The Somatosensory System

dorsal column-medial lemniscal pathway

Ascending sensory pathways in the spinal cord that carry information about fine touch, vibration, pressure, and proprioception (sense of body position) from the peripheral nervous system to the central nervous system.

• Gracile Fasciculus: Medial portion of the posterior column that carries information from the legs and lower trunk.

• Cuneate Fasciculus: Lateral portion of the posterior column that carries information from the upper trunk, arms and neck.

Functions:

• Fine Touch (Tactile Discrimination): The pathway enables the ability to detect small, detailed textures and identify objects by touch.

• Vibration Sense: It allows the detection of vibration on the skin, such as the sensation felt when holding a vibrating object.

• Pressure Sensation: It carries information about deep pressure applied to the skin.

• Proprioception (Sense of Body Position): The pathway conveys information about the position and movement of different parts of the body, contributing to balance and coordination.

Pathway:

• First-Order Neurons (Peripheral to Spinal Cord): Sensory receptors detect sensory information and send their axons through the dorsal roots of the spinal nerves and enter the ipsilateral posterior columns of the spinal cord to ascend to the dorsal column nuclei of the medulla. 

• Second-Order Neurons (Spinal Cord to Medulla): The axons of the first-order neurons synapse with second-order neurons in the medulla. These second-order neurons then cross to the opposite side (decussate) in the medulla as internal arcuate fibres to form the medial lemniscus and continue to ascend.

• Third-Order Neurons (Medulla to Thalamus and Cortex): The medial lemniscus axons terminate in the ventral posterolateral nucleus (VPL) of the thalamus where they synapse to the third-order neurons which send their axons to the primary somatosensory cortex (S1) in the parietal lobe trough the posterior limb of the internal capsule.

Somatotopic Organisation: In the posterior columns the feet are medial to the arms (think of fibres adding on laterally from higher levels as the posterior columns ascend) while for the medial lemniscus the feet are lateral to the arms (humunculus lying down)

internal arcuate fibres

Bundle of decussating nerve fibres in the medulla that originate in the nucleus gracilis and nucleus cuneatus - relay nuclei of the dorsal column pathway. After crossing, the internal arcuate fibers converge and ascend as the medial lemniscus, carrying sensory information to the thalamus (specifically the VPL).

medial lemniscus

Major ascending sensory tract in the brainstem that forms part of the dorsal column-medial lemniscal (DCML) pathway. It transmits sensory information about fine touch, vibration, and proprioception (body position awareness) from the nucleus gracilis and nucleus cuneatus in the medulla to the ventral posterolateral (VPL) nucleus of the thalamus via second-order sensory fibres.

anterolateral pathway

Sensory pathways that ascend in the anterolateral column of the spinal cord and carry information related to crude touch, pressure, temperature, and pain from the peripheral nervous system to the central nervous system.

• Spinothalamic Tract

  • Neospinothalamic Tract (sharp localisable pain, temperature)

  • Paleospinothalamic Tract (slow burning/aching pain, crude touch, pressure)

• Spinoreticular Tract (emotional response to pain)

• Spinomesencephalic Tract (pain modulation/inhibition)

Functions:

• Crude Touch and Pressure: Transmit sensations of general touch and pressure that lack fine discrimination.

• Pain Sensation (Nociception): Transmit information about tissue damage, allowing the body to perceive pain and initiate protective responses.

• Temperature Sensation: Convey information about changes in temperature, such as detecting heat or cold.

Pathway:

• First-Order Neurons (Peripheral to Spinal Cord):

  Sensory receptors detect sensory information and send signals via neurons which send their axons through the dorsal roots to enter the spinal cord.

• Second-Order Neurons (Spinal Cord to Thalamus):

  These axons immediately synapse with second-order neurons in the dorsal horn of the spinal cord which then decussate in the anterior commissure of the spinal cord within a few segments of entry. After crossing, the fibres ascend in the anterolateral white matter of the spinal cord as part of the spinothalamic tract.

• Third-Order Neurons (Thalamus to Cortex): The second-order neurons terminate in the ventral posterolateral nucleus (VPL) of the thalamus. The thalamus then relays the information to the primary somatosensory cortex (S1) in the parietal lobe, where conscious perception of the sensory stimuli occurs. Some fibres also project to other thalamic nuclei and regions involved in the emotional and autonomic aspects of pain, such as the reticular formation and periaqueductal grey (PAG).

Somatotopic Organisation: In the anterolateral pathways the feet are most lateral (picture fibres from the anterior commissure adding on medially as the anterolateral pathway ascends in the spinal cord).

Neospinothalamic Tract

Subdivision of the spinothalamic tract responsible for carrying acute, sharp, localised pain (fast pain) and temperature sensations to the thalamus for rapid and conscious perception.

Pathway: Signals travel via the dorsal root ganglion into the dorsal horn of the spinal cord where they synapse in the lamina I and V in the grey matter. The axons then decussate to the contralateral side of the spinal cord trough the anterior white commissure and ascend in the anterolateral column of the spinal cord as part of the spinothalamic tract.

Synapse: Axons project through the internal capsule to the primary somatosensory cortex (postcentral gyrus) for conscious perception and localization of pain and temperature.

paleospinothalamic tract

Subdivision of the spinothalamic tract that transmits slow, dull, poorly localised pain and crude touch to the thalamus.

Pathway: Signals travel via the dorsal root ganglion into the dorsal horn of the spinal cord where they synapse in the lamina I, II and V in the grey matter. The axons then decussate to the contralateral side of the spinal cord trough the anterior white commissure and ascend in the anterolateral column of the spinal cord as part of the spinothalamic tract.

Synapse: Projects diffusely to the limbic system, including the cingulate gyrus and prefrontal cortex. This tract does not have the precise somatotopic organization of the neospinothalamic tract.

spinoreticular tract

Ascending sensory pathway in the anterolateral system that is involved in alertness and the emotional response to pain. It terminates in the medullary-pontine reticular formation in the brainstem and helps the brain process the wakefulness and behavioural arousal associated with painful stimuli by projecting diffusely to the entire cerebral cortex.

spinomescencephalic tract

Component of the anterolateral system that transmits pain and temperature information to the midbrain, specifically the periaqueductal gray (PAG) and superior colliculi. It is primarily involved in pain modulation and can activate descending pain inhibition pathways.

Pain Modulation: The tract sends signals to the periaqueductal gray (PAG), which is involved in the descending pain control system. The PAG activates brainstem nuclei that release neurotransmitters like serotonin and norepinephrine, inhibiting pain transmission at the spinal cord level.

Pathway: Signals travel via the dorsal root ganglion into the dorsal horn of the spinal cord. Axons then decussate to the contralateral side via the anterior white commissure and ascend in the anterolateral column of the spinal cord. Axons then synapse in the periaqueductal grey (PAG).

anterior (white) commissure

Thin band of white matter in the anterior portion of the spinal cord hat forms the crossing point for components of the anterolateral system.

thalamus

Pair of structures located deep in the brain, serving as the brain's sensory relay station by processing and transmitting sensory information to the cerebral cortex.

• Relay Function: The thalamus receives sensory inputs from the peripheral nervous system and lower brain regions, relaying them to the primary cortical areas for conscious perception (e.g., visual, auditory, somatosensory).

• Modulation and Filtering: It filters and prioritises sensory signals, enhancing important stimuli and suppressing irrelevant ones based on attention and context.

• Multimodal Integration: The thalamus integrates sensory inputs across different modalities, ensuring a coherent perception of the environment.

Each sensory modality - except for olfactory input - has a different nuclear area where synapses occur before the information is relayed to the cortex (reciprocal circuits).

Damage to the Thalamus: When the thalamus is damaged, particularly the bilateral thalamus (affecting both sides), it can result in a coma. This is because the disruption impairs the relay of sensory and arousal signals to the cortex, preventing the brain from achieving a state of consciousness.

ventral postlateral nucleus (VPL)

Sensory relay nucleus in the thalamus that processes and transmits somatosensory information from the body to the primary somatosensory cortex (S1) in the parietal lobe.

This nucleus receives sensory input for the body via two major pathways: the dorsal column-medial lemniscal pathway and the spinothalamic tract.

Output: The VPL projects sensory information to the primary somatosensory cortex (postcentral gyrus of the parietal lobe) through the posterior limb of the internal capsule. Different regions of the VPL correspond to specific parts of the body (somatotopic organisation), ensuring precise localisation of sensory input in the somatosensory cortex.

lateral geniculate nucleus

Sensory relay nucleus in the thalamus that receives visual information from the retina via the optic nerve (CN II) and projects to the primary visual cortex (V1) in the occipital lobe. It is a critical component of the visual pathway, enabling conscious perception of visual stimuli. Each nucleus processes information from the contralateral visual field (e.g., the right nucleus processes the left visual field).

Each LGN processes information from the contralateral visual field (e.g., the right LGN processes the left visual field).

Retinotopic Organisation: Visual information is organized based on the spatial arrangement of the retina.

medial geniculate nucleus (MGN)

Sensory relay nucleus in the thalamus that transmits auditory information from the ear to the primary auditory cortex (A1). It receives input from the inferior colliculus and sends signals to the primary auditory cortex via the auditory radiations. It is essential for conscious auditory perception and tonotipic organisation.

pulvinar nucleus

Association nucleus in the thalamus that is involved in visual attention and orientation and integrates sensory information for higher-level processing. It is also involved in the dorsal stream, helping locate where objects are (spatial information processing).

reticular nucleus

Thin layer of inhibitory neurons (GABAergic neurons) that surrounds the thalamus like a shell. It is unique in that it does not send signals directly to the cerebral cortex but instead communicates exclusively with other thalamic nuclei. It receives input from both the thalamus and the cerebral cortex, allowing it to act as a regulatory filter for information travelling through the thalamus (overstimualtion → filter not functioning well).

nociceptive pain

Type of pain that occurs in response to actual or potential tissue damage. It results from the activation of nociceptors (A-delta and C fibres) in response to harmful stimuli which generate action potentials that travel to the spinal cord via A-delta (sharp, fast pain) or C fibres (dull, slow pain). From there, signals are relayed trough the spinothalaic tract and thalamus to the somatosensory cortex. This type of pain can be classified as somatic (sharp and localized → neo) or visceral (deep and poorly localized → paleo), depending on the source.

• Mechanical: Physical trauma, pressure, or stretching.

• Thermal: Exposure to extreme heat or cold.

• Chemical: Inflammatory mediators like prostaglandins or bradykinin released during tissue damage.

Neuropathic Pain

Type of pain caused by abnormal neural signaling due to damage or dysfunction of the nervous system—either the peripheral nerves or the central nervous system (CNS).

• Ectopic activity: Spontaneous nerve firing in damaged neurons.

• Sensitization: Heightened response to stimuli in the nervous system.

• Loss of inhibitory control: Reduced ability of the CNS to suppress pain signals.

Prefrontal Cortex

Descending pain modulation system responsible for the higher-order regulation of pain. It processes the emotional and cognitive aspects of pain, allowing individuals to evaluate the meaning or context of a painful experience. It works closely with the limbic system to regulate the affective dimension of pain, such as fear or distress.

• Cognitive Reappraisal: Modulation of the emotional experience of pain by reframing or shifting focus away from the pain stimulus.

• Activation of Descending Pathways: Modulation of pain perception by activating the PAG, which then initiates descending inhibitory pathways

periaqueductal gray

Descending pain modulation system that projects to the raphe nuclei in the medulla, which release serotonin into the spinal cord to suppress pain signals. Serotonin inhibits pain by activating spinal inhibitory interneurons and directly reducing the excitability of nociceptive neurons. It is also rich in opioid receptors, making it a critical site for both endogenous (endorphins) and exogenous (e.g., morphine) opioid-mediated pain relief.

locus coeruleus

Descending pain modulation system that is a major source of norepinephrine in the central nervous system, which acts to suppress pain at the spinal cord level. It sends descending fibres to the dorsal horn of the spinal cord, where norepinephrine reduces pain transmission by two mechanisms:

• Directly inhibiting nociceptive neurons by suppressing the release of excitatory neurotransmitters

• Activating inhibitory interneurons that release GABA or glycine, further dampening pain signals.

Its activity is also influenced by stress and arousal, helping to reduce pain perception during situations requiring heightened focus or survival responses.

gate control theory

Theory of pain modulation that proposes that a gate in the spinal cord regulates the transmission of pain signals to the brain. Pain signals from A-delta and C fibres open the gate, increasing pain peception, while non-painful stimuli from A-beta fibres can close the gate, inhibiting pain.

a delta fibres

Small, lightly myelinated nerve fibers that transmit sharp, localized pain and cold temperature signals quickly to the central nervous system

a beta fibres

Large, heavily myelinated nerve fibers that carry non-painful sensory information, such as touch, vibration, and proprioception, at high speeds.

c fibres

Small, unmyelinated nerve fibers that transmit slow, diffuse pain (e.g., burning or aching) and warmth sensations.