TWIN PATHWAYS INTO THE BRAIN
The phrase "Twin Pathways into the Brain" generally refers to the two main types of nerve fiber tracts that carry information into the Central Nervous System (CNS—the brain and spinal cord) from the body and the environment: the Sensory (Afferent) Pathways and the Special Sense Pathways.
However, in the context of general brain function, this phrase is often used specifically to describe the two major ascending sensory pathways that bring touch, pain, temperature, and body position information from the body up to the Somatosensory Cortex in the parietal lobe.
1. Sensory Pathways (Somatic Sensation)
These two primary pathways are responsible for all touch, pain, temperature, and joint position information.
A. Dorsal Column-Medial Lemniscal Pathway (DCML)
Information Carried: Fine touch (discriminative touch), vibration, and proprioception (awareness of body position and movement).
Speed: This is the faster pathway, necessary for rapidly transmitting precise, detailed information.
The Cross: The fibers travel up the spinal cord on the same side (ipsilateral) they entered until they reach the medulla in the brainstem, where they cross over (decussate) to the opposite side.
Function: Essential for tasks requiring high spatial resolution, like recognizing an object by touch without looking (stereognosis).
B. Anterolateral (Spinothalamic) Pathway
Information Carried: Pain, temperature, and crude (non-discriminative) touch.
Speed: This is the slower pathway, as the information is not as urgently detailed.
The Cross: The fibers cross over (decussate) immediately upon entering the spinal cord at that level, and then travel up the opposite side (contralateral) to the brain.
Function: Critical for survival and reflexes, alerting the brain to potentially harmful stimuli.
2. General Input Pathways
More broadly, all information reaching the brain falls into these two categories:
A. Somatic and Visceral Sensory Pathways
This includes the DCML and Spinothalamic pathways mentioned above, carrying information from:
Soma (Body): Skin, muscles, and joints (touch, pressure, pain).
Viscera (Internal Organs): Information about internal states (e.g., blood pressure, digestive discomfort).
B. Special Sensory Pathways
These are dedicated pathways from the specialized sense organs, each with a unique neural route:
Vision: Information travels via the Optic Nerve to the Occipital Lobe.
Hearing and Balance: Information travels via the Vestibulocochlear Nerve to the Temporal Lobe.
Smell (Olfaction): Information travels via the Olfactory Nerve directly to the Limbic System and olfactory cortex.
Taste (Gustation): Information travels via several cranial nerves to the Parietal Lobe.
Diagram of the Human Nose
The centimeter-long human vomeronasal organ (VNO), shown sliced in two in the photograph below acts as a sensor for airborne human pheromones-odorless molecules that influence sexual desire and other feelings. The molecules set off signals that are transmitted through the VNO nerves (red) directly to the hypothalamus. Smell messages travel through the parallel olfactory system (blue) to a different part of the brain...(diagram of the human nose)
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