Peripheral levels of injury
Mononeuropathic type: A single peripheral nerve is affected (e.g., due to trauma or tunnel compression). The area of sensory loss is usually smaller than the anatomical area due to the overlap of innervation from neighboring nerves. It is accompanied by pain, allodynia, and paresthesias. Clear examples include:
- Lesion of the radial nerve (n. radialis) results in "wrist drop" and numbness on the dorsum of the radial half of the hand.
- Lesion of the ulnar nerve (n. ulnaris) produces a "claw hand" and numbness of the fifth digit (little finger).
- Lesion of the median nerve (n. medianus) leads to an "ape hand" and loss of sensation in the radial half of the palm.
Polyneuropathic type: Multiple symmetrical nerve involvement (often due to diabetes, alcoholism, or genetics). Disorders begin in the distal parts of the legs and arms, forming classic "glove" and "stocking" zones.
Plexopathic type: Damage to an entire nerve plexus (trauma, ischemia, metastases). Pain and sensory deficit strictly encompass the zones of several nerves originating from the affected plexus.
Radicular and spinal levels
Radicular type (radiculopathy): Occurs when posterior spinal roots are compressed, most commonly by a herniated intervertebral disc. On the trunk, sensation is lost in a transverse band (within the corresponding dermatome). In the limbs, pain and numbness spread distally. Important topical landmarks:
- Root L4 — anteromedial part of the thigh and lower leg.
- Root L5 — lateral part of the lower leg and dorsum of the foot.
- Root S1 — posterior surface of the lower leg and lateral margin of the foot.
Myelopathic syndrome: Injury to the spinal cord itself. When the grey matter (posterior horns) is involved, a segmental-dissociated type of disorders arises. Only pain and temperature sensation disappear on the affected side, while tactile and deep sensation remain preserved.
Methods for examining sensitivity
In clinical practice, pain sensation is tested by alternating pricks with the sharp end of a needle and touches with the blunt end. Stereognostic sensation (object recognition) is tested with eyes closed. Instrumental methods are used for precise quantitative assessment:
- Electroneuromyography (ENMG): evaluates the amplitude of the biopotential and the nerve conduction velocity along peripheral nerves.
- Somatosensory evoked potentials: assess the state of the brain's receptive structures.
- Quantitative sensory testing: analyzes the function of thin nerve fibers (cold, heat, and pain sensation).
Reflex regulation and muscle tone
The assessment of deep reflexes is inextricably linked with topical diagnosis. Skeletal muscles are controlled by feedback systems. Muscle spindles respond to stretch thanks to intrafusal fibers with a nuclear bag. Signals from them travel via fast afferent fibers (group IA) to the spinal $\alpha_1$-motor neurons, triggering a reflexive muscle contraction. The Golgi tendon organ and nuclear chain fibers are responsible for tension control.
A strike with a neurological hammer artificially stretches the muscle and triggers this monosynaptic arc. Loss of a reflex indicates the level of the lesion. For example, the knee reflex arc (n. femoralis) is integrated at the L2–L4 level, while the Achilles reflex (n. tibialis) is at the S1–S2 level. Simultaneously, reciprocal antagonistic inhibition comes into play: the contraction of the target muscle is accompanied by the automatic relaxation of its antagonists via interneurons.
Briefly: answers to topical questions
Which types of sensitivity belong to deep (proprioceptive) sensitivity?
Deep (proprioceptive) sensitivity includes joint-muscle and vibration sensation. These types of sensation are provided by receptors located in muscles, tendons, ligaments, and joint capsules.
- Joint-muscle sensation — allows determining the position of the body and limbs with eyes closed.
- Vibration sensation — allows feeling the vibration from a tuning fork placed on a bony prominence.
Where are the cell bodies of the first neurons of the superficial and deep sensory pathways located?
The cell bodies of the first neurons of the superficial and deep sensory pathways are located in the dorsal root ganglia, and for the head and facial region — in the trigeminal ganglion.
- Spinal ganglia — contain the cell bodies of the first sensory neurons for the trunk and limbs.
- Trigeminal ganglion (Gasserian ganglion) — is the site of the cell bodies of the first sensory neurons providing innervation to the head and face.
Which dermatomes on the trunk correspond to the level of the nipples and the umbilicus?
The thoracic dermatomes Th4 and Th10 correspond to the level of the nipples and the umbilicus on the trunk, respectively.
- Th4 dermatome — runs at the level of the nipples.
- Th10 dermatome — runs at the level of the umbilicus.
The innervation of the trunk has a transverse (circular) character, so the thoracic segments are arranged as horizontal bands, sequentially descending downwards.
What is the difference between primary astereognosis and secondary astereognosis (astereomagnosia/astereoplenesthesia)?
The main difference lies in the state of elementary sensory modalities in the patient.
| Characteristic | Primary astereognosis | Secondary astereognosis (stereoanesthesia) |
|---|---|---|
| Nature of the disorder | Inability to recognize an object by touch | Inability to recognize an object by touch |
| Elementary sensation | Tactile, thermal, and deep sensation are preserved | Tactile and proprioceptive sensation are impaired |
| Level of lesion | Parietal lobe cortex, posterior to the postcentral gyrus of the contralateral hemisphere | Primarily caused by impaired tactile and proprioceptive sensation |
Through which tract in the spinal cord are pain and temperature sensation conducted?
Pain and temperature sensation in the spinal cord are conducted via the lateral spinothalamic tract.
- Lateral spinothalamic tract (tractus spinothalamicus lateralis) — is the main pathway for conducting pain and temperature impulses.
The fibers of this tract are formed by axons of second-order neurons, which decussate via the anterior white commissure and ascend in the lateral funiculus of the opposite side of the spinal cord.
Associations for hand deformities in mononeuropathies: Radial nerve — "wrist drop" (inability to extend), Ulnar nerve — "claw hand" (hyperextension of the phalanges), Median nerve — "ape hand" or "predator's hand" / preacher's hand sign.
Class and exam questions
What is secondary astereognosis (stereoanesthesia)?
This is the inability to recognize an object by touch, primarily caused by impairment of elementary modalities of sensation (tactile and proprioceptive).
Why is the area of sensory loss smaller than the anatomical area in mononeuropathy?
This occurs due to partial overlap of the innervation zones of adjacent peripheral nerves.
What is the difference between monosynaptic and polysynaptic reflexes?
Monosynaptic reflexes form feedback loops to regulate muscle length and tone. Polysynaptic arcs provide protective reflexes (e.g., the flexor withdrawal reflex from pain).
What else to study on this topic
- Structure of the muscle spindle: intrafusal fibers with a nuclear bag and a nuclear chain.
- Mechanism of the gamma loop and reflex regulation of set muscle length.
- Sensitization of pain receptors in acute and chronic pain.
- Deep reflex arcs of the upper extremity (biceps and triceps reflexes).
- Role of Renshaw cells and descending pathways (pyramidal, reticulospinal) in tone control.
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