Brain Dissection Part 1

Brain Dissection Orientation: Terms (different than humans) Dorsal Cranial Caudal Ventral External Observations PART 1 Unless the directions specifically tell you to do so! Preliminary observations Measure the brain from the cranial to caudal end. Rinse sheep brain and pat dry. Weigh the brain and calculate size of the donor animal. (Sheep brain is ~ 0.5% of body weight; human brain is ~2% body weight) Record your observations. The Meninges “Mater” • Examine the dura mater*, the • The arachnoid mater, another toughest layer of the layer of the meninges, is an meninges covering and ultra-thin spongy layer that protecting the brain. Note may appear as a delicate how the dura mater wraps “webby” material spanning around the structures of the the fissures or holding the brain dura mater to the brain. • The meningeal coverings are • The pia mater, another layer of named descriptively: the meninges, is the thin – Mater = mother membrane that adheres tightly – Dura = tough to the surface of the brain, – Arachnoid = spider dipping into each sulcus. If – Pia = gentle you pull away a small amount of the pia mater, you can see the depth of the sulcus. Optic chiasma Pituitary gland Olfactory bulb This image shows the ventral surface of the sheep's brain; some of dura mater has been removed. The pituitary gland and the optic chiasma (part of the cranial nerves) are still intact. (A = pituitary gland, B = optic chiasma, C = olfactory bulb) Using a ruler approximate the sizes of the lobes • You can approximate their volume by measuring length, width and height. Parietal Lobe Occipital Lobe Frontal Lobe Temporal Lobe • Place the brain dorsal side up in the dissecting pan. Using scissors, gently cut through the dura mater between the cerebellum and the temporal lobe with the scissors pointing down toward the spinal cord. • Once the dura mater is free you should be able to carefully peel back the rest of the dura mater from the dorsal and lateral surface. Leave it in place on the ventral surface (SEE NEXT SLIDE FOR IMAGE). • You can now examine the dorsal and lateral structures of the brain. • The largest structure in the dorsal view is the cerebral cortex, the outer layer of the cerebrum. The cortex is folded upon itself, with a number of gyri (hills; singular = gyrus) and sulci (valleys; singular = sulcus). Leave dura mater in place as shown outlined above. • Locate the four lobes of the cerebral cortex: • • • • Frontal Parietal Temporal Occipital Sheep brain dorsal side; dura mater removed • Locate the dividing lines between these lobes: the longitudinal fissure separates the 2 hemispheres of the cerebral cortex, the central sulcus separates frontal lobe from parietal lobe; the lateral sulcus separates parietal and temporal lobes (not shown in this photo). • Take a picture and label it. A – longitudinal fissure B - central sulcus C - frontal lobe D - parietal lobe E - occipital lobe F - temporal lobe G – cerebellum H – vermis of cerebellum I - medulla • Anterior to the central sulcus is the primary motor cortex. Posterior to the central sulcus is the primary sensory cortex. These correspond to the areas in the human brain. • Identify the cerebellum (little brain). In the human brain, this structure is divided longitudinally, whereas you will notice that in the sheep it is not. The fissures are also oriented differently in the sheep brain. This lateral view shows the parietal lobe and cerebellum. If you were not careful when you removed the dura mater you may have accidentally pulled the entire cerebellum away from the brain. Note the transverse fissure, which separates the cerebrum from the cerebellum. The convolutions of the brain are also visible as bumps (gyri) and grooves (sulci). • Gently separate the 2 lobes of the cerebral cortex. DO NOT CUT! • Answer 4 in part 1 by probing the longitudinal fissure between the two hemispheres be careful to not pierce the soft tissue. • Place your finger on the probe where the top meets the brain, withdrawal the probe and measure the distance between the end of the probe and the location you marked on the probe with your finger Parietal Temporal Occipital Cerebellum Frontal lobe Medulla Olfactory bulb & tract Optic chiasma Lateral View Sheep brain, lateral view; most of dura mater has been removed You can also push the cerebellum down to show the pineal body from the side. It appears as a small, oval, slightly pink protrusion in the midline. The pineal body secretes melatonin, a hormone important in establishing day/night cycles, sleep induction and seasonal behavior (e.g., hibernation).