Sensory Pathways and Diencephalon

• Information is processed on an ascending pathway from sensory receptors up to the spinal cord and brain stem within the somatic nervous system.
• Somatosensory pathways divide into cranial nerves for the head and neck and spinal nerves for the rest of the body.

Sensory Pathways

• The dorsal white column system starts with the axon of a dorsal root ganglion neuron, which enters the dorsal root and joins the dorsal column white matter in the spinal cord.
• The spinothalamic tract starts with neurons in a dorsal root ganglion; these neurons extend their axons to the dorsal horn, synapsing with a second neuron in the thalamus.

Diencephalon

• The hypothalmus controls the somatic, autonomic, and limbic systems.
• The thalamus serves as a communication relay or "gateway" between the cerebrum and the rest of the nervous system.
• Most special senses and ascending somatosensory tracts send sensory input via the thalamus.
• Each sensory system relays through a particular nucleus in the thalamus.
• Sensory information ascends via a 3-neuron relay (sometimes 2): multineuron pathway that includes decussation (crossover); relay of 2-3 neurons; somatotropy to the brain; symmetry
• First order neuron: A sensory neuron with a cell body in the dorsal root ganglion or cranial nerve ganglion goes to spinal cord: dorsal horn from dorsal rootlet/root (somatic sensory or visceral sensory).
• The first order neuron synapses with the second order neuron in the dorsal horn, from the spinal cord to the thalamus through an interneuron that may cross the spinal cord sides via commissural fibers.
• The second order neuron synapses with a neuron in the thalamus that goes to the cerebral cortex somatosensory cortex for a specific region

Cortical Processing

• The sensory homunculus maps sensory axons positioned similarly to their corresponding receptor cells in the body.
• The cortex contains specific regions responsible for processing specific information, such as the visual, somatosensory, and gustatory cortices, resulting in a continuous perception of the world.
• Sensory processing in the cerebral cortex begins in the primary sensory cortex, proceeds to an association area, and finally to a multimodal integration area.
• There are two main regions associated with visual information processing.

Three Ascending Pathways

• The dorsal column medial lemniscal pathway carries somatosensory information related to touch, vibration, and proprioception to the sensory cortex via the thalamus.
• The first order neuron goes to the dorsal white column, synapses with the second order neuron in the medulla oblongata, which decussates and goes to the thalamus
• Third order neuron goes from the thalamus to the primary somatosensory cortex (sensory homunculus).
• The spinothalamic pathway carries somatosensory information related to coarse touch, pain, pressure, and temperature to the sensory cortex via the thalamus.
• The first order neuron goes to the dorsal horn, synapses with the second order neuron, decussates to the ventral column, and goes to the thalamus.
• The third order neuron goes from the thalamus to the primary somatosensory cortex (sensory homunculus).
• The spinocerebellar tract carries somatosensory information related to tendon stretch and proprioception to the cerebellum and has tracts on the dorsal and ventral sides.
• The first order sensory neuron goes to the dorsal horn, synapses with the second order neuron in the medulla oblongata, which decussates and goes to the cerebellum.

Cortical Processing

• Motor functions are largely controlled by the frontal lobe.
• The prefrontal areas of the frontal lobe are important for executive functions, including higher cognitive processes like working memory.
• Motor processing follows a descending pathway.
• Primary Motor Cortex, located in the precentral gyrus of the frontal lobe, receives input to plan movement and stimulates spinal cord neurons to stimulate skeletal muscle contraction.
• Secondary Motor Cortices, including the premotor cortex, aids in controlling skeletal movements and maintaining posture.

Descending Pathways

• The anterior corticospinal tract controls the muscles of the body trunk.
• The lateral corticospinal tract, composed of fibers that cross the midline at the pyramidal decussation, controls appendicular muscles.
• Descending connections between the brain and the spinal cord outside the corticospinal pathway are called the extrapyramidal system.
• The ventral horn contains lower motor neurons that control skeletal muscle contraction, with axons extending to join the emerging spinal nerve.
• Descending pathways involve two motor neurons: an upper and a lower.
• The upper neuron leaves the primary motor cortex and goes to the viscera or skeletal muscle via the lateral or ventral horn.
• Direct Motor Pathway includes the corticospinal tract (lateral and ventral tracts).
• The upper neuron leaves the precentral gyrus and goes down to the spinal cord via the lateral or ventral tract.
• The ventral tract goes to the spinal cord ventral horn, decussates, and synapses with the lower motor neuron.
• The lateral tract decussates in the lateral column and synapses with interneurons in the lateral horn to synapse with the lower motor neuron.
• The indirect motor pathway regulates axial muscles for balance and posture and muscle tone; the upper neuron originates in the brainstem, decussates in the pons, synapses in the spinal cord with an interneuron, and finally connects with the lower motor neuron, which goes to target muscle.

Lesions

• Upper motor neuron lesion
• Lower motor neuron lesion
• Paralysis results from damage to outgoing motor neurons, leading to loss of motor function.
• Flaccid paralysis is due to damage to the lower motor neuron, resulting in no muscle contraction on the same side.
• Spastic paralysis results from damage to the upper motor neuron, leading to muscle reflex activity but no voluntary control and paralysis on the opposite side of the injury.
• Transection of the spinal cord results in total motor and sensory loss inferior to the region cut.