I've wondered about sense of smell for a long time.

CTenaLouise discussed zinc. And that has a lot to do with it, but not everything. But a lot . . . so if there is any chance you're zinc-deficient, try taking some. However, Nature loves a zinc:copper ratio of about 10:1 (10 mgs zinc, 1 mg copper) so don't go overboard on the zinc and forget the copper.

This discussion reminded me of a brief comment I saw. Here it is:

E-MOVE reports from the 57th Annual Meeting of the American Academy of Neurology, 9-16 April 2005, Miami.
Poster, session and page numbers are from Neurology 2005;64(suppl 1).

Olfaction is normal in essential tremor and can be used to distinguish it from Parkinson’s disease
M Shah, L Findley, N Muhammed, C Hawkes
S27.001, A261

Two forms of smell tests were administered to 59 ET patients, 65 PD patients, and 74 controls. Tests used were the 40-odorant UPSIT and olfactory evoked potential in response to hydrogen sulfide.

Mean UPSIT scores were 33 for controls, 32 for ET patients, and 18 for PD patients.

OEP values were normal in all but 4 ET patients (which included at least one patient who didn’t receive testing). Forty percent of PD patients had no recordable OEP, and the remainder had significantly prolonged latency with normal amplitude.

“A normosmic patient with tremor is more likely to have ET than PD,” the authors conclude.

- - - - - - - - -

Also, here's an abstract from a review article.

Adv Otorhinolaryngol. 2006;63:133-51.
Olfaction in neurodegenerative disorder.
Hawkes C.
Essex Centre for Neuroscience, Oldchurch Hospital, Romford, UK.

There has been gradual increase of interest in olfactory dysfunction since it was realised that anosmia was a common feature of idiopathic Parkinson's disease (IPD) and Alzheimer-type dementia.

It is an intriguing observation that a premonitory sign of a disorder hitherto regarded as one of movement or cognition may be that of disturbed sense of smell.

In this review of aging, IPD, parkinsonian syndromes, tremor, Alzheimer's disease (AD), motor neuron disease (MND), Huntington's chorea (HC) and inherited ataxia, the following observations are made:

(1) olfactory senescence starts at about the age of 36 years in both [genders] and accelerates with advancing years, involving pleasant odours preferentially;

(2) olfactory dysfunction is near-universal, early and often severe in IPD and AD developing before any movement or cognitive disorder;

(3) normal smell identification in IPD is rare and should prompt review of diagnosis unless the patient is female with tremor-dominant disease;

(4) anosmia in suspected progressive supranuclear palsy and corticobasal degeneration is atypical and should likewise provoke diagnostic review;

(5) subjects with hyposmia and one ApoE4 allele have an approximate 5-fold increased risk of later AD;

(6) impaired sense of smell may be seen in some patients at 50% risk of parkinsonism, and possibly in patients with unexplained hyposmia;

(7) smell testing in HC and MND where abnormality may be found is not likely to be of clinical value, and

(8) biopsy of olfactory nasal neurons reveals non-specific changes in IPD and AD and at present will not aid diagnosis.

PMID: 16733338 [PubMed - in process]

- - - - - - - - - - -

And Harvard's "Massachussetts Eye and Ear Infirmary" says:

Our sense of smell begins when an odor enters the nose and reaches the special smell (olfactory) nerves in the roof of the nasal cavity.

The nerves then send signals to the brain where we recognize a smell. These nerves can be damaged, but luckily as in all animals, these nerves can be replaced with new ones.

When there is an interruption in the ability of an odorant to travel from the outside to inside the nose and from the inside of the nose to the smell nerves of the nasal cavity and from the smell nerves to the brain, a smell disorder occurs.

We call a decreased sense of smell hyposmia, and a total loss of smell anosmia.

Phantosmia is the term given when someone thinks they can smell something when no one else in the room smells the same thing. This can occasionally indicate an infection.

http://www.meei.harvard.edu/patient/tasteandsmell.php