Chapter 49 Nervous Systems 49 1 NERVOUS SYSTEMS CONSIST OF CIRCUITS OF NEURONS AND SUPPORTING CELLS The ability to sense and react originated billions of years ago in prokaryotes enhancing survival and reproductive success in changing environments Evolution modification of simple recognition and response processes provided a basis for communication between cells in an animal body By the Cambrian explosion animals were able to sense and respond Cnidarians have a diffuse nerve net which controls the contraction and expansion of the gastro vascular cavity Complex animals have NERVES which are bundles of axons from various neurons Animals with bilateral symmetry have a more specialized nervous system and this reflects CEPHALIZATION a trend towards clustering sensory neurons towards the anterior end of the body A major evolutionary trend in animals is that more active animals have a more sophisticated nervous system while sessile and slow moving animals have relatively simple nervous systems Invertebrates have ventral nerve cords containing Ganglia which are segmentally arranged clusters of neurons Vertebrates have the brain and spinal cord form the CNS while the nerves and ganglia form the PNS GLIA The nervous systems of vertebrates and most invertebrates also include glial cells or glias which are cells that necessary for development Schwann Cells are glia that produces myelin sheaths in the PNS and Oligodendrocytes are glia that forms myelin sheaths in the CNS Glia called ASTROCYTES participates in formation of the blood brain barrier that restricts entry of most substances from the blood into the CNS ORGANIZATION OF THE VERTEBRATE NERVOUS SYSTEM During embryonic development in vertebrates the central nervous system develops from the hollow dorsal nerve cord a hallmark of chordates The cavity of the nerve cord gives rise to the CENTRAL CANAL of the spinal cord as well as the ventricles of the brain which fill with CEREBROSPINAL FLUID Cerebrospinal fluid forms from filtered arterial blood and supplies nutrients and hormones to the CNS while also carrying away wastes The Brain and Spinal Cord also have GRAY MATTER made up of neuron cell bodies and WHITE MATTER Bundled Axons White Matter links the CNS to PNS and forms the outer layer of the Spinal Cord and interior of the Brian In vertebrates the spinal cord rungs lengthwise inside the vertebral column and produces reflexes an automatic response to stimuli THE PERIPHERAL NERVOUS SYSTEM The PNS transmits information to and from the CNS and plays a large role in regulating both an animal s movement and its internal environment AFFERENT NEURONS carry sensory information from the PNS to the CNS EFFERENT NEURONS carry away sensory information from the CNS to the PNS The PNS has 2 components the Motor System which controls the skeletal muscles and the AUTONOMIC NERVOUS SYSTEM which is involuntary and is made up of 3 divisions SYMPATHETIC PARASYMPATHETIC AND THE ENTERIC The Autonomic Nervous system controls smooth muscles cardiac muscles and glands The Sympathetic Division and Parasympathetic Division counteract each other with the sympathetic division corresponding to arousal and energy generation while the parasympathetic division promotes calming Both these divisions have a pathway of information involving pre ganglionic and a postganglionic neuron with the preganglionic neurons releasing acetylcholine as a neurotransmitter However the postganglionic neurons in the parasympathetic division release acetylcholine while those in the sympathetic division release norepinephrine and it is this difference that allows them to counteract each other 49 2 THE VERTEBRATE BRAIN IS REGIONALLY SPECIALIZED Each region of the brain is specialized in function The FOREBRAIN contains the olfactory bulb and cerebrum which function in smell sleep learning and complex processing The Midbrain is located centrally in the brain and coordinates routing of sensory input The HINDBRAIN part of which forms the cerebellum controls involuntary activities such as blood circulation and coordinates motor activities such as locomotion Evolution has altered the brain regions and sizes for various organisms For example active fish tend to have a much larger cerebellum functions in motor movement then non active fish AROUSAL AND SLEEP The Brainstem and Cerebrum regulates states of mental alertness and sleepiness Arousal is a state of awareness of the external world while sleep is a state in which external stimuli are received but not consciously perceived Sleep is actually an active state and EEG reveals electrical activity called brain waves Sleep is hypothesized to function in consolidating learning and memory Furthermore Arousal and Sleep are controlled by the reticular formation a diffuse network formed primarily by neurons in the midbrain and pons These neurons control the timing of sleep periods BIOLOGICAL CLOCK REGULATION Cycles of sleep and wakefulness are an example of a circadian rhythm a daily cycle of biological activity The circadian rhythm relies on a Biological Clock which is a molecular mechanism that directs periodic gene expression and cellular activity The SUPRACHIASMATIC NUCLEUS SCN links the biological clock to environmental cycles of light and dark The SCN responds to sensory information form the eyes and the SCN acts a pacemaker to correlate with the length and time of day EMOTIONS The generation and experience of emotions depends on many brain structures such as amygdala hippocampus and parts of the thalamus which form the limbic system Generating emotions requires many interactions between these regions of the brain Emotional experiences can also be stored as memories The AMYGDALA an almond shaped mass of nuclei is the most important brain structure for memory 49 3 THE CEREBRAL CORTEX CONTROLS VOLUNTARY MOVEMENT AND COGNITIVE FUNCTIONS The Cerebrum is the region of the brain that is essential for language cognition memory consciousness and awareness of surroundings The Cerebrum is the largest structure in the human brain and the cerebrum exhibits regional specialization INFROMATION PROCESSING