Download CMSC 828D – Human Level AI and Computational Cognitive

Report #3 – Monosynaptic Stretch Reflex
CMSC 828D – Human Level AI and Computational
Cognitive Neuroscience
Prof. Don Perlis
Hua He
University of Maryland, College Park
Reflex is an automatic response to a stimulus, or changes within or outside our body. Reflex is
typically fast and subconscious, because most reflexes do not require much brain activities.
Reflexes can maintain heart rate, blood pressure, and can carry out automatic actions such as
swallowing, coughing and vomiting. The monosynaptic stretch reflex, which is the topic of this
report, is one of the most straightforward types, mainly focus on maintaining balance and posture,
for example, it can control trunk and limb muscles of our human body.
Studying the monosynaptic stretch reflex circuit is beneficial, because this unique model can allow
us to have a better understanding of how a real neuronal circuit develops. In this circuit, its
synaptic connections are highly precise, and all of its components can be easily accessed for
further functional investigations. [1]
A. Simple Reflex Arc
The reflex circuit, or reflex arc, has two general components: one is for sensory purpose, and the
other is for the motor/response purpose. The reflex arc governs the operation of reflexes, and is
used to transmit the nerve impulses through the nervous system. It works as follows: a part of the
sensory neuron, called receptor, is made sensitive to specific types of internal and external
changes, and can detect a stimulus. Then a sensory neuron can transmit the impulse to the spinal
cord (for the case of monosynaptic stretch reflex) from its receptor. Within the spinal cord, there is
a type of neuron called interneuron, which can serve as processing center because this neuron can
relay the nerve impulses and conduct those impulses from the sensory neuron to a motor neuron.
Then a motor neuron transmits a nerve impulse out from the spinal cord to an effecter in a
peripheral region. This whole process can be shown below in Figure 1 below. [2,4]
FIGURE 1 – A Simple Reflex Arc: Five Step Process. [7]
We use the word ‘monosynaptic’ because the integration center (As shown in Figure 1, the gray
matter of the spinal cord) only involves one synapse. A more complicated version is called
‘polysynapse reflex arcs’ where one or more interneurons are involved in the integration center
part, but for the case of the stretch reflex, we are just interested in ‘mono’ type for now. [2]
An effector is a muscle or gland that receives the impulse from the motor neuron. In somatic
reflexes, the effector is skeletal muscle. In autonomic (visceral) reflexes, the effector is smooth or
cardiac muscle, or a gland. [3, 6]
B. Real Examples
We can easily get a good ‘feeling’ of such monosynaptic stretch reflex, for example, when we are
seated on a doctor’s examination table, and a doctor uses a small rubber hammer to tap the tendon
below our kneecap. At that moment, our leg kicks outward. This kick is called a knee-jerk reflex
or patellar reflex. This process of our leg reflex can be shown below in Figure 2.
FIGURE 2 - A Knee-Jerk Reflex [3]
The monosynaptic stretch reflex can be described below by [5]: stretching the muscle spindle
activates the annulospiral endings of the spindle. Axons from the annulospiral endings (la fibers)
enters the dorsal root of spinal cord and synapse with alpha motor neurons. Simulation of the
alpha motor neurons causes the extrafusal muscle fibers to contract. Basically the above
descriptions are the same as what we have seen in the first part of this report.
For another example, Suppose now we want to get off the examination table and stand. The
gravity exerts pressure on our legs, and our legs should go away from our body. If this pressure is
not contracted, we will fall and our leg will buckle. In this case, the patellar reflex again kicks in,
and helps produce an opposing muscle contraction to allow us remain standing rather than fall. [4]
In a conclusion, the study on monosynaptic stretch reflex is useful and interesting, it can give us
more insights on how our body reflexes really work, and help us obtain better understanding of
more complicated neural structures.
Bibliography
1.
2012, Wikipedia on Stretch reflex, url: http://en.wikipedia.org/wiki/Stretch_reflex
2.
2012, Cliff Notes on Reflexes. url: http://www.cliffsnotes.com/study_guide/Reflexes.topicArticleId-277792,articleId-277641.html
3.
2012, Image on reflexes. url : http://blade.is/modules/com_yvcomment/?monosynaptic-stretch-reflex-1025
4.
2006, Biological Psychology, Stephen B. Klein, B. Michael Thorne. Mcmillian,
5.
2011, Principles of Neural Science, Fourth Edition, Eric Kandel, James Schwartz, Thomas Jessell
6.
2012, Neural Science Online. Chapter 6. Synaptic Transmission in the Central Nervous System,
url: http://neuroscience.uth.tmc.edu/s1/chapter06.html
7.
2007, Lecture Notes on Reflex Physiology, Dr. Ali Ebneshadi:
url: http://www.lamission.edu/lifesciences/lecturenote/AliPhysio1/Chap%206%20Reflexes.pdf