Sensory System: Receptors & Neural Pathways

The Sensory System

• A component of the nervous system responsible for processing sensory details.
• It consists of sensory receptors, neural pathways, and the brain.
• A stimulus will cause a sensory receptor to become stimulated. Nerve signals are then generated and travel to the brain where signals are interpreted. The root cause for the initial simulation is then perceived.

General Senses

• Sensory receptors are categorized based on the stimuli they respond to.

Sensory Receptor Categories

• Mechanoreceptors: respond to changes in pressure or body movement. Examples include pressure receptors in the skin, proprioceptors in muscle, and mechanoreceptors in the inner ear.
• Thermoreceptors: respond to changes in temperature and are located in the skin and internal organs.
• Pain receptors: respond to damage or oxygen deprivation to tissues, such as those in the skin.
• Chemoreceptors: respond to changes in the chemical concentration of substances and are found in taste buds of the tongue and olfactory receptors in the nose.
• Photoreceptors: respond to light energy and are located only in the eyes.
• Sensory receptors in muscles, joints, tendons, some internal organs, and skin send action potentials to the spinal cord.
• These action potentials travel up the spinal cord to the thalamus, and the information is relayed to the somatosensory areas of the cerebral cortex.
• General sensory receptors are categorized into proprioceptors, cutaneous receptors, and pain receptors.

Proprioceptors

• Mechanoreceptors that are involved in reflex actions to maintain muscle tone, equilibrium, and posture.
• Help to know the position of limbs by detecting the degree of muscle relaxation, the stretch of tendons, and the movement of ligaments.
• Two forms are muscle spindles and Golgi tendon organs, both detect the degree to which a muscle or tendon is stretched.
• Motor nerves increase the degree of muscle contraction in response to signals from muscle spindles.

Cutaneous Receptors

• Exists in the deepest layer of epidermis and the entire dermis.
• They make the skin sensitive to touch, pressure, pain, and temperature.
• Three types are sensitive to fine touch: Meissner corpuscles, Merkel disks, and Root hair plexus.
• Three types are sensitive to pressure: Pacinian corpuscles, Ruffini endings, and Krause end bulbs.

Pain Receptors (Nociceptors)

• Exists in the skin and many internal organs.
• Somatic nociceptors from the skin and skeletal muscles respond to mechanical, thermal, electrical, or chemical damage.
• Visceral nociceptors respond to excessive stretching of internal organs, oxygen deprivation, or chemicals released by damaged tissues.
• A sensation of pain from the skin as well as the internal organs is referred to as referred pain; stimulation of internal receptors causes this.

Referred Pain Occurence

• Somatic nociceptors travel in the same spinal cord pathway as visceral nociceptors.
• Signals from both sets of neurons converge on the same nervous pathway.
• The brain is unable to distinguish between the two.
• Symptoms of a heart attack can be accompanied by referred pain in the left shoulder and arm.

Receptor Potential

• A system by which sensory receptors start signal transmission.
• It begins with a stimulus, such as a light stimulus for receptors in the eye.
• It can be weak or strong, unlike an action potential.
• Receptor potentials can add together, as they do when a presynaptic neuron synapses with a postsynaptic neuron.
• Receptor potentials do not generate action potentials but are parts of neurons or synapse with other neurons that do create action potentials.

Sense of Taste

• The taste buds are primarily on the tongue along the walls of peglike projections called papillae in the tongue mucosa that make the tongue surface slightly abrasive.
• Taste buds are a chemical sense because its receptors are sensitive to molecules in the food we eat. The sensory receptors are primarily in taste buds.
• Each taste sensation is scattered throughout the tongue; isolated taste buds are present on the soft palate, inner surface of cheeks, pharynx, and epiglottis.

Three Kinds of Papillae With Taste Buds

• Fungiform papillae are mushroom-shaped, scattered over the entire tongue, and have one to five taste buds each.
• Vallate papillae are the largest and least numerous; have many taste buds, and eight to 12 form an inverted V at the back of the tongue.
• Foliate papillae are located laterally on the tongue, contain many taste buds during childhood, but fewer with age.

Gustatory Epithelial Cells

• Gustatory cells comprise long microvilli called gustatory hairs, which project from their tips. Gustatory hairs represent the sensitive receptor membranes of the gustatory epithelial cells.
• Sensory dendrites that represent the initial part of the gustatory pathway to the brain coil intimately around the gustatory epithelial cells.
• At least three types of gustatory epithelial cells exists: one forms traditional synapses with the sensory dendrites and releases the neurotransmitter serotonin. The other two lack synaptic vesicles, but one releases ATP that acts as a neurotransmitter.
• Basal epithelial cells act as stem cells, dividing and differentiating into new gustatory epithelial cells.
• The new replacement of taste bud cells is approximately every seven to ten days.

The Gustatory Pathway

• Transmits impulses from taste receptors in the anterior two-thirds of the tongue is a branch of the facial nerve (VII), the chorda tympani.
• The lingual branch of the glossopharyngeal nerve (IX) services the posterior third and the pharynx just behind.
• Taste impulses from the few taste buds in the epiglottis and the lower pharynx are conducted primarily by the vagus nerve (X).
• These afferent fibers synapse in the solitary nucleus of the medulla.
• Impulses stream from the solitary nucleus to the thalamus and ultimately to the gustatory cortex in the insula.
• Fibers project to the hypothalamus and limbic system structures, regions that determine our appreciation of what is being tasted.

Basic Taste Sensations

• Taste sensations can be grouped into five basic modalities: sweet, sour, salty, bitter, and umami.

The Five Primary Modalities

• Sweet is produced by many organic substances including sugars, saccharin, alcohols, some amino acids, and some lead salts.
• Sour is produced by acids, specifically their hydrogen ions (H+).
• Salty is produced by metal ions (inorganic salts); table salt (sodium chloride) is the "saltiest".
• Bitter is produced by alkaloids (such as quinine, nicotine, caffeine, morphine, and strychnine) as well as several nonalkaloid substances, such as aspirin.
• Umami is produced by the amino acids glutamate and aspartate, responsible for the "beef taste" of steak, the characteristics tang of aged cheese, and the flavor of the food additive monosodium glutamate (MSG).
• There appears to be a sixth sense to also taste long-chain fatty acids.

Sense of Smell

• The receptors are sensitive to odor molecules in the air being breathed.
• The sense of smell is dependent on olfactory cells located within olfactory epithelium (in the roof of the nasal cavity).
• Each cell ends in a tuft of five olfactory cilia bearing receptors for odor molecules, and has only one type of receptor
• Olfactory cells communicate with olfactory bulb neurons.
• Afterward, olfactory bulb neurons communicate with the olfactory areas in the brain.

The Olfactory Pathway

• Axons of the olfactory sensory neurons form the olfactory nerves that synapse in the overlying olfactory bulbs, which are the distal ends of the olfactory tracts.
• There, the filaments of the olfactory nerves synapse with mitral cells, second-order sensory neurons, in complex structures called glomeruli.
• Axons from neurons bearing the same kind of receptor converge on a given type of glomerulus, each glomerulus represents a single aspect of an odor, and each odor activates a unique set of glomeruli.
• The mitral cells refine the signal, amplify it, and then relay it.
• When the mitral cells are activated, impulses flow from the olfactory bulbs via the olfactory tracts to the piriform lobe of the olfactory cortex.

Information Pathway of The Olfactory Tract

• One pathway brings information to part of the frontal lobe just above the orbit for interpreting and identifying smells.
• The other pathway flows through the thalamus, hypothalamus, amygdaloid body, and other regions of the limbic system; there, emotional responses to odors are elicited.

Sense of Vision

Accessory Organs of The Eye

• The eyebrows are short, thick hairs positioned transversely above the eye and shade the eyes from the sun, prevent perspiration or debris on the forehead from falling into the eye.
• The eyelids are a continuation of the skin, help keeps the eye lubricated and free of debris. The orbicularis oculi muscle closes the eyelid, and the levator palpebrae superioris muscle raises the lid.
• The inner surface is lined by conjunctiva, a transparent mucous membrane which folds back to cover the anterior of the eye, except for the cornea.
• The eyelashes trap debris and keep it from entering the eyes
• The associated sebaceous glands produce an oily secretion that lubricates the eye; inflammation of one of the glands is called a sty.

The Lacrimal Appratus

• The lacrimal gland produces tears that are secreted in the eye in the lacrimal sac.

Extrinsic Eye Muscles

• The medial rectus muscle moves the eye medially and is controlled by the oculomotor nerve (III).
• The lateral rectus moves the eye laterally and is controlled by the abducens nerve (VI).
• The superior rectus muscle elevates the eye and turns it medially and is controlled by the oculomotor nerve (III).
• The inferior rectus muscle depresses the eye and turns it medially, also controlled the oculomotor nerve (III).
• The inferior oblique elevates the eye and turns it laterally, and the superior oblique depresses the eye and turns it laterally. Both are controlled by the oculomotor nerve (III).

Eye Anatomy

• Layers of the Eye: Sclera (outer layer), Choroid (middle layer), and Retina
• Fovea centralis is where the light is focused when one looks directly at an object.
• Attached to the ciliary body by the suspensory ligaments. Divides eye into an anterior compartment filled with aqueous humor and posterior compartment filled of vitreous humor.

Function of the Eye

Functions of the parts of the eye:

• Sclera - Protects and supports the eyeball
• Cornea - Bends incoming light
• Choroid - Absorbs stray light
• Ciliary body - Holds the lens in place
• Ciliary muscle - Accommodation; changes the shape of the eye
• Iris - Regulates entrance of light into the eye
• Pupil - Entrance of light into the Retina
• Retina- Sensory reception for light.
• Rods - Black and White vision, peripheral vision, dim light vision
• Cones - Color vision, bright conditions.
• Fovea Centralis - Concentration for cone cells. Lens refracts and focuses lightrays. Suspensory ligaments support the lens Aqueous humor supports anterior compartment Vitreous humor supports posterior compartment

Eye Focus

• The lens, assisted by the cornea and the humors, focuses images on the retina.
• Light rays from each point on an object are bent in such a way that an inverted and reversed image of the object forms on the retina.
• The lens is flat and ciliary muscle is relaxed with a taut suspensory ligament when focusing on a distant object.
• The lens becomes rounded, ciliary muscle contracts, which relaxes the suspensory ligament when focusing on nearby objects.

Corrective Lenses

• A concave lens helps focus light rays on the retina for nearsighted people.
• A convex lens helps focus light rays on the retina and helps farsighted people.
• Specialized lenses for astigmatism helps focus unevenly focused light to the retina.

Vision Pathway and Visual Pigments

• Visual pigment in rods called rhodopsin, is made of the protein opsin and retinal.
• There are B (blue), G (green), and R (red) visual pigments in cones. Also, made of opsin and retinal but a slight difference of opsin in each.
• Rods are plentiful throughout retina and helps people with vision in peripheral locations in the retina. Cones can be located primarily at the fovea centralis. Helps people with detailed vision of the cones in the sharper detailed image of object.

Function of the Retina

• Ganglion cells can be located here and their fibers create the optic nerve.

Layers of the retina

• The retina's deepest layer is closest to the choroid and contains the rod and cone cells. The middle layer contains bipolar cells, and the innermost layer contains ganglion cells The sensory fibers of ganglion cells become the optic nerve.
• Multiple rods converge onto one ganglion, compared to only certain cones cells that connect to only one ganglion bell.

The Journey From Retina to Visual Cortex

• Optic nerves carry signals to the optic chiasma and the optic chiasma is formed where the optic nerve cross over.
• Fibers from right and left half of each retina converge into optic tract.
• The fibers synapse at neurons in the thalamus and forms the optic radiation. Finally creating action potentials at the primary area of the occipital lobe for interpretation.

Eye Diseases

• Scarring of the cornea Causes include injury/infection and can often be restored by corneal transplant
• Cataracts-Lens become cloudy that can replaced artificially

Eye problems

• A buildup of aqueous humor can lead to blindness, also known as glaucoma
• Macular Degeneraration causes distorted visual field and will have areas in the scene that appear as gray or wrinkled. Whereby objects may appear smaller and colors are dimmer

Hearing Senses

• The ear has two sensory functions: hearing and equilibrium (balance). The sensory receptors for both hearing and equilibrium reside in the inner ear.
• Hair cells respond to mechanical stimulation or movement.

The Three Divisions of Ear

• The Outer, Middle, and Inner.
• Outer ear includes: Pinna and auditory canal.
• Middle ear includes: Tympanic membrane, Auditory Tube, and ossicles: malleus, incus, stapes
• Inner ear includes: Semicircular canal Vestible and Cochlea

Auditory Waves Pathway

1. Sound waves vibrate tympanic membrane
2. Auditory ossicles vibrate, pressure is amplified 3.Stapes push pressure at end of oval window as the perilymph move through scala vestibule.
3. (a) Sounds past the hearing range will travel to helicotrema and does not excite hairs (b) sounds in the hearing range will vibrate to the vibrating base membrane

Hearing and the Cochlea

• Consist of the Vestibular, Cochlear, and Tympanic. The vestibular and typanic are filled by perilymph and the vestibular is filled with endolymph.
• Also consist of a basilar layer and a tectoral gelatinous material and the stereocilia that is embedded above From the ear drum to inside the cochlea.

The Inner Ear and Signal Pathway

Inner sound comes that that base member causing steriocia to signal Which causes a change of the signalling from the hair cells to the associated neurons Then signals go to brainstem and then relaid via the thelamus towards the cortex Signals are interreted as sound Action goes in coclea pass spiral where auitory bipolar cell bodies reside along afferner of auitory toward coclear nuclei

General ear diagram description

Acoustic sound turns into mechanical action in inner ear. Then to mechanical vibration