Neurophysiology

·         The response of rod cells to light causes hyperpolarization, whereas olfactory stimuli result in depolarization.

The olfactory epithelium and rod cells are two examples of signal transduction that bypass a protein kinase system.

In the case of the olfactory epithelium, an odorant molecule binds to an odor-specific transmembrane receptor found on the modified cilia at the apical surface. The binding activates an odorant-specific G protein (Golf), which binds GTP. The resulting dissociation of the alpha subunit stimulates adenylate cyclase to produce cyclic AMP. Cyclic AMP directly stimulates the opening of the cation channels on the membrane of the bipolar olfactory receptor cells, leading to Na+ influx. The resulting change in membrane potential (depolarization) is transmitted from the modified cilia to the olfactory vesicle through the neuron to the basal axon. Axonal processes traverse the lamina propria as the olfactory nerve and pass through the cribriform plate of the ethmoid to terminate in the olfactory bulb. In the case of the rod, the cyclic nucleotide involved is cGMP.

Visual transduction involves closing of the Na+ channel in rod cells in response to photons of light. Rhodopsin is the visual pigment of rod cells and is composed of retinal, a vitamin A derivative, bound to opsins. Photons reaching rhodopsin isomerize retinal to the all-trans form from 11-cis retinal. The result is bleaching, which represents the dissociation of retinal from the opsins. The bleaching process results in a fall in cGMP within the cytosol. Transducin is a G protein that couples bleaching to cGMP through the action of a phosphodiesterase enzyme that cleaves cGMP to GMP. The closing of the Na+ channel results in a reduction in permeability to sodium ions and hyperpolarization of the cell membrane. The signal spreads to the inner segment and through gap junctions to nearby photoreceptor cells. In the presence of cGMP, the Na+ channel remains open; in its absence, the channel closes and the cell hyperpolarizes. Therefore, the rods and cones differ from other receptors in that hyperpolarization of the cell membranes occurs rather than the depolarization that occurs in other neural systems. Closing the channel slows down the release of the visual transmitter.

·         The anatomic arrangement allows frequency analysis of sounds:

1.       The basilar membrane responds to high frequencies at its base and to low frequencies at its apex.

2.       The hair cells in the base of the cochlear duct have short and fat stereocilia, which are stimulated by high frequencies.

3.       The hair cells in the apex of the cochlea have long and thin stereocilia, which respond best to low frequencies.

·         POSTURES:

·      Decorticate posture → upper extremities flexed, lower extremities extended ( CAR )

Decorticate posturing most often occurs in comatose patients with lesions below the thalamus but above the red nucleus.

·      Decerebrate posture → full extention of ALL extremities ( SCOOTER )

Comatose patients with lesions below the red nucleus but above the vestibular nucleus may have decerebrate posturing.

·         High-frequency sounds produce a vibration of the basilar membrane at the base of the cochlea (near the oval and round windows); low-frequency sounds produce a vibration of the basilar membrane at the apex of the cochlea (near the helicotrema).

·         Audiologists refer amplification phenomenon of middle ear bones as impedance matching.

·         Even extensive lesions of the visual cortex will not affect the center of the visual field, which is conveyed to the cortex by nerve fibers projecting from the macula. This is called macula sparing.

·         The inner ear is divided into three chambers (scala vestibuli, scala media, and scala tympani).

The scala vestibuli is separated from the scala media by Reissner’s membrane; the scala media and the scala tympani are separated by the basilar membrane.

The stapes is attached to the membrane of the oval window, which separates the middle ear from the scala vestibuli.

The scala tympani is separated from the middle ear by the round window.

The organ of Corti sits on the basilar membrane.

The fluid within the scala vestibuli and scala tympani (perilymph) is similar to interstitial fluid; the fluid within the scala media (endolymph) resembles intracellular fluid in that it contains a high concentration of K+.

Vibration of the stapes causes the fluid within the scala tympani to vibrate, which in turn causes the basilar membrane to vibrate. Vibration of the basilar membrane causes the cilia to bend back and forth. Bending the stereocilia toward the kinocilium causes K+ channels on the hair cells to open; bending the stereocilia away from kinocilium causes K+ channels to close. Auditory hair cells are unusual because they are depolarized by the flow of K+ into the cell. K+ can flow into the hair cells because the endolymph surrounding the apical portions of the hair cells contains a high K+ concentration.

The basilar membrane is most stiff at the base of the cochlea (near the middle ear) and most compliant at the apex of the cochlea. High-frequency sounds cause a greater vibration of the stiff portion of the cochlea, and, therefore, the hair cells located near the base of the cochlea transmit information about high-frequency sounds to the auditory cortex. Similarly, low-frequency sounds are transmitted to the auditory cortex by the hair cells near the base of the cochlea, which are located on the more compliant portions of the basilar membrane.

·         Light is detected by the rods and cones contained in the retina of the eye. The retina contains five types of neurons: photoreceptors (rods and cones), bipolar cells, ganglion cells, horizontal cells, and amacrine cells. Light rays from distant objects are normally focused on the photoreceptors by the cornea and the relaxed lens. When objects are brought closer to the eye, they are kept focused on the retina by the accommodation reflex, which causes the refractive power of the lens to increase. The rods and cones contain a visual pigment, called rhodopsin, which absorbs light energy. Rhodopsin contains two components: opsin, which determines the wavelength of light absorbed by rhodopsin, and retinal, which undergoes isomerization by light.

The photoreceptors are unusual because they hyperpolarize when they are stimulated by light. When the rods and cones are not stimulated, they are depolarized by the flow of Na+ into the cell through Na+ channels held in the open state by cGMP. The photoisomerization of retinal from its 11-cis form to its all-trans form activates rhodopsin, which in turn activates a G protein called transducin. Activated transducin activates a cGMP esterase. Hydrolysis of cGMP causes Na+ channels on the rod and cone outer segments to close, which produces the membrane hyperpolarization.

The neurotransmitter keeps the bipolar cells and, therefore, the ganglion cells in a polarized and relatively quiescent state. Hyperpolarization of the photoreceptors stops the release of an inhibitory neurotransmitter, which in turn causes bipolar cells to depolarize. The bipolar cells stimulate ganglion cells, which in turn convey information about the light stimulus to the visual cortex. The ganglion cells are the only cells in the retina to produce an action potential. Their axons form the optic nerve.

·         The gate-control hypothesis of pain states that pain transmission is suppressed by innocious signals in thick myelinated afferents (group II), whereas the pain sensation is enhanced by signals in thin afferents. Inhibitory interneurons in the dorsal horn of the spinal cord perform the gate-control through a special type of presynaptic inhibition called primary afferent depolarization (PAD), and the receptors on the cell body of the secondary neuron is the gate.

·         The reticular activating system (RAS) transmits facilitatory signals to the thalamus. The thalamus excites the cortex, and the cortex then excites the thalamus in a reverberating circuit. Such a positive feedback loop is what wakes us up in the morning. During the day external stimuli and internal factors including inhibitory interneurons balance the different activity levels.

·         The 3 cone types are uniformly distributed in the retina except in the fovea. The fovea has no cyanolab cones and no rods.

·         Movements in the visual scenery are depicted as opposite movements on the retina. Convergent inputs from the two eyes result in depth perception (ie, stereopsis or stereoscopic vision). Stereopsis depends upon the medial longitudinal fasciculus and the corpus callosum. These structures co-ordinate the movements of the two eyes.

·         The medial geniculate nucleus and the inferior collicle can increase its tone selectivity by dampening other sound signals. This explains the cocktail party effect.

·         CBF is normally 55 FU in humans at rest. One Flow Unit (FU) is one ml of blood per min per 100 g of brain tissue. This resting CBF and the oxygen uptake of the brain can double during cerebral activity and triple in active brain regions during an epileptic attack. Brain vessels are metabolically regulated. Increased PaCO2, and reduced PaO2 dilatate brain vessels and increase CBF. 

·         POMC peptides are neuroregulatory hormones: ACTH, endogenous opiates, b-endorphin,  b-lipoprotein, a-MSH and b-MSH.

·         CSF differs from blood in having a lower concentration of K+, glucose, and protein and a higher concentration of Na+ and Cl-; CSF normally lacks blood cells.

·         Many ganglion cells in primates can be classified as P, M, or W cells. P cells with small receptive fields and tonic and linear responses signal fine detail and wavelength. M cells have nonlinear responses and signal motion. Most W cells are difficult to activate.

·         The upper layers of the superior colliculus are involved in visual processing. The deep layers produce eye movements directed at visual targets that move into the field of vision or that are sources of somatosensory or auditory stimuli.

·         Sound waves are combinations of pure tones, and the composition of a sound can be determined by Fourier analysis. A pure tone is characterized in terms of its amplitude, frequency, and phase.

·         Specific thalamocortical afferent fibers terminate mainly in the middle layers of the neoocortex; diffuse thalamocortical afferent fibers synapse in layers I and VI.

Cortical efferent fibers from layers II and III project to other areas of the cortex; those from layer V project to many subcortical targets, including the spinal cord, brainstem, and striatum, as well as to non-specific thalamic nuclei; and layer VI distributes to the appropriate specific thalamic nucleus.

·         The cortical structure varies in different regions. Agranular cortex is found in the motor areas, whereas granular cortex (koniocortex) occurs in the primary sensory receiving areas. Homotypical cortex is found elsewhere in the neocortex.