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11 spinal reflexes sr2002 2013 al
1. Motor units in movements:
Spinal reflexes
SR2002
October 31, 2013
Dr. Arimantas Lionikas
2. Spinal reflexes
Plan
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•
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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.
3. α Motor neuron
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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.
4. The Reflex Arc
Integration
Centre
Muscle
(Effector)
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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)
10. 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
11. 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
12. 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
14. 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
15. 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)
16. 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
19. 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
Editor's Notes
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.).
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.
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)
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).
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).
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).
We will focus on a couple of reflexes which are also relevant for various athletic activities.
Muscle spindle is a sensor organ for the length and the rate of change in length of the muscle.
Golgi tendon organ is a sensor for tendon tension.
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).
Reciprocal inhibition assist coordination of antagonists in various motor tasks.
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.
