General Principle of Response Organization
The reaction of the nervous system to visual and auditory stimuli is always conceptually divided into two independent streams with distinct ultimate goals and relay centers.
- Unconscious response (automatic): Formed primarily at the midbrain level. It is responsible for basic orienting reflexes — for example, a sudden turn of the head and eyes toward the source of a loud sound or an unexpected flash of light.
- Conscious response: Requires the mandatory participation of the cerebral cortex. The signal reaches the cortex only after a transit relay in specialized diencephalic structures. Following the conscious perception of the stimulus, a motor response is initiated via the pyramidal pathways with the involvement of the extrapyramidal system.
Auditory System: Afferent Link
The signal from the receptors of the inner ear undergoes a complex and multilevel chain of neuronal relays on its way to subcortical centers:
- First neuron (peripheral link): Located in the ganglion spirale (spiral ganglion). The axons of these cells form the auditory component of cranial nerve VIII and travel toward the pons.
- Second neuron (primary centers): Located within the pons, where the primary associative auditory nuclei are represented.
- Third neuron and ascending pathways: Axons from the pontine nuclei ascend to the midbrain and diencephalon.
An important feature of the auditory pathway is the decussation of fibers: some of them cross to the opposite side, forming the lemniscus lateralis (lateral lemniscus). For some fibers, this decussation is primary (single), while for others it is secondary (double).
Visual System: Pathway Characteristics
The visual pathway has a number of fundamental differences from the classical reflex arc scheme. The main anatomical feature lies in the order of the neurons.
- First neuron: Photoreceptor cells of the retina.
- Second neuron: Local associative (bipolar) cells.
- Third neuron: Retinal ganglion cells.
It is the axons of the third neurons that form the optic nerve. These fibers undergo partial decussation (chiasma opticum) on the ventral surface of the brain, even before entering the brain tissue proper. Following the chiasma, the axons of the ganglion cells travel in transit, without any additional relays, directly to the subcortical centers.
Integration and Efferent Output
The separation (divergence) of sensory streams occurs at the level of subcortical centers. This is where it is determined whether the signal will proceed to the cortex for conscious perception or be integrated into a reflex arc.
Path to Consciousness Follows the general rule of sensory systems: all afferent pathways must relay in the diencephalon before entering the cortex.
- Auditory signals: Relayed in the corpus geniculatum mediale (medial geniculate body), from where they are directed to the superior temporal gyrus.
- Visual signals: Relayed in the lateral geniculate body and the pulvinar of the thalamus, and then projected to the occipital lobes of the cortex.
Path to Reflex and Efferent Link To produce an unconscious automatic reaction, fibers are directed to the midbrain, specifically to the nuclei of the tectum mesencephali (tectal plate / corpora quadrigemina). The descending motor limb of the reflex originates from the tectal nuclei — the tractus tectospinalis (tectospinal tract). It descends to motor neurons of the brainstem and spinal cord (passing in its anterior funiculi, adjacent to the anterior median fissure). The final effectors are skeletal muscles that ensure a rapid turning of the head and eyes.