Chapter 48: Nervous Systems
Command and Control Center
* The human brain contains an estimated 100 billion nerve cells, or neurons * Each neuron my communicate with thousands of other neurons * Functional magnetic resonance imaging (fMRI) is a technology that can reconstruct a 3-D map of brain activity * The results of brain imaging and other research methods reveal that groups of neurons function in specialized circuits dedicated to different tasks 48.1: Nervous Systems consist of circuits of neurons and supporting cells * All animals except sponges have some type of nervous system * What distinguishes the nervous systems of the different animal groups is how the neurons are organized into circuits * The simplest animals with nervous systems, the cnidarians have neurons arranged in nerve nets * Sea stars have a nerve net in each arm connected by radial nerves to a central nerve ring * In relatively simple cephalized animals, such as flatworms a central nervous system (CNS) is evident * Annelids and arthropods have segmentally arranged clusters of neurons called ganglia * These ganglia connect to the CNS and make up a peripheral nervous system (PNS) * Nervous systems in molluscs correlate with the animals’ lifestyles * Sessile molluscs have simple systems while more complex molluscs have more sophisticated systems * In vertebrates the central nervous system consists of a brain and dorsal spinal cord * The PNS connects to the CNS
* Nervous systems process information in three stages: sensory input, integration, and motor output * Sensory neurons transmit information from sensors that detect external stimuli and internal conditions * Sensory information is sent to the CNS where interneurons integrate the information * Motor output leaves the CNS via motor neurons which communicate with effector cells * The three stages of information processing are illustrated in the knee-jerk reflex * Most of a neuron’s organelles are located in the cell body * Most neurons have dendrites, highly branched extensions that receive signals from other neurons * The axon is typically a much longer extension, that transmits signals to other cells at synapses and may be covered with a myelin sheath * Neurons have a wide variety of shapes that reflect their input and output interactions * Glia are supporting cells that are essential for the structural integrity of the nervous system and for the normal functioning of neurons * In the CNS, astrocytes provide structural support for neurons and regulate the extracellular concentrations of ions and neurotransmitters * Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) are glia that form the myelin sheaths around the axons of many vertebrate neurons 48.2: Ion pumps and ion channels maintain the resting potential of a neuron * Across its plasma membrane, every cell has a voltage called a membrane potential * The inside of a cell is negative relative to the outside * The membrane potential of a cell can be measured
* The resting potential is the membrane potential of a neuron that is not transmitting signals * In all neurons, the resting potential depends on the ionic gradients that exist across the plasma membrane * The concentration of Na+ is higher in the extracellular fluid than in the cytosol while the opposite is true for K+ * By modeling a mammalian neuron with an artificial membrane we can gain a better understanding of the resting potential of a neuron * A neuron that is not transmitting signals contains many open K+ channels and fewer open Na+ channels in its plasma membrane * The diffusion of K+ and Na+ through these channels leads to a separation of charges across the membrane, producing the resting potential * Gated ion channels open or close, in response to membrane stretch or the binding of a specific ligand and in response to a change in...