11 spinal reflexes sr2002 2013 al
TRANSCRIPT
Motor units in movements: Spinal reflexes
SR2002
Dr. Arimantas Lionikas
October 31, 2013
Spinal reflexesPlan
• 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.
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
• 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)
IntegrationCentre
Muscle(Effector)
Receptor
Muscle sensory receptors (proprioceptors)
Muscle spindles
Golgi tendonorgans
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
• 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
(1.)
(3.) (4.)
(2.)
• Monosynaptic means ONE synapse is involved
McArdle et al. 2001
Monosynaptic stretch reflex
• Sequence of events:• 5. AP reaches the motor neuron
through the dorsal roots • 6. 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
(5.)
(6.)
(7.)
(8.)
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) motor neurons innervating hamstrings
• Functional significance: Coordination of antagonists - hamstrings are relaxed when quadriceps contracts
Autogenic inhibition reflex
Employing Stretch Reflex: Countermovement jump
• 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
Vertical Force
Velocity
Jump Height
Power
Exc
entr
icE
xcen
tric
Con
cent
ricC
once
ntric
Flig
htF
light
Land
ing
Land
ing
Sta
ndin
g st
illS
tand
ing
still
Sta
ndin
g st
illS
tand
ing
still
Muscles are stretched
Countermovement jumpBeneficial 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
• Plyometric exercise (explosive jump training) is used to improve power in sports
• Stretch reflex assists recruitment of motor neurons by exciting or “facilitating” them
Muscle damage can be induced in this phase
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