Sensory Neuron Orders - Extents and Locations

1st Order Neuron (Primary Afferent)

• Cell body location: Dorsal root ganglion (DRG) of the spinal cord (or trigeminal ganglion for facial sensation)
• Extent: From the sensory receptor in the periphery (skin, muscle, joint) → enters the spinal cord via the posterior root → synapses in a relay nucleus in the spinal cord (dorsal horn) or brainstem
• In the somatosensory and olfactory systems, the receptor IS the first-order neuron (no separate synapse needed between receptor and primary afferent)
• Exceptions exist: in the visual, auditory, and taste systems, specialized epithelial receptor cells first synapse onto the first-order neuron

2nd Order Neuron

• Cell body location: Relay nucleus in the spinal cord or brainstem
• Extent: From the relay nucleus → axon crosses the midline (decussates) → ascends all the way up to the thalamus

Key distinction: In the spinothalamic tract, the 1st order neuron synapses and the 2nd order crosses immediately at the spinal cord level. In the DCML pathway, the 1st order ascends ipsilaterally all the way to the medulla before synapsing on the 2nd order neuron, which then crosses.

3rd Order Neuron

• Cell body location: Relay nuclei in the thalamus - specifically the Ventral Posterior Lateral (VPL) nucleus for body sensations, and the Ventral Posterior Medial (VPM) nucleus for facial sensations (from the trigeminal nerve)
• Extent: From the thalamus → axons project through the posterior limb of the internal capsule → ascend to the primary somatosensory cortex (postcentral gyrus, parietal lobe)
• Many 2nd-order neurons converge on a single 3rd-order neuron in the thalamus

4th Order Neuron

• Cell body location: Primary somatosensory cortex (postcentral gyrus, parietal lobe - Brodmann areas 3, 1, 2)
• Extent: These are the cortical neurons where conscious sensory perception occurs. They process, integrate, and distribute the sensory information further within the cortex (to association areas)
• The receptive field of a 4th-order neuron is the largest of all the orders - it represents a wide area of the body surface

Summary Table

Important Clinical Note - Where Lesions Produce Contralateral Deficits

• A lesion above the decussation causes contralateral sensory loss
• Spinothalamic: decussation is in the spinal cord - so a lesion above the cord affects the opposite side
• DCML: decussation is in the medulla - a lateral medullary stroke can therefore produce ipsilateral face loss (trigeminal, which crosses in brainstem) + contralateral body loss (DCML, which already crossed at medulla)

Intention Tremor vs. Resting Tremor

Resting Tremor

Definition

Characteristics

• Frequency: 5-7 Hz (in Parkinson's disease)
• Distribution: Typically distal - fingers, hands, wrists; can involve lower extremities, chin, lips, jaw
• Pattern: Asymmetrical at onset; often described as "pill-rolling" (rolling motion between thumb and fingers)
• Behavior: Increases with anxiety, stress, and cognitive distraction; suppressed by voluntary movement; disappears during sleep
• Patients may conceal it by holding one hand with the other, sitting on the affected hand, or crossing their legs

Underlying Pathology - Basal Ganglia Dysfunction

• The classic cause is Parkinson's disease - loss of dopaminergic neurons in the substantia nigra pars compacta
• Dopamine deficiency disrupts the balance between the direct and indirect pathways of the basal ganglia
• This leads to abnormal oscillatory activity in thalamo-cortical circuits at rest
• Other causes: Holmes tremor, palatal tremor, drug-induced parkinsonism

Associated Features (Parkinson's Disease)

• Bradykinesia (slowness of movement)
• Cogwheel or lead-pipe rigidity
• Postural instability
• Shuffling gait, stooped posture, masked facies

Intention (Ataxic) Tremor

Definition

Characteristics

• Frequency: 2-4 Hz (lower than resting tremor)
• Movement: Irregular, oscillating movements in multiple planes that increase as the target is approached
• Test to elicit: Finger-nose-finger test, heel-shin test - the limb overshoots or oscillates wildly just before reaching the target
• Absent at complete rest; appears only when movement is initiated

Underlying Pathology - Cerebellar Dysfunction

• Caused by lesions of the cerebellum or its connections (superior cerebellar peduncle, dentate nucleus)
• The cerebellum normally fine-tunes ongoing movements by comparing intended vs. actual motor output and making real-time corrections
• When this error-correction circuit is damaged, limb movements become uncoordinated and oscillate, especially near the endpoint of movement
• Also called ataxic tremor because it is a feature of cerebellar appendicular ataxia

Associated Cerebellar Signs (DANISH mnemonic)

Common Causes

• Multiple sclerosis (demyelination of cerebellar pathways)
• Cerebellar stroke or infarct
• Cerebellar tumors
• Alcohol toxicity (acute and chronic)
• Friedreich's ataxia
• Holmes tremor (combined rest + postural + intention tremor from superior cerebellar peduncle lesions)

Side-by-Side Comparison

Memory Hook

"Rest = Parkinson's (basal ganglia) | Intention = Cerebellum"

• Resting tremor: the hand shakes when you're NOT trying to use it
• Intention tremor: the hand shakes ONLY when you TRY to reach something