Summation of excitation and inhibition Reflex arc Stretch reflex Reciprocal inhibition Reflexes in sports and exercise • Reading list: 1. Enoka R. Neuromechanics of human movement. 2008. Publishers: Human Kinetics, p. 249-253, 257-264, 309-313 2. McArdle W.D. et al. Exercise Physiology: energy, nutrition, human performance. 2007. Publisher: Lippincott Williams & Wilkins, p. 411-415.
  • α Motor neuron • • • • • Motor neuron (MN) receives thousands of contacts (synapses) on its dendrites and soma from other neurons Many of these synapses are excitatory (secreted mediators excite MN) Other synapses are inhibitory (secreted mediators inhibit MN) Spatial and Temporal Summation of excitatory postsynaptic potentials, EPSP, on MN may elicit an action potential (if certain threshold of EPSP is reached) or make the MN more excitable, facilitated (meaning that membrane potential of MN is nearer the threshold for firing action potential than normal) Inhibitory postsynaptic potentials, IPSP , reduce excitability of MNs.
  • The Reflex Arc Integration Centre Muscle (Effector) • • • • Receptor The reflex arc consists of receptor, integration centre and effector Receptors: e.g. prioprioceptors (muscle spindles, Golgi organs), chemoceptors Integration center: sensory nerves, interneurons (not shown here), α Motor neurons Effectors: skeletal muscle (or other organs, depending on the type of reflex)
  • Muscle sensory receptors (proprioceptors) Muscle spindles Golgi tendon organs
  • Muscle spindles Muscle spindle
  • Spindles in mouse soleus
  • Proprioceptors (cont.) Golgi tendon organ
  • Reflexes at work http://neuroscience.uth.tmc.edu/s3/chapter02.html
  • Monosynaptic stretch reflex • Monosynaptic means ONE synapse is involved (3.) (4.) (2.) (1.) McArdle et al. 2001 • Sequence of events: • 1. Tendon is stretched when hit by the hammer • 2. Muscle is stretched • 3. Action potential (AP) is generated in a muscle spindle (stretch receptor) • 4. AP travels by an afferent fibre of the sensory neuron
  • Monosynaptic stretch reflex (5.) (6.) (8.) (7.) • Sequence of events: • 5. AP reaches the α motor neuron through the dorsal roots • 6. α motor neuron is depolarized and AP is generated • 7. AP travels by efferent fibre • 8. AP arrives at the neuromuscular junction and causes muscle contraction McArdle et al. 2001
  • Reciprocal inhibition • Stretch of the knee extensors (quadriceps muscle) inhibits knee flexors (hamstrings) • Ia (fast conducting) afferents activate interneurons that inhibit (hyperpolarize) α motor neurons innervating hamstrings • Functional significance: Coordination of antagonists hamstrings are relaxed when quadriceps contracts
  • Autogenic inhibition reflex
  • Employing Stretch Reflex: Countermovement jump Muscles are stretched Power Standing still Jump Height Landing Velocity Standing still Excentric Concentric Flight Vertical Force • Muscles can do a greater positive work when they are stretched before the contraction • Athletes use muscle pre stretching to increase power in the movements
  • Countermovement jump Beneficial mechanisms • Storage of elastic energy (Some energy is stored in elastic structures, for ex. tendons, during the stretch phase and then released during the contraction phase) • Stretch reflex (Stretch reflex facilitates voluntary muscle activation)
  • Plyometric exercise Muscle damage can be induced in this phase • Plyometric exercise (explosive jump training) is used to improve power in sports • Stretch reflex assists recruitment of motor neurons by exciting or “facilitating” them
  • Muscles and flexibility
  • Activation of stretched muscles contributes to mechanical resistance Magnusson et al 1996
  • Spinal Reflexes Summary • Motor neurons are affected by summation of excitatory and inhibitory postsynaptic potential, EPSP and IPSP, respectively • The reflex arc consists of receptors, integration centre and effectors • Sometimes reflexes can assist to athletic performance, whereas in other circumstances they can hamper it