Overview
This lecture covers the anatomy and function of sensory and motor processing in the brain, focusing on the major sensory systems and key neural pathways for sensation, perception, and movement.
Cerebral Cortex and Sensory Processing
- The cortex has four main lobes: frontal, parietal, temporal, and occipital.
- Smell (olfactory) and taste (gustatory) are processed deep within the frontal lobe.
- Hearing (auditory cortex) is in the temporal lobe.
- Touch (somatosensory cortex) is in the parietal lobe.
- Vision (visual cortex) is in the occipital lobe.
Stimulus, Receptors, and Transduction
- A stimulus is internal or external energy (sound, scent, memory) that triggers a response.
- Sensory receptors convert stimuli to electrochemical signals via transduction.
- Each sensory system has specialized receptors (e.g., mechanoreceptors for touch).
Reflex vs. Instinct and Pathways
- Reflex: fast, involuntary response, processed by spinal cord or brainstem.
- Instinct: learned, experience-based response, involves limbic system and prefrontal cortex.
- Reflex arc: stimulus → receptor → sensory neuron → interneuron → motor neuron → muscle.
Contralateral and Ipsilateral Processing
- Contralateral: sensory processing on opposite side of the brain (e.g., left hand → right cortex).
- Ipsilateral: same side processing (e.g., proprioception to cerebellum).
Ascending & Descending Pathways
- Ascending: sensory info sent from receptors up to the cortex.
- Descending: motor commands sent from cortex to effectors (muscles).
Sensory System Pathways
Olfactory (Smell)
- Odorants activate olfactory receptors, sending info to olfactory bulb and then to cortex, amygdala, and hippocampus (linked to emotion and memory).
- Olfactory pathway bypasses the thalamus, unique among senses.
Gustatory (Taste)
- Taste buds on tongue connect to cranial nerves VII (facial), IX (glossopharyngeal), and X (vagus).
- Signals relay via solitary tract nucleus to thalamus, hypothalamus, amygdala, and gustatory cortices.
Tactile (Touch)
- Fine touch uses the dorsal column system: receptor → dorsal root ganglion → medulla → thalamus → somatosensory cortex.
- Crossing over (decussation) occurs in the medulla.
Auditory (Hearing)
- Sound info: ear → auditory nerve → cochlear nucleus → superior olivary nucleus → inferior colliculus → thalamus (medial geniculate) → auditory cortex.
Visual (Sight)
- Retina processes inverted visual fields; fibers cross at optic chiasm.
- Left field → right visual cortex, right field → left visual cortex via the lateral geniculate nucleus and optic radiations.
- Reflex visual processing takes a shortcut via the midbrain.
Motor Processing Pathways
- Primary motor cortex sends commands via corticospinal (limbs) and corticobulbar (face) tracts.
- Crossover occurs at the pyramids in the medulla (decussation).
- Upper motor neuron: cortex to spinal cord; lower motor neuron: spinal cord to muscle.
- Basal ganglia initiate intended movement; cerebellum coordinates ongoing movement.
- Superior parietal lobule and postcentral gyrus integrate visual and proprioceptive signals for movement.
Key Terms & Definitions
- Transduction — conversion of stimulus to electrochemical signals.
- Reflex — involuntary, automatic response to stimuli.
- Instinct — learned, experience-based response requiring brain processing.
- Contralateral — processing on the opposite side of the CNS.
- Ipsilateral — processing on the same side.
- Ascending pathway — carries sensory info to the brain.
- Descending pathway — carries motor commands from the brain.
- Decussation — point where neural pathways cross sides.
Action Items / Next Steps
- Review diagrams of sensory pathways for olfactory, gustatory, tactile, auditory, and visual systems.
- Study key neural structures and their functions in sensory and motor processing.