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Chapter 7

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Human Biology

MEERA MEHTA Chapter 7: Functional Organization of the Nervous System - Ancient Greeks and Egyptians thought brain to be unimportant; Egyptians tossed brain away while preparing mummy + Aristotle thought brain acted as cooling system for soul, which he said was located in heart. - Galen concluded that mental activity occurred within the brain - Until 17 century, physiologists + anatomists thought brain worked as whole and didn’t have regional specialization. - The case of Phineas Gage helped anatomists assign functions to various parts of the brain and provided insight to how the brain works. - London’s streets are not laid out in a grid pattern, so London taxi drivers really need to know how to navigate. They have an enlarged hippocampus, a region of the brain involved in spatial relationships and memory. - MRI technology o MRI machine emits powerful magnetic field that can be directed at the brain (or at other parts of the body). This magnetic field causes the H atoms in water molecule to realign with magnetic field. o Sends out a pulse of radio energy, which briefly knocks the H atoms out of alignment. o As H atoms return to aligned position, emit energy which MRI machine can detect and interpret.  Because the amount of water (and H atoms) varies in different structures of the brain, an MRI machine can provide detailed brain images. - fMRI o Parts of the brain that are working harder need more oxygen. MRI signal is a little different between oxygenated and deoxygenated blood, so the MRI signal changes as a subject uses different parts of the brain. o Can observe changes in blood flow (and thus changes in activity) in different parts of brain. Overview - We have: o Afferent (sensory) neurons— (PNS) detect incoming stimuli.  Stimulus reaches sensory receptors, which convert stimulus into signal; afferent neuron then conducts these signals in form of Aps to integrating centres like brain or ganglia. o Interneurons— (CNS) make up integrating centres in brain that form synaptic connections among neurons to integrate info. o Efferent neurons— (PNS) send signal to effector organs including skeletal muscles, glands, internal organs.  Cause change in behaviour. Organization of Nervous Systems - Most nervous systems are organized into 3 functional divisions: afferent sensory division, integrating centres, efferent division. o Only cnidarians don’t have this general plan. - Cnidarians are radially symmetrical animals with nervous systems that are interconnected into a nerve net with neurons distributed throughout the body. o Neurons aren’t specialized; can function as sensory/interneurons/efferent neurons and can communicate synaptically at several points along their length. o Neurons often from en passant synapses, allowing info to be passed in either direction across the synapse. o Many cnidarian neurons are functionally bipolar in that any stimulus at any point on the organism triggers an impulse that radiates out from the stimulus sit in every direction. o Apparent simplicity of cnidarian nerve net hides substantial complexities. - In some species, the nerve net is broken down into several pathways with characteristic conduction speeds that control different behavioural responses. In addition, in some species, neurons are concentrated around the oral opening or into clusters in other locations. o These groupings of neurons may act as integrating centres, providing additional layers of functional complexity to the nervous system. - Many species of cnidarians can also generate APs and are connected via gap junctions, adding another layer of complexity. EVOLUTION OF NERVOUS SYSTEMS - Most animals are bilaterally symmetrical—have anterior/posterior sides + right/left sides - Sense organs in bilaterally symmetrical animals tend to be concentrated at the anterior end of the body, close to the mouth. Have complex groupings of neurons. MEERA MEHTA o Typically have 1+ ganglia—groupings of neuronal cell bodies connected by synapses.  Ganglia function as integrating centres for the nervous system.  In many species, ganglia clustered together to form brain  Within brain, groupings of neuronal cell bodies are called nuclei, which are the functional equivalent of ganglia, and groupings of neuronal axons are called tracts.  Outside of the integrating centres, axons of afferent/ efferent neurons organized into nerves— functional equivalent of tracts in integrating centres. - Structure of a Nerve o Bundles of myelinated and unmyelinated axons enclosed in several layers of connective tissue  Endoneurium – wraps each axon  Perineurium – wraps a bundle (fascicle) of axons  Epineurium – wraps the entire nerve  Mixed nerves – contain both afferent and efferent neurons  Each neuron is either afferent (sensory) or efferent (motor) Bilaterally symmetrical animals exhibit cephalization - Cephalization—pattern of locating sense organ and nervous integrating centres at the anterior end of the body. o more apparent in more complex nervous systems. o Most species of bilaterally symmetrical invertebrates have a well-developed brain, several ganglia and 1+ nerve cords. o In invertebrates, bundles of axons that connect ganglia or run between a ganglion and the brain are called connectives or commissures. - Nervous system complexity varies among molluscs but most species have a series of large ganglia including the cerebral ganglia, the buccal ganglia and the pedal ganglia. - Octopus had brain that’s much larger relative to its body size than the brain of a fish or a reptile, suggesting the possibility of substantial intelligence. o Another important integrating centre in octopus: each arm has a large ganglion that controls arm movements independently of brain. - Echinoderms are one of few exceptions to the general trend of increasing cephalization in animals. o Radially symmetrical animals o Lack an obvious brain; have series of ganglia and several nerve rings. o Lost bilateral symmetry + cephalization during transition to radially symmetrical body plan o Have bilaterally symmetrical larvae that become radially symmetrical during metamorphosis to adult form. - In general, organisms with more complex nervous systems have more neurons than organisms with less complex nervous systems. o However, total number of neurons is not always larger in species with more complex integrating centres. The vertebrate central nervous system is enclosed in a protective covering - Vertebrates among most highly cephalized organisms + unique in possessing hollow dorsal nerve cordm rather than solid ventral nerve cord. MEERA MEHTA - It’s been suggested that protostome (worm, mollusk, arthropod) and deuterostome (vertebrate, echinoderm) nervous systems evolved independently from common ancestor with a nervous system similar to that of flatworms. - Portion of vertebrate nervous system encased in cartilaginous or bony covering. o This portion is the CNS, and is composed of the brain (within skull) and spinal cord (within spine) - Remainder of nervous system is found throughout body called PNS The cranial and spinal nerves form synapses in the CNS - Cranial nerves—exit directly from braincase o Often refer to 12 pairs of cranial nerves (labeled with Roman numerals), but most vertebrates have 13 pairs o Some bring in afferent into from sense organs; others send efferent signals to effector organs. Others are a combo of both afferent and efferent. - Spin
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