Download Lesson 34 - Zoology, UBC

Lesson 34
Lesson Outline:
Nervous System Structure and Function
Central Nervous System
Brain
Embryonic Origin
Phylogenetic Origin/Differences
Form and Function
Telencephalon
Diencephalon
Mesencephalon
Metencephalon
Mylencephalon
Objectives:
References:
Chapter 16 : 387-428
Reading for Next Lesson:
All Chapters - Begin to Review for Final Exam!!!!
Nervous System Structure and Function
Central Nervous System
Brain
Embryonic Origin
The neural tube, neural crest and
notochord form just as we have
previously discussed.
Formation of prosencephalon
mesencephalon and
rhombencephalon (or forebrain,
midbrain and hindbrain).
At the anterior end of the
neural tube, prechordal
mesoderm induces the
swelling of the
prosencephalon, which in
turn induces the
formation of the
mesencephalon and
rhombencephalon.
These three regions subsequently differentiate to form the Telencephalon Diencephalon,
Mesencephalon, Metencephalon and Mylencephalon.
As the brain forms, the hollow centre of the neural tube becomes the ventricles of the
brain, which fill with the cerebrospinal fluid (CSF).
Formation of Meninges
The meninges are the membrane
that surrounds the brain. It is derived in
part from the neural crest.
In fishes there is a single
membrane, the meninx.
In terrestrial vertebrates there is
further development of the meninges into
two and subsequently three layers (the pia mater, arachnoid and dura mater).
Formation of CSF
The CSF is the fluid that flows through the ventricles of the brain, the central
canal of the spinal cord and between the brain and the meninges
The fluid is produced by filtration
from arterial blood in the choroid plexus,
and is reabsorbed ultimately by venous
sinuses. Because of the filtration, it lacks
blood cells and other large molecules. It has
some nutritive function but in terrestrial
vertebrates it also serves to cushion and
protect the nervous system.
Phylogenetic Origin/Differences
The extent to which the different parts of the brain are developed in different vertebrate
groups, including the sharks, reflects the relative importance of different sensory and
motor systems in the lifestyles of the species. The sizes of the sensory nuclei, motor
nuclei and integrating areas increase/decrease with the demands for information
processing required by a particular habitat or mode of life.
Form and Function
Telencephalon (Cerebral Hemispheres and Olfactory lobes)
Reception of olfactory information is one important role of the telencephalon.
Formation of Olfactory Organs
The olfactory system begins as a
pair of olfactory placodes,
thickenings of ectoderm that
invaginate toward the neural tube.
The lateral walls of each placode
form the olfactory epithelium
lining the nasal passages. Olfactory
sensory cells differentiate within
the epithelium and send axons to
meet the forming telencephalon.
These axons induce the
telencephalon to produce the olfactory bulb that is connected to the rest of the
telencephalon by the olfactory tract. The term olfactory nerve applies only to the very
short axons from the olfactory sensory cells.
The remainder of the telencephalon is composed of a dorsal pallium and a ventral
subpallium, which make up the cerebrum. These areas receive olfactory, auditory, lateral
line, somatosensory and visceral inputs relayed from other sites in the brain. They
process this information, integrate it and transmit responses to lower areas of the
brainstem. Thus, they indirectly control locomotion and other motor functions.
Diencephalon (Hypothalamus, Thalamus)
The diencephalon consists of three regions, the epithalamus, the hypothalamus, and the
thalamus.
The epithalamus includes the pineal gland which
serves as a third eye for monitoring photoperiod
and regulating both skin colour and biological
rhythms.
The hypothalamus in the floor of the
diencephalon houses a collection of nuclei that
regulate homeostasis, processes pertaining to
temperature, water balance, appetite, metabolism,
blood pressure, sexual behaviour alertness and
some aspects of emotional behaviour.
It controls the pituitary gland that is situated
immediately below it.
Formation of the Pituitary
The pituitary has two embryonic
sources, the infundibulum that is an
outgrowth of the floor of the
diencephalon and Rathke's pouch, which
is a diverticulum from the stomodeum.
These two structures fuse and the
infundibulum retains its connection to
the brain and becomes the neurohypophysis while Rathke's pouch becomes pinched off
from the stomnodeum and becomes the adenohypophysis
The thalamus receives sensory input indirectly from all parts of the body. Almost all
sensory inputs ultimately project here (except the olfactory tracts). This area integrates
the information and relays it to the cerebral cortex.
It receives direct input from the eyes.
Formation of Eyes
The eye is a composite structure formed from neural ectoderm, ectoderm and
mesenchyme.
The optic vesicles first appear as paired
outgrowths of the diencephalon. As the
optic vesicles approach the overlying
ectoderm, it thickens and becomes the optic
placode. This invaginates and forms the lens
primordium. It pinches off and settles into
the optic cup, which is the extension of the
optic vesicle. Mesenchyme surrounding the
developing eye condenses to produce the
coats of the eye.
Thus, the outer ectoderm gives rise to the eyelid, cornea and lens.
The optic vesicle forms the iris and the retina, which retains its connection to the brain as
the optic stalk. This carries the axons that project to the optic areas in the optic area of the
diencephalon and the optic tectum of the midbrain. This stalk or tract is the optic or
second cranial nerve.
The mesenchyme forms the choroid, and sclera.
Mesencephalon (Tectum and Tegmentum)
The roof of the midbrain is the tectum, which receives sensory information while the
floor is the tegmentum, which initiates motor output.
The tectum is divided into an optic tectum, receiving visual information, and a torus
semicircularis, receiving vestibular and lateral line input. In higher vertebrates, it also
receives auditory input.
Thus, the optic tectum receives direct input from the eyes as does the thalamus of the
diencephalon. While this
information is integrated
with other inputs into a
pattern of sensation in the
thalamus that project to the
cerebrum, it is integrated
into a spatial map in the
optic tectum.
The tectum also receives
indirect input from the
lateral line system, the
vestibular apparatus, the
cerebellum and the
olfactory system.
The tegmentum is an
integrating site and contains
nuclei for two of the cranial
nerves that control the
movements of the eye.
Metencephalon (Pons and Cerebellum)
The pons is a swelling in the floor of the rostral hindbrain, which is an important
crossroads for the flow of information, both up and down the brain. It houses the nuclei
for cranial nerves V-VII.
The cerebellum is also derived from the rostral hindbrain. This area modifies motor
output to help maintain equilibrium and to refine motor activity. . It is relatively large in
sharks, receiving extensive input from the lateral line sensory system.
Mylencephalon (Medulla)
The medulla oblongata has three major functions:
1) It contains centres for visceral auditory and proprioceptive functions including
respiration, circulation and digestion.
2) It is the major route through which ascending and descending fibre tracks must
run between the spinal cord and higher brain centres.
3) It houses the primary nuclei of many of the cranial nerves (VII through X in
sharks, VII to XII in mammals).
Formation of Otic Capsules
The vestibular system is a balancing organ that
arises as part of the lateral line system. It forms
from the otic placodes, which sink inwards and
differentiate to form hair cells, which are
modified neuromast cells.
These form the sensory cells in the semicircular
canals that respond to rotation or angular
acceleration.
They also form the sensory cells in the two
chambers at the base of the semicircular canals,
the sacculus and utirculus (the sensory cells are
the macula and otoliths), which respond to
changes in orientation.
Together these form the vestibular apparatus,
which conveys information about movement and
orientation.
Remember- in fishes this is an organ of
equilibrium. “Hearing” is accomplished by the
lateral line system.