Brain of vertebrates

1. By- Sanju Sah
St. Xavier’s college, Maitighar, Kathmandu
Department of Microbiology
Comparative Anatomy of
Vertebrate Brain

3. Development of Brain
 The anterior end of embryonic
ectodermal neural tube is
thickened known as
‘Encephalon’/embryonic brain.
 Encephalon undergoes differential
growth and two constrictions
appear dividing it into a linear
series of three primary cerebral
vesicles:
- Prosencephalon (fore brain),
- Mesencephalon (mid brain) and
- Rhombencephalon (hind brain).

4. Development of Brain
Prosencephalon:
 Prosencephalon is
divided into anterior
Telencephalon and
posterior diencephalon.
 Telencephalon grows
and forms paired cerebral
hemispheres.
 From anteroventral part
of telencephalon paired
olfactory lobes
(Rhinencephalon) grow
anteriorly.
 Cerebral hemispheres
enclose lateral
ventricles (1st and 2nd )

5.  Diencephalon has
upper epithalamus,
middle thalamus
and lower
hypothalamus.
 Dorsally ,
diencephalon gives
out unpaired parietal
body and pineal
body.
 Ventrally
hypothalamus forms
infundibulum that
fuses with pituitary
gland.
 Diencephalon

6. Development of Brain
Mesencephalon:
 Mesencephalon has thick
roof forming two optic
lobes (4 in mammals)
and two longitudinal bands
of nerve fibres called
cerebral peduncles or
crura cerebri.

7. Rhombencephalon:
- Anterior dorsal
part forms
metencephalon or
cerebellum.
- Remaining part
forms myelencephalon
or medulla oblongata.
 In higher vertebrates
(mammals) the floor of
metencephalon
thickens due to the
tracts of many nerve
fibres and forms
distinct pons varolii.
Rhombencephalon

8. Development of Brain
Generalized Pattern of Vertebrate Brain
Foramen of
Monro
Iter or
Cerebral
aqueduct

9. Development of Brain
Divisions Subdivisions Parts Cavity
PROSENCEPHALON 1. Telencephalon Rhinencephalon Rhinocoel
Cerebral
Hemispheres
Lateral
ventricles (1st
and 2nd)
2. Diencephalon Epithalamus III ventricle
Thalamus
Hypothalamus
MESENCEPHALON - Optic lobes & Crura
Cerebri
RHOMBENCEPHAL
ON
1. Metencephalo
n
Cerebellum IV ventricle
2.
Myelencephalon
Medulla Oblongata

11. Neural
Plate
Neural
Tube

12. Comparative Account of Vertebrate Brain
a. Cyclostomes:
 Brain is very primitive without
well marked subdivisions.
 Olfactory lobes are distinct.
 Cerebral hemispheres are small.
 Lateral ventricles are
rudimentary.
 Parietal and pineal bodies are
well developed.
 Two optic lobes are imperfectly
differentiated.
 Cerebellum is small transverse
dorsal band - sluggish animals.
 Medulla oblongata is very well
developed.

13. b. Fishes
i. Elasmobranchs/Chondrichthyes
 Olfactory organs are enormous, so olfactory lobes of brain are
proportionately large, attached to cerebrum by short olfactory
peduncle.
 Cerebrum is undivided.
 Diencephalon is narrow with well developed dorsal pineal body.
 Mid brain is very large with two oval optic lobes; centre for nervous
coordination.
 Cerebellum is especially large due to active swimming habit.
 To assist cerebellum in maintaining equilibrium, Restiform bodies
are present in antero-lateral angles of medulla oblongata.

14. ii.Teleosts/Osteichthy
es:
 Olfactory lobes are large
without olfactory
peduncles.
 Cerebral hemispheres
and diencephalon are
smaller.
 Optic lobes and
cerebellum are larger
than elasmobranch.
 The antero-lateral sides
of medulla oblongata
shows unusual bulging
or vagal lobes for taste.

15. c. Amphibians:
 Forebrain is large. Olfactory lobes are
smaller but cerebral hemispheres
are larger in accordance with more
complex activities of locomotion,
hibernation, breeding etc.
 Cerebral hemispheres are separated
by a median longitudinal fissure.
 Diencephalon is short with vestigial
pineal body.
 Mid brain is small but well developed
forming two large optic lobes
(corpora digemina). Optic lobes are
the probable dominant coordinating
centre in amphibian brain.
 Cerebellum is poorly developed with
just a transverse band on dorsal side.
 Medulla oblongata is short.

16. d. Reptilians:
 Reptilian brain shows advancement
in size and proportions over that of
amphibians because of complete
terrestrial mode of life.
 Cerebral hemispheres are larger
enclosing lateral ventricles.
 Two long olfactory lobes are
connected to cerebral hemispheres
which are larger than that of
amphibians.
 Diencephalon is small and rounded.
 Mid brain has large optic lobes, may
be the important coordinating centre.
 Cerebellum is small, simple and
some what pear shaped; larger than
that of amphibians.

17. e. Birds
 Brain of birds is relatively larger but short,
broad and rounded in shape.
 Olfactory bulbs are small and
degenerate due to poor sense of smell.
 Cerebral hemispheres are very large and
smooth; responsible for intelligent
behavior and to control reflex behavior.
 Diencephalon is covered by cerebrum
and cerebellum; bears a small pineal
body.
 Mid brain is highly developed with large
rounded optic lobes. Optic lobes are
very well developed correlated with
keen sight and are laterally placed.
• Cerebellum is large with several superficial folds (flocculi)
extending forwards. It is greatly enlarged due to many activities
involving muscular coordination and equilibrium such as flight
and perching.

18. f. Mammals:
 Brain is proportionately larger.
 Cerebral hemispheres are immense,
projecting forward above olfactory lobes
and behind above the diencephalon and
mid brain.
 In Prototheria: Cerebral hemispheres
are small and smooth.
 In Metatheria: They are larger but
smooth.
 In Eutheria: they are greatly enlarged
with convoluted surface. The elevations
of convolutions are called ‘Gyri’ and the
depressions are called ‘Sulci’.
• The convolution/folding increases surface area of cerebral cortex
resulting in greater intelligence.
• Two cerebral hemispheres are separated by median longitudinal
fissure and are joined internally by transverse band of fibres –
Corpus callosum.

19. Mammals (Contd.)
 Olfactory lobes are relatively smaller.
 Diencephalon and mid brain are also
covered by cerebrum.
 Mid brain bears 4 optic lobes (corpora
quadrigemina).
 Cerebellum is also very large, highly
folded bearing ‘folia’.
 Pons varolii (present only in
mammals) bears crossing fibres
connecting opposite sides of cerebrum
and cerebellum.
 Medulla oblongata lies ventrally and is
much thickened. It bears involuntary
centres controlling respiration, heart
rate, diameter of blood vessels etc.

20. Cranial Nerves
 Anamniotes: 10 pairs
 Amniotes: 12 pairs