Babinski reflex – how does it work and what does it indicate?

For the neuromodulation tests on patients, we use certain neurological examinations which give us direct indications of certain weaknesses or disorders of the brain and spinal cord, etc.

There are simply things that I somehow didn’t understand well or didn’t remember well from my studies.

Even during my studies, I only remembered the Babinsky as “indicating damage to the pyramidal tract” – but what exactly is all that? Yes – of course – it goes through the pyramid and that is the intersection – but what exactly is that and what exactly does it indicate?

Summary up front

Babinski = sign of a dampened excessive muscle reflex because there is damage somewhere along the long stretch between the cortex = primary motor center and the part of the spinal cord for the toes.

In anatomy, you learn about this path as the TRACTUS CORTICO-SPINALIS = path from the cortex (cerebral cortex) to the muscle-corresponding spinal segment of the spine.

Whenever this first motor neuron is damaged, a positive Babinski sign appears and usually also spastic muscles, since the normal inhibition of reflexes by the central nervous system is missing.

These long nerve fibers are “white matter” in the brain – therefore “a positive Babinski is damage to the white matter” = axons – that’s what Petros always says – but in fact I have the same problem when the gray matter = cell nuclei / cell bodies are damaged.

In any case, there is damage to the line from the brain down to the spinal cord and this line has several names and they are called alternately and that actually confused me.

Confusing nomenclature

Actually it is the same thing:

Tractus corticospinalis – the path from the primary motor cortex in the precentral gyrus down to the spinal cord segment of the muscle

Pyramidal tract = the same, different name – because this path runs crossed down in the pyramid on the brain stem (medulla) on the front.

first motor neuron – is the same, different name – is the functional description – because it is the path from the brain down to the muscle segment of the spinal cord, where it is connected to the second motor neuron and this transmits the signal from the spinal cord to the muscle

white matter – are the long extensions of the nerve cells, which run together in “paths” and transmit “the signals”. Of course there are also other “white matter” paths apart from the corticospinal tract

Complicated control of the muscles

Muscle self-reflex – spasticity

the muscles have a “self-reflex” which is formed by stretch receptors in the muscle –> any change in muscle tension or stretching is immediately reported back to the spinal cord and triggers a reflex that leads to “opposite” muscle activity.

If a muscle is stretched, the reflex immediately causes it to contract – up to a maximum tension that is preset from above, then the muscle suddenly relaxes.

We use this pre-setting in kinesiology because it is easy to “test” and thus obtain clues about stressors in the brain stem: what causes stress weakens the maximum strength (of the kinesiological test), what relieves stress and brings balance increases the maximum strength of the muscle being tested.

This “monosynaptic” self-reflex runs from the stretch receptors into the spinal cord and is switched directly to the muscle-activating outgoing nerve, so only a single synapse in between, which is why it is so fast, hence the “monosynaptic reflex”.

This nerve, which goes from the spinal cord out into the muscle (efferent) is “the second motor neuron“.

If this efferent muscle-own nerve is damaged –> no more information comes from / via the spinal cord to the muscle —> flaccid paralysis (with fasciculation = twitching).

Modulation of the muscle reflex by the first motor neuron

The first motor neuron (in the motor cortex) has an important influence on the muscle reflex by exerting a dampening or regulating effect on the reflex arc in order to keep the muscle movements smooth and controlled. Despite this regulating role, the first motor neuron also enables voluntary movements.

– The first motor neuron (also known as the upper motor neuron) is located in the primary motor cortex and is responsible for targeted motoric signals to the spinal cord.

– These signals travel via the pyramidal tract and modulate the activity of the lower motor neurons.

– The modulation of reflexes by the first motor neuron occurs mainly via the interneurons in the spinal cord, which act as “filters” or “dampers” for reflex signals. This prevents reflexes from running uncontrolled.

Balance between reflex activity and voluntary motor activity

– The first motor neuron partially suppresses spontaneous reflex activity in order to enable targeted and voluntary movement. This means that reflex activity is not completely switched off, but rather regulated in such a way that muscle tone remains, but the muscles can still be targeted controlled.

– This means that the muscle reflex can still provide stability (for example when standing), while at the same time voluntary commands from the first motor neuron (such as bending or stretching a joint) can be carried out without the reflexes interfering.

Voluntary muscle movement through the interaction of both systems

– For a voluntary movement, the first motor neuron sends signals to the lower motor neurons in the anterior horn of the spinal cord.

– These lower motor neurons (cell bodies in the spinal cord) then innervate the skeletal muscles directly and initiate a targeted muscle contraction.

– During this process, the signals from the first motor neuron also dampen reflexive activities in the muscles so that the muscle is not disturbed in its movement by unwanted reflexes.

– The conscious, targeted control by the first motor neuron ensures that the movements are coordinated and smooth. At the same time, reflexes that could be triggered by external stimuli (such as sudden stretching) are reduced to a level that is useful for the movement.

Fine control and adaptation

– During a voluntary movement, the first motor neuron and various other regions of the brain (such as the cerebellum and the basal ganglia) are involved in the fine control of the movement.

– This not only activates the muscles, but also adapts the movement accordingly and adjusts it to external influences. The muscle reflex also plays a role here by making short-term adjustments to muscle tension in order to react to external stimuli, such as sudden resistance.

Summary of movement control

The first motor neuron regulates reflex activity to enable targeted, voluntary movements by sending

• inhibitory signals that keep the intrinsic reflexes in balance.

For a voluntary movement, the first motor neuron sends

• activating signals to the lower motor neurons, which then control the muscles.

This creates a controlled muscle contraction in which reflexes are reduced but not completely switched off.
This enables both targeted movement and the maintenance of stability and protective mechanisms through the intrinsic muscle reflex.

Pyramidal tract = first motor neuron = tractus corticospinalis

these are the very long nerve endings that run from the motor center of the brain (gyrus precentralis) down to the spinal cord, where they are then switched to the muscle nerve (the second motor neuron). So when activating signals come from above in the brain, the monosynaptic self-reflex must be partially or completely turned off, which happens via the dampening interneurons.

a first motor neuron can address different structures

I was wondering how the brain blocks the self-reflex and initiates voluntary movement at the same time – does this happen via the same first motor neuron or via separate channels downwards?

Answer: there is a frequency modulation and a summation modulation:

at high action potential frequency rates, the inhibitory interneurons that suppress the muscle reflex are addressed more, at low to medium frequencies, the motor neurons are addressed more.

then there is convergence (summation): many motor neurons pass on their signals to the target structures at the same time, if more signals come from above, i.e. via several motor neurons, the reflex is blocked and the voluntary movement is carried out.

In diseases that affect the first motor neuron = tractWhen the corticospinal nerve is cut off = the pyramidal tract, the modulation and dampening of the first motor neuron is lost. As a result, the muscle reflex is undamped and therefore overactive.

• Stroke
• Multiple sclerosis
• Spinal cord damage
• Brain tumors
• Amyotrophic lateral sclerosis (ALS) – “upper disorder” – in the “lower ALS” which (also) affects the 2nd motor neuron, flaccid paralysis occurs, in practice you often see mixed parts: on the one hand spastic, on the other hand flaccid + atrophy

With Parkinson’s I have RIGOR – no spasticity

The first motor neuron is intact, but the fine programming in the basal ganglia no longer works correctly.

The increased tone in Parkinson’s is not caused by reflexes (as in classic spasticity, where there is excessive intrinsic reflex activity), but by a disturbed regulation of the agonistic and antagonistic muscle groups.

• If the first motor neuron is damaged, such as in a stroke or multiple sclerosis, there is a direct interruption of the pyramidal tract. This leads to disinhibition of the reflex arcs and resulting hyperreflexia and spasticity.
• With Parkinson’s, on the other hand, the reflexes are usually normal. (says ChatGPT, I can’t always confirm this from practice).  The muscle tone is increased, but not due to overactive reflex activity, but rather to a disturbed modulation of the central movement control by the basal ganglia.

Babinski and Hofmann

A positive Babinski reflex (Babinski sign) usually indicates a disorder of the central nervous system, in particular damage to the pyramidal tract, which controls voluntary motor skills. The reflex is described as “positive” if, when the sole of the foot is stroked along the outer edge, the big toe is stretched upwards (dorsally) and the other toes spread out like a fan.

If there is no disorder, it also has a nice name – namely FND = functional neurological disorder (= not a disease, just a functional change that is present here).

The Hoffmann sign is often referred to as the “Babinski sign of the upper extremity”, as it also indicates a pathology of the first motor neuron (upper motor neuron), but in the upper extremities.

Just as the Babinski sign is a pathological sign of pyramidal tract damage in the lower extremities, the Hoffmann sign indicates a lack of inhibition of reflexes in the upper extremities, often due to damage to the cervical spine (C-spine) or the upper motor neurons.

A positive Babinski test is normal in infants (since the nervous system is not yet fully developed), but pathological in adults. Possible causes are: