Nervous system Key points

• Nervous systems consist of cells called neurons that process and transmit information, along with supporting cells called glial cells.
• Sensory cells transduce information from the environment and the body. Neurons receive this information and transmit it to effectors such as muscles or glands.  The nervous systems of different species vary, but all arecomposed of neurons.
• In vertebrates, the brain and spinal cord form the centralnervous system, which communicates with the rest of the body via the peripheral nervous system.
  Neurons generally receive information via their dendrites and transmit information via their axons.
• Where neurons and their target cells meet, information is transmitted across synapses by the release of neurotransmitters.
  Glial cells physically support neurons and perform many housekeeping functions. Schwann cells and oligodendrocytes produce myelin, which insulates neurons. 
• Astrocytes create the blood–brain barrier. Neurons work together in networks.
  Neurons: Generating and Conducting Nerve Impulses Neurons have an electric charge difference across their plasma membranes. This resting potential is created by ion pumps and ion channels.
  The sodium–potassium pump concentrates K+ on the inside of a neuron and Na+ on the outside. Potassium channels allow K+to diffuse out of the neuron, leaving behind unbalanced negative charges. A potassium equilibrium potential exists when the tendency of K+ ions to diffuse out of the neuron is balanced by the negative charges pulling them back in. This potential can be calculated using the Nernst equation. The resting potential is perturbed when ion channels open or close, changing the permeability of the plasma membrane to
  charged ions. Through this mechanism, the plasma membrane can become depolarized or hyperpolarized. An action potential is a rapid reversal in charge across a portion of the plasma membrane resulting from the sequential opening and closing of voltage-gated sodium and potassium channels. These changes in voltage-gated channels occur when the plasma membrane depolarizes to a threshold level. Review
  
  Action potentials are all-or-nothing, self-regenerating events. They are conducted down axons because local current flow depolarizes adjacent regions of membrane and brings them to threshold. Patch clamping allows the study of single ion channels.
  
  In myelinated axons, action potentials appear to jump between nodes of Ranvier, patches of axonal plasma membrane that are not covered by myelin.Neurons, Synapses, and Communication
  Neurons communicate with each other and with other cells at specialized junctions called synapses, where the plasma membranes of two cells come close together.
  The classic chemical synapse is the neuromuscular junction, a synapse between a motor neuron and a muscle cell. Its neurotransmitter is acetylcholine, which causes a depolarization of the postsynaptic membrane when it binds to its receptor. When a nerve impulse reaches an axon terminal, it causes the release of neurotransmitters, which diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane. Synapses between neurons can be either excitatory or inhibitory. A postsynaptic neuron integrates information by summing excitatory and inhibitory postsynaptic potentials in both space and time.  
• Synapses that form between the axon terminals of one neuron and another can influence the release of neurotransmitter
  by the second cell by presynaptic excitation or presynaptic inhibition. 
• Ionotropic receptors are ion channels. 
• Metabotropic receptors are G protein-linked receptors that influence the postsynaptic cell through various signal transduction pathways and result in the opening of ion channels. 
• The actions of ionotropic synapses are generally faster than those of metabotropic synapses. Electrical synapses allow electric signals to pass between cells without the use of neurotransmitters.
  There are many different neurotransmitters and even more types of receptors. The action of a neurotransmitter depends on the receptor to which it binds. With repeated stimulation, a neuron can become more sensitive to its inputs. Since this increased sensitivity can last a long time, it is called long-term potentiation, or LTP. The properties of the NMDA glutamate receptor appear to explain LTP.  
• In chemical synapses, the transmitter must be cleared rapidly from the synapse. Some poisons and drugs act by blocking or slowing the clearance of transmitter from the synapse.