~

. l ; I :

E~'"EllALL

The eyeball is formed of three coats ;/~

(_ - .- - .,. .' . ,. ----,./ I ~

1. ')

Fibrous coat c:<.ternal.

Vascular and muscular coat : intermediate .

Nervous coat : internal.

c::: -- r:.. ~- 9 .

~ 0 ..._; ffv) ~.· ~k

~t-; '& C{,p..f.t I

..... 3.

Fibrous Coat

is formed of the cornea and sclera.

. •• I'

A. Cornea : transparent and forms the anterior sixth of the fibrous coat.

B. Sclera : is formed of dense white fibrous tissue (white of the eye). The optic nerve pierces the sclera about 3 mm. to the

1 "~ro-medial side of the posterior pole of the eyeball. ~ (L.i wtb \,\5) .

The cornea-scleral junction presents a circular canal cal-led the sinus venosus sclerae (canal of Schlem) into which ~ --------------® tE.e aquous humour is absorbed from the anterior ch'a:-~ber.

Vascular and Muscular Coat

is formed by the iris, ciliary body and choroicL

A. Iris : ~ular·~~~· behind the cornea. It presents a central hole called the pupil. It contains the constrictor (sphincter) and dilator pupillae muscles.

The co lo~r of the iris va r ies in different individuals due to presence of pigments. The space between the iris and cornea is called anterior chamber. The space between the · iris and lens is called posterior cham,ber. The two chambers communicate .. thr­ough the pupil.

B. Ciliary body : com posed of several parts :

a) ciliary m.1(,$cle : which forms a muscular ring around the iris. It is formed of smooth muscle fibers :arranged t;i.rcu­larly and radially.

b) ci.liary p?·oces;es .: which aie irregular projections· 'deep to the ciliary muscle. They lie lateral to the posterior chamber between the margins of the iris and lens.,~

secrete the a_ uous humour. , c) ciliary ring : which is a narrow vascular zone at the

junction with the ch0roid.

I

i I I I I

I I

I I

. -·

N·& ft9ueous -Btw'l\au.r 0 ~_CA..efe.d. infu tW_ foSft.ridY CAttMbt!.IL

(}tam t0- CtxjJ iL(r~. ric?.s cfb tl~ c.<.l.r"a..ty ~roce sse 5 ¥ ci.A..eu.J M:e 5 ; n fo thz_ c,.\nft(lll'(' CkrlMblr d..rrr\An.J -lj...t_ fJtWtl--==;:. /{-0""-" ~ cin.f<Atc.lt. ,.,~_ L. I'.J- • _} • ' , 'V'~ I ~,A ~rl~r><r ~ h A ctfi/1 n ea t 111-o =tt..t_ c"'.n ftrit.JI u..K r'.i.ry ve.//t.r -(.4 lJ Cttnt'L.{ rlJ sc-Memm ~ -r- /) . r<7Li..911 -u~

-u {) ..J-Ylfe.rre_re.ttCC W!1J.... d r1Jin.lu1 rJ) M -q_riA.(.ov..s --o\uru.our tflf.o tL~ uot~ d6 Qd_1e111/.fiA. 5~ .-o ~

r f\ Crlt1f(. db t.b..<_ in tr't:fo c uidr PA 2,.._, (an ~ 1 n r~,., f ~ J-~ r·- ~JUAe. ;;Kd(,(~ct) -?- Tfc · n 1 ~{).,t\.l. rL-V\.. ~ -a~ l't/)i.-tq_ c ?..us. L /.' -A i.VJ I rtJcv..<ec/) I

~ 'Cj {}UflC{f'ti~ .

1J(f;,V/.1~~· ~~-·=~

. .

C. Choroid : is the largest part -of the middle coat, lying bet­ween the sclera and retina. The choroid is formed of deli­cate areolar tissue which is highly pigmented and rich in blood vessels. Posteriorly, it is pierced by the optic nerve.

Nervous Coat · 0 v.:t~ P{J!YI.e.fltel. / IJ.-der

This coat is mainly formed by th~"'- ; nneA nen,rov.~ lLJ'er The retina is supplied by branches of the c~ntral' artery of

the retina which never anastomose together or with other arter-ies in the eyeball( i-e.. f:k.~ ~ ~~- tVL:tu-~s) ·

The retinal veins collect into a central vein/ U.s obst rv<ch c..'"YI. ~~ ~b~iSiM

Lens

transparent, solid, elastic and bivonvex.

lies between the iris and vitrious body.

Its equator is blunt.

(t.fld$ -to sv-J.d~~"~ b\it'\J.t'l<..~S)

The suspen,sory ligamera.t of the lens is. attached to the an: terior surface of the capsule of the lens· close to the the equa- . · to~. Some fibers of the ligament are attached to . the equator and posterior surface close to the equator.

frhe suspe~sory ligaments of th=e)~~~fixes the lens ·ir?: .position ' ·and connects it to the ciliary muscle. Th~refore the. curva­ture of the lens is affected by the contraction of . the ciliary muscle and the ~ree of tension of :the suspensory.liga1nent. *.

{JDuring looking to a near object the ciliary muscle refiexiY contracts, the su~pens6ry ligament ge~s loose and: the __ curva- . ture of the lens 1nc~ease (accomoda~~op). _·: ::' .. ·: · . . _: · ·,·1

- f'The elasticity of the lens . begins to diminish after the age of 1

. \. fpurty years.~~~P~1~ ct) . _ · . ·: . .. · . .. . .l . . . . . . . • . . . . . . I

·The transparency of the lens begins to diminish in old people, a I condition known as cataract.

· · Vitreous Body

This is a transparent, structurelss, colourless gel-like sub­stance which fills the concavity -of the retina. It occupies E.:.bout four-fifths of the eyeball and lies behind the lens.

~-------.. (J'-) ' '-._.....- J

~~ ·P

I I I

-;~Optic n.

Dura mt~tor

f I Fig. 174 Sagittal section of the ¢yebali.

~ @ oypos)t--e f:k ~r\tr~nce <f1 d'f'ti"- Y\<!YV'(_ (in/ero­'YYleJc'oJ. tp ~ fosteri6f fo~e) it~ clrcu.J~tr elrrct 06 1·5 1-!-l,"' ~~~~(__{.:_~'_ -~ _!5now~ _ M i c. _ a;: fi--resse~-~refl- d6 -~ crpfl'<::. cUSc io ctLfl_e.J. a~ p JS( o-& ( CN!.. 7'- u. Covd-ct i tU

1'\o re.te.p+<rrs ~no. rods ~ c6Ytes) & 0 TJ:..eA.fj<Jr(. ;11 s~s/n'w ~ Lt'~-t ( E_~s~~a~c~ _t_!_rnd_ Sfo_!) CD ll-t -(}~ fosteri6'1" P.c-1< d6 ~ ~e. ( '5 rtY\W\' llt:te.r~ $ ~ err/c. disc)-'=7- ~r<.. .~ ~nether d..tfrU:.St'On. db s;m~1r Stz..e c~ille.d. -f.h.. m,1cdfl... Ldea 1 <..c- 0 clV(?S((.(J

clrtcl. ~ ·~.V\ c.o-Po.LJuA ~ ~ ,centre. elf,, rn1,riea..l/l. c%J -A<.r+A~r dlfNSJe) t;::. fufM ~ foy_tJt Cf?t'l.fr~S +.~~ h ~ "Uintt(Sf, fttrt o'(>

• LA • • ~() r AIVI.( s. o 1 v.:; ~ S 1 t e. dJ rv\ .q ,X. 1 M u. t ... _" rt.httll.. C6YL,.Lfnr'""q -~ v- . , , -o .

, ,Jl \1 . . : ~ LIJ tf...h'n!i. ~Y!~t5t s .. cf6 _ilh o'v~er ';)~fYitnm ; 0 ... w, ty . --- ~ { . ·' 6")-1. & . ~ . .. . . . . .. _ .. : ___ .... :.:-. ._.1~.!5!!.,{}_}2 ~ ~ · c1r~ [']!3!r · nerv·a(.)/:J .11 , er ( cts ow:er . .)u_rf?ce 0 in .f4l1f1a-w/-(;(,.._ ~ .c.f,('IYqict fL (ts Inner Sur/~tce t<J vv, Ctrvtftlcf wt~ ~ V(ftt.o U.Y fs-o~) · ln 'Rt:hV',J de..ft1(Vtwt ev1t h1 o~'bl ~ oul.e...- ?i.Jn~~e.n-teJ lP....jer r~~~-~ cJ...qyo/cl 6u...t ~ inn(r ntrvou.~ :J./7...-.!j.f.r S~c?rd-te_s Ou.f- fy(J}pt, fk..e. ?r!Jrvt.e.A.fec/. itt&e.r c:tr-cl d.ist);lce-~ ;'rt WdYd @) ~ F'<-h,fl{ V:> 0stnpo<;<d ~ {lOJ t"-H e.rs ~'-~~ ~N k Y s lttj e.rs db m ~ ~: . .r-r r1e tArcvts ~ f2e_cetlnr>(re;ds o/-Qm~5) <~---<O.e €) ,

(])§1- ardt.~~... bi('~<tr 3'"1~//611 Ofhc ne.riK. '11 e uraYJ s Ce..L1. t c W s ~

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Or{( CZJ:? ~)~ . uu>Jta:r~ . f)~~~~~

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f\CCO"W\odttt)OI'\ 0:~1 (~~(;. ~ incre«-Fes & ~fter1:Jti.. ~ tf'~-L lens ftr, 'fletlA. V/S/oh

(j) Tk ~irUt.J~ 06 ~ ~~s d~ends ffi1 C:ts sR&fe / w.«,~ in f-UAn ~ /S HLjultt-tf.d b-::f f:i.z cLft'cfJ 0'WV(sct~ ,

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@ rw~ fk_ @irlr:J: 'YIAU.fcJj) ~ tda..x ~ tk StASf.etlSrry

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Gt-.s ~~trertce_ cle.crea..ses, t::iWKt':J _tl...t:... ~St<:f){ /n. ,S,\AS £7 o'Y .J 0 iL WLertts --7' th- 1 c,;, s le ((J)l.'(( s m oY( 5p J,<ri c<R. ( ,.. rrfe

to e.l) + U:s S1T~tA. increases C({u.s;"j ~ett:J:tr ~~ d6 ~ rn.y.s. d3i ~

~

~ c.<lidry "YY\. \,(_ s c .t ~ "' . .

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:>

~ Outer P'e;xifo:m layer

:5" Inner nuClear tayer

f.:. lnne' plexriorm laye'

~ T Ganglion C1!~ layer

~CQ //~ - I ..

g 0 ~ .., u ~ 6

FIGURE 6-5. (Jrgamzati(ln oi the retina. Photoreceptors converge on b1pol,or Cells, wh1ch converge on g<mKhon celb. All of the receptors that convey information to a ganglion celt are part of thilt g;:mglion cell'< r~eptrve field - > · · -· - .. ,_ • . '

@±g <Zld:1'

~~-cv.t4t

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cg) 1 n k_.., "',J 1~ IV!; n'c1..j '"nil i:: w·J> r .=n11 e

(l energy transduction. The rods and cones I Figure 1). 3 J are the photoreceptors oi the eye @c -1. Morphology. Both cell types consist oi· ...-- ,.. ___ _

a. An inner segment containing the nucfeus,. a.~undant mitochondria, and synaptic vesicles ~~w· .-.· ..,. · ~

b. An outer segment containing membranous disks

r-'"'-

/·.::;y '--~...-:- \

c-·~'"" ( 1 J The membranous disks are continuously formed at the base of the cuter seg­ment and migrute toward the apex, where they are sloughed off.

(2 ) The membranous d isks contain a visual ~igment, called rhodopsin, wh ich ab­sorbs light rays.

'"';:::,~ 'o. ~-·

(a) Rhodopsin consists of a protein called opsin and a light-absorbing analogue of vitamin A (retinol) called 11 -cis retinal (f igure 6-4}.

(b ) The am ino acid comPQsi tion of opsin determinE'S the wavelength of light absorbed by the photopigme~- - • - · - --··--- - ·- ·

ROD

(i) Rods contalnasingl e1ype of opsin. The gene encoding ior rod opsin is located on chromosome 3.

(ii) Cones contain three·t ypes of opsins (blue, green, or red, depending on the portion of the visual spectrum they absorb best).

Outer segment

Inner segment

Cell body

Inner fiber

CONE FIGURE 6-3. Morphology of rod anrl cone receptor cells. Cones. whrch .:~ re re­sponsible for color perception Jnd high visual acuity, are found in the fovea. Rods, which are responsrble for night vr ­sion, are locJted in the penpherJI reuna . g; ;Y1 6 r'c S ~Z n.s ,-t/oJ-.<. "(;"_; ~f,t

-th. ,) r~ CGI'\eS; Ke1po'lif b(t f.o-r i1~1jht \) ;' c;. i (')'l . C., 1'114 ; n I'll._~ r·<: ,-A ...,J 'J Js, ~~~ in_ ~h e..._·-.. c ~,.:...i c r .. f~iYI <' , ) f . c."•l •" dc f~ ci { _i) h t- e n f er,·,~ t t..._ ~:,.e f.r..,...,"" cj "'-dJ-d ,· •'"ten ~-;)., t.,J t.. c rr rt s c ..... Yl ( s H .s,1)" -n/ ;,- . ;

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~ '1 ~ 3 ci' f.f'e ;· ('11 f f l~r..· h.'f /_j i-1'1 t' 11 f ,_,

R 0 d s ~- Co11 ~ s- 1 C't re. l) E: P 0 LA i<.i -z ,;}) / 11 -tf'-<- bl} J<.t::- ? &· H~J'j' e r pofr?ri :zr.J i"n -t-h. . .L{g 1Jt . 7Jt:. OJ't..~ i"'fCt:.j" lwrs {-: ift,-t.(, tc'5jJc'Yir._-(

-t'~ t:e\ ~ if~ 5 r'ec/('c S hiYl (,tiLts t~ ~j pt.r?crf·~ r, ?, r~ f> ~~, a. Darkness. Rods and cones are depolarized in the dark. Their resting membrane po­

tential is low , approximately - 40 mV. (1) The low resting membrane potential results fro~ the high Na + conductance of

the outer seo.,ment (see Figure 6-4A). (a) Na + flows into the cell through Na + channels in the outer segment and is

transported out of the inner segment by Na + -K + pumps. (i) Na + channels are maintained in the open state by cyclic guanosine

monophosphate (cGMP), which is synthesized from guanosine triphosphate (GTP) by guanylate cyclase. When cGMP binds to the Na + channel, the channel opens. T_hat is, in this case, cGMP acts by activating the channel directl y, not by activating a protein kinase.

(ii) The numerous mitochondria in the inner segment provide the large quantities of adenosine triphosphate (ATP) required to maintain the high Na + -K + pump activity.

(b) The large flow of current into the cell through the outer segment and out of the cell through the inner segment is called the dark current.

(2) The low resting membrane potential allows continuous release of synaptic transmitter.

b. Light. The photoreceptors hyperpolarize when stimulated by light. Absorption of liRht by rhodopsin in itiates a series of react ions resulting in the hydrolysis of cGMP,

• • o . o 0 - --~• - - = --•:~~ ,..{ tho r-<>1 1 I<.PP Fi l!llrP.

,..-: -;? ·..___... :~;

L./

~--------------------------->

- ,o .. 9)­r1 :~ " ---~l.~~Jfl {,t)\~/

FiGURE 10-25 Th feS'j50nse··. of the cllowr~c~t~ a:1d -::ones) t

1 li~ht is falwaysh\'pen)oTamatjon. ~ A The bipolaicerrTe/'in t ts pat, way 1s excnc · ~y the decrease m transmiw:r rck<tsc fror:-t ihe photoreceptors. that is. the transminer had an mhibilOr)' effect that was turned f,tT The t·1polar excitation increases the number of acuon poten­\iE!s in the gan!!lion cell (G) and therefore this is l:nown as the ON pathwa~ B Hyperpolarizau~m of the photoreceptors de­creases transmmer release , ~-.hich mhibits the bipolar cell. This etTeCl indicates that receptor cell transmiuer has an excitatOry etTect on this class of bipolar cells. The mhibited bipolar cell releases less transmitter at its terminals on the ganglion cell. and this resuhs in a decreased number of action potentials f~om ganglion cells in this pathway. This is therefore the OFF path· way. ------------

Produces more action poreniiols

Rele::~ses more tr::.nsmirter

when ::lepolori=ed

Depolori=cd when

less transmitter :ele-:>sed

by receptors

Hyper­polarized

in response

to ligh1

On pathway

A

Produces fewer ocfion pot;r.tiais

Relecscs less transmitter when

I hyperpola~i=ed

rL Hype~~!~riz.ed

less 1 ronsmitter rel::!cs2d

by recepTors

l_j Hyper·

polori=ed m

response to

light

Off pathway -- /) .~,~ . ... 0;7);_1 :--~~~{{_( B

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VISUAl. fiEI.D

DEFECTS

1 .h .!t

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I Optic Rodlotion

~()

~

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FICVRE 38. 111c visunl pnthwny. On the right lire mnps of thu vlsunl field a wilh nrcns ! · · . ?. : ' y . • • •

of Lllnuneu t.brkcnud to ahuw tho effects of lnjurle• In various locations. 1

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1

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;_---' . _..,_

the chiasm disconnects fibers

optic radiation, will also produce left homonymous hemianopia. Vis­ual acuity of the parts of the retinae whose functions remain is not affected, and the patient may not be aware of the presence of hemi-anopia. ___ ....,

The @:unc•us,J which is the gyrus above the calcarine fissure, re­ceives visunl impulses from the dorsal, or upper halves, of the reti­~; the~ below the calcarine fissure, receives impulses that arise hom the ventral, or(lower halvej) Thus a lesion that is confined to the right lingual gyrus cuts off visual impulses from the lower part of t.l-te right half of each retina . . This produces a. loss o.f : vision in one quadrant, rather than hemianopia. Since the · images which are focused on the lower part of the r~tina come from objects above the horizon line, there is, in this instance, an upper left quadrant defect (see Fig. 38). The visual impulses which go to the iingual gyrus travel in the ventral part of the optic radiation. Con­sequently, a lesion of the ventral fibers of the right optic radiation has the same effect as a lesion of the right lingual gyrus. - Lesions of the (~iddle part of the optic chiasrii)are frequently

produced by compression of these fibers from a tumol"of the pituitary gland, or a craniopharyngioma which lies near the midline imme­diately behind the chiasm. Th i:iecussatin fi ers f the o tic nerves are injured and visual impulses om ther~ of each retina are blocked. As a result, the left eye does not perceive images in the left half of its visual field, and the right eye does not record images in the right half of its field of vision .. The defect is in the temporal field of each eye and is therefore called heteronomous bitemporal hemi­anop£a.

I I I

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+ t fhe pathway for the accommodati.on-convergence reflex is

rhus difft:rent from that of the light reflex. This is supported clinically by a condition known as the Argyll Robertson pupil, Tn which the light reflex is lost while the accommodation-convergence reflex persists. The site of the lesion in this condition has not been established with certainty, but its . etiology is known to be syphilis or the nervous system.

L~l

l'o~ruanglionic ~h url c ili :u lc~ lo ir is n11tl ci liary body

\ Lateral reclus

Oculolllolor nerve ~ -.o Lateral geniculate b o~

(1-GB)

Oculo111olor nucleus

E.dingcr-\\'cstphaluucleus

~Lt,,._, 0

Posterior commiscurc I

ACCOMMODATION REFLEX -~ Rccp tin:~ tit ickcni 11g of the lcm, narrowing of the pupil, <tlld·cor'lvel'gcnce it:a.Qidcr to sec ncarobjcc~ ~~~r!f. The \'i~u:tl cortical sti111ulus relayed to the frontal cYcficlds is. sent via the intcrnai" capsule lO the prclr.claluuclci and a tnidhr:tintcgmcntal reticular "vcl'j;el'lce·ccntcr".111c prctc'tlum: organizes I he I"<'! JIIin:d p:u·:t~)'lll p<~thct.ic stimulus lo the ·smooth muscle·ofthe c~lia,.y body and the fris through lite Edingcr-We~tphalnuclcus. The vergence cctficr otc:hcstrarcs'bilatcral stimulation of the medial

r~c ti (ami i11 hibitiun or the latcr:d rcc_t~) ~~ -rough its cont'lcc.tions with the MLF yckiug sytem. ~,. .I

When the eyes are directed to an object close at hand,

tl1ree different reflex responses are brought into cooperati ve

ac ~i .on (Nea.r-.. pon'l-t Hc1CfJ.(I')'\) •'

1 •

2.

3.

Convergence: The medial recti ~uscles contract to

move the eyes into alignment so that images in each

eye focus on the same part of the retina. Otherwise

the two images cannot be fused and diplopia will .

result. I

Accommodation: The lenses are thickened as a result

of Co nkr.1c+io11 af ciliary muscles in order to

maintain a sharply focused image.

Pupillary Constriction: The pupils are 'n arrowed as

an optical aid to regulat~ the depth of focus. The

constriction do~s not depend on any change in

illumination and is separate from the light reflex.

j

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":'<~. .r VOLUNTARY EYE MOVEMENTS

The area of L.~e cerebr21 cortex tha t ccntrols voluntary eye movements is the frontal eye fi e ld, located anterior to the motor cortex. Elecoical stimulation of the frontal eye field results in conjugate deviation of the eyes to the opposite side. A destructive lesion there causes both eyes to deviate to the same side-looking ,

a way from the par<J:yzed side of the body if[he 1

../

motor con ex has been damaged by the sar.Je rt,t{) 1

lesion. There are probably no direct cor-~ ticobulbar fibers from any part of the ce:-etral 1~,-­conex to the nuclei of cranial nerves III, tV, 111.-, and VI. Inste<Jd, the volunta ry control of eye ....__ movements is mediated by a polysynaj)tic pathway th<J t involves the frontal cortex, supe-

~ rior colliculus, pretecta l area, accessory oc-..z­lomotor nuclei, and, finally, oculomotor, r rochlet~ r, and <~bduccns nuclei (Fig. 8 -4). (The - .

Frontal -~:~~~-:-Ri~;~-

~ ~~ (~~.

Supenorcolhculus - - --.1:.... 1 \ Dorsal tegmental ~ dccussat1on

Oculomotor nucle1 T<'ctobulbar tract right l left

Inhibitory internuclear--­

neuron

dRo~;ht

a tcral ectus

_) Abduccm nerve

lnhibotory ontcrnuclear neuron

Abducens nucleus

~

' <JiiD.

\ I

. "I' 11@

1.4.1. @

From vesltbulor nuclei

Paramedian pontine reucular formation

I I , Right. supranuclear ophtt>~~mc­

fYie..J• ~

PA RAL YSIS OF CONJUGAT E L ATERAL GAZE

Cortical Gaze Control

From frontal and occipi tal(gaze control centers) the ~ide of the brain turns the eyes conjugately to the right. The frontal eye field initiates voluntary saccadic movements of the eyes ("Look to the right!"), while the occipital eye field initiates slower automatic pursuit movements ( .. Follow my finger!").

Ischemic stroke and cerebral hemorrhage arc .the most common causes for conjugate gaze paralysis because gaze paralysis from cerebral lesions only occurs immediately after acute lesions, disap­pears then, and can be reyisualized only under special circun-;­stanccs. I

~ .. I ·g~

~

Internuclear Ophth-almoplegia

'~ To understand so-called internuclear

oph~moplegia one must recall ce1·tain information.

~

~ Three types of conjugate movement,

i.e., convergent, parallel vertical, and par­~lel horizontal, were described previously.

·The conjugate convergent as well as the vertical movements involve t·...,o pairs of nuclei that are situated close together, i.e., the oculomotor nuclei and the trochlea1· nuclei. The conjugate horizont.a.l or lateral gaze movement involves a pair of nuclei which .are far apart from each other, i.e., the abducens (right or left) and the oculo­motor (left or right) (Fig. 13-7). It appears

I. that the cortical descending motor fi­bers stimulate the su peri•) r colliculus and that the superior colliculus sends fibers to a nucleus of the opposite side, i.e., the parabducens nucleus, which is

.If·:· :·located in the paramedian pontine re- i · ' . ticular formation (PPRF), close to the 1

ab<:fuccn~ nucleus; and

right left lateral 1

j rectus . ~' Q' ~:· muscle mod;al~ ( /? rectus -:::;t i muscle

oculomotor nerve & nucleus ----flo 0] . . . . . .

abducens nerve

·-superior ::olliculus

tectobulbar fibers

parabducens &

I

I A · .. ~ ~-=

(~ ~ -~-. )

B nystagmus

Fig. 13-7. (A) Pathway for lateral conjugate gaze; (f?) right internuclear ophthalmoplegia.

I

2. tha t the parabducens r.udeus stimu­-'; lates the near-by abducens nucleus -\ -· -(concertJcd with the lateral rectus mus-! ~ •

; i:· cle) and also, through a path with oth.:!r different fibers, the medial longitudinal fasciculu s (MLF), the portion of the op­posite oculomotor nucleus concerned with th(' 111cdial rectus muscle.

A lesion of the MLF (affecting the path between the parabducens and oculomotor _nuclei) prod uces internuclear ophthalmo­,plegia. Tilt: most common cause is multi­ple sclerosis. *

If a Ic.:sion occurs in one MLF, e.g., the right MLF as in Figure 13-7, it is manifest when the p:1tiem tries to look laterally to the side opposite of the lesion. The medial rectus on the side of the lesion does not adduct; the nbducting left eye moves later-

' :illy and displays horizontal nystagmus in ·lateral gaze. These signs of internuclear :9phthalmo plegia are also known as medial ; ~ongitudinal fasciculus syndrome, which f\1sually is bilateral, affecting both MLF. ~- }?lis is an important syndror.ne as i~ verifi­i·.~tion pinpoints the cau~al I~ion very pre­

· ~c~ely in a specific regi'on of ~e _brain stem, ·: ~.e., the r<:gion of the ML~: in the upper

__,?? ;;;'! _/!

right

• :-"'='~~ ~~

~) ..

.-,~~

-~)

• ~~~

-~) convergence

• ~~-==!::... left /~-

(~

• ~.....--.. (@-

• ~----(~~

-'1-<'·

Fig. 13-8. Bilateral internuclear ophthalmo­plegia.

pt.illS between the abducens and oculomo- \

t~~ ~~an:~e ~n:sis :;:_ tcrnuclear ophthalmoplegia, when the de­scribed signs have appeared, consists in verifying that the patient is able to con­verge the eyes and make vertical move­ments of the eyes. A case of internuclear

·ophthalmoplegia affecting both MLF is il-

lustrated in Figur~l44t'"~·

Auditory cortqx

\ Medial geniculate

Inferior --co lli culu~

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body

nucleu~

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~~r~· I \

COCHLEAR NERVE

'- . ,_,

SPI RAL GANGLION

Figure 7.6. Schematic diagram of the auditory pathways.

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• AGURE 6- 36 Transmission of Sound Waves (a) Fluid movement within the perilymph set up by \'ibr<Jtion of the oval window lollo·ns two pathways; (I) through the scala vestibuli, around the helicotrema, and through the scala tympani, causing the round window to vibrate; and (2) a "KIJ!Jrlt:llt" front the l!Ciafil VC.'>IilnJii thrtllll!h !111: hiL,ilolf lllt:llll tWIU' lll lht• !ot ollol tyoupn111 '!lot• lw.t Jtotlhw,ty Jtt:'OH11'· .. -.op.oh-, -..1111td t'ltl'IH)', but lltt• ~n·ooh l

pathway trigger.; activation or the receptor.; lor sound by bending the hair.; of the hair cells as the organ of Coni on top of the vibrating basilar mem· brane is dtsplarcd tn relation to the ovcrlyinR tectorial mcmhran<~. (h) fliHcr<'nt r<'gioct~ <)flht• h;u;il;•r nwmbr;m1• vihrall' llli'll:imally ,1t difft•n•nt lrt• quencies. (c) The narrow, stiff end of the basilar membrane nl!arest !h .. oval window vibrdtes best with high·lrequeucy pitches. '11te w1de, tlc:xible end of the basilar membrane by the helicotrema vibrates best with low·frcq11ency pitches.

Incus Malleus

Tympanic membrane

~3.000

Wtde, flexible end of basilar membrane ~--=u:::::!'- £U by helicotrema ~ ~

Narrow, stllf end of basilar membrane by oval window

I 20,000

(II)

Scala

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Round wtndow

Cochlear duct

tympani

~" '

(a)

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)

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) Medtum frequency

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(t')

The numbers indicate the frequencies w11h which c1tflerenl re()tons of the b~s•lrtr mP.mlmme m<Jxtm:ll'f vthr:~ta

Peripheral Nervous Sys1em: Allerenl Division. Special Senses 183

In ear

Sound waves

~ Vibration of tympanic membrane

~ Vibration of middle ear bones

~ Vibration of oval window

~ Ruid movement within cochlea

~ Vibration of basilar membrane

~ Bending of hairs of receptor hair cells of organ of Corti as basilar membrane movement d isplaces these hairs in relation to the overlying tectorial membrane in which the hairs are embedded

~ Graded potential changes (receptor potential) in receptor cells

l Changes in rate of action potentials generated in auditory nerve

~ Propagation of action potentials to auditory cortex in temporal lobe of brain for sound perception

Vibration of round window

~ Dissipation of energy (no sound perception)

I I I ,

Pressure waves of frequencies associated with sound re- \ ception take a "shortcut." Pressure waves in the upper com­partment are transferred through the thin vestibular mem­brane, into the cochlear duct, and then through the basilar membrane into the lower compartment. where they cause the round window to alternately bulge outward and inward. . The main difference in this pathway is that transmission of ; pressure waves through the basilar membrane causes this membrane to move up and down, or Yil2r:.a_t~. in synchrony with the pressure wave. Since the organ of Corti rides on the basilar membrane, the hair cells also move up and down as the basilar membrane oscillates. Because the hairs of the re­ceptor cells are embedded in the still. stationary tectorial membrane. they are bent back and forth when the oscill;l!ing basilar membrane shifts their position in relationship to the tectorial membrane ( ' Fig. 6-37) . This back-and-forth me­chanical deformation of the hairs alternately opens and closes mechanically gated ion channels (seep. 81) in the hair cell. resulting in alternating_ depolarizing and hyperpolarizing potential changes-the receptor potential-at the same fre­quency as the original sound stimulus.

Thus. the ear converts sound waves in the air into oscillat­ing movements of the basilar membrane that bend the hairs of the receptor cells back and forth. This shifting mechanical deformation of the hairs alternately opens and closes the re­ceptor cells' channels, which bring about graded potential changes in the receptor that lead to changes in the rate of action potentials propagated to the brain. In this way. sound waves are translated into neural signals that can be perceived by the brain ac; sound sensations ( Fig. 6-38).

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PPER SURFACE

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ig. 30-9 Two views of the primary (41, 42) and secondary (22)

udilory cortex.

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Fig. 30-2 Spiral g~nglion ~nd cochlear nerve. The nerve lerminales in dorsal (OCN) and ventral (VCN) cochlear nuclei.

~~

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IUioll II Tectorial membrane

Scala rnetlla "'f"...1f!'U 'I! (cochlear duel)

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(;'l)

• AGURE 6-35 Middle Ear and Cochlea (il) Gro'i.~ annlomy nf lht• luitldh· t·:u and nwhlt·:e. w ilhl lll' n wltlt•a " ueuullt •t l." ( II) C'rns.-: scclion or cochlea. (c) E.nl;~rgemenl or organ or Corti.

l111id fillt•d t'Odllt •<l. /\.-; lilt• lympanit· tllt'tllhraiH' vihr;•lt•s in response to sound waves, the chain of bones is set into mo­tion ill Ill<' salll<' frPqll<'n<·y, 1ra11smittin!-! this freCJucm:y of movement from the tympanic membmne to the oval window. llre resultant pressure on the ovnl window with each vibra­tion produces wnvelikc movements in the inner ear fluid at lhl' snm<> lrC'C")Uf'ncy ilS tlrC' nri~innl sound waves. llowevcr, ns noted earlier, wenter pres.c;urc is reCJuired to set fluid in mo­tion. Two tnl!dranistns rclatcu to the ossi<.:ulur systc111 a111plify the pressure of the airborne sound waves to setup flu id vibra­tions in the cochlea. First, because the surface area of the tym­panic mcmhra11c is much !urger than tlwt of the oval window, prCl>surc is illC'fC'ilsC'd os fnr~.:e l'XC'flC'd on I hC' lyrnpan ic mcm-

. brane is conveyed to the oval window (pressure= force/unit 1 are<1). Second, the lever uc·tion of the ossicles provides an ad­ditional nll•t·hanit'ill ;ulv:utlil!-W· To~l'lhc-r, thcosc> tnC'c-hilnisms increast> the force <'Xerlt'd on the ovul window by twenty tillll'S whllt il would hl' if I he sound w;rvc struck the ovul win­dow direclly. llris additional pressure is sufficient to set the cochleilr fluid in motion.

Several tiny muscles in the middle car contract reOexly in response to loud sounds (over 7D dH), causing the tympanic membrane to tighten and limiting movement of the ossicular chain. This reduced movement of middle ear structures di­minishes the transmission of loud sound waves to the inner ear to protect the delicate sensory apparatus from damage. TI1is rcfll'x re.c;ponse is rei;'! lively slow, however, happening ut least 40 msec after exposure to a loud sound. It thus provides protection only from prolonged loud sounds, not from sud­den loud sounds like an explosion.

182 Ch~pter 6

~ Vestibular membrane

Tectorial membrane

Scala Scala

vestibull 7~

(IJ)

Norvo lll.>crs Basilar mombmno

(c)

Hair cells In the organ of Corti transduce fluid movement5 Into neurnl signals.

The snail-shaped cochlear portion of lhe inner ear is a coiled tubular system lying deep within the temporal bone (Fig. 6-32). It is easier to understand the functional compo­nents of the cochlea by "unrolling" it, as shown in Figure 6-35a. Tile cochlea is divided throughout most of its length into three fluid-filled longitudinal compartments. A blind­ended cochlear duct, which is also known as the scala media, constitutes the middle compartment. It tunnels lengthwise

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FIGURE 2 5. 5 The Weber-and Rinne tuning fork tests. (a) The Weber test to evaluate whether the sound remains centralized (normal) or lateralizes to one side or the other ~ndicative of some degree of conduction or sensorineural deafness). (b and c) The Rinne test to compare bone conduction and air conduction.

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(b) 12 I. 1'\ VH~ 1: I? Sf (c)

Weber Test to Determine Conduction . .. and ~sorineural Deafness ( ~J ~ Yv~ d ~-l t 1'\~.,s) ._. Strike a tuning fork and place the handle of the tuning fork

_medially on your partner's head ee Fi.e:ure 25.6a . Is the tone * ~u~?:ua lD ~ih e~or is it ouder in one earrx--- - .

~ ··. -

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@~{In 1. Strike the tuning fork. and place its handle on your part­ner's mastoid process (Figure 25.6b).

2. When your partner indicates that the sound is no longer audible, hold the still-vibrating prongs close to his auditory canal (Figure 25.6c). If your partner hears the fork again (by air conduction) when it is moved to that position, hearing is not impaired and the test result is to be recorded as positive ( + ). (Record below step 5.)

3. Repeat the test on the same ear, but this time test air­conduction hearing first.

A,-"r- ~d'>1cf tAch·~.,., { erl ;-/'j

4. After the tone is no longer heard by air conduction. hold the handle of the tuning fork on the bony mastoid process. If the subject hears the tone again by bone conduction after hearing by air conduction is lost, there is some conduction deafness and the result is recorded as negative ( - ).

5. Repeat the sequence for the opposite ear.

Right ear: Left ear:-------

Does the subject hear better by bone or by air conduction?

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\.i1<li "'11 ~ ~j ue.flj· S"v.tre;.I,J:j

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r.M~J..,-,J_~ -Y ~nJ V(vrtl{ ic.R.t

Hypothclcm ~LU-lr•J..aj- (oH: · ./ _ _i, . .,. ~ co sulcus IHt e y,~~,J/. C~?J..:..(c.

I "fr J r.rr s J. -7 fc.""' 1 "();_,_ /t n:r . S<A.rfq,c ~ ctG f4 .b ... ti; .. .,.;~_)2

1/f!.n tY/d-t.

-- - -- i1J,~-: ~~;~

f'ig":.: 7 ~- Sch~!1131ic d.ag;r~m shvwing lh~ subdivision' of lh~ dicnc:ph~lon ~s ~r~n in a ~omr-o~ile coronal sc:clion.

5 ~-I:. fA-&_~ fl h.t . --7. •• \1\ ~ . ur .i\td c.-tf!J,_

ws,tJJ..,..., +'-'.~<~."'« '(

v~ .. ,..tf.~~ ; •"t-. Functiollnl Sl.l.b.Jn.:is i!lM or ~r-<1.{5 - ' _ c,>:r~~ 1 Specific 1111clei are reciprocally connected to localized areas of the cerebrnl cortex. They are said lu be 'cortically dependent', because they degenerate when the target area of cortex is removed.

~ A vd.e.n· a 1 \l A , \) L , \J p} M G cs ) L G-g

Pulvinar

ig. 17-1 Thalamus from above, with nuclear subdivisions. lML, olcrn~! ml'l.lull~ry l~mina.

.nntomicnl SUbdiv;s/011 ,-_~ JJ 71 T l.M.«..J(I{_HA/.J! • -· .

he Y-shaped inlemn/ medullary lnminn g/'vides the 1alamus into 1'~nterior, ~&ediodorsal, and'' ateral cell ·cups. The lateral group has dorsal and ventral tiers, I 1ch containing three nuclei. Oclow the hindmost ember of the dorsal tier (!he pulvinar) lie the medial ' 1d lateral geniculate nuclei.

2 Non-specific nuclei project to wide areas of the cortex and influence their level of ilclivity. They are not cortically dependent because they give abundant sustaining coUaterals to one another. . ... . . 4

~t<,~<\1\A.i\'\a.y J f_u-.·eu.JM

3 7\.ssoctntton nuclei have reciprocal linkages- with associatio,.., areas o.f the cerebral cortex (Table 17·1).

'

~ M~·odo'HJ J..i'H:t;r.J dcrvs.J.. l kkrc&

l !') (1 • I~.AXNtflttr

('oHo·,·c;

g. 17-6

Superior \

;. 17-7 Hnriwntalt<t.'<.iionthruu~;h internal c.psulc. AL, .criur limb; G. );l'IIU; I'L. puslt•riur limh. c~utl.llt• nudt•u• (CN) J lcnlifnmlnuclci (P, put~ men; GP, );lubu~ p<1lliuus) ~rc shuwn. anlcrim nudL•us; MO, nwdiu .. htrs.1lnud,•us; VL, vt•nlmiiJll'r.ll deus; GN, geniculate nuclei. Arrows indicate thal.lmic liatiuns.

~- ~.ryu rc\cl; ~tl~""

C,.(\ t-1 eo pN.. t-i~

[t\ fl'c.o bulb A. r

- -~Ci. 0

• 00 0 - · - · · - -cpqo-----:r /.. .p'if;): r «Ji<f• .mr !f:iJ u!f!/u) ' ~ 1fp . 0

-t~Clk. R E C itR 0 C fJ h~Cfc h'~t.s

~ t:.Pl) ~vttck -4-<- ~i/..wtl,d ~ ce..rtfrtfli 4->t,t ~

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St.Afe.ri ar pel c.Anc!_'- ·. ~s tiA.rr/VIQf,...

fost(..ri(Y- L::b 1 {k ;n1· C"-fJt.J...t..

Ql.w}... ~\1\tds /l'r~-->Lf;~ \..VIt-t..... \poH c~t-r~--.,.l,l,1..

0 -1).-u_

ll ". • j_\} A. ~ . ::J::tr• , vL

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{9 2_ ~--~!_tMfur~ Peduv--J(__ ~~s~ dr~ -tL'-(, sukkh· ~ ~ "1 ..:V..t- c~rvJ,t.. S.\1\.c~Jes ~*-hl'-j _rt{,cLitt.h.(}./1 {Yef'lv--M Cr a~ -hit.O\S \It--1~ f<..,.fcl\.,.J_ j,yrv.s

Head of caudate---1-­nucleus

Lentiform nucleus

-"\ o\ J. Head ? ) ..:J·, t.vl

Neck •

Upper limb and trunk

Thalamus --r--

Medial geniculate body Lateral geniculate ----.1, body

{: .:::::j Thalamocortical fibres

~ Corticopontine fib res -

Lower limb

1- -I--- Inferior thalamic (audilory) radiation·

..--···~

~ ~

I I

\

'' - - · .r lf"'""":-;·r - - • -- - ·- · --

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~ ~f- ~. ~d:u_f -t. 01..'-L S"'ftf-t'4( f:s:J d.f\ckJ ob t-V~

<1. vtl· <.-r i t"( c h c:rc; d. J. t!VL~ ~

.,..- ,.··) : • ,. 1""· -

l ..... ..

\~:~· .. . ;_ . ~! ..... ~;-;,1

Olfactory tract

j ., . ·.

O lfactory bulb

Optic tract

Anterior ..,.....,._.. ~ - perforated •

substance / -

Meta l retractor

Figure 17-3. Some components of the olfactory system seen on the ventral surface of the brain. The right temporal pole has been cut away to give a clear view of the olfactory trigone, anterior perforated substance, and limen insulae. ( x 1 )

(~)

®

~ oR )Mh.rr <-~ bulb ~ ol/- (Yal.i: ~ cJ../- ty ig~ / 'bf~o,f olt•)(fuy -J v \ SJr,_c\ .J)

7/J • {!_ tf /. "IYI&J, ,,P off. s.~y ;, -+ J.Aie-ca2 okfa cA"::J s-Joa --?- - ·lkr. o} , ~ rf<'!.'i a~} ·

~( fnM•"Ir_j o.tlf.,uc"':j .et~ ) \,.

err~."' <J 1-. \. t-l n '-!.< y Cc_l k!. o2:. 141_-frJ r /..;" .J ' 4sY k -x. ;,.. (~~ Cill'l ~H IITY fi! r t +"- .

c ... 1 • !J -~ rtf l-t{,),., l" 't _r"-ntpfoCi'-~~.Af"-X 0 At.

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lr~ri~~ c~-t-tx \) ~ ,khe . D

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:")') e \ \K_i c.',\,/ c .\.u.s i ':J _ 0 n 0~ M i A

~~~ S•~ irtj"'~ ""'j_ rU.b .:.._ . Je,JI<t)~ c(,_ ( .5P t""' Sivtti\(r\(. ~~ct d S[>il(t 1nh i?H~ (~l)lfj l CS'F K.J..III~J"YtA~·~,)

;f.,,(.{(_( ~ ?\ ""~r ~ rd::t='lct:. tyY1e; ·li;Je1·~ 1~,1il1 •-"'

• • f) tl'li/~V(J

ol~do':j

-Uuz.

--­TASTE

-RECEPT :·;::: · j =' 3.~.?,!:3 & ?.~T:-i'N;.:.'·i ~

Taste Bucis The taste: buds. the sense organs for taste, are

0 void bodieS mc:asuring 50-70 Jl.ffi. Each t<lste bud is made up .:.f .:1 ;yyes of cells !Fig 10-5): basal cells: ;ype 1 and : :ells, which are sustentacular cdls; and . ._..,e 3 cells. which are the gustatory receptor cells -.. r • . ihat make synaptiC connec:!c•ns w sensory nerve fibers. The type l. 2. ~nd 3 cells have !:lJC'r·:iViili. ·.;:hich proje.:t into :h~ taste pore an opening in the linaual epilbe!ium. The necks of a!l these cells are co~ected to each ocher a..,d to :he surrounding epi­ibelial cells by tlght ;unctions, so that the only part c·f the gustatory receptor exposed to the fluids in the oral cavity is its apical crown of microvilli . Each usee bud is innervated by about 50 nerve fibers. and conversely, e:~ch nerve fiber receives input from an average of 5 Laste buds. The basal cells arise from the epithelial cells surrounding the tlste bud. They dif­ferentiate into new receptor cells, and the old recep­tor cells are continuously repl:J.ced with a half time of about 10 days. If the sensory nerve is cut. the taSte buds it innervates degeneraie and evenru:.l~y disap­pear. However. if the nerve regenerates, the cells in :!1~ :1:!~hborhood become organized i-lto new t::~ste buds, presumably as a result of some sore of chemical inductive effect from the regenerating fiber.

In humans, the usre buds are located in the mu-

n:::· FUNGIFORM

" VALlATE

Figure 1o-s. Taste bud, showing type 1, 2. and 3 cells. (MOdified and reproduced, w1th permission, from Shep­herd GM· Neurobiology. 2nd ed. Oxford Univ Press, 1988.)

~---­/:1/ l

( /. i 6 '/,z._

--v~'-7 . ""0~­

~~£' ·;osa of :he epiglot;is. p::~l:tte, and ;:-haryr.x and .n the ·.••a!is 0f :he fungifcrm :md vailate papillae :.f the tongue. The fur.g1forrn papiil:le :tre rounded struc­tures most :1umerous nenr the tip d L'le tongue; the ·1all:ue papillae :Ire pror.inent srnJctuies a.'Tanged in a Von the back of the tongue. There are up ro 5 taste buds per fungifvrm p<ipilla, and the)' ";e usual!y 1.:>­cated at ~he top of the papilia r.Fig '0-5). Toe i:u-ger -.·a!late papillae each c0ntain up :o 100 taste buds. usually located along the sides of the papillae. The small co.Ucal filiform papillae ~h:n cover the dorsum of the congue do not usually contain Lis te buds. There are a total of ab-:~ut 10,000 ta.sre buds .

-:-asta PG:fh JVe:y::. The sensory nerve fibers from t.1e t::sre buds on :he

anterior two-thirds of Lf}e tongue travel in the chorda tympani branch of the facial nerve, :md those from the posterior third of the tongue reach che brain stem via the glossopharyngeal nerve (Fig 10-6). The fibers from areas other than L~e tongue reach the brain stem via the vagus nerve. On each side, the myelinated but relatively slow-conducting taste fibers in these 3 nerves unite in the medulla oblongata to enter the nu­cleus of the tractus solitar ius (Fig 10--). There they synapse on second-order neurons, the axons of which cross the midline and join the medial lemnis­cus, ending with LI-Je fibers for touch, pain. and tem­perature sensibility in the specific sensory relay nu­clei of the thalamus. Impulses are relayed from there to the taste projection area in the cerebral cortex at the foot of the postcentral gyrus. Taste does not have a separate cortical p<vje~:ion. ~ea but is represented in Li)e portion of the postcentral gyrus that subserves cutaneous sensation from the face. ,......?, II

-{/t( v';:;t.,~·:r::u:{_f ~

------------------_------------------ \ General

sersat•on ~-·-~ . .;::-.. -:,-

,._:-·=··

Figure 10-6. Sensory innervation of the tongue. The numbers refer to cranial nerves.

.f 1\ C..e. i.V.W..

.· ··!ofpost­central gyrus

Tractus solitarius

Thalamus

M IDLINE

Nucleus of the tractus solitarius

Taste pore

Taste fiber to brain via facial, I glossopharyngeal, and vagus

nerves

Figure 1D-7. Diagram of taste pathways

(9; 1:< v . \C=V ·· - .···-;u .- . ' .-;_}::r , .. c/){c -. .....-IJ._)~t-~·

\

~ nC:t IRF (j -<l C"ottlpn.-i<~con or Pt'ln !Julunlh <' t\hllll y or H('14hm~ with Swnll 'c•t-,cttM I Jtqlt' Hc•<'t'l•ll vt• Flc·ldM I Itt' u·l:\hH·Lu tile• ;u·uih· nl.t "'i .,.,,,, l!lnu \ :n1

,,,. df'!Pittlitlf'd b\' thl' (\\'U-pufc.tt tht't•Jlhtlhl U f

t.llfl<'l hnhu•llon 11.'~1 lliht• twol'oirtls nf a pair ttf •·;rlir•t'l" "l'l'lil· lit• lht• •;ud:rn• t•l till' ~I in''"" d.oll' 1w11 olil!l'l"lllll'<'l'l'h'·<' lil'lds. two SPIHifill<' poilllS •·· ill I •I ' h·h lllho• l\\'l>(lllilll' llnll'h lht• sanu• If' l'f'(lli\'1 h<•l· I . lht'\' will hP I"'"Ti\'t'tl n< nnh· nno· J•Pi111 I h e ;uliu•:lhll! lht• dl•.l,\lll"t'IH'IWt'PIIIh .. t:HitJH'I

l'"iul<. •llll'' 0111 dt;h•nniut•th,.. 111111irn'll disliult:l! ill ~.\.·}1if'h lilt" IWO J'(tiii!C: t':lf\ hP lf'\'('L~lli7.•'•\ il..;. {\VII

onllu•r than Pllf', whieh IS" rPIIl•t·lion ,,f lhP s;ze n( llu· tt~t'PJ'\In' !it~ld..:; ill tht• U'Hlnn \Vtllt lh·~ ,,., hno pH'. il i.~ pno.silolt•l•> 1 ·I ''lit I' ~!i~en•:•iu;,ti ··~ al•ilil\ • of lh~· hotly surlnt'f' ·n .. •rwopn•ntllm•,lutld t:"lltt:t·~ (H'I11:! l!Ull in tilt• lin~c·llip tf'n:lbliu~ trn•.! h) II'·'" 1\railh.'. \Yht'lt liH' r,·,i~·d lo,L-i 4:1\! '\;':lt'!.'d

:>;. mu1 apnrl) 111 1!-lunu 1111ln· I"''""' dis•·•iouinalivc sl·in olthe eall . tn) Rt•!liorr willr s111all n·c·epllve liPids. tl>_) Region w~th large rE'CPplivc lieJ,Is

l'lccr[)lor entlong-; ol alloronl neurons

_) /) i {~ ~

Two receptive fields slimulaled by ll1c! lwu IY.rlnl5 of stunulallon:

---""~~ Two points felt .......- -

Only one receptive liel::l stimulaicd by tho two polniG of :;limulullon

""-> lhc same distance a;:>arl as in (a): ----.;:;-y-.. One point lelt

~

~

(a)

tkuity is Influenced hy rece{)tive field sEze and !nte:-al inhibition.

E.nch sensory neuron resronds to stimulus information only wrllarrr u !2_~nht.:d rq;u111 of the :-.kin stu !ut:l! :.u:ru:rmlirt!{ il: 1 his r<'ginn is known as ris ret·<-ptivc field. '!.J H! siw Qf a rcc!:plive lielu varies inversely witlr the dcusity or rc<.:ept'C>%­ir"ltht-;-;:egio n; tl w rnom dost~yret·t•ptnrs of n parfil'iiln7Wj1t! ;;rt~-;;,;~H:t•cl, tlrt• snrallt•r tlrt• iiH'a of :-.ltiu t:adr IIHtllilurs. '1111' smaller the receptive liclll in a region, !lie grl!atcr its ucully or cll,..·rhnhmtlv(! uhlllly. luurp;trt• iitt• l;u·lilt• (lou.-ir) disnirni­nutiull ill your fingertips with that in your elbow uy "feeling"

ill<c> s:1r11t> object with bolh. You are able to discern more pre­visl· infnnrwtion :!Ullllllhe oi>jet.:l wrlh your ridtly rnm•rvalcd lirrL!t'rlips ~~~~ llil' ll'ceplivt• ltl!ltls there urc SlllUII; as a resull. each neuron si~nals information about small, tliscrete portions of lilt' tlhjt•ct's surf:l{'t'. In contrast. tire skin over the cll>uw is served l>y rclutivcly few sensory crrlliugs will I larger

rcceptiv<> fielrls. Subtle differences within each larRe recep­tive lield cannot be detected (• Fig. 6- G). The distorted corti­cal representation of various body parts in the sensory hmrrtruculu~ (s_t~c!_ p. I 1!1) corrC'.o;poncls precist~ly with the

(iun:_rvat~~~~.-~SJ~l'~ more cortical sp~e is allotted for sen­sory reception from areas with smalhfr receptive fielcjs and, uc<.:ortlingly, greater tuctile discriminative ability. ~

...._.. -(b)

~

~

Skin surface

:':

Receptor pathways

f

I Stimulated less l _]''I Stimulated less j y

I Stimulated ,mostJ

(a)

L.l.\

~

I .Tronsmlssion stopped I I

} Lutc ral inhibition

[Transmission stopped

) , J..+ ·r I I I

). T

l:;rrar:'a~lsslon,contlnuos, j (IJ)

l'roprrt ies of rcccjltnrs

Heceptors have the properties of ... •H.IeLLttatl! 1.. stimulus, cxt:it;lhility ami ;lllaptation. - -......___,._.._ ~ ~

-""-' .:2_ "3 \l}!tdc>cfliO/e. sti111ulus Each type of receptor is most '""

sensitive to e1 specific form of energy, which is called its auequ;lte stimulus, ;mtl is almost non­responsive to the normal intensities of other forms of e nergy; c.b. li~ht is the adequate stimulus lor the rods and cones of the eyes hut they do not n.:~poml to hc;lt or enid I Fig. 17. 1 OJ.

[P•tin rcccplt>r~larc not stimulateu by a blunt object touching the skin, but they discharge as

~---..-

soon as the blunt object is pushed with enough force to ~l"!:l~c tissues:__

The sensation pcrccivctl as a result of stimu­l;ttion of <1 receptor is called the modr7lity of sensation. Thus, cold, warmlh, tm~ch ;mu p;iin -....._._.400.--..J ;m: different modalities of scns;ttinn.

• A

Y\..f 'l~ • FlGURE 6- 7 LnterallnhiblUon (a) Tite receptor at the site olmc~ inlcn.o;c stimul<~ l imt i.~ activilh.•d lot he Hrcai~'SI cxtcnL !iunounding receptors arc also slimulatc<l uut to a lt'SSCr dcwcc. (It) "lloc must iutco~-:. ,ly ;"·tival<."<l rc,:cplor p<~lhway halt.s lt.~IL~IUis­sion of impulses in the lcs:~ iutcnscly slimulah..-tl(klthwa~ tluuugh lateral inhibition. l11is prot:t!:SS f<lt:ilit<•tcs lo..:illi:tatiun of the site of stimulation. ..,._,......- ' __,.._...._._., t lko.;}d~-\{~~~;,;-;j-;;-,~ity;r.l-;~coll cl factor innucncing

acuity is l"tcral lnhl bitlon. You can appreciate the impor­talll:c of this pheuuntcuoll IJy slightly intlc.:uting the surface o( your skin with the poin t of a pencil (• FiF(. G-7a). The recep­tive field is excited immediately under the center of the pencil point where U1e stimulus is most intense, but the surrounding receptive fields are illso stimulated, only to a ieS.ser extent be­ciluse they are less d\storted. If information from these mar­ginally excited aff~t fibers in the fringe of the stimulus area were to reac;h the ~ex, localization of the pencil poinl would be bluned. To facilitate local i:t.ation and~~­trast, lateral inhibit~NS (Fig. 6-75): The ·;nc;sr stronHIY activiltcd signal pathway originilting from the center of U1e stimulus area inhibits the less exci ted pathways from the frin~e areas. Tit is u~ via inhibi tory interneurons that pasS laterally between ascending fibers serving neighbor­ill!{ receptive lich.ls. l.ll01..:kage uf further transmission in the weaker inputs increases the contrast between wanted and un­wanted information so U1at the pencil point can be precisely localized. ·n1e extent of lateral inhibitory connections within seusory pathways varic:; fur uiflereut tnodaliti~. Those wjUl

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. the~ost l~eml i~ib ition-ft;;uch and visi-o!J-bring about J the most accurate locali7..ation.

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QUALITIES OF I'AIN

Classically, the sensation nf .pain ts suhdtvid~.:d into two components: --~0\St C Pr·,~~·~ (')

"-SI<NJ \ ~cf,.,..:l r) 1 J>ricl<itH; [Win. This is oftcn rdcrn.:u LO as~ pain. It is a fnst ncute sensation, which occurs WithTn 0.1 s after applic~tion of a painful sumu­lus. It is usually well localized, the kind of sen­sation felt when n pin is stuck into the skin or thc skin is cut with a knife. Pricking pain is usu;11ly superficial and is not felt in most of the dceptr tissues. It is transmitted via type Ab fibres. 2 nurning or aching pain. This is often referred to as second pain. It is 41 slow pain, which incrcasc.:s slowly over a period of many s~comls or tninutes.

_This component is the type that is difficult to

~iidure and c:~n occur both in the skin and in the deeper tissues. A good example is intestinal colic, toothache or ~ burn. Slow pain is transmitted by unmyelinated tyreC fibres.

The two qualities rcflec·t not only the dual n:tturc of the input li.c. Ab and C fibres), httt also the -two sets of connections within the nervous

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