Chapter 48: The Nervous System Nervous Systems in General Simple animals like hydra have a nervous system arranged in a diffuse network called nerve netsThere is no centralization (no part of the network is any more important than any other part) More complex animals have nerve nets as well as nerves, which are bundles of fiberlike extensions of neurons. Sea stars, for example, have a central nerve ring tat has nerves that extend out to nerve nets in each arm The most complex animals have a Central Nervous System (CNS) usually located at the anterior.The development of this primary, controlling part of the nervous system is called cephalization Nerves that connect the CNS with the rest of the body are part of the peripheralnervous system (PNS) The Brain and Spinal Cord in humans is an example of a CNS Information Processing In general there are three stages of information processing by nervous systems 1. Sensory Input: sensory neurons transmit information from external stimuli (light, sound, touch, heat, smell, and taste) and internal conditions (blood pressure, blood CO2 level, and muscle tension, for example). This info is sent to the CNS for processing.2. Integration: In the CNS, interneuron analyze and interpret the sensory input taking into account the immediate context and what happened in the past. The CNS the decides how to respond. 3. Motor Output: leaves the CNS via motor neurons to effector cells which can be either muscle cells or endocrine glands, which then do something (move or release hormones, for example) Neuron Structure The ability of neurons to receive and transmit information depends on their elaborate structure.• The main part of a neuron is the cell body.o usually contains the nucleus and other organelles• Dentrites are branched extensions of the cell body that receive signals from other neurons.o They are typically more numerous and shorter than axons • A neuron typically has only one Axon, and extension of the cell body that transmits signals to other cells. o Axons can be a meter long or more o the region where the axon meets the cell body is called the axon hillockit is here where signals that travel down the axon are typically generatedo Axons are usually covered in sheaths of a protein called myelin These function as insulation for electrical signals o The ends of axons usually branch and end with a synaptic terminalThis is where signals pass from one cell to the next via specialized chemicals called neurotransmittersAction Potentials The signal that moves along an axon in a neuron is called an action potential • This is an all-or-nothing response to a stimulus that passes a certain threshold o If the stimulus is less than the threshold, then there is no response o If the stimulus is just a little bit higher than the threshold, the neuron’s response is the same as if the stimulus was tremendously higher than the threshold (this is the all-or-nothing response) • The neuron cell has an electric potential when it is not conducting signals; this is called the resting potential and it is generally around -60 to -80 mV o It is negative because of the action of the Na+/K+ Pump that transports Na+out of the cell and K+ inside the cell o The details of how this pump generates a negative resting potential in the neuron will be saved for a later course, however, the details are in the textbook if you are interested • When a stimulus is strong enough to bring the cell’s potential above a threshold, the neuron cell rapidly depolarizes by opening voltage gated ion channels that allow Na+ to rush into the cell. o The cells potential becomes briefly positive, generating an electrical pulse that regenerates itself as it moves from the cell body along the axon a wave of depolarization along the axon keeps it moving and strongCommunication between Neurons When an action potential reaches the branched ends of an axon (terminals) it stops there. Signals typically transmit between neuron via chemical signals called neurotransmitters. Some neurons can transmit electrical signals to other neurons, but most don’t• The area where a chemical signal is transmitted between neurons is called a chemical synapse• The sending neuron is called the presynaptic neuron and the receiving neuron is called the postsynaptic neuron • The terminals of the presynaptic neuron can send signals to the postsynaptic cell body or its dentrites. • When a signal (action potential) reaches the presynaptic terminal, it depolarizes the plasma membrane, opening voltage-gates Ca2+ channels.o An influx of Ca causes vesicles in the presynaptic terminal to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft (the area between the presynaptic terminal and the post synaptic membrane) o Neurotransmitters bind to receptors on ligand-gated ion channels in the postsynaptic membrane, opening the channels o Na+ and K+ diffuse through the channels into the post synaptic membrane o If the depolarization in the postsynaptic neuron is higher than a threshold, a new action potential is generated and the signal moves on o Neurotransmitters release from the receptors and the channels close o Neurotransmitters then diffuse out of the synaptic cleft or are degraded by enzymes The Brain The brain is a very complex organ that has many specialized functions. The major parts of the brain are o Brainstem “lower brain” (evolutionarily older part of the brain) functions in automatic and homeostatic mechanisms coordinates conduction of information to “higher” brain centers o CerebellumFunctions in coordination and error checking during motor, cognitive, and perceptual activities Thinking, consciousness and integrated sensory awareness of surroundingso DiencephalonIncludes the pineal gland, the hypothalamus, and the thalamus Thalamus is the input center for information going to the cerebellum and the output center for information leaving the cerebrum • Also functions in emotion and arousal Hypothalamus is the major endocrine gland and functions in sexual behavior, fight-or-flight response, and pleasure. o Cerebrum Divided into two hemispheres (the left and right) and that some of the specialized functions of one hemisphere are not processed in the other.Largest part of the brain Cerebral Cortex is the outermost part of the brain ? Please know the four general regions of the cerebral cortex and the major functions that are processed there. o Frontal Lobe