Sensory Pathways and Pain

Sensory Pathways and Pain

Peripheral nerves and spinal roots
 Peripheral nerves carry all modalities of sensation from either free or specialized nerve endings to dorsal root ganglia and thus to the cord
Lesions Of The Sensory Pathways
 Altered sensation (paraesthesia), tingling, clumsiness, numbness and pain are the principal symptoms of sensory lesions. The pattern usually suggests the site of pathology. Peripheral nerves and spinal roots

Peripheral nerve lesions

 Section of a sensory nerve is followed by complete sensory loss
 Nerve entrapment causes numbness, pain and tingling. Tapping the site of compression sometimes causes a sharp, electric-shock-like pain in the distribution of the nerve, e.g. Tinel's sign in carpal tunnel syndrome
 Neuralgia refers to local pain of great severity in the distribution of a damaged nerve. Examples are:  trigeminal neuralgia postherpetic neuralgia)

Spinal root lesions (Root pain)

 The pain of root compression is felt in the myotome supplied by that root, and there is also a tingling discomfort in the dermatome. The pain is made worse by maneuvers that either stretch the nerve root (e.g. limitation of straight leg raising in lumbar disc prolapse) or increase pressure in the spinal subarachnoid space (coughing and straining

Spinal cord lesions

 Transverse lesion of the spinal cord produce loss of all modalities below that segmental level,. If the transverse lesion is vascular in origin (e.g. due to anterior spinal artery thrombosis), the posterior one-third of the spinal cord (and therefore the dorsal column modalities) may be spared


Lesions damaging one side of the spinal cord will produce sensory loss for spinothalamic modalities (pain and temperature) on the opposite side and for 2 dorsal column modalities (joint position and vibration) on the same side as the lesion. This is the pattern seen in the Brown-Séquard syndrome
 Lesions in the centre of the spinal cord (e.g. syringomyelia,) spare the dorsal columns but affect the spinothalamic fibres crossing the cord from both sides over the length of the lesion. The sensory loss is therefore dissociated (in terms of the modalities affected)

Pontine lesions 

Since lesions in the pons (e.g. an MS plaque) lie above the decussation of the posterior columns,
 and the medial lemniscus and spinothalamic tracts are close together, there is loss of all forms of sensation on the side opposite the lesion.

Thalamic lesions

 Thalamic pain, also called thalamic syndrome, usually follows a small thalamic infarct. A patient has a stroke (hemiparesis and sensory loss). The weakness recovers partially or completely but there develops constant very severe deep-seated burning pain in the paretic limbs. This continues night and day. Extreme anguish is usual and the secondary depression may lead to suicide. Thalamic lesions may also produce loss of sensation on the opposite side of the body; this is a less usual clinical picture

Parietal cortex lesions  Sensory loss, neglect of one side, apraxia and subtle disorders of sensation occur. Pain is not a feature of destructive cortical lesions. Irritative phenomena (e.g. partial sensory seizures from a glioma) arising in the parietal cortex cause tingling sensations in a limb, or elsewhere. Regional Cerebral Dysfunction:

Regional Cerebral Dysfunction: Higher Mental Functions

Focal lesions of the cerebral cortex,, cause symptoms and signs by two processes:
1--Suppression or destruction of neurons and surrounding structures ( most common process - part of the system fails to work. 3
2--Synchronous discharge of neurons by irritative lesions, e.g. cerebral cortex lesions cause epilepsy, either partial or generalized.

Frontal lobe Function

Personality
Emotional control
Social behavior
Contra lateral motor control
Language
Micturition Disinhibition
Dysfunction
Lack of initiation
Antisocial behavior
Impaired memory
Expressive dysphasia
Incontinence ,Impaired smell Contra lateral hemi paresis Frontal release signs1Versive seizures Focal motor seizures (Jacksonian march) Continuous partial seizures (epilepsia partialis continua)


Dominant parietal lobe Function
Language
Calculation
Dysfunction
Dysphasia
Contra lateral hemisensoryloss
Focal sensory seizures
Dyscalculia Astereognosis Dyslexia Agraphaesthesia Apraxia

Non-dominant parietal lobe Function

Spatial orientation
Constructional skills
Dysfunction
Neglect of contra lateral side
Contra lateral hemi sensory loss
Astereognosis
Focal sensory seizures
Spatial disorientation Agraphaesthesia Constructional apraxia

Temporal: dominant Function

Auditory perception
Language
Verbal memory
Smell
Balance Dysfunction
Receptive aphasia Dyslexia Impaired verbal memory Contra lateral homonymous upper quadrantanopia Complex hallucinations (smell, sound, vision, memory)


Temporal: non-dominant Function
Auditory perception Melody/pitch perception Non-verbal memory Smell Balance
Dysfunction
Impaired non-verbal memory Impaired musical skills (tonal perception) Contra lateral homonymous upper quadrantanopia Complex hallucinations (smell, sound, vision, memory)

Occipital Visual processing

Dysfunction
Visual inattention
Visual loss
Visual agnosia
Homonymous hemianopia (macular sparing)
Simple visual hallucinations (e.g. phosphenes, zigzag lines)