Optokinetic nystagmus (OKN) is an involuntary eye movement which occurs when visually tracking a moving object across the visual field. It serves to stabilize images on the retina by keeping the object centered on the fovea, allowing for clear vision [1][3]. OKN has two phases – a slow tracking phase in the direction of motion,…
Author: lewj2012
Laughter
Laughter is a complex vocal expression of positive emotion which is ubiquitous in human behavior. It serves several social functions like diffusing tension and strengthening bonds. Laughter is also pleasurable and activates ancient brain circuits related to reward and social affiliation [1][2][6]. Neuroimaging studies show that humorous stimuli engage key nodes of the brain’s mesolimbic…
Itch and scratching
Itch is transmitted to the spinal cord dorsal horn by small diameter C and Aδ fibers responding to pruritogens like histamine [2]. These first order neurons synapse onto second order spinothalamic projection neurons that ascend to the thalamus and cortex to generate the sensation of itch [3][4]. Scratching initiation involves spinal interneurons that form part…
Ocular convergence
Ocular convergence refers to the inward rotation of the eyes to maintain binocular fixation as objects approach closer to the viewer [1][2]. This allows the object’s image to fall on corresponding retinal regions in each eye to achieve unified vision [3]. The degree of convergence required depends on the object’s distance, with nearer objects eliciting…
Phototransduction
Phototransduction is the process by which light is converted into electrical signals in photoreceptor cells such as rods and cones in the retina. It involves a cascade of biochemical reactions which ultimately lead to hyperpolarization of the photoreceptor cell membrane and a decrease in the release of the neurotransmitter glutamate to bipolar cells [2][4][5]. The…
Fluid homeostasis of the ocular surface
The ocular surface comprising the cornea and conjunctiva must maintain adequate hydration for optical transparency and refractivity. Tear film covers the ocular surface, providing lubrication and nutrition while protecting against desiccation [1]. Tears are secreted by the lacrimal glands and distributed by blinking and drainage through the lacrimal apparatus [1][3]. The aqueous layer is secreted…
Pupillary constriction
Pupillary constriction represents one of the most fundamental and clinically significant reflexes in human neurophysiology. The pupillary light reflex (PLR) describes the constriction and subsequent dilation of the pupil in response to light as a result of the antagonistic actions of the iris sphincter and dilator muscles (1, Abstract). This reflex not only serves as…
Postural balance and control
Maintaining upright stance requires continuous integration of sensory information and motor output [1]. The size of the base of support influences postural stability, with a larger area (e.g. lying prone) easier to balance over than a small area (e.g. standing) [2]. Postural adjustments involve both reflexive and feedforward control mechanisms [3][4]. Sudden external perturbations elicit…
Lens accommodation
Lens accommodation refers to dynamic changes in the shape and optical power of the lens to maintain focus on objects at varying distances [1]. The primary mechanism involves the ciliary muscle and zonular fibers altering lens curvature and thickness [2]. In the unaccommodated state for distant vision, parasympathetic input to the ciliary muscle is low….
Snout reflex
The snout reflex is a primitive reflex observed in infants that facilitates rooting and suckling behaviors [1]. When the infant’s cheek is stroked or tapped near the mouth, it responds by turning its head in that direction and opening its mouth, appearing to search for the stimulus [2]. This reflex emerges as early as the…