Spindles Versus GTOs, and Withdrawal Reflexes

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From the course by Duke University

Medical Neuroscience

1042 ratings

Duke University

Medical Neuroscience

1042 ratings

Medical Neuroscience explores the functional organization and neurophysiology of the human central nervous system, while providing a neurobiological framework for understanding human behavior. In this course, you will discover the organization of the neural systems in the brain and spinal cord that mediate sensation, motivate bodily action, and integrate sensorimotor signals with memory, emotion and related faculties of cognition. The overall goal of this course is to provide the foundation for understanding the impairments of sensation, action and cognition that accompany injury, disease or dysfunction in the central nervous system. The course will build upon knowledge acquired through prior studies of cell and molecular biology, general physiology and human anatomy, as we focus primarily on the central nervous system. This online course is designed to include all of the core concepts in neurophysiology and clinical neuroanatomy that would be presented in most first-year neuroscience courses in schools of medicine. However, there are some topics (e.g., biological psychiatry) and several learning experiences (e.g., hands-on brain dissection) that we provide in the corresponding course offered in the Duke University School of Medicine on campus that we are not attempting to reproduce in Medical Neuroscience online. Nevertheless, our aim is to faithfully present in scope and rigor a medical school caliber course experience. This course comprises six units of content organized into 12 weeks, with an additional week for a comprehensive final exam: - Unit 1 Neuroanatomy (weeks 1-2). This unit covers the surface anatomy of the human brain, its internal structure, and the overall organization of sensory and motor systems in the brainstem and spinal cord. - Unit 2 Neural signaling (weeks 3-4). This unit addresses the fundamental mechanisms of neuronal excitability, signal generation and propagation, synaptic transmission, post synaptic mechanisms of signal integration, and neural plasticity. - Unit 3 Sensory systems (weeks 5-7). Here, you will learn the overall organization and function of the sensory systems that contribute to our sense of self relative to the world around us: somatic sensory systems, proprioception, vision, audition, and balance senses. - Unit 4 Motor systems (weeks 8-9). In this unit, we will examine the organization and function of the brain and spinal mechanisms that govern bodily movement. - Unit 5 Brain Development (week 10). Next, we turn our attention to the neurobiological mechanisms for building the nervous system in embryonic development and in early postnatal life; we will also consider how the brain changes across the lifespan. - Unit 6 Cognition (weeks 11-12). The course concludes with a survey of the association systems of the cerebral hemispheres, with an emphasis on cortical networks that integrate perception, memory and emotion in organizing behavior and planning for the future; we will also consider brain systems for maintaining homeostasis and regulating brain state.

From the lesson

Movement and Motor Control: Lower and Upper Motor Neurons

We come now to another pivot in Medical Neuroscience where our focus shifts from sensation to action. Or to borrow a phrase made famous by C.S. Sherrington more than a century ago (the title of his classic text), we will now consider the “integrative action of the nervous system”. We will do so in this module by learning the basic mechanisms by which neural circuits in the brain and spinal cord motivate bodily movement.

Overview of the Motor System14:13

Overview of Motor Units20:19

Muscle Spindles and Myotatic Reflexes, part 118:13

Muscle Spindles and Myotatic Reflexes, part 211:24

Golgi Tendon Organs7:24

Spindles Versus GTOs, and Withdrawal Reflexes9:24

Central Pattern Generation8:52

Lower Motor Syndrome5:34

Meet the Instructors

Leonard E. White, Ph.D.
Associate Professor
Department of Neurology, Department of Neurobiology, Duke University School of Medicine; Department of Psychology & Neuroscience, Trinity College of Arts & Sciences; Director of Education, Duke Institute for Brain Sciences; Duke University

So, let's look at how muscle spindles and Golgi tendon organs might operate in a

coordinated fashion during muscle activity.

And let's begin by considering what happens when a muscle is stretched

against some kind of load that's being applied.

So here the muscle is lengthening, and so there's going to be an increase in

activity in our muscle spindle afferents as this muscle is stretched, okay?

Now, while that stretch is happening, there's really not much change and the

afferent activity derive from the Golgi tendon organ.

Because again, the Golgi tendon organ is not so much sensitive to a change in

muscle length, it's sensitive to the active contraction of the muscle.

And so far, this is a passive stretch of a muscle.

So that's going to activate the muscle spindle afferent and it's not going to

activate much at all, the Golgi tendon afferent.

However, if we were to now actively contract that muscle by stimulating the

alpha motor neuron, what we find is that the muscle is now contracting against

that load, it's getting shorter. And if we didn't fill-in that muscle

spindle afferent by also activating the gamma motor neuron, notice that this

muscle spindle now relaxes. And as the muscle is getting shorter,

there is a drop in afferent activity derived from the muscle spindle.

So, this active contraction is going to unload the muscle spindle.

However, this active contraction provides exactly the stimulus that will drive

afferent activity in the Golgi tendon organ.

So, as this muscle is getting shorter, there is now a dramatic increase in the

output from the Golgi tendon organ. So, we can see how during muscle stretch

and muscle contraction, the spindles and the Golgi tendon organs are tuned to

complimentary sources of information about the movement.

The muscle spindle is sensitive to length but not tension.

The golgi tendon organ is sensitive to tension but not length.

We need them both and they both operate to allow that muscle to function within

the context of the goal that's been established by our descending upper motor

neuronal systems. And lastly, in this part of our tutorial,

let's consider yet a third reflex, and this is the most complicated of the ones

that we've considered. But it's really not so bad once we think

about what has to happen here. So, this reflex is the one that would be

engaged if we were to step on a tack, or perhaps we were to extend our hand and

inadvertently touch a hot surface. the immediate response is to withdraw the

limb from the damaging stimulus. So, this is called deflection withdrawal

reflex or sometimes, it's called the crossed extensor reflex.

And while the circuitry involved here looks rather daunting at first glance,

I'm going to suggest that it really makes perfect sense if we consider the behavior

and what must be coordinated in the central nervous system in order to

produce the desired result, which is the withdrawal of the limb from the

potentially harmful stimulus. Now let's consider, as is illustrated

here, the example of stepping on a tack, okay?

So, the cutaneous receptor in question here is going to be a free nerve ending

supplied by, let's say, our A delta fiber.

So, this is going to be an example of a first pain signal, coming from an A delta

fiber that's transmitted into the central nervous system and notice what happens.

There is the distribution of this incoming sensory signal to a set of

interneurons. Well, why does this need to be so

complicated? well, for very good reason, think of what

would be a desired outcome of this sensory stimulus being transmitted into

the central nervous system. What we would want to do is we would want

to increase the activity of the flexor muscles on the side that encountered the

tack. And in order to facilitate the withdrawal

of the limb, we are going to want to decrease the activity of the extensor

muscles on that very same limb. Now, if we have any hopes of maintaining

a standing posture while we withdraw our foot from the tack, we're going to want

to do exactly the opposite on the other side of the body.

That is, we’re going to want to increase the activity to the extensor muscle and

decrease activity to the flexor muscle. Now, if you just understand what’s going

on with the limbs, then I think you can infer what the circuitry must be like in

the spinal cord to achieve these goals. Knowing that there is what we call,

polysynaptic activation in the circuitry of the spinal cord, that simply means we

have a number of synapses involving several different kinds of neurons that

are interposed between the incoming sensory signal via the A delta fiber and

the outgoing motor signal being derived from the alpha motor neurons.

And these interneurons need to be some of them need to be excitatory, some of them

need to be inhibitory. In order to activate the appropriate

muscles, there needs to be excitatory interneurons linking the sensory signal

to the alpha motor neuron. We need to excite the flexor muscles in

the ipsilateral limb, that is the limb that encountered the attack.

Meanwhile, we need to excite the extensor muscles that are on the opposite side of

the body to maintain posture. So, that must mean that some of these

interneurons must actually expand the midline and allow incoming information

from one side of the spinal cord to impact the output on the opposite side of

the spinal cord. Now, let's look at the actual circuitry

that we find here and see what elements that we can detect.

So, with respect to the flexor muscle on the ipsilateral side of the body, what we

have is basically disynaptic excitation. So, we have one interneuron, a second

interneuron, and then the activation of the alpha motor neuron that supplies the

flexor muscle. So disynaptic excitation to the

ipsilateral flexor. With respect to the extensor muscle that

we want to suppress, we have disynaptic inhibition that is, we have two synapses

first an excitatory interneuron. And then, the signal is inverted by

virtue of the fact that we now have an inhibitory interneuron that is

responsible for the second synapse between the sensory signal and the motor

output. So, with that disynaptic inhibition, the

output to the extensor muscle is suppressed.

Now, on the opposite side of the body, we have polysynaptic excitation, again just

meaning multiple synapses perhaps at least two or three are present here, that

convey an excitatory signal, eventually to the alpha motor neuron that supplies

the extensor muscle. And that allows us to maintain our

posture even while we are disturbing posture by withdrawing the limb from the

tack. Meanwhile, we have polysynaptic

inhibition, that is applied to the antagonistic muscle on the opposite side

of the body. So, we have some number of interneurons,

maybe two or three, that ultimately lead to an inhibition of the alpha motor

neuron that innervates the flexor. So, this is a beautiful example of how

circuitry can coordinate the output of our alpha motor neurons.

And that circuitry can do the job of either increasing the output of the alpha

motor neuron, or decreasing it depending upon if the circuitry interposes an

exitatory interneuron or an inhibitory interneuron between the incoming sensory

signal, which is always going to be an excitatory signal.

And then, the output from the alpha motor neuron, which likewise, is always going

to be an excitatory signal. So, the way that we increase or decrease

the output of the spinal cord to our muscle is by either exciting the alpha

motor neuron or inhibiting it through the activity of an interneuron.

In the next part of this tutorial, we're going to consider one particular kind of

circuitry that will be similar in nature to what we've just described here.

It's one that will coordinate the activity of flexor and extensor muscles

on both sides of the spinal cord, and it's the kind of circuitry that's

essential for generating the rythmical behavior that supports locomotion.

We call this kind of circuit essential pattern generator.

So, that's the topic that we come to next.

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