Chapter 12 – Part 1Somatic and Special Senses

PowerPoint Presentation to accompany Hole’s Human Anatomy and Physiology, 10th edition, edited by S.C. Wache for Biol2064.01

 You are responsible for the following figures and tables:

Tab. 12.1 - Information flow.Read TB, p.424 - Somatic Senses.Fig. 12.4 - stretch receptors / Golgi tendon organs.Fig. 12.2 - Define 'referred pain'. Read TB, p.430 - Special Senses - We will focus on the ear and eye.Fig. 12.9, 12.11, Fig. 12.15 (locate hair cell receptors)Fig. 12.16 - Auditory nerve pathway.Fig. 12.24 - Extrinsic muscles of the eye.Fig. 12.25, 12.28, 12.31Fig. 12.32 - photoreceptors - rods and cones. Clinical Application 12.5 - far-sightedness (hyperopia); near-sightedness (myopia).Fig. 12.41 - Visual nerve pathway.

Receptors

• Chemoreceptors: sense change in chemicals

• Nociceptors: stimulated by tissue damage

• Thermoreceptors: sense temperature changes

• Mechanoreceptors: sense mechanical forces including proprioceptors, baroreceptors, and stretch receptors

• Photoreceptors: respond to light

Sensory Impulses• Stimulation of sensory receptors cause local changes in

membrane potentials (receptor potentials)• Sensory impulses are generated directly or indirectly to the

CNS• Sensation: a feeling that occurs when the brain interprets

sensory impulses• Projection: pinpoints region of stimulation

Sensory Adaptation• Continuous stimulation leads to the receptor becoming less

responsive to the stimulus

• Adaptation: Eventually receptors stop sending signals.

• Increasing the strength of stimulus will trigger impulses

• Sensory nerve fibers: free nerve endings in epithelium respond to touch and pressure

• Meissner’s corpuscles: found in lips, palms, soles, nipples, fingertips, genitalia, respond to sensation of light touch

(Fig. 12.1)

• Pacinian corpuscles: deep subcutaneous tissues, respond to heavy pressure and vibrations

Temperature Senses

There are two groups of free nerve endings located in

the skin:

• Heat receptors respond to temperatures between 25 and 45 oC

• Cold receptors respond between 10 and 20 o C

• Outside of these ranges, pain receptors are stimulated

Sense of Pain

• Free nerve endings sense pain.

• Pain receptors may respond to more than one stimulus or be sensitive to only one type of change.

• Pain receptors adapt very little if at all.

Visceral Pain (Fig. 12.2)

• Internal organs have only pain receptors.• Referred pain is pain that feels like it comes from a different location

than the site stimulated.• Referred pain may come from common nerve pathways.

Fig. 12.3

Pain Nerve Pathways

• Acute pain fibers, A-delta fibers: thin, myelinated fibers that conduct impulses rapidly. These cause sharp pain

• Chronic pain fibers, C-fibers, are thin, unmyelinated nerve fibers that conduct impulses more slowly. These cause dull, aching pain

• Awareness of pain occurs at the thalamus.

Postsynaptic Potentials

Neuropeptides can block pain signals by

inhibiting presynaptic nerve fibers:

• Enkephalins suppress acute and chronic pain.

• Serotonin stimulates enkephalin release.

• Endorphins are morphine like substances released in response to extreme pain.

Stretch Receptors

Stretch receptors called muscle spindles are found

in skeletal muscles where they join tendons.

• Function: these are sensory organs that are sensitive to stretch in the muscle, consisting of small striated muscle fibers richly supplied with nerve fibers, and enclosed in a connective tissue sheath

• The stretch reflex opposes muscle lengthening.

• Golgi tendon organs are found in tendons which allows attaching to

muscles.• They stimulate a reflex that opposes the stretch reflex.

(Fig.12.4)

Sense of Smell (Fig. 12.5 and 12.6)

• Olfactory receptors: chemoreceptors that work closely with sense of taste• Olfactory receptor cells: bipolar neurons surrounded by columnar epithelium• Odorant molecules enter as gases. The molecules bind to receptors which change

in structure causing a depolarization of the memebrane.

Fig. 12.5

Olfactory Nerve Pathways

• Olfactory receptor fibers synapse with neurons in the olfactory bulbs (cranial nerve I).

• Impulses travel along the olfactory tracts to the limbic system.

• Impulses are interpreted in olfactory cortex.

• Olfactory receptor neurons are in direct contact with the environment and can be replaced if damaged.

Sense of Taste (Fig. 12.7)

• Taste buds are found on tongue papillae. They are spherical with a taste pore.

• They contain taste cells / taste receptors which are modified epithelial cells that act as receptors.

• Taste hairs, the sensitive parts of the cell, are microvilli that protrude from the taste cells.

• Molecules dissolved in fluid bind to the taste hairs.

Fig. 12.7

Taste Sensations

• Sweet: respond to carbohydrates and some inorganic salts

• Sour: respond to acids, intensity is proportional to hydrogen ion concentration

• Salty: respond to ionized inorganic salts

• Bitter: respond to many compounds including alkaloids

Taste Nerve Pathways

• Sensory impulses travel on the nerve fibers of following cranial nerves:

facial (VII), glossopharyngeal (IX), and vagus (X)

to the medulla oblongata.

• Impulses travel to the thalamus and to the gustatory cortex of the cerebrum for interpretation.

• Taste sensations are affected by olfactory sensations.

External Ear (Fig. 12.9) – The outer auricle or pinna collects sound waves.External auditory meatus: canal that passes into the temporal bone, directs sound waves to the tympanic membrane (eardrum).Ceruminous glands line the canal and secrete ear wax.

Middle Ear (Fig. 12.10)Note the entrance to the cochlea, the oval window right at the stapes. Note the exit at the round window.

Middle Ear (Fig. 12.10)• Tympanic cavity separates external and internal ears.• Tympanic membrane has a layer of skin on the outer

surface and mucous membrane on the inner surface.• Auditory ossicles (malleus, incus, stapes) are small bones

attached to the tympanic cavity that transmit vibrations to the oval window.

Tympanic Reflex• Two middle ear muscles are attached to the auditory

ossicles.• In response to loud or long sounds, the ossicles are pulled and vibrations are transmitted efficiently to the hearing

receptors, but with decreased protective levels of intensity.

Auditory Tube

• The auditory or eustachian tube connects the middle ear to the throat.

• It maintains equal air pressure on both sides of the eardrum / tympanic cavity.

• Rapid changes in pressure may result in an inability of the eardrum to vibrate.

• When pressure is released, it results in a “popping” sensation – such as when jet landing.

Inner Ear (Fig.12.11)Note the cochlea. Note the vestibule with utricle and saccule in front of the cochlea.

The inner ear is a complex system of inter-

communicating chambers and tubes.

• Osseous labyrinth: bony canal in the temporal bone

• Membranous labyrinth: tube within osseous labyrinth filled with endolymph

• Perilymph: fills the space between osseous and membranous labyrinth

Inner Ear (Fig.12.11)

• Cochlea: functions in hearing

• Structure: shell-shaped, coiled around a bony core (modiolus) with a bony shelf (spiral lamina)

Note the path the vibrations will travel within the cochlea.

(Fig. 12.12)

Fig. 12.14

Note the organ of corti.

Cochlea

• Scala vestibuli leads from the oval window to the spiral organ of Corti.

• Scala tympani extends from the cochlea to the round window

• Cochlear duct lies between the bony compartments, filled with endolymph.

• The vestibular membrane separates the cochlear duct from the scala vestibuli

• The basilar membrane separates the cochlear duct from the scala tympani.

• Function: Vibrations enter the perilymph at the oval window, travel along the scala vestibuli, pass though the vestibular membrane to the endolymph where they move the basilar membrane.

Organ of Corti

• Contains hearing receptor cells (hair cells)

• Above the hair cells is a tectorial membrane

• Different vibration frequencies move different regions of the basilar membrane and cause hair cells to shear against the tectorial membrane

• Depolarization of hair cells transmits impulse along the cochlear branch of the vestibulo-cochlear sensory nerve (cranial nerve VIII)

Fig. 12.13

Note the hair cells and cochlear nerve.

Fig. 12.16

Auditory Nerve Pathways

• The cochlear sensory branch of the vestibulocochlear nerve (VIII) extends to the medulla oblongata and proceeds to the thalamus.

• From the thalamus impulses are transmitted to the auditory cortex of the cerebrum.

• Fibers cross over /decussate so that impulses are interpreted on both sides of the brain.

Sense of Equilibrium• Static equilibrium: sense position of the head, maintain

stability and posture• Dynamic equilibrium: sense motion and rotation and

maintain balance

Static Equilibrium• Vestibule: bony chamber between the semicircular canals

and the cochlea. Inside the vestibule are two chambers: the utricle and the saccule.

• Macula acustica, a specialized epithelium: consists of hair cells and supporting cells in the utricle and saccule. Note: This epithelium is unlike the hair cells in the organ of corti.

• Movement of the head allows the hair cells to contact the otolithic membrane with crystals of calcium carbonate (otoliths).

Fig. 12.19

Static Equilibrium• Otoliths of calcium carbonate maintain static equilibrium by

adding weight to the otolithic membrane in the macula acustica of the utricle and saccule.

• Gravity causes gelatinous mass of the macula to move and the hairs on the hair cells to bend.

• Nerve impulses travel along the vestibular branch of the vestibulo-cochlear (VIII) sensory nerve

• Information is used to maintain balance.

Dynamic Equilibrium• Three bony semicircular canals (Fig. 12.11) lie in three planes

in space within the inner ear (Fig. 9) and function to maintain the dynamic equilibrium.

• The membranous semicircular canal ends in an ampulla that communicates with the utricle of the vestibule

• The ampulla contains a crista ampullaris with sensory hair cells and supporting cells which extend into the cupula

Protection of the Eye: Eyelids (Fig. 12.22)

The eyelid consists of 4 layers of tissue:• Skin.• Eyelid muscles

– orbicularis oculi, levator palpebrae superioris

• Connective tissue– tarsal glands secrete oily substance

• Conjunctiva– mucous membrane that covers eyelids and

anterior eyeball except the cornea

Protection of the Eye: Lacrimal Glands

• Lacrimal gland secretes tears which contain an antibacterial enzyme called lysozyme.• Ducts carry tears into the nasal cavity.• Fluid moves into the lacrimal sac and into the nasolacrimal duct.

Fig.12.23

Extrinsic Eye Muscles

• Superior rectus: rotates eye up and in

• Inferior rectus: rotates eye down and in

• Medial rectus: rotates eye to midline

• Lateral rectus: rotates eye from midline

• Superior oblique: rotates eye down and out

• Inferior oblique: rotates eye up and out

Fig.12.24

Eye Structure• Outer fibrous tunic onsists of the sclera which

extends over the lens as the cornea.– Cornea: transparent, focuses light rays

– Sclera: white,protection and muscle attachment

• Middle vascular tunic– Choroid: absorbs scattered light

– Ciliary body: holds lens and aids in focusing

– Iris: colored part of the eye, diaphragm of connective tissue and smooth muscle

• Inner nervous tunic– Retina

Fig. 12.26

Note the aqueous and vitreous humor. Note the iris.

• Lens: clear, elastic structure held by suspensory ligaments • Ciliary muscles relax and tense the suspensory ligaments to change lens shape• Accommodation: lens shape changes when the eye focuses on a close object

Fig. 12.28

• Iris (Fig. 12.31): Composed of connective tissue and smooth muscle fibers; divides space between cornea and lens into anterior and posterior chambers

• Aqueous humor: watery fluid in the anterior chamber• Pupil: circular opening in the center of the iris; size controlled by smooth muscles

Retina

Retina: contains the photoreceptors (rods, cones)

• Five groups of neurons– direct impulses to brain: receptor cells, bipolar neurons,

ganglion cells– modify impulses: horizontal and amacrine cells

• Macula lutea: contains fovea centralis, region of sharpest vision

• Optic disk: where nerves and vessels leave the eye, lacks photoreceptors (blind spot)

Fig. 12.32

Fig. 12.34

Fig. 12.34

Posterior Cavity

• Space closed off to the anterior cavity by the lens, ciliary body, and the retina

• Filled with vitreous humor– jelly-like fluid– supports internal structures– maintains shape

Function of the Lens: Light Refraction

• Bending of light• Occurs when light

passes at an oblique angle from a medium of one optical density into a medium of a different density

• Convex surfaces cause light to converge

• Concave surfaces cause light to diverge

Fig.12.35

Function of the Lens: Bend and focus parallel waves of light (Fig. 12.A, p. 457).

Note that, on the retina, the image is reversed by the action of the lens.

Visual Receptors (Fig. 12.38)

• Stimulated when light reaches the receptors• Rods - sensitive to light, provide vision in dim light

- produce black and white outline images• Cones - provide color vision and sharp images

- fovea centralis of the macula lutea has cones and lacks rods

Visual Pigments

• Rods and cones contain light-sensitive pigments

• Rods contain rhodopsin embedded in membranous disks in the photoreceptors

• Rhodopsin– opsin: colorless protein– retinal: vitamin A

Response to Light• In darkness, sodium channels in the cell membrane of the

photoreceptors are kept open by cGMP.

• When rhodopsin absorbs light, it changes shape and re-aligns opsin which activates transducin.

• Transducin activates phosphodiesterase which breaks down cGMP.

• Sodium channels close resulting in hyperpolarization of the receptor.

• In bright light, almost all the rhodopsin decomposes, reducing the sensitivity of the rods.

• Cones function in bright light resulting in color vision.

• Rods function in dim light: rhodopsin is regenerated from opsin and retinal.

• Cones are not stimulated in dim light.

Visual Nerve Pathways

• Ganglion cell axons leave the eye to form the optic sensory nerve (cranial nerve II).

• The nerve fibers cross anterior to pituitary gland, the optic chiasma.

• Nerve fibers travel to the thalamus and to reflex centers.

• Optic radiations are nerve pathways that lead to the visual association area in the occipital lobe.

Fig.12.41

Note cranial nerve II, decussating at the optic chiasma, and interpretation in the visual cortex of the cerebrum.