SMOKING
THESENSEO F SMELL
The smell receptors are specialized nerve cell endings that are
situated in a small patch ofmucous membrane lining the roof
ofthe nose. The axons (fibres) ofthese sensory cells pass up
through tiny perforations in the overlying bone to enter the
two elongated olfactory bulbs lying on top ofthe bone. These
bulbs are swellings at the ends ofthe olfactory nerves; the
nerves contain millions ofnerve fibres and enter the brain
on its lower surface. The olfactory nerves carry sensory
information to smell centres (parts ofthe limbic system
and frontal lobes) situated within the brain.
Physiological basis of smell
The receptor cell bodies that are situated in
the nasal cavity are swollen at their lower
ends; each one has several cilia that extend
down to the surface ofthe mucous membrane.
The cilia contain the receptor sites at which
stimulation bythe molecules ofodorous
substances gives rise to nerve impulses.
These nerve impulses pass from the olfactory
bulb along the olfactory nerve to smell centres
in the brain (which include parts ofthe limbic
system and frontal lobes). Although we know
that we are able to distinguish between as
many as several thousand different odours,
the exact basis ofthis high degree of
specificity is uncertain. No microscopic
difference in structure can be detected
among the different receptors.
The smell process is probablybased on a physical
“fit” between the odour molecules and the receptor
sites. For example, the receptors on some cells may
fit onlywith ether molecules, otherswith molecules
of bleach. The molecules must dissolve in the
mucus before they can stimulate the receptors.
The sensitivity ofthe system is remarkable; as few
as four molecules can give a recognizable smell.
S
L o s s o f s e n s e o f s m e l l m a y r e s u l t f r o m i n -
f l a m m a t i o n o f t h e n a s a l m e m b r a n e , a s i n
a c o m m o n
c o ld ;
c i g a r e t t e
s m o k in g ;
h y p e r -
t r o p h i c
rh in itis ,
i n
w h i c h
a
t h i c k e n e d
n a s a l m e m b r a n e o b s c u r e s o l f a c t o r y n e r v e
e n d i n g s ; a t r o p h i c r h i n i t i s , i n w h i c h t h e
n e r v e s w a s t e a w a y ; h e a d i n j u r y t h a t t e a r s
t h e n e r v e s ; o r a t u m o u r o f t h e
m e n in g e s
( m e m b r a n e s
c o v e r i n g
t h e
b r a i n )
o r
n a s o p h a r y n x ( u p p e r p a r t o f t h e t h r o a t ) .
T h e p e r c e p t i o n o f i l l u s o r y , u n p l e a s a n t
o d o u r s m a y b e a f e a t u r e o f
d e p r e s s io n
,
s c h iz o p h re n ia ,
s o m e f o r m s o f
e p ile p s y ,
o r
a l c o h o l w i t h d r a w a l .
s m e llin g s a lt s
A p r e p a r a t i o n o f
a m m o n ia
t h a t w a s o n c e
u s e d t o r e v i v e a p e r s o n w h o f e l t f a i n t .
S m ith ’s fra c tu re
A b r e a k t h a t o c c u r s i n t h e l o w e r p a r t o f
t h e r a d i u s ( o n e o f t h e t w o b o n e s o f t h e
f o r e a r m ) i n w h i c h t h e f r a g m e n t u n d e r -
n e a t h t h e f r a c t u r e i s d i s p l a c e d f o r w a r d s .
T h e w r i s t j o i n t i s d e f o r m e d , b u t i n c o n -
t r a s t t o a
C o lle s ’ fra c tu re
( a t y p e o f w r i s t
f r a c t u r e ) , t h e h a n d a n d w r i s t a r e m o v e d
f o r w a r d o f t h e i r n o r m a l p o s i t i o n . T h i s
t y p e o f f r a c t u r e u s u a l l y r e s u l t s f r o m a
f a l l o n t o t h e b a c k o f t h e h a n d , w h i c h
c a u s e d b e n d i n g o f t h e w r i s t j o i n t f o r -
w a r d s s o t h a t i t b r e a k s .
s m o k in g
I n h a l a t i o n o f
to b a c c o
s m o k e f r o m c i g a -
r e t t e s ,
c i g a r s ,
o r
p i p e s .
M
o r e
t h a n
1 0 0 , 0 0 0
d e a t h s p e r y e a r i n t h e U K a r e
a t t r i b u t e d t o s m o k i n g . I t c a u s e s s e r i o u s
d i s e a s e s
s u c h a s
lu n g c a n c e r, b ro n c h it is ,
e m p h y se m a , c o ro n a ry a rte ry d ise a se ,
a n d
p e rip h e ra l v a s c u la r d is e a se ,
a n d
a l s o
i n -
c r e a s e s t h e r i s k o f
m o u th c a n c e r
,
lip c a n c e r
,
a n d t h r o a t c a n c e r ( s e e
p h a ryn x, c a n c e r o f ).
699
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