Lecture Final Exam - Chapter 18: General and Special Senses

Central Processing and Adaptation

Adaptation is a reduction in sensitivity in the presence of a constant stimulus. Tonic receptors are slow-adapting receptors Phasic receptors are fast-adapting receptors 1. Peripheral (sensory) adaptation - when a receptor or sensory neuron alters its level of activity 2. Central adaptation - a process involving nuclei along the sensory pathways within the CNS. - central adaptation restricts the amount of information arriving at the cerebral cortex.

Mechanoreceptors: Baroreceptors and Proprioceptors

Baroreceptors are stretch receptors that monitor changes in the stretch of organ walls. Proprioceptors monitor the position of joints, the tension in tendons and ligments, and the extent of muscular contraction.

Introduction

Every plasma membrane functions as a receptor for the cell because it responds to changes in the extracellular environment. 1. Sensory receptor - a specialized cell or cell process monitoring conditions in the body or external environment. - stimulation of the receptor directly or indirectly alters the production of action potentials in a sensory neuron. 2. Sensation - the sensory information arriving at the CNS. 3. Perception - a conscious awareness of a sensation. 4. General senses - sensations of temperature, pain, touch, pressure, vibration, and proprioception (body position). 5. Special sense - smell, taste, equilibrium (balance), hearing, and vision. - these receptors are localized within complex sense organs, such as the eye or ear.

Vision: Accessory Structures of the Eye - Eyelids

Eyelids - act like windshield wipers - continual blinking keeps the surface lubricated and free from dust and debris - palpebral fissure - the gap between the upper and lower eyelids - medial and lateral angels connect the upper and lower eyelids - tarsal glands - secrete a lipid-rich produce that keeps the eyelids from sticking together - lacrimal caruncle - a mass of soft tissue at the medial angel of the eye. Within it, glands produce thick secretions that form the gritty deposits sometimes found at the edge of the eye after a good night's sleep - tarsal plate - supports and strengthens the eyelids - palpebral conjuctiva - covers the inner surface of the eyelids - bulbar conjuctiva - covers the anterior surface of the eye

Receptors

Free nerve endings - the simplest receptors in the human body such as dendrites of sensory neurons. Receptor sensitivity - characteristic receptor sensitivity. For example, the receptors of the eye are surrounded by accessory cells that normally prevent their stimulation by anything other than light. Receptive field - the area monitored by a single receptor cell. The larger the receptive field, the harder it is to localize the stimulus.

The General Senses

General senses are divided into three classes: 1. Exteroceptors - provide information about the external environment. 2. Proprioceptors - monitor body position 3. Interoceptors - monitor conditions inside the body Detailed classification of general senses: 1. Nociceptors - respond to a variety of stimuli associated with tissue damage (sensation of pain). 2. Thermoreceptors - respond to changes in termperature. 3. Mechanoreceptors - stimulated or inhibited by physical distortion, contact, or pressure on their plasma membrane. 4. Chemoreceptors - monitor the chemical composition of the body fluids and respond to the presence of specific molecules.

Gustation (Taste)

Gustation, or taste, provides information about the foods and liquids we eat and drink. Gustatory epithelial cells, or taste receptors are found in taste buds that are distributed over the tongue surface. The tongue surface has numerous epithelial projections called lingual papillae. Four types of lingual papillae: 1. Fiiform papillae 2. Fungiform papillae 3. Vallate papillae 4. Folicate papillae

Vision: Accessory Structures of the Eye - The Lacrimal Apparatus

Lacrimal apparatus - produces, distributes, and removes tears. Each lacrimal apparatus consists: 1. lacrimal gland 2. superior and inferior lacrimal canaliculi - where the tears drain 3. lacrimal sac 4. nasolacrimal duct Superior and inferior lacrimal puncta drain the lacrimal lake, the medial angle where tears collect.

General Senses: Mechanoreceptors

Mechanoreceptors are sensitive to stimuli that stretch, compress, twist, or distort their cell membranes. Three classes of mechanoreceptors: 1. Tactile receptors - detect sensations of touch, pressure, and vibration - fine touch and pressure receptors: provide detailed information about exact location, shape, size, texture, and movement of the stimulation - crude touch and pressure receptors: provide poor localization and little additional information about stimulus. 2. Baroceptors - detect pressure changes in the walls of blood vessels and in portions of the digestive, reproductive, and urinary tracts. 3. Proprioceptors - detect the positions of joints and skeletal muscles.

General Senses: Nociceptors

Nociceptors (pain receptors) are free nerve endings with large receptive fields. They are common in the skin, joint capsules, and periostea of bones and around the walls of blood vessels. Three types of nociceptors: 1. receptors sensitive to extreme temperature 2. receptors sensitive to physical damange 3. receptors sensitive to dissolved chemicals, such as those released by injured cells Fast pain (pricking pain) - sensations produced by deep cuts or similar injuries. Slow pain (burning pain) - sensations result from the same types of injuries as fast pain. However, sensations of slow pain begin later and persist longer. Referred pain - pain sensations originating in visceral organs

Interpretation of Sensory Information

Sensory coding - provides information about the strength, duration, variation, and movement of a stimulus. Tonic receptors - sensory neurons that are always active. For example, receptors of the eye and receptors monitoring the body position. Phasic receptors - normally inactive receptors, but become active for a short time whenever there is a change in the conditions they are monitoring. For example, touch and pressure receptors in the skin.

Mechanoreceptors: Tactile Receptors

Tactile receptors are subdivided into two groups: unencapsulated receptors and encapsulated receptors. 1. unencapsulated receptors - free nerve endings: free nerve endings of the root hair plexus detect distrotions and movements across the body surface. - in sensitive areas, the dendritic branches penetrate the epidermis and contact Merkel cells 2. encapsulated receptors - large, oval tactile corpuscles are found where the sense of touch is well developed, such as the eyelids, lips, fingertips, nipples, and external genitalia - tactile corpuscles detect light touch, movement, and vibration - bulbous corpuscles are located in the dermis and are sensitive to pressure and distortion of the skin - bulbous are tonically active and show little to no adaptation - lamellar corpuscles are large encapsulated receptors. The dendritic process lies within a series of concentric cellular layers. - layers in lamellar corpuscles shield the dendrite from sources of stimulation other than direct pressure. It is most sensitive to pulsing or vibrating stimuli

The External Ear

The external ear includes the outer fleshy auricle, the external acoustic meatus, and the tympanic membrane (eardrum). The tympanic membrane is a thin, semitransparent connective tissue sheet separating the external ear from the middle ear. In the external acoustic meatus, hairs project and trap debris and provide tactile sensitivity through their root hair plexuses. Ceruminous glands line the external acoustic meatus and secrets waxy substances called cerumen which slows the growth of microorganisms and reduces the chances of infection.

The Eye

The is divided into two cavities: the posterior cavity and anterior cavity. The anterior cavity is divided into two chambers: 1. the anterior chamber (between the cornea and the iris) 2. the posterior chamber (between the iris and the lens) Layers of the eye: 1. the fibrous layer (outermost layer) - sclera - white of the eye 2. the vascular layer 3. the inner layer

The Middle Ear

The middle ear contains an air-filled space called the tympanic cavity where the auditory ossicles are located and the auditory tube. The auditory ossicles connect the tympanic membrane with the receptor complex of the internal ear. They transfer sound vibrations from the tympanic membrane to a fluid-filled chamber within the internal ear. The three auditory ossicles are the: 1. Malleus - attaches to the interior surface of the tympanic membrane 2. Incus - connects the medial surface of the malleus to the stapes. 3. Stapes Two small muscles inside the tympanic cavity protect the eardrum and ossicles from violent movements under very noisy conditions. 1. Tensor tympani 2. Stapedius

Olfactory Sensory Neurons

The olfactory system is very sensitive. As few as four molecules of an odor-producing substance can activate an olfactory sensory neuron. Olfactory sensory neurons have a life span of 4 to 6 weeks.

The Internal Ear

The sensory receptors for equilibrium and hearing are located within the internal ear. The receptors are found within a membranous labyrinth filled with fluid called endolymph. The bony labyrinth is located within the temporal bone and protects the membranous labyrinth. Perilymph - fluid that flows between the bony and membranous labyrinth The bony labyrinth is divided into the vestibuled, semicircular canals, and the cochlea. Vestibular complex provides equilibrium sensations by detecting rotation, gravity, and acceleration.

General Senses: Thermoreceptors

There are two types of thermoreceptors: cold and warm. Temperature sensations are conducted in the spinothalamus tracts. Temperature sensations are sent to the reticular formation, the thalamus, and the primary somatosensory cortex. Thermoreceptors are phasic receptors.