Local Function of the Spinal Cord: Segmental Spinal Reflexes

Define alpha efferent neuron.

Alpha efferent neuron — with its cell body in the ventral grey horn of the spinal cord, innervates the extrafusal muscle fibre.

Define Gamma efferent neuron.

Gamma efferent neuron - with its cell body in the ventral grey horn of the spinal cord, innervates the polar region of the intrafusal muscle fibre.

Define LMN (Lower motor neuron).

LMN - lower motor neuron - neurons associated with motor function that have their cell body in the CNS and their axon leaves the CNS in a cranial or spinal nerve to synapse via a neuromuscular junction with striated, smooth or cardiac muscle. They could be considered to be peripheral motor neurons.

Define UMN (Upper motor neuron). 

UMN - upper motor neuron - neurons confined to the CNS that are associated with motor function. They could be considered to be central motor neurons

Define Reflex arc.

Reflex arc - involves both the peripheral nervous system and the CNS. Sensory/afferent nerve brings information into the CNS where the input is linked to a motor/efferent nerve that connects back to a muscle. Hard-wired into the nervous system.

Define or explain the response/reaction of UMN and LMN.

Response/reaction - cortically mediated. Sensory/afferent information is integrated and interpreted in the cerebral cortex and a motor/efferent response is initiated as a consequence.

What is the clinical relevance of neurological examination?

to localize the lesion within the nervous system

What regions can lesions of the SC be localized to?

What is the importance of the interconnectedness of neurons?

Constant interconnection among sensory (afferent) and motor (efferent) neurons

• Always a receptor 

• Afferent path → CNS (Brain + SC)

• Effector Path

• Effector

Describe the differences between a response and a reflex.

• Response must arrive and integrate into forebrain (cerebral cortex)

What is a reflex arc and what comprises it?

A neural circuit which comprises:

• Receptor: collect the sensory information.

• Afferent pathway: sensitive neurons which transmits the information from the receptor to the next neuron located at the CNS.

• CNS center: process the information and elaborate an involuntary reaction.

  • - In the context of the spinal reflexes, the CNS region is the SC

    - In the context of the response, the CNS region is the forebrain

    - In the context of the cranial reflexes, the CNS is the brainstem

• Efferent pathway: motor fibers which transmit the reaction.

• Effector: implement the response. They are the muscles and glands.

What is the key feature of a REFLEX.

INVOLUNTARY/ UNCONSCIOUS = Do not cross the brain cortex

What happens when we stimulate the patellar reflex?

1. Tapping below patellar tendon → stimulates the stretch receptors as touching tendon stretches the quadriceps

2. la Afferent sensory nerve (femoral nerve) carries stimuli to the dorsal root of spinal cord

3. Stimulates efferent motor nerve in the ventral horn, which innervates muscles causing contraction of the quadriceps

   1. At the same time, inhibits the alpha motor neuron to antagonistic muscles, ensuring they are relaxed to initiate movement

What are some types of spinal reflexes (or segmental reflexes)?

- Pelvic limbs

- Thoracic limbs

- Cutaneous trunci reflex

- Perineal reflex

What are the parameters for testing of spinal reflexes, specifically:

• How should pt be restrained?

• What tools should be used?

• Patient restrained in lateral recumbency (but also accepted in stand-up position)

• Only test "upside" limbs

• Extreme care or even abandon if vertebral column instability suspected

• Instrument:

• Rubber mallet

• Instrument handle

What nerves contained within the lumbosacral plexus do the following functional tests evaluate?

• Flexor (Withdrawal reflex)

• Patellar Reflex

• Perineal Reflex

• Flexor (withdrawal) reflex

• Evaluates sciatic + femoral nerve and L4-S1 spinal cord segments

• Patellar reflex

• Evaluates femoral nerves + L4-L6 spinal cord segments

• Perineal reflex

• Evaluates pudendal nerve, caudal nerves + S1-Cd5 spinal cord segments

How should the patellar reflex test be performed?

• What anatomy is involved?

Clinical examination

- Hold patient's limb such that stifle is semi-flexed

- Percussion of patellar tendon (muscle stretch receptors)

- The result of this reflex is a brisk, controlled extension of stifle

Anatomy involved (afferent & efferent pathways)

• Femoral nerve

• Spinal cord segments L4-L6 (cranial aspect lumbar intumescence)

How should the pelvic limb withdrawal test be performed?

• What anatomy is involved?

Clinical examination

- Lateral recumbency

- Pinch digit with fingers (stimulate nociceptors)

- Varying degree of stimulus

- The result of this reflex is the flexion of hip, stifle and tarsus

Anatomy involved (Afferent & efferent pathways)

• Sciatic nerve (= flexion of the stifle and tarsus)

• Femoral nerve (= flexion of the hip)

• L4-S1 spinal cord segments

How should the perineal reflex test be performed?

• What anatomy is involved?

Clinical examination

- Pinch perineum (haemostat) to both sides

- Sensory receptors perineum

- The result of this reflex is a contraction of the anus (anal sphincter) and flexion of the tail

Anatomy involved (Afferent & efferent pathways)

• Pudendal nerve (anus)

• Caudal nerves (tail)

• S1-Cd5 spinal cord segments

What nerves contained within the brachial plexus do the following functional tests evaluate?

• Flexor (Withdrawal)

• Cutaneous Trunci Reflex

• Flexor (withdrawal) reflex

• Evaluates brachial plexus + C6-T2 spinal cord segments

• Cutaneous trunci reflex

• Evaluates sensory nerves of the dorsal trunk (T2-L5) + CTL spinal cord segments + lateral thoracic nerve

How should the thoracic limb withdrawal test be performed?

• What anatomy is involved?

Clinical examination

- Lateral recumbency

- Pinch digit with fingers (stimulate nociceptors)

- Varying degree of stimulus

- The result of this reflex is the flexion of carpus, elbow and shoulder

Anatomy involved (Afferent & efferent pathways)

• Brachial plexus

• C6-T2 spinal cord segments

How should the cutanous trunci reflex test be performed?

• What anatomy is involved?

Clinical examination

- Pinch skin over dorsum (1 inch off midline from level of ileal wings to T2)

- The result of this reflex is the bilateral contraction of the cutaneous trunci muscle, whichever side stimulated

- Anatomy involved (Afferent & efferent pathways)

• Sensory nerves of the dorsal trunk (T2-L5)

• C8-L5 spinal cord segments

• Lateral thoracic nerve (level of C8- Т1)

What is the grading scheme for spinal reflexes?

• Absent (0)

• Decreased (1)

• Normal (2)

• Increased (3)

• Clonus (4): repetitive/oscillating reflex following one single stimulation

Broadly speaking, what are UMN and LMN?

All motor functions require the interaction between two motor systems: the UMN and the LMN

• A functional classification of the neurons

• Found in the ventral aspect of the spinal root

UMN - modulates and controls the LMN

What are the key features of the lower motor neurons?

The LMN system is the motor system connecting the CNS with the muscle to be innervated.

• Cell body at the ventral horn of the spinal cord grey matter (or within the cranial nerve nucleus of the brainstem).

• Its axon leaves the CNS by the ventral nerve roots to join, successively, a spinal nerve and then it synapses with an effector muscle.

• The LMN is the last neuron in the chain of neurons that produces the muscular contraction necessary to maintain posture, support weight and provide the gait

  • Involved in voluntary and involuntary reflexes, movements and basal muscle tone

What are the clinical signs of a lower motor neuron lesion?

- Affection of the voluntary movement = weakness or complete paralysis

- Affection of the involuntary movements = reflexes reduced or absent

- Reduced/absent basal muscle tone = flaccidity

- Denervation of the muscles = quick atrophy

Describe the differences between flaccid and spastic paralysis.

Flaccid paralysis occurs when there is damage to the lower motor neurons (LMNs) — the neurons that directly connect the spinal cord or brainstem to skeletal muscle. Because these neurons provide the final pathway for muscle contraction, their loss leads to complete disconnection between the nervous system and the muscle. As a result, the affected muscles become weak or paralyzed, lose tone (flaccid), and reflexes disappear (areflexia or hyporeflexia). Over time, the muscles undergo severe atrophy due to denervation, and fasciculations (visible muscle twitches) may appear as dying motor neurons fire irregularly.

In contrast, spastic paralysis results from damage to upper motor neurons (UMNs) — the neurons in the brain and descending motor tracts that normally control and inhibit LMNs. When these pathways are lost, the LMNs and reflex circuits in the spinal cord remain intact but become overactive because they are no longer properly regulated by the brain. This leads to increased muscle tone (spasticity), exaggerated reflexes (hyperreflexia), and stiff or jerky movements. Muscle atrophy may occur over time, but it is usually mild and due to disuse, not denervation.

What are the key features of the upper motor neurons?

The UMN system is the motor system that is confined to the CNS (brain and spinal cord).

- Its cell body lies within the brain. It travels through the brain and/or spinal cord white matter and synapses indirectly (via an interneuron) with an LMN to modulate its activity (via inhibition). It is responsible for the initiation and maintenance of normal movements.

Involved in: initiation and control (by inhibition) of voluntary movements.

What are the clinical signs associated with an UMN lesion?

Clinical signs observed when damage in UMNs:

- Affection of the voluntary movement = weakness or paralysis

- Spinal reflexes are intact (as LMN is not affected) or spinal reflexes are increased (as there is not inhibition to the LMN).

• A release (or lack) of inhibition from UMNs to the LMN = may results in an increase (vigorous) reflex (involuntary movement).

- Stiffness / spasticity

- When atrophy appears is very slow and due disuse

Why may an UMN lesion cause hyper-reflexia?

A release (or lack) of inhibition from UMNs to the LMN = may results in an increase (vigorous) reflex (involuntary movement).

• Normal UMN function = proper modulation/control of reflexes

• Clonus - UMN affection = there is not inhibition of reflexes (= hyperreflexia)

What are the features of pseudo-hyper-reflexia.

Pseudo- Hyper-reflexia: Reduction in action of antagonistic pair (lack of opposing forces to the reflex).

• Exaggerated (unopposed reflex)

• LMN sciatic dysfunction

• Disproportionately large and uncontrolled stifle extension

• Due to failure of "antagonistic pair"

Summarize the criteria for differentiation from LMN & UMN signs.

• LMN activity modulated by predominantly inhibitory action of the UMN

• Damage to UMN pathways = release of LMN from inhibition = increased extensor tone, increased reflexes & spasticity

• Criteria → help localize the lesions in the patient

Describe the functional regions the spinal cord can be divided into, based upon where the UMN are located and where the LMN are located. 

1. C1–C5 (Cervical Region)

• Contains: only UMN pathways to the limbs (no LMN cell bodies for limbs here).

• Function: UMN tracts descend through this region to synapse lower down.

• Lesion signs:

  • Tetraparesis/tetraplegia (since all limbs affected)

  • UMN signs in all four limbs (increased tone, exaggerated reflexes)

  • Possible proprioceptive deficits.

➡ So yes — C1–C5 = UMN region (to both thoracic and pelvic limbs).

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2. C6–T2 (Cervicothoracic or Intumescence Region)

• Contains: LMN cell bodies for the thoracic limbs, and UMN tracts descending to the pelvic limbs.

• Function:

  • LMNs here form the brachial plexus, innervating thoracic limb muscles.

  • UMN tracts to pelvic limbs pass through this region.

• Lesion signs:

  • Thoracic limbs: LMN signs (flaccid paresis/paralysis, decreased reflexes, atrophy).

  • Pelvic limbs: UMN signs (spasticity, hyperreflexia).

  • Often called a “two-engine gait” — short-strided thoracic limbs + long, spastic pelvic limb gait.

➡ So C6–T2 = LMN to thoracic limbs, UMN to pelvic limbs.

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3. T3–L3 (Thoracolumbar Region)

• Contains: UMN tracts to pelvic limbs, but no LMN cell bodies for limbs.

• Function: Controls pelvic limb movement via descending UMN pathways to lumbosacral LMNs below.

• Lesion signs:

  • Thoracic limbs: normal (since LMNs for them are above, in C6–T2).

  • Pelvic limbs: UMN signs (spasticity, hyperreflexia, increased tone).

  • Often accompanied by loss of proprioception and possible urinary retention.

➡ So T3–L3 = UMN to pelvic limbs only.

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4. L4–S3 (Lumbosacral Intumescence)

• Contains: LMN cell bodies for pelvic limbs and perineum.

• Lesion signs:

  • Pelvic limbs: LMN signs (flaccid paresis/paralysis, reduced reflexes).

  • Tail/bladder/anus: LMN signs (decreased tone, incontinence).

➡ L4–S3 = LMN to pelvic limbs.

It is important not to confuse deep sensation with withdrawal, explain how to make this distinction.

• Only a behavioural response to noxious stimulus (turning of the head, attempt to bite) indicates conscious pain perception

• Withdrawal of the limb is only the flexor reflex and should not be taken as evidence of nociception

Lack of nociception (deep pain) = bad prognosis

(may be associated to UMN or LMN lesion - so a lesion at any level - C1-C5 / C6-Т2 / T3-L3 /L4-S3) (nociception is also absent in a comatose stage)

Monosynaptic = _____ reflex

Polysynaptic = ______ reflex

patellar, withdrawal