A pathway of the human nervous system is the series of neurons or other structures used to transmit an item of information.
In general, we consider two major types of pathways
• the general sensory pathways
• the motor pathways
Page 2 The General Sensory Pathways
The general sensory pathways ascend through the neuraxis to the brain.
The motor pathways descend through the neuraxis from the brain.
The neuraxis includes both the spinal cord and the brainstem.
The pathways are included in various fiber tracts of the neuraxis.
Page 3 Decussation
At some specific level in the neuraxis, all of these pathways cross to the opposite side of the midline of the CNS.
Each crossing is called a decussation.
Thus, the right cerebral hemisphere of the brain communicates with the left half of the body.
The left cerebral hemisphere communicates with the right half of the body.
Page 4 The General Senses
The general senses detect those specific stimuli which are received throughout the body (general distribution).
When these general senses are perceived at the conscious level (in the cerebral cortex), they are known as sensations.
The general senses of humans include pain, touch, temperature, and proprioception ("body sense").
Page 5 Neurons of a General Sensory Pathway
A general sensory pathway extends from the point where the stimulus is received to the postcentral gyrus of the cerebral hemisphere.
The postcentral gyrus is the site of conscious sensation of a stimulus.
Between the point of stimulus reception and the postcentral gyrus, there is a minimum of three neurons in series.
1. The first neuron is the afferent (sensory) neuron. It picks up the information from the sensory receptor organ and carries it to the CNS via the appropriate peripheral nerves.
2. The second neuron is the interneuron, located within the spinal cord or brainstem. It crosses the midline of the CNS to the opposite side, it then ascends the neuraxis to the forebrainstem, where it reaches a mass of gray matter called the thalamus. In the thalamus, the interneuron synapses with the cell body of the third neuron.
3. The axon of the third neuron projects up through the cerebral hemisphere to the appropriate location in the postcentral gyrus.
Page 7 Homunculus of Conscious Sensations
There is a specific location in the postcentral gyrus which corresponds to each location in the body.
For example, a location in the postcentral gyrus near the midline of the brain (at the top of the cerebral
hemisphere) receives information from the hip region.
On the other hand, information from the tongue and the pharynx projects to the lowest part of the postcentral gyrus, just above the lateral sulcus.
The homunculus is a translation of the body’s sensory map into human form. This sensory map is not drawn to scale; instead it is distorted to reflect the amount of neural processing devoted to different regions.
Page 8 Visceral Sensory Inputs
Visceral sensory inputs follow pathways different from those of other general sensory pathways.
The inputs for visceral reflex actions usually travel via the parasympathetic nerves.
The visceral inputs for pain usually travel via the sympathetic nerves.