BIOL 2304
Special Senses
General Senses
General senses:
Respond to
Temperature
Pain
Touch
Pressure
Vibration
Proprioception
Receptors
Located throughout body
Structurally simple
General Senses
Sensation – transduction of stimulus energy
Perception – conscious awareness of stimulus energy
Receptive field – area on body monitored by a single receptor cell
Adaptation – reduction in sensitivity in the presence of a constant stimulus
Peripheral (receptor activity change)
Central (inhibition of sensory pathway)
Sensory Receptors
Classified by Location:
Exteroceptors – sensitive to stimuli arising from outside the body
Located at or near body surfaces
Include receptors for touch, pressure, pain, and temperature
Interoceptors – receive stimuli from internal viscera
Located in digestive tube, bladder, and lungs
Monitor a variety of stimuli
Changes in chemical concentration
Taste stimuli
Stretching of tissues
Temperature
Proprioceptors
Located in skeletal muscles, tendons, joints, and ligaments
Monitor degree of stretch
Send inputs on body movement to the CNS
Sensory Receptors
Classified by Stimulus Detected
Mechanoreceptors respond to mechanical forces (touch, pressure, stretch, vibration, and itch)
Baroreceptors monitor blood pressure
Thermoreceptors respond to temperature changes
Chemoreceptors respond to chemicals in solution
Photoreceptors respond to light
Nociceptors respond to harmful stimuli that result in pain
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Sensory Receptors
Classified by Structure
Free Nerve Endings – naked dendrites
Merkel’s – stratum basale (basal layer) of epidermis, light touch
Hair follicle receptors – reticular layer of dermis
Encapsulated Nerve Endings – dendrites enclosed by connective tissue
Meissner’s – dermal papilla (papillary layer of dermis), discriminative touch
Pacinian – hypodermis, deep touch
Ruffini – dermis and hypodermis, deep touch
Proprioceptors – in muscles/tendons, monitor stretch
Major Somatosensory Pathaways
Spinothalamic Pathways
Anterior spinothalamic tract carries crude touch and pressure sensations.
Crude touch to right side of body activates receptor that synapses in spinal cord
2nd neuron decussates anteriorly and ascends medially to thalamus, where it synapses.
Thalamic neuron ascends to primary somatosensory cortex (post-central gyrus)
where it synapses, and perception occurs.
Lateral spinothalamic tract carries pain and temperature sensations.
Painful stimulus to right side of body activates receptor that synapses in spinal cord.
2nd neuron decussates anteriorly and ascends laterally to thalamus, where it synapses.
Thalamic neuron ascends to primary somatosensory cortex (post-central gyrus)
where it synapses, and perception occurs.
Posterior Column Pathway
Posterior column tract carries fine touch sensations.
Fine touch to right side of body activates receptor that ascends dorsally and synapses
ipsilaterally in the medulla.
2nd neuron decussates and ascends medially to thalamus, where it synapses.
Thalamic neuron ascends to primary somatosensory cortex (post-central gyrus)
where it synapses, and perception occurs.
Spinocerebellar Pathway
Spinocerebellar tract carries proprioceptive sensations.
Proprioceptive sensations from the right and left sides of the body activate receptors
that synapse in the spinal cord.
One of the 2nd order neurons decussates and ascends to the cerebellum.
The other 2nd order neuron ascends ipsilaterally to the cerebellum.
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Sensory Homunculus
Each region of the sensory cortex corresponds to a body regions.
The amount of cortical space dedicated to that body region is proportional to the number of sensory
receptors located in that body region.
This is the sensory homunculus
Motor Homunculus
Corticospinal pathway provides voluntary control over skeletal muscles.
Neurons in primary motor cortex (pre-central gyrus) descend, decussate in the medulla and
synapse onto anterior spinal cord neurons.
The 2nd order neuron is a motor neuron that synapses onto a skeletal muscle
There is a corresponding motor homunculus
Sensory and Motor Maps
Mapping is plastic
Changes with experiences
Changes with use/disuse
Changes with injury
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Olfactory Anatomy
Olfactory receptors are part of the olfactory epithelium
Olfactory epithelium is pseudostratified columnar and contains three main cell types:
Olfactory sensory neurons
Supporting epithelial cells
Basal epithelial cells
Cell bodies of olfactory sensory neurons
Located in olfactory epithelium
Have an apical dendrite that projects to the epithelial surface
Ends in a knob from which olfactory cilia radiate
Olfactory cilia act as receptive structures for smell
Mucus captures and dissolves odor molecules
Pathway for Olfactory Perception
Axons of olfactory epithelium
Gather into bundles—filaments of the olfactory nerve
Pass through the cribriform plate of the ethmoid bone
Attach to the olfactory bulbs and synapse with mitral cells
Mitral cells transmit impulses along the olfactory tract to
Limbic system
Piriform lobe of the cerebral cortex
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Anatomy of Gustation
Taste receptors
Occur in taste buds
Most are found on the surface of the tongue
Located within tongue papillae
Two types of papillae (with taste buds)
Fungiform papillae
Vallate papillae
Anatomy of Gustation
Collection of 50 –100 epithelial cells
Contain two major cell types
Gustatory epithelial cells supporting cells
Basal epithelial cells gustatory cells
Contain long microvilli – extend through a taste pore to the surface of the epithelium
Cells in taste buds replaced every 7–10 days
Anatomy of Gustation
Pathway for Perception of Taste
Taste information reaches the cerebral cortex
Primarily through the facial (VII) and glossopharyngeal (IX) nerves
Some taste information through the vagus nerve (X)
Sensory neurons synapse in the medulla
Located in the solitary nucleus
Impulses are transmitted to the thalamus and ultimately to the gustatory area of the
cerebral cortex in the insular lobe
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Pathway for Perception of Taste
Accessory Structures of the Eye
Eyebrows – coarse hairs on the superciliary
arches
Eyelids (palpebrae) – separated by the
palpebral fissure
Meet at the medial and lateral angles
(medial & lateral canthi)
Lacrimal caruncle – reddish elevation at
the medial canthus
Tarsal plates – connective tissue within
the eyelids
Tarsal glands – modified sebaceous
glands
Conjunctiva –
transparent mucous membrane
Palpebral conjunctiva
Bulbar conjunctiva
Conjunctival sac
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Accessory Structures of the Eye
Lacrimal apparatus – keeps the surface of the eye moist
Lacrimal gland – produces lacrimal fluid
Lacrimal sac –
fluid empties into nasal cavity
Lacrimal puncta (punctum) – minute opening of the nasolacrimal duct on the margin of each
eyelid near the medial commissure; drains tears from the eye, through the nasolacrimal duct
and into nasal cavity
Extrinsic Eye Muscles
Six muscles that control movement of the eye
Originate in the walls of the orbit
Insert on outer surface of the eyeball
Annular ring – origin of the four rectus muscles
The six extrinsic eye muscles are:
Lateral rectus and medial rectus
Superior rectus and inferior rectus
Superior oblique and inferior oblique
Extrinsic Eye Muscles
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Anatomy of the Eyeball
The Fibrous Layer – most external layer of the eyeball
Composed of two regions of connective tissue
Sclera – posterior five-sixths of the tunic
White, opaque region
Provides shape and an anchor for eye muscles
Cornea – anterior one-sixth of the fibrous tunic
Limbus – junction between sclera and cornea
Scleral venous sinus – allows aqueous humor to drain
The Vascular tunic (layer) – the middle coat of the eyeball
Composed of choroid, ciliary body, and iris
Choroid – vascular, darkly pigmented membrane
Forms posterior five-sixths of the vascular tunic
Brown color – from melanocytes
Prevents scattering of light rays within the eye
Choroid corresponds to the arachnoid and pia maters
Ciliary body – thickened ring of tissue, which encircles the lens
Composed of ciliary muscle
Ciliary processes – posterior surface of the ciliary body
Ciliary zonule (suspensory ligament) - attached around entire
circumference of the lens
Iris
Visible colored part of the eye
Attached to the ciliary body
Composed of smooth muscle
Pupil – the round, central opening
Sphincter pupillae muscle
Dilator pupillae muscle – act to vary the size of the pupil
Anatomy of the Eyeball
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Anatomy of the Eyeball
The retina – the deepest tunic, the inner layer (sensory tunic)
Composed of two layers
Pigmented layer – single layer of melanocytes
Neural layer – sheet of nervous tissue
Contains three main types of neurons
Photoreceptor cells (rods & cones)
Bipolar cells
Ganglion cells
Microscopic Anatomy of the Retina
Humors of the Eye
Posterior segment (cavity)
Filled with vitreous humor
Clear, jelly-like substance
Transmits light
Supports the posterior surface of the lens
Helps maintain intraocular pressure
Anterior segment
Divided into anterior and posterior chambers
Anterior chamber – between the cornea and iris
Posterior chamber – between the iris and lens
Filled with aqueous humor
Renewed continuously
Formed as a blood filtrate
Supplies nutrients to the lens and cornea
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Humors of the Eye
Pathway for the Perception of Light
Pathway begins at the retina.
Light activates photoreceptors
Photoreceptors signal bipolar cells
Bipolar cells signal ganglion cells
Axons of ganglion cells exit eye as the CN II
(optic nerve)
Optic nerve signals lateral geniculate
nucleus of the thalamus
Thalamus signals primary visual cortex
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Anatomy of the Ear
The ear – receptor organ for hearing and equilibrium
Composed of three main regions:
Outer ear – functions in hearing
Middle ear – functions in hearing
Internal ear – functions in both hearing and equilibrium
Anatomy of the Outer Ear
The auricle (pinna)
Helps direct sounds
External acoustic meatus
Lined with skin
Contains hairs, sebaceous glands, and ceruminous glands
Tympanic membrane
Forms the boundary between the external and middle ear
Anatomy of the Ear
Anatomy of the Middle Ear
Composed of
The tympanic cavity
A small, air-filled space
Located within the petrous portion of the temporal bone
Medial wall is penetrated by
Oval window
Round window
Pharyngotympanic tube (auditory or eustachian tube)
Links the middle ear and pharynx
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Anatomy of the Middle Ear
Ear ossicles – smallest bones in the body
Malleus – attaches to the eardrum
Incus – between the malleus and stapes
Stapes – vibrates against the oval window
Anatomy of the Middle Ear
Anatomy of the Inner Ear
Membranous labyrinth
Series of membrane-walled sacs and ducts
Fit within the bony labyrinth
Consists of three main parts
Organs of equilibrium
Semicircular ducts
Utricle and saccule
Organ of hearing
Cochlear duct
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Anatomy of the Inner Ear
Anatomy of the Inner Ear
Hair cells in the cochlea
Inner hair cells are the receptors that transmit vibrations of the basilar membrane
Outer hair cells actively tune the cochlea and amplify the signal
Pathway for Perception of Sound
Vibrations of basilar membrane signal hair cells
Hair cells signal Cranial nerve VIII (vestibulocochlear nerve)
Cranial nerve VIII signals a nucleus (superior olivary) in the medulla/pons
SO nucleus signals the inferior colliculus
Inferior colliculus signals medial geniculate nucleus of thalamus
Thalamus signals primary auditory cortex (temporal lobe)
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Pathway for Perception of Sound
Anatomy of Equilibrium
Utricle and saccule – suspended in perilymph
Two egg-shaped parts of the
membranous labyrinth
House the macula – a spot of sensory
epithelium
Macula – contains receptor cells
Monitor the position of the head
when the head is still
Contains columnar supporting
cells
Hair cells synapse onto vestibular
nerve.
Anatomy of Equilibrium
Semicircular duct – snakes through each
semicircular canal
Membranous ampulla – located within bony
ampulla
Houses a structure called a crista
ampullaris
Cristae contain receptor cells of
rotational acceleration
Epithelium contains
supporting cells and
receptor hair cells
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Pathway for Equilibrium
Vibrations of semicircular ducts, utricle, and saccule signal hair cells
Hair cells signal Cranial nerve VIII (vestibulocochlear nerve)
Cranial nerve VIII signals the vestibular nuclear complex of the pons
The pons signals the cerebellum
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