Peripheral Nervous System
There are 31 pairs of spinal nerves: 8 cervical spinal nerves, 12 thoracic, 5 lum- bar, 5 sacral, and 1 coccygeal
spinal nerve. Each can be identified by its associa- tion with adjacent vertebrae.
cervical nerves take their names from the vertebra immediately following them. In other words, cervical2nerve C
precedes vertebra C2, and the same system is used for the rest of the cervical spinal nerves.
o This transition changes between the last cervical and first thoracic vertebrae
o The spinal nerve lying between the two vertebra is called C8
o So, there are seven cervical vertebrae but eight cervical spinal nerve
Spinal nerves caudal to the first thoracic vertebra take their names from the vertebra immediately preceding them.
Thus, the spinal nerve 1 emerges immediately caudal to vertebra1T , spinal ner2e T follows verteb2a T , and so
Each spinal nerve forms through the fusion of dorsal and ventral nerve roots as those roots pass through an
intervertebral foramen; the only exceptions are 1t C and1Co , where some people lack dorsal roots
the spinal nerve divides into several branches. All spinal nerves form two branches, a dorsal ramus and a ventral
ramus. For spinal nerves 1 to 2 there are four branches: a white ramus and a gray ramus( collectively known as
the rami communicantes -communicating branches), a dorsal ra- mus, and a ventral ramus
o The rami communicates carry visceral motor fibers to and from a nearby autonomic ganglion associated
with the sympathetic division of the ANS. Because preganglionic axons are myelinated, the branch
carrying those fibers to the ganglion has a light color, and it is known as the white ramus (ramus, branch).
o Two groups of unmyelinated postganglionic fibers leave the ganglion. Those innervating glands and
smooth muscles in the body wall or limbs form a second branch, the gray ramus, that rejoins the spinal
The dorsal ramus of each spinal nerve provides sensory innervation from, and motor innervation to, a specific
segment of the skin and muscles of the neck and back.
The relatively large ventral ramus supplies the ventrolateral body surface, structures in the body wall, and the
Each pair of spinal nerves monitors a specific region of the body surface, an area known as a dermatome
o clinically important because damage to either a spinal nerve or dorsal root ganglion will produce a
characteristic loss of sensation in specific areas of the skin.
the ventral rami of adjacent spinal nerves blend their fibers to produce a se- ries of compound nerve trunks. Such
a complex interwoven network of nerves is called a nerve plexus Nerve plexuses form during development as
small skeletal muscles fuse with their neighbors to form larger
―nerves‖ that innervate these compound muscles in the adult contain sensory and motor fibers from the ventral
rami that innervated the embryonic muscles. Nerve plexuses exist where ventral rami are converging and
branching to form these compound nerves. The four major nerve plexuses are the cervical plexus, brachial plexus,
lumbar plexus, and sacral plexus
Conditions inside or outside the body can change rapidly and unexpectedly. A reflex is an immediate involuntary
motor response to a specific stimulus
The neural ―wiring‖ of a single reflex is called a reflex arc. A reflex arc begins at a receptor and ends at a
peripheral effector, such as a muscle or gland cell.
o STEP 1. Arrival of a Stimulus and Activation of a Receptor.
o STEP 2. Relay of Information to the CNSinformation is carried in the form of action potentials along
an afferent fiber
o STEP 3. Information Processingthis happends when a neurotransmitter reaches postsynaptic
membrane of either motor neuron or interneuron
o STEP 4. Activation of a Motor Neuron.
o STEP 5. Response of a Peripheral Effector. Activation of the motor neuron causes a response by a
peripheral effector, such as a skeletal muscle or gland.
Can have two kinds of responses:
o Somatic felexes control skeletal muscle contractions, include superficial and stretch reflexes
o Visceral reflexes control actions of smooth and cardiac muscles, glands
components of the peripheral nervous system that connect to the brain rather than to the spinal cord.
Twelve pairs of cranial nerves on ventrolateral surface
Each has a name related to its appearance or function
Numbered according to their position along the longitudinal axis of the brain
Each cranial nerve attaches to the brain near the associated sensory or motor nuclei.
o sensory nuclei act as switching centers, with the postsynaptic neurons relaying the information either to
other nuclei or to processing centers within the cerebral or cerebellar cortex.
o motor nuclei receive convergent inputs from higher centers or from other nuclei along the brain stem.
Cranial nerves have five general functions:
o Special sensory, somatic sensory, visceral sensory, somatic motor, veceral motor
Nerves in Order Modality Function
Olfactory Special Sensory Smell Special Sensory
Oculomotor Somatic Motor Levator palpebrae, superioris, superior, medial & inferior recti muscles
Parasympathetic to ciliary & pupillary constrictor muscles
Trochlear Somatic Motor Superior oblique muscle
Branchial Motor Muscles of mastication
General Sensory Sensory for head/neck, sinuses, meninges, & external surface of tympanic membrane
Abducens Somatic Motor Lateral rectus muscle
Facial Branchial Motor Muscles of facial expression
Visceral Motor Parasympathetic to all glands of head except the parotid
General Sensory Sensory for ear and tympanic membrane
Special Sensory Taste anterior two-thirds of tongue
Vestibulocochlea Special Sensory Hearing and Balance
Branchial Motor Stylopharyngeus muscle
Visceral Motor Parotid Gland
Glossopharyngea Visceral Sensory Carotid Body
General Sensory Sensation posterior one-third tongue & internal surface of tympanic membrane.
Special Sensory Taste posterior one-third tongue
Branchial Motor Muscles pharynx & larynx
Visceral Motor Parasympathetic to neck, thorax, & abdomen
Visceral Sensory Sensory from pharynx, larynx & viscera
Special Sensory Sensory from external ear
Spinal Branchial Motor
Accessory Trapezius & sternocleidomastoid muscles
Hypoglossal Somatic Motor Tongue muscles except palatoglossal
Autonomic Nervous System:
Innervates visceral effectors
Has afferent and efferent neurons
o Afferent sensory information processed in the CNS while the efferent pathways are sent to receptor
o The afferent pathways originate in visceral receptors and the efferent pathways connect to visceral
The axon of a visceral motor neuron within the CNS innervates a second neuron located in a peripheral ganglion
Preganglionic neurons send their axons called preganglionic fibers to synapse on ganglionic neurons whose cell
bodies are leaved outside the CNS in autonomic ganglia
o The axons that leave the ganglia are unmyelinated and called postganglionic fibers
Carry impulses away from the ganglion Innervate peripheral tissues and organs
Two major subdivisions: sympathetic and parasympathetic
Usually have opposing effects ( symp. Causes excitation then parasymp. Causes inhibition)
o Sometimes the two divisions work independently but with some structures innervated by only one
o Sometimes the two division may work together, each controlling one stage of a complex process
In general: parasympathetic works under resting conditions and sympathetic kicks in during times of exertion,
Both divisions affect their target organs through release of neurotransmitters by postganglionic fibers
o Depending on response of receptor the target organ may be stimulated or inhibited
Three general statements about ANS neurotransmitters and effects:
o All preganglionic autonomic fibers release acetylcholine at their synaptic terminals. The effect is always
o Postganglionic parasympathetic fibers also release Ach but the effects may be either stimulatory or
inhibitory depending on the nature of the receptor
o Most postganglionic sympathetic terminals release the neurotransmitter norepinephrine. The effect is
This division originates from preganglionic neurons located b/w segments t1 and L2 of he spinal cord
o The cell bodies of these neurons occupy the lateral gray horns and their axons enter the ventral roots of
Also has two types of ganglionic neurons in ganglia near the vertrebral colomn
o Paravertebral and prevetebral
the ventral roots of spinal segments1T to2L contain sympathetic preganglionic fibers. Each
ventral root joins the corresponding dorsal root, which carries afferent sensory fibers, to form a
spinal nerve that passes through an intervertebral foramen. As it clears the foramen, a white
ramus, or white ramus communicans, branches from the spinal nerve .The white ramus carries
myelinated preganglionic fibers into a nearby sympathetic chain ganglion. Fibers entering a
paraveterbral ganglia may have one of three destinations:
They may synapse within the sympathetic chain ganglion at the level of entry
they may ascend or descend within the sympathetic chain and synapse with a ganglion at
a different level
they may pass through the sympathetic chain without synapsing and proceed to one of the
One preganglionic fiber synapsin on as many as 32 ganglionic neurons
Unmyelinated postganglionic fibers then leave the sympathetic chain and proceed to their
peripheral targets within spinal nerves and sympathetic nerves. Postganglionic fibers that
innervate structures in the body wall, such as the sweat glands of the skin or the smooth muscles
in superficial blood vessels, enter the gray ramus (gray ramus communicans) and return to the
spinal nerve for subsequent distribution. However, spinal nerves do not provide motor innervation
to structures in the ventral body cavities. Postganglionic fibers innervating visceral organs in the
thoracic cavity, such as the heart and lungs, proceed directly to their peripheral targets as
Main function in general is that the target cell reponses help prepare the individual for a crisis that
will require sudden intensive physical activity
o Prevertebtral Ganglia:
Preganglionic fibers that regulate the activities of the abdominopelvic viscera originate at preganglionic neurons in the inferior thoracic and superior lumbar segments of the spinal cord.
These fibers pass through the sympathetic chain without synapsing, and converge to form the
greater, lesser, and lumbar splanchnic nerves in the dorsal wall of the abdominal cavity. These
ganglia are most often single, rather than paired, structures.
The general patters is
A reduction of blood flow, energy use and activity by visceral organs that are not
important to short term survival ( digestive tract)
Release of stored energy reserves
Summary of Sympathetic Division
The sympathetic division of the ANS includes two sympathetic chains resembling a string of beads, one on each
side of the vertebral column; three collateral ganglia anterior to the spinal column; and two suprarenal medullae.
Preganglionic fibers are short because the ganglia are close to the spinal cord. The postganglionic fibers are
relatively long and extend a considerable distance before reaching their target organs. (In the case of the
suprarenal medullae, very short axons from modified ganglionic neurons end at capillaries that carry their
secretions to the bloodstream.)
The sympathetic division shows extensive divergence; a single preganglionic fiber may innervate as many as 32
ganglionic neurons in several different ganglia. As a result, a single sympathetic motor neuron inside the CNS can
control a variety of peripheral effectors and produce a complex and coordinated response.
All preganglionic neurons release ACh at their synapses with ganglionic neurons. Most of the postganglionic
fibers release norepinephrine, but a few release ACh.
The effector response depends on the function of the plasma lemma receptor activated when epinephrine or
norepinephrine binds to either alpha or beta receptors.
Has preganglionic neurons located in the brain stem and in sacral segments of the spinal cord
o Associated with cranial nerve 3,7,9,10
Also has neurons in peripheral ganglia locater close to/withing target organs
o Ganglionic neurons in psd found in terminal ganglia
o A typical preganglionic fiber synapse on 6-8 ganglionic neurons
o The effects of parasympathetic stimulation are more specifi anc localized than those of the sympathetic
parasympathetic preganglionic fibers leave the brain in cranial nerves III (oculomotor), VII (facial), IX
(glossopharyngeal), and X (vagus)
The function of this division centers around relaxation, food procesiins, and energy absoptions
o Lead to a general increase in the nutrient content of the blood
All of the preganglionic and postganglionic fibers in the parasympathetic divi- sion release ACh at their synapses
and neuroeffector junctions.,
o The parasympathetic division includes visceral motor nuclei in the brain stem associated with four cranial
nerves (III, VII, IX, and X). In sacral segment2 S4–S , autonomic nuclei lie in the lateral portions of the
anterior gray horns.
o The ganglionic neurons are situated in intramural ganglia or in ganglia closely associated with their target
o The parasympathetic division innervates structures in the head and organs in the thoracic and
o Release of ACh by preganglionic neurons stimulates nicotinic receptors on ganglionic neurons, and the
effect is always excitatory. The release of ACh at neuroeffector junctions stimulates muscarinic receptors,
and the effects may be either excitatory or inhibitory, depending on the nature of the enzymes activated
when ACh binds to the receptor5
o The effects of parasympathetic stimulation are usually brief and restricted to specific organs and sites. Special Sensors: Eye
Humans rely on vision more that any other special sense
Visual cortex is several times larger that any other area
Accessory Structures :Eyelids
Also known as the palpebral are continuation of the skin
Act as windshield wipers
The free margins of the upper and lower eyelids are separated by the palpebral fissure but the two are connected at
the medial canthus and the lateral canthus
At the medial canthus, glands within the lacrimal carnucle produce thick secretions
o These various glands are subjected to occasional invasion and infection by bacteria-
The visible surface of the eyelid is covered by a thin layer of stratified squamous epithelium
The muscles of the orbicularis ocu muscle and the levator palpebral superiors muscle are responsible for closing
the eyelids and raising the upper eyelid
The epithelium covering the inner surface of the eyelids and the outer surface of the eye is called the conjunctiva
o It is a mucus membrane
o The inner surface is covered by palpebral conjunctiva
o The anterior surface is covered by ocular conjuctive
o Produces distributes and removes tears
o A lacrimal gland provides the key ingredients an most of the volume of the tears that bath the conjuctival
surfaces. It produces at a rate of 1ml/day
o The blinking of the eye sweeps the tears across the ocular surface, and they accumulate at the medial canthus
in an area known as the lacus lacrimalis, or ―lake of tears.‖
o Two small pores, the superior and inferior lacrimal puncta, drain the lacrimal lake, emptying into the lacrimal
canaliculi that run along grooves in the surface of the lacrimal bone. These passageways lead to the lacrimal
sac, which fills the lacrimal groove of the lacrimal bone. From there the nasolacrimal duct extends along the
nasolacrimal canal formed by the lacrimal bone and the maxilla to deliver the tears to the inferior meatus on
that side of the nasal cavity
Extrinsic Eye Muscles
There are six extra-ocular muscles, sometimes called the oculomotor or extrinsic eye muscles
originate on the surface of the orbit, insert onto the sclera of the eye just posterior to the cornea, and control the
position of each eye. These muscles are the inferior rectus, medial rectus, superior rectus, lateral rectus, inferior
oblique, and superior oblique muscles
The rectus muscles move the eyes in the direction indicated by their names. Additionally, the superior and inferior
rectus muscles also cause a slight movement of the eye medially, whereas the superior and inferior oblique muscles
cause a slight lateral movement. Thus, to roll the eye straight up, one contracts the superior rectus and the inferior
oblique muscles; to roll the eye straight down requires the inferior rectus and the superior oblique muscles. The extra-
ocular muscles are innervated by the third (oculomotor), fourth (trochlear), and sixth (abducens) cranial nerves.
Structure of the Eye
The eyeball shares space within the orbit with the extra-ocular muscles, the lacrimal gland, and the cranial nerves and
blood vessels that supply the eye and adjacent portions of the orbit and face
The wall of the eye contains three distinct layers, or tunics:
o an outer fibrous tunic, an intermediate vascular tunic, and an inner neural tunic.
The fibrous tunic, the outermost layer of the eye, consists of the sclera and the cornea
The fibrous tunic:
o (1) provides mechanical support and some degree of physical protection o (2) serves as an attachment site for the extra-ocular muscles
o (3) contains structures that assist in the focusing process
Most of the ocular surface is covered by the sclera
o Is ―white of the eye,‖ consists of a dense, fibrous connective tissue containing both collagen and elastic
o Is thickest at the posterior portion of the eye, near the exit of the optic nerve, and thinnest over the
o The 6 ocular muscles insert into on the sclera and the collagen fibers of their tendons are interwoven
into the collagen fibers of the outer tunic
o The anterior surface has blood vessels
the transparent cornea is continuous with the sclera.
o surface is covered by a delicate stratified squamous epithelium continuous with the ocular conjunctiva.
o Deep to that epithelium, has a dense matrix containing multiple layers of collagen fibers.
The transparency of the cornea results from the precise alignment of the collagen fibers within
The cornea is structurally continuous with the sclera. The cornea is avascular, and there are no blood vessels
between the cornea and the overlying conjunctiva.
is the most sensitive portion of the eye. This sensitivity is important because corneal damage will cause
blindness even though the rest of the eye—photoreceptors included—is perfectly normal.
The Vascular Tunic
contains numerous blood vessels, lymphatics, and the intrinsic eye muscles.
The functions of this layer include
o (1) providing a route for blood vessels and lymphatics that supply tissues of the eye
o (2) regulating the amount of light entering the eye
o (3) secreting and reabsorbing the aqueous humor that circulates within the eye
o (4) controlling the shape of the lens, an essential part of the focusing process.
The vascular tunic includes the iris, the ciliary body, and the choroid
o can be seen through the transparent corneal surface
o contains blood vessels, pigment cells, and two layers of smooth muscle cells that are part of the intrinsic
eye muscles. Contraction of these muscles changes the diameter of the central opening of the iris, the
pupil. One group of smooth muscle fibers forms a series of concentric circles around the pupil. The
diameter of the pupil decreases when these pupillary sphincter muscles contract.
o A second group of smooth muscles extends radially from the edge of the pupil. Contraction of these
pupillary dilator muscles enlarges the pupil. These antagonistic muscle groups are controlled by the auto-
nomic nervous system; parasympathetic activation causes pupillary constriction, and sympathetic
activation causes pupillary dilation.
o The body has a connective tissue whose posterior surface is covered by an epithelium containing
pigment cells. Pigment cells may also be present in the connective tissue of the iris and in the epithelium
covering its anterior surface. Eye color is determined by the density and distribution of pigment cells.
When there are no pigment cells in the body of the iris, light passes through it and bounces off the inner
surface of the pigmented epithelium. The eye then appears blue. Individuals with gray, brown, and black
eyes have more pigment cells, respectively, in the body and surface of the iris.
The Ciliary Body
o At its periphery the iris attaches to the anterior portion of the ciliary body.
o begins at the junction between the cornea and sclera
o The bulk of it consists of the ciliary muscle, a muscular ring that projects into the interior of the eye.
o The epithelium is thrown into numerous folds, called ciliary processes. The suspensory ligaments, or
zonular fibers, of the lens attach to the tips of these processes. These connective tissue fibers hold the
lens posterior to the iris and centered on the pupil. As a result, any light passing through the pupil and
headed for the photoreceptors will pass through the lens.
o Oxygen and nutrients are delivered to the outer portion of the retina by an extensive capillary network
contained within the choroid.
o The innermost portion of the choroid attaches to the outer retinal layer.
Sensory Tunic The sensory tunic, or retina, consists of two distinct layers, an outer pigmented layer and an inner neural layer
The pigment layer absorbs light after it passes through the retina and has important biochemical interactions
with retinal photoreceptors.
The neural retina contains
o (1) the photoreceptors that respond to light,
o (2) supporting cells and neurons that perform preliminary processing and integration of visual
o (3) blood vessels supplying tissues lining the posterior cavity.
The neural retina and pigmented layers are normally very close together, but not tightly interconnected.
The pigmented layer continues over the ciliary body and iris. The neural retina thus forms a cup that establishes
the posterior and lateral boundaries of the posterior cavity
two types of photoreceptors: rods and cones.
o Rods do not discriminate between different colors of light. They are very light-sensitive and enable us to
see in dimly lit rooms, at twilight, or in pale moonlight.
o Cones provide us with color vision. There are three types of cones, and their stimulation in various
combinations provides the perception of different colors. Cones give us sharper, clearer images, but they
require more intense light than rods.
o The rods and cones synapse bipolar cells. Stimulation of rods and cones alters their rates of
neurotransmitter release, and this in turn alters the activity of the bipolar cells.
o Bipolar cells in turn synapse within the layer of ganglion cells that faces the vitreous chamber. The
ganglion cells are the only cells in the retina that generate action potentials to the brain.
o Axons from an estimated 1 million ganglion cells converge on the optic disc, penetrate the wall of the
eye, and proceed toward the diencephalon as the optic nerve There are no photoreceptors or other retinal
structures at the optic disc. Because light striking this area goes un- noticed, it is commonly called the
Chambers of the Eye
The eyeball is hollow, and the interior is divided into two cavities.
o The anterior segment is subdivided into anterior and posterior chambers
o The posterior segment is called the virteous
The anterior and posterior chambers are filled with aqueous humor.
Aqueous humor forms continuously as interstitial fluids pass between the epithelial cells of the ciliary processes and
enter the posterior chamber
The virteous body has the that lens lies posterior to the cornea, held in place by the suspensory ligaments that originate
on the ciliary body of the choroid.
o This chamber contains the vitreous body, a gelatinous mass sometimes called the vitreous humor.
o helps maintain the shape of the eye, support the posterior surface of the lens, and give physical support to the
retina by pressing the neural layer against the pigment layer.
The primary function of the lens is to focus the visual image on the retinal photoreceptors.
It accomplishes this by changing its shape. The lens consists of con- centric layers of cells that are precisely organized
A dense, fibrous capsule covers the entire lens.
Each rod and cone cell monitors a specific receptive field. A visual image results from the processing of information
provided by the entire receptor population. A significant amount of processing occurs in the retina before the
information is sent to the brain because of interactions between the various cell types.
The two optic nerves, one from each eye, reach the diencephalon at the optic chiasm The Ear and Equilibrium
The ear is divided into three anatomical regions
o the external ear
collects and directs sound waves to the eardrum.
o the middle ear
Structures within the middle ear amplify sound waves and transmit them to an appropriate portion of
the inner ear.
o inner ear
contains the sensory organs for equilibrium and hearing.
The External Ear
The external ear includes the flexible auricle, or pinna, which is supported by elastic cartilage.
The auricle of the ear surrounds the external acoustic meatus. The auricle protects the external acoustic meatus and
provides directional sensitivity to the ear by blocking or facilitating the passage of sound to the eardrum, also called the
The tympanic membrane is a thin, semitransparent connective tissue sheet that separates the external ear from the
o is very delicate.
The auricle and the narrow external acoustic meatus provide some protection from accidental injury to the tympanic
ceruminous glands distributed along the external acoustic meatus secrete a waxy material, and many small, outwardly
projecting hairs help deny access to foreign objects or insects. The waxy secretion of the ceruminous glands, called
cerumen, also slows the growth of microorganisms in the external acoustic meatus and reduces the chances of
The Middle Ear
The middle ear consists of an air-filled space, the tympanic cavity, which contains the auditory ossicles
o separated from the external acoustic meatus by the tympanic membrane,
o communicates with the nasopharynx through the auditory tube and with the mastoid sinuses through a number
of small and variable connections
The auditory tube or the Eustachian tube.
o 4.0 cm in length
o penetrates the petrous part of the temporal bone within the musculotubal canal. The connection to the
tympanic cavity is relatively narrow and supported by elastic cartilage. The opening into the nasopharynx is
relatively broad and funnel-shaped.
o serves to equalize the pressure in the middle ear cavity with external, atmospheric pres- sure. Pressure must be
equal on both sides of the tympanic membrane or there will be a painful distortion of the membrane.
Unfortunately, the auditory tube can also allow microorganisms to travel from the nasopharynx into the
tympanic cavity, resulting in an ―ear infection.‖.
The tympanic cavity contains three tiny ear bones collectively called auditory ossicles.
o the smallest bones in the body, connect the tympanic membrane with the receptor complex of the inner ear
o The three auditory ossicles :
These bones act as levers that transfer sound vibrations from the tympanum to a fluid-filled chamber
within the inner ear.
The tensor tympani muscle is a short ribbon of muscle whose origin is the petrous part of the temporal bone, within
the musculotubal canal, and whose insertion is on the ―handle‖ of the malleus
o When the tensor tympani contracts, the malleus is pulled medially, stiffening the tympanum. This increased
stiffness reduces the amount of possible movement. The tympani muscle is innervated by motor fibers of the
mandibular branch of the trigeminal nerve
The stapedius muscle, innervated by the facial nerve (NVII), originates from the posterior wall of the tympanic cavity
and inserts on the stapes o Contraction of the stapedius pulls the stapes, reducing movement of the stapes at the oval window.
The Inner Ear
The senses of equilibrium and hearing are provided by the receptors of the inner ear
The receptors are housed within a collection of fluid-filled tubes and chambers known as the membranous labyrinth
o contains a fluid called endolymph
The receptor cells only function when exposed to the unique ionic composition of the endolymph.
The bony labyrinth is a shell of dense bone that surrounds and protects the membranous labyrinth.
o Between the bony and membranous labyrinths flows the perilymph a liquid whose properties closely resemble
those of cerebrospinal fluid.
o The bony labyrinth can be subdivided into the vestibule, the semicircular canals, and the cochlea
capture and transmission of sound to the cochlea.
The vestibule and semicircular canals together are called vestibular complex
o the cavity within the vestibule contains a
pair of membranous sacs, the utricle and the saccule
Receptors here provide sensations of gravity and linear acceleration.
The sensory receptors of the inner ear are called hair cells
These receptor cells are surrounded by supporting cells and are monitored by sensory afferent fibers.
The hair cells are clustered in the oval maculae
The hair cells are embedded in a gelatinous mass which contains densely packed calcium carbonate
crystals known as statocnia. ( whole structure called oto