NA2- Ascending pathways

Ascending Pathways

• Somatosensory pathways transmit sensory information from the body to the brain.
• General somatic pathways are responsible for conscious sensation like proprioception, vibration, and discriminative touch.
• Conscious transmission travels through the medial division, and lateral corticospinal tract.
• Unconscious transmission pathways travel through tracts like the spinothalamic and spinocerebellar tracts, involved in pain and thermal sensations, and proprioception transmission.
• Visceral pathways transmit information related to the internal organs.
• The arrangement of grey and white matter is different in various spinal cord levels. White matter is primarily composed of axons and dendrites.
• Cervical and lumbosacral enlargements of the spinal cord innervate the limbs, and contain abundant white matter for motor and sensory pathways.

Types of Neurons

• Interneurons are the smallest neurons (5-20µm) and are located within the spinal cord, interconnecting different spinal cord segments, participating in spinal reflexes.
• Medium-sized neurons (20–50µm), are relay neurons receiving inputs from sensory fibers and projecting their axons to the brain.
• Motor neurons are the largest (50–100µm) and are used for the supply of skeletal muscles. Scattered among them are γ motor neurons, that supply muscle spindles. Renshaw cells, found in the ventral horn of the spinal cord, provide tonic inhibition to alpha motor neurons.

Sensations

• Conscious sensations are perceived at the cerebral cortex categorized as exteroceptive or proprioceptive.
• Exteroceptive sensations originate from external stimuli (touch, pressure, heat, cold, and pain).
• Proprioceptive sensations come from receptors in muscles, joints, bones, and vestibular labyrinth, providing positional and kinesthetic sense.
• Unconscious sensations are not perceived but transmitted to the cerebellum involved in proprioceptive and interoceptive pathways. Smooth motor coordination relies on these unconscious proprioceptive inputs. Visceral reflexes are regulated through interoception.

Somatic Sensory Pathways

• Two major pathways: dorsal column-medial lemniscal (DCML) and spinothalamic (ventrolateral) tracts.
• Both have first, second, and third-order neurons.
• Somatosensory cortex is a specific area that process these sensations.
• These tracts play a role in somatic sensory perception.

Dorsal Column-Medial Lemniscal Pathway (DCML)

• Carries sensory modalities like fine touch, vibration and proprioception.
• First-order neurons travel in the posterior columns of the spinal cord, ascending to the medulla oblongata and crossing over to the opposite side.
• Second-order neurons extend in a tract known as medial lemniscus to synapse with third-order neurons in the thalamus.
• Third-order neurons project to the primary somatosensory cortex (SI).
• The pathway organizes somatic senses—such as touch, pressure, vibration, and proprioception (body position)—from the body to the brain in a somatotopic manner.

Anterolateral System (ALS)

• Carries sensory modalities like pain, temperature, crude touch and itch.
• First-order neurons enter through the dorsal root ganglia and form synapses with secondary neurons in the posterior horn.
• Second-order neurons cross the midline in the spinal cord and ascend in either the anterior or lateral spinothalamic tracts.
• Synapse with the third-order neurons within the thalamus, which ascend to the postcentral gyrus (SI), involved in processing somatic sensation of pain, temperature, and crude touch.

Spinoreticular Pathway

• Originates in laminae V-VII of the spinal cord and terminates at levels of the brainstem, not somatotopically arranged.
• Plays a role in arousing cerebral cortex and mediating emotional responses to stimuli.

Unconscious Pathways

• Proprioceptive information is often relayed directly to the cerebellum via the dorsal spinocerebellar, cuneocerebellar, ventral spinocerebellar, and rostral spinocerebellar tracts.
• These pathways manage reflexes and motor coordination without reaching conscious awareness.

Trigeminal Pathway

• Sensory information from the head is transmitted by the trigeminal nerve.
• First-order neurons are gathered in the Gasser's ganglion.
• Information is then channeled to three different nuclei in the brain stem: spinal, main sensory, and mesencephalic nuclei, depending on the type of sensation.
• Transmission to thalamus, then to the postcentral gyrus (SI).

Pain Reception

• Pain information ascends via the spinothalamic and spinoreticular tracts.
• The spinothalamic tract carries acute pain and is part of the neospinothalamic pathway.
• The spinoreticular tract carries chronic pain (paleospinothalamic tract).
• Ascending pain pathways communicate with descending pathways for pain inhibition or modulation.
• Both pathways feature three neuron types transmitting pain information through multiple synapses in the spinal cord and brainstem.

Gated Theory

• Sensory signals can be modulated through an inhibitory pathway in the dorsal horn, where tactile sensation can inhibit pain sensation, closing the “gate” for the pain signal.