physiology lecture #7 (ns2)

8/6/2019 Physiology Lecture #7 (NS2)

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SPINAL CORD REFLEXES

Today, we will talk about the spinal cord in general and spinal cord reflexes, and a few

points about what happens if you get complete transection of the spinal cord and also some points

about consequences of cutting a nerve; What happen to that nerve and so on.

This is a cross section of the spinal cord.

In the spinal cord, we have many types of neurons such as sensory, motor and interneurons.

In the dorsal aspect; we have sensory fibers coming from the periphery with different types of

sensation. And we have a lot of interneurons. And if you remember from first year lectures, when

we talk about the Central Nervous System (CNS), most of neurons inside our CNS are actuallyinterneurons, and few sensory ganglia.

And you can see here that the interneuronsare performing a very important function

because they are part of the neuronal circuits which serve many functions. And we have this motor

neuron ,its cell body is found in the anterior aspect (gray matter) of spinal cord. And we have one of

interneuron which is called Renshaw inhibitory cell, this neuron is excitedby certain neurons and

it inhibits other neurons.

Just to give you an idea about this Renshaw inhibitory cell. Lets take this circuit. This is

sensory neuron, this is interneuron and this is motor neuron, and from this motor neuron, we can

see an impulse going through Renshaw inhibitory cell to inhibit either the same interneuron orother interneurons.

So by this way, the excitation will go on this neuron as if we are concentrating the excitation

only in one neuron or group of neurons.


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SPINAL REFLEXES

The spinal cord is the central part of what we call spinal reflexes. There aremany types of

spinal reflexes.

Reflex:Automatic rapid response to different stimuli of whatever typesuch as pain, mechanical, and

so on.

And these are the components of a reflex:

1. Receptor2. Sensory fiber3. Integrating center (spinal cord)4. Motor fiber5. Effector (mostly skeletal and smooth muscle)

And we have 2 types of reflexes depending on how many synapses are involved.

1. Monosynaptic (e.g. stretch reflex)2. Polysynaptic (e.g. Golgi tendon reflex, withdraw reflex, and cross-extensor reflex)

By the way, all reflexes are polysynaptic except the stretch reflex.

There is a question from a student, but I cant hear it (I suppose it was something like if

monosynaptic is stronger than polysynaptic or not?). So, the doctor asnwered:

It is not stronger or not, but depends on strength of stimulus, if the stimulus is strong, the impulse

will be strong. So doesnt mean it is strong, but definitely it is faster. It takes short time to finish

because you have only one delay. If you have two delays, definitely the time will be more.

STRETCH REFLEX

It is the only spinal reflex that has 1 synapse, and because of that, the time for reflex to be

excited and finish is short if you compare it with other polysynaptic spinal reflexes.

This reflex is very important. Just to remind you about this reflex, very simple and used all

the time as we will see in just few minutes.

As we said, reflexes consist of a receptor, sensory fiber, integrating center, motor fiber, and

an effector. So lets apply this equation together here. This is skeletal muscle, and the receptor here

is called muscle spindle. The stimulus here is stretching, so in order to excite or start this reflexyou have to stretch the muscle.


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So this is muscle spindle that responds to stretching. The sensory fiber going to spinal cord,

and as you can see there is only one synapse. Then, the motor fiber will go to the same muscle that

has been stretched. About the response, because it is excitation, there will be contraction.

Stretching > stimulation > response > contraction.

There are 2 (sensory and motor) neurons and 1 synapse. Then, the sideway or collateral of

this reflex, of course we have inhibition of the antagonist muscle. For instance if you are stretching

the biceps, you have to inhibit the triceps.

So if you apply it here there is a collateralneuron (inhibitory) going to the triceps. If

we assume this is biceps. So we have what we call collateral but the basic components of

this reflex are two neurons, one synapse and so on. The fiber going to excite the muscle is

alpha motor neuron (thickest nerve fiber).

MUSCLE SPINDLE

Where are the muscle spindle fibers?. This is a diagram of a skeletal muscle. These dark

lines are muscle fibers, responsible for contraction. And you can see the muscle spindle

here*. They are between the muscle fibers.

@


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Now the muscle spindle has some contraction fibers, so we have to differentiate

these fibers from the muscle fibers (responsible for muscle contraction).So we have 1)

Extrafusal muscle fibers> responsible for whole muscle contraction , and 2) Intrafusal

muscle fibers> responsible for the contraction of the muscle spindle fibers.

Another important point is the arrangement of these receptors (muscle spindles), itisparallel to the longfibers (extrafusal). And we will see why it is parallel.

Two types of nerve fibers coming to the muscle spindle receptors sensory and

motor-. So in addition to being a sensory receptor, muscle spindle is also a contractile

element(motor).

The motor fiber going toIntrafusal is gamma. The nerve fiber going the Extrafusal is

alpha. The other point from this diagram, you can see that the gamma motor fiberis

goingto the peripheral part (the two ends) of the receptor.

So we can say -from this picture- that the contractile element (actin and myosin)is

actually found in the periphery. So muscle spindles are sensory receptors and they have a

contractile element(actin and myosin) which is found in the periphery or at the edge.

So what is the function of this contraction (referring to muscle spindle)?

The muscle spindles are the stretch receptors (excited by stretching). So if there is

excitation of the motor fiber to the two ends, we are pulling the central part of the receptor.

Of course this stretching is not enough to excite but because it leads to elongation of musclespindle, it means the sensitivity is increased.

So the contraction of the both ends will increase the sensitivity. In other words, if

you want to increase the sensitivity of the muscle spindle to stretching, you can stretch the

two ends.


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The central part is sensitive to stretching. If you stretch it too much, you may excite

the sensory fiber. This central part sensitivity can be increased if you stretch it alittle bit.

This stretching is done by the contractile element here. As if you are pulling this central

part in opposite direction. So if this central part is a little bit stretched, you need less

strong stimulus to excite it.

Again. The central part is the sensory-sensitive to stretching-, while the peripheral

part is the contractile element. Contracting the muscle from opposite direction will

stimulate the stretch receptor at the center.

So the idea of this gamma motor fiber is to increase the sensitivity of the muscle

spindle. Consequently, you are increasing the contraction of the skeletal muscle because at

the end, the aim of stretching is to cause contraction of the skeletal muscle and these motor

fibers (gamma) are so important.

30% of the motor nerve fiber in the anterior aspect of the spinal cord are of gamma fiber.This gives you an idea about how important this stretch receptor is.

Muscle spindle is working all the time to keep the muscle in certain strength/tone.

Sensory nerve fibers for the muscle spindle are of two types: 1. Primary ending (Ia)

2.Secondary ending (II). There are different sensory fibers, providing certain functions.

Let us see the slide.

This is again the muscle spindle showing a little bit more detail. We have two types of

muscle spindles; 1. Nuclear bag, 2. Nuclear chain.

The central part resembles a bag and a chain respectively. So just to differentiate

between two points. These are the contractile elements*.

Also notice the two types of sensory fibers, and the gamma motor fibers.


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So if you contract the two contractile ends, the center will be stretched. 30% of

fibers in ventral roots are gamma fibers. It means that these muscle spindles are very very

important for function of motor activities and so on.

By the way, there are two types of stretching; 1. Static, 2. Dynamic

1. StaticIf you are stretching muscle once and stop. [hold it stretched]

2. DynamicIf you are stretching -during the process of stretching.

Dynamic and static stretching will excite certain types. Both primary and secondary

are excited when muscle spindle is elongated. It means that if the muscle fiber is still

elongated, both fibers (primary and secondary) will be stimulated. But during the process

of excitation the primary is responsible (dynamic response).

So we have two response; 1. During stretching 2. When the muscle is stretched.

Muscle contraction can result form 1) direct excitation of alpha motor. For instance,

impulses from motor area going down to the alpha motor supplying the skeletal muscle.

2)indirect through excitation of muscle spindle. If you excite the sensory nerve fiber, this

will go through the spinal cord and to reflex going directly again to the muscle to cause

excitation.

ACTIVATION OF MUSCLE SPINDLE SERVES MANY FUNCTIONS

So what is the function of this muscle spindle?

First of all these muscle spindles actually they are making our motor activity more

smooth. To explain this, normally if there is excitation to the alpha motor fiber it makes

excitation, then we will have relaxation. If this happens, you get like this response

(excitation-relaxation-excitation-relaxation). This leads to oscillation (jerky movements).

If you want to move, your motor activity will be fragmented(left figure). But because of

this stretch reflex, during stretching and relaxation there is a little bit more excitation to

muscle spindle leading to partial contraction. So the contraction will be like smooth

course (Right figure).


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Secondisstabilization of bodyposition (joints). Usually during movement there is

stretching-relaxation. Stretching will lead to excitation and so on. In general we will get

the stabilization of the joint. So muscle spindle is very important for this thing.

Third is maintaining muscle length. Follow the above figure. For example: If you

stretch skeletal muscle this will lead to stretching of muscle spindle. This will lead to

excitation of stretch reflex and leads to contraction of skeletal muscle.

Stretching/elongation, in order to retain the normal muscle length through excitation of

stretch reflex the muscle will be shortened by contraction.

If there is a decrease in muscle length during contraction, it means less stretching to

muscle spindle, and there is less excitation to this reflex. In other words, there is muscle

relaxation. So we can see both during contraction and relaxation there is some sort of

inhibition or excitation of muscle spindle reflex and this will make our movement and the

tone of the muscle almost kept constant.

Sometimes, in certain position we need to increase it or decrease it. If we want to

increase it simply you stimulate the gamma fiber and this is from the higher centers (e.g.:

vestibular nuclei/reticular formation/red nucleus). All these sending impulses down to the

gamma motor fiber.

If it is excitation, it means the muscle tone will be increased. Sometimes we increase

it and sometime we decrease it.

GOLGI TENDON REFLEX

Golgi is the person who describes this reflex. Actually I can say this reflex is exactly

opposite to stretch reflex.

So let us see first of all the circuit of this reflex. This is the receptor and you can see

it is found in the tendon or at the junction between the tendon and the muscle fiber and it is

called Golgi according to the first describer.


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This receptor is in series with the muscle fiber. If you remember the muscle fiber

and the muscle spindle are arranged in parallel. Muscle spindle is excited when there is

stretching, while Golgi tendon is excited when there is contraction (Increase in tension).

So during contraction of muscle, there will be excitation of this one and will be

conveyed by sensory fibers to the center and then through this motor fiber (it is

inhibitory). It inhibits that muscle.

So let us say it again. Contraction of the muscle leading to excitation of the tendon

receptor and this will lead to relaxation or inhibition of contraction. So as if it is increasing

the tension leads to relaxation of that muscle. So this is exactly opposite to the stretch

reflex. The difference is obvious here.

Both stretch reflex and Golgi tendon are necessary to keep muscle tone. And this is

very important and sometimes confusing.

What is muscle tone?

It is the partial/basal contraction of skeletal muscle. It means all muscles have some

contraction in it. This tone is due to the activity of muscle spindle and Golgi tendon.

If you cut the sensory fiber, the muscle tone will be decreased. This is called

hypotonia. It is not paralysis because you are decreasing the tone- especially the muscle

spindle-. Muscle tone is increased when there is impulses coming from the brainstem(eg:

reticular formation) through the gamma, increasing the sensitivity of stretch receptor

hence increasing the muscle tone.

A student asked: why sensory destruction leads to hypotonia?


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Answer: a muscle tone actually depends on these reflexes which work all the time.

Put it in this way. Just imagine this is stretch reflex. Always there is muscle contraction

even you are not using your muscle due to some sort of excitation of this stretch reflex. If

you cut this one, there is no stretch reflex which leads to less contraction of muscle. That is

why muscle becomes hypotonic/flaccid. But this is not paralysis. Paralysis is due to

cutting of the motor neuron.

Another response from the student which I couldnt here it well.

But the doctor responded: mostly stretch reflex is responsible for muscle tone. They

are helping each other in order to maintain certain muscle tone. Muscle tone is mainly due

to gamma which is belonging to stretch reflex. This (Golgi) is helping but not essential.

WITHDRAWAL REFLEX

I think you can figure out what is meant by this. The response is flexion when the

stimulus is painful. So let us see what this reflex is made of.Stimulus (painful), sensory

through the dorsal rootof spinal cord and you can see through series of interneurons you

will get certain response. The response is flexion.

In addition to that, you can see that we have collateral. One of them is this one which

goes to the antagonist muscle. If we stimulate this (the flexor), we have to inhibit that (the

extensor).

In addition to that, at the same time, this reflex will excite the other side of the body

in the opposite manner. Here (side of pain) the flexion is stimulated, there (opposite side)

the flexion is inhibited. While here the extensor is inhibited and there the extensor is

stimulated. That is why it is called cross extensor reflex.


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Actually every time when we withdraw something (e.g.: leg), the other side will be

extended in order to keep the balance. So this is called cross extensor reflex. Original one

is withdrawal/flexor because it involves the flexors.

Other important thing is if stimulus is painful your response is multi. It is causing

correction of flexor that is why it is called flexor reflex and sometimes it is calledpolysynaptic reflex. (!!)

Question: is the same pain stimulus causing the reflex and the perception of pain?

Answer:We are talking about reflex, at the same time we have collateral tract here,

but it is painful of course if we go this direction it will cross. Again it will ascend through

the anterolateral tract to the brain. But we are talking about reflex. You are withdrawing

your hand before perceiving the pain.

SPINAL CORD TRANSECTION

Let us talk a few points about what happens if there is a complete transection in the

spinal cord. I think you are familiar now about the motor and sensory tracts. What

happens if there is hemisection in one side? How it affects motor and sensory activity. I

want to concentrate here on complete transection of the spinal cord and how it affects the

reflexes.

** Here just to give you an idea about what happens to the voluntary movement if

you get transection below cervical enlargement of the spinal cord. It means almost at the

level of C6/C7. So you will get paralysis of the lower part of the body leads to paraplegia

(paralysis of the lower part of the body).

If it is cervical transection -say at cervical segment 4/5- you will get maybe

quadriplegia because nerve fibers going to upper extremities will be there.

Transection in sacral region will cause paralysis of the lower limb similar to

paraplegia.

** With this actually you will get loss of all sensations because you are cutting all the

nerve fibers going up and down.

**The other thing you can see is that there is complete loss of all reflexes [somatic

(stretch reflex) and autonomic (vasomotor, blood pressure, sweating, bladder, defecation)

reflexes].

This areflexia (complete loss of reflexes) after transection is not well-explained.

But it seems that there is cutting of all fibers coming from the higher centers especially


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from the brainstem and so on. This areflexia is called spinal shock. It lasts for a few

weeks. It depends on many factors.

Recovery:

Because we are talking about reflexes which are not correlated to the highercenters. So it seems that the reflex must be there. Actually the reflex is retained back

gradually (after about four weeks).

First reflexes to recover are the bladder and rectal, after that maybe stretch reflex.

Not only recovering but also getting exaggerated. So, as it seems that the inhibitory impulse

comes (used to come) from the higher center.Cutting the inhibitory impulse leading to

stretch reflex alone, so it is exaggerated.

Mass reflex : activation of many reflexes at the same time in persons with spinal

cord transaction.

If you stimulate certain point the lower inner aspect of the thigh you will cause

excitation of all reflexes.

It will cause :

Strong muscle contraction Evacuation of bladder and rectum Increased blood pressure Increased sweating

So, you are reversing all these things (areflexias) by mass reflexes and this is done

actually by some research when there is spinal shocks and after one week there is still no

evacuation of bladder or whatever, they do this mass reflex by excitation of somewhere.

You know what is LOWER MOTOR NEURON (LMN) or UPPER MOTOR NEURON (UPN)

lesion??

A student asked regarding respiratory muscles.

Doctor answered: if you cutting the fibers going to respiratory muscle, it means DEATH. If

you cut some maybe C3, C4 or C5 and there is some of respiratory nerve fiber it will be

affected.

But if you cut all the nerve that go to respiratory muscle it means DEATH.


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LOWER MOTOR NEURON LESION

We have 2 types of motor neurons that come from the higher center.

Pyramidal

Extrapyramidal

They control all motor neuron.

If you cut motor neuron, this is called LMN lesion

If you cut higher center neurons, it is called UMN lesion

What happens if there is LMN lesion.

Flaccid paralysis of muscle or paresis Muscle atrophy Appearance of spontaneous contraction (fasciculation or fibrillation) Absence or decrease in muscle stretch reflex Muscle involve individually.

EFFECT OF MUSCLE DENERVATION

FASCICULATION FIBRILLATION

MUSCLE ATROPHY

FASCICULATIONIs spontaneous muscle contraction of group of muscles fibers

In the slide : visible spontaneous muscle contraction involved muscle fiber in motor

unit.


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(The damage occured at middle of axon)

There is possibility that aspontaneous action potential will be started at this point

(refer to figure). So, it will be propagated to these muscle fibers.

Thats why fasciculation can be seen because the group muscle fibers depend on the

size of motor unit.

This is the one mechanism that leads to spontaneous muscle fiber contraction.

FIBRILLATIONInvolving single or individual muscle fiber, that is cannot be seen by naked eye.

In the slide :

Contraction (spontaneous) of individual muscle fiber, no visible because it is small

contraction, can be detected by EMG.

The mechanism (see slide)

If there is a cut in the nerve fiber and degeneration, then there is a possibility that

action potential will be liberated involving individual fibersand this is difficult to be

seen.


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MUSCLE ATROPHYThe muscle fiber is receiving trophic substances from the motor nerve. So if you cut this

one, you are depriving the muscle from these trophic substances. So the muscle fiber

will be atrophied.

Or if you dont use muscle this will lead to muscle atrophy too.

In the slide: when motor fibers to a muscle are completely degenerated. This muscle

becomes silent and undergoes atrophy called denervation atrophy which is a very severe

atrophy.

As your colleague said, denervation will increase the sensitivity of the ACH receptor,(compensation for what has been happened) cutting the nerve means there is no activity in

muscle fiber.

So, this leads to increased ACH sensitivity , if there is liberated ACH near muscle fiber this

will lead to

Fasciculation ( involving many fiber) Fibrillation (one muscle only)

So, ACH receptors in large number.

If there is no ACH for a long time or certain time and no regeneration, this will silent the

ACH receptors and they will disappear.

Done by Faiz AFA.

Special thanks to ArifHakimi and Faiq Nabil for helping me doing this tfree3. By the way this is

my first one. Please forgive me for any mistake. And please, do correct me.

Thank you

physiology lecture #7 (ns2)

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