Nervous System
Nervous System
Brain In Mammals:
Fore brain—includes olfactory lobes, cerebrum and Diencephalon.
Olfactory Lobes (Rhinencephalon)
- These are the pair of solid club shaped smaller structures present along the lower surface of cerebrum; each consists of an olfactory bulb and an olfactory tract to relay sense of smell from olfactory epithelium to temporal lobes of cerebrum.
Cerebrum (Telencephalon):
- The main and largest part of brain that has undergone largest degree of advancement during evolution consists of left and right Cerebral hemispheres which are extremely large and cover the diencepthalon and midbrain from dorsal side.
- Each cerebral hemisphere is divided dorsolaterally by Sylvian fissure into frontal and temporal lobes. Ventrally the hippocampal fissure divides it into outer lateral lobe and inner hippocampal lobe. Its anterior part is divided as rhinal lobe (olfactory tract) by rhinal fissure.
- The outer layer cerebral cortex of grey matter has large number of gyri and sulci on the surface to increase its area.
- The dorsal thick roof is called as pallium which in mammal is derived differently hence called as neopallium.
- On the roof of the telocoel the two cerebral hemispheres are connected by fibrous tract, the corpus callosum (characteristics of mammal). Its anterior free end is called genu while the posterior free end is splenium. The lateral strip of nerves connecting corpus callosum to the hippocampal lobe is fornix.
- On the floor of telocoel, anteriorly, there is a pair of strip of nerves corpus striatum connected by anterior commissure and posteriorly, a pair of hippocampi, connected by hippocampal commissure (or lyra).
Diencephalon:
- The small posterior part of fore brain that remains dorsally overlapped by cerebrum.
- Its roof is called epithalamus, having anterior choroid plexus (ACP) with blood vessels and on its dorsal surface there is epiphysis (or pineal body). In lower vertebrates there is parietal body or paraphysis also.
- The floor is hypothalamus with ventral hypophysis (or pituitary gland) attached through infundibulum which is less prominent in mammals.
- The thick lateral walls are the two optic thalami connected anteriorly by middle or soft commissure (massa intermedia) and a Habenular commissure in the antero-dorsal part of the diocoel.
- Thalamus is the key link of communication between the sensory inputs and the part of cortex that receives the major incoming signals. Thalamus determines the source of signals, evaluates their importance and integrates them before passing them to the cerebrum.
- In front of the hypophysis there is crossed tract of nerves called optic chiasma that connects the left optic lobe with right cerebrum and right optic lobe with left cerebrum. In frog only cross fibres are present while in mammals straight fibres are also present beside cross-fibres. Hence, both the optic lobes are connected with both the cerebral hemispheres.
Sub Topics of Nervous System
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Class VIII