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

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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

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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