Topic 5 – The Nervous System, Brain, Hormones, and BehaviorWhat is an animal? Taken together these characteristics define “animal”Multicellular EukaryoticHeterotrophicMobility at some point in life (Nerves and muscles)Sexual ReproductionEmbryonic Development including Hox genesNervous system – Sensory, Interneurons, and Motor neuronsElectric signaling (3 types of membrane potentials):Resting potential – balance of electrostatic and diffusion forces involving Na+, K+, Cl-, and Anions- (large, negatively charged molecules, usually proteins) on either side of the phospholipid bilayer. Ions CANNOT pass directly through the phospholipid bilayer but movement is controlled by ion channels which can prevent or, by changing conformation, enable the passage of ions. At rest, when there is no net flow of ions into or out of the neuron, the charge on the inside relative to the outside is generally around -70mV.Graded potential – These changes in the membrane potential are due to opening of ligand-gated Na+ channels in the dendrites and soma. The degree to which depolarization occurs depends on the number and timing of Na+ channel opening.Action potential – If a depolarization reaches “threshold” and this is detected by the voltage-gated Na+ channels in the axon hillock, an action potential will be generated. Although both channels allow the influx of Na+, the Na+ channels are different from the channels in the dendrites and soma. Na+ floods intothe cell, depolarizing the membrane potential past 0 so that the intracellular space is positive relative to the extracellular space. At the peak of the action potential, Na+ channels close and K+ channels open. K+ flows out of the cell and restores the neuron membrane potential to a negative charge. The potential actually overshootsthe resting potential, thus hyperpolarizing the cell. The proper balance of Na+ andK+ is restored in part through the action of the Na+/ K+ pump which is an active transport system requiring ATP to operate.Chemical signaling (actions at the synapse): Voltage-gated Ca++ channels open at the end of the axon allowing Ca++to flow into the cell. Through a series of biochemical reactions, vesicles containing neurotransmitter substances release their contents into the synaptic cleft. Receptor sites (proteins) on the post-synaptic side are channels that change conformation when attached to neurotransmitters allowing the entrance of ions (usually Na+) which causes a graded potential. The neurotransmitters are functioning as ligands to open the channels in the dendrites and/or soma. Note thatthe neurotransmitters do NOT enter the post-synaptic neuron! Mental illnesses and drugs commonly used for recreational purposes usually affect normal functioning of synaptic transmission. For instance, cocaine blocks the dopamine reuptake mechanism in the pre-synaptic neuron, and curare blocks the ACh receptors on the post-synaptic neuron in the neuro-muscular junction.Reflex Arcs – sensory neurons stimulate and motor neurons respond to behaviorally remove or change a body part away from dangerous situations without the intervention of the brain. Examples include nocturnal insects reflexively changing their flight paths when detecting ultrasound to avoid becoming a bat’s prey. Habituation, or a reduced reflex response, occurs at the level of the neuron. So neural responses (and behaviors) can be modified due to repeated exposure to a stimulus. Example: the gill withdrawal reflex of the Aplysia.Evolution animal nervous systemsRadially symmetrical animals, such as sea anemones have nerve nets.ALL terrestrial animals are bilaterally symmetrical and show a progression of increased complexity from flatworms to vertebrates which is proposed as the hypothetical evolutionary progression of the nervous system. By examining extant animal species, biologists can suggest possible trends in the evolution of animal nervous systems.BrainsThe RELATIVE proportions of sensory and motor processing areas of animal brains usually reflects the importance of that sensory modality or motor movement to the animal’s fitness (survival and reproduction). For example, sharks rely on their sense of smell and this is reflected by relatively large olfactory lobes compared to the rest of the brain. Humans and other primates have a considerable portion of their brains involved in visual processing of information.Subcortical regions of the brain that have received particular attention from animal behavior researchers are: hypothalamus (behavioral involvement include: control of food and water intake, hunger and thirst; control of sexual behavior and reproduction; control of daily cycles in physiological state and behavior also known as circadian rhythm; mediation of emotional responses), amygdala (the integrative center for emotions, emotional behavior, and motivation), and hippocampus (important role in the consolidation of new memories about experienced events i.e., episodic or autobiographical memory AND a brain system responsible for spatial memory and navigation)The Cortex in mammals is 6 cell layers thick in all areas although the neurons lookdifferent and synapse on each other differently in the different parts of the cortex. Different areas of the cortex are responsible for different functions including sensory processing and organizing appropriate movements which we see as behavior. Although a pseudoscience, the phrenologists were the first people to recognize that the cortex has a variety of different functions.Auditory transductionOne major question in behavior, specifically sensory physiology, is how neurons transduce information from the environment into the “language” of the nervous system which is action potentials.The ear is an example of a mechanosensory mechanism to transduce air vibrationsinto action potentials. In a mammal, transduction occurs in the cochlea, specifically through the mechanical interaction of the basilar membrane, hair cells (where transduction actually occurs), and the tectorial membrane. Simply moving the stereocilia in a particular direction causes K+ channels to open mechanically, thus depolarizing the hair cell.Lateral lines in fishes have “hair cells” which detect vibrations in the water and are anatomically very similar to the hair cells in the cochlea. This is considered a good example of convergent evolution.Animals tend to be able to detect vibrations in the air or water that are