SOMATOSENSORY AND SPECIAL SENSES

Chemoreceptors

Sensory receptors that detect chemicals in the mouth nose and body fluids

Mechanoreceptors

Sensory receptors that are sensitive to mechanical stimuli such as the deformation stretching or bending of cells

Nociceptors

Sensory receptors that respond to painful stimuli resulting from physical or chemical damage to tissue

Photoreceptors

Sensory receptors that respond to light striking the receptor cells and are necessary for vision

Thermoreceptors

Sensory receptors that detect changes in temperatures

Free nerve ending

Branching structure with no capsule that detects pain itch tickle temperature joint movement and proprioception

Merkel disk

Consists of flattened expansions at the end of axons associated with a Merkel cell to detect light touch and superficial pressure

Hair follicle receptor

Wrapped around hair follicles or extending along the hair axis where each axon supplies several hairs and each hair receives branches from several neurons resulting in considerable overlap to detect light touch through very slight bending of the hair

Pacinian corpuscle

Onion-shaped capsule composed of several cell layers with a single central nerve process to detect deep cutaneous pressure vibration and proprioception

Meissner corpuscle

Several branches of a single axon associated with specialized Schwann cells and surrounded by a connective tissue capsule involved in two-point discrimination

Ruffini end organ

Branching axon with numerous small terminal knobs surrounded by a connective tissue capsule that detects continuous touch or pressure and responds to depression or stretch of the skin

Muscle spindle

Three to ten striated muscle fibers enclosed by a loose connective tissue capsule striated only at the ends with sensory nerve endings in the center involved in proprioception associated with the detection of muscle stretch important for control of muscle tone

Golgi tendon organ

Surrounds a bundle of tendon fasciculi enclosed by a delicate connective tissue capsule with branched nerve terminations and small swellings applied to individual tendon fasciculi for proprioception associated with the stretch of a tendon important for control of muscle contraction

Cutaneous Receptors

Sensory receptors associated with the skin providing information about the external environment

Visceroreceptors

Sensory receptors associated with internal organs providing information about the internal environment

Proprioceptors

Sensory receptors associated with joints tendons and other connective tissue providing information about body position movement and the extent of stretch or force of muscle contractions

First-order neuron

Sensory neurons that conduct nerve impulses from somatic sensory receptors into the brainstem or spinal cord

Second-order neuron

Neurons that conduct nerve impulses from the brainstem or spinal cord to the thalamus where axons decussate as they course through the brainstem or spinal cord before ascending

Third-order neuron

Neurons that conduct nerve impulses from the thalamus to the primary somatosensory cortex on the same side

Spinothalamic tract

Ascending tract allowing conscious perception of pain and temperature information as well as light touch and pressure tickle and itch sensations decussating within the level of the spinal cord

Dorsal-column medial-lemniscal system

Ascending tract carrying sensations of two-point discrimination proprioception pressure and vibration decussating at the level of the medulla oblongata

Fasciculus gracilis

Tract transmitting sensations from the lower part of the body below the midthoracic level terminating and synapsing in the nucleus gracilis of the medulla oblongata

Fasciculus cuneatus

Tract transmitting sensations from the upper part of the body above the midthoracic level terminating and synapsing in the nucleus cuneatus of the medulla oblongata

Sclera

The white of the eye giving shape to the eyeball

Cornea

Transparent coat that covers the iris

Pupil

Hole in the center of the iris

Iris

Colored portion of the eyeball consisting of melanocytes and circular and radial smooth muscles that regulates the size of the pupil

Lens

Located behind the pupil and iris helping focus images on the retina to facilitate clear vision

Rods

Bipolar photoreceptor cells involved in noncolor vision responsible for vision under conditions of reduced light

Cones

Bipolar photoreceptor cells responsible for color vision and visual acuity

Aqueous humor

Fluid in the anterior chamber

Vitreous humor

Fluid in the posterior and vitreous chambers

Accommodation

The process of changing the shape of the lens

Refraction

Bending of light as it passes from air to a denser substance slowing its speed and bending the rays

Reflection

Bouncing of light from the surface when they strike an object that is not transparent

Convergence

Medial rotation of the eyes as an object moves closer so that the object is kept focused on corresponding areas of each retina

External ear

Consists of the auricle or pinna external auditory canal and tympanic membrane

Auditory ossicles

Three bones in the middle ear consisting of the malleus incus and stapes

Eustachian tube

Connects the middle ear to the pharynx and equalizes pressure

Bony labyrinth

Canal system within the temporal bone containing perilymph and membranous labyrinth

Cochlea

Spiral-shaped canal within the temporal bone divided into scala vestibuli scala tympani and cochlear duct responsible for hearing

Utricle and saccule

Two otolith organs located in the vestibule containing maculae with hair cells embedded in a membrane containing otoliths evaluating position of head relative to gravity and detecting linear acceleration and deceleration

Macula

Small thickened region of otolith organs attached to their inner wall where hair cells are bundled together

Otolith

Layer of dense calcium carbonate crystals pulled down by gravity to detect head tilt

Semicircular canals

Three canals at right angles to one another evaluating movements of the head with ampullae containing crista ampullaris and cupula

Anterior semicircular canal

Semicircular canal that detects forward and backward head movement

Posterior semicircular canal

Semicircular canal that detects upward and downward head movement

Lateral semicircular canal

Semicircular canal that detects left and right head movement

Olfactory neurons

Bipolar neurons in the olfactory epithelium whose distal ends are enlarged as olfactory vesicles with long cilia

Facial nerve

Cranial nerve VII carrying taste sensations from the anterior two-thirds of the tongue

Glossopharyngeal nerve

Cranial nerve IX carrying taste sensations from the posterior one-third of the tongue

Vagus nerve

Cranial nerve X carrying taste sensations from the epiglottis