Chapter 43: Animal Nervous System

43.1 Principles of Electrical Signaling

• The diffuse arrangement of cells is called a nerve net, is found in cnidarians (jellyfish, hydra, anemones) and ctenophores (comb jellies).
• A central nervous system (CNS) includes large numbers of neurons aggregated into clusters called ganglia.
• Many sensory cells are sensory neurons, which carry information to the CNS.
• Cells in the CNS called interneurons, which pass signals from one neuron to another, perform this integration. 
• Motor neurons and sensory neurons are bundled together into long strands of nervous tissue called nerves.
• All neurons and other components of the nervous system that are outside the CNS are considered part of the peripheral nervous system, or PNS.
• A cell body, or soma, contains the nucleus. 
• A highly branched group of relatively short projections called dendrites.
• One or more relatively long projections are called axons.
• A difference in charge between any two points creates an electrical potential or voltage.
  • When an electrical potential exists across a plasma membrane, the separation of charges is called a membrane potential.
  • The volt (V) is the standard unit of electrical potential, and a millivolt (mV) is 1/1000 of a volt. 
•  When a neuron is not communicating with other cells, the difference in charge across its membrane is called the resting potential.
• Neurons are said to have excitable membranes because they can generate action potentials that propagate rapidly along the length of their axons. 

43.2 Dissecting the Action Potential

• The action potential depends on voltage-gated channels-membrane proteins that open and close in response to changes in membrane voltage.
• Voltage clamping allows researchers to hold the voltage of a cell’s plasma membrane at any desired value and record the electrical currents that occur at a voltage. 
• Studying individual ion channels became possible in the 1980s when Erwin Neher and Bert Sakmann perfected a variation of voltage clamping known as patch clamping.
  • ==Using this technique, Neher and Sakmann were able to document for the first time the currents that flowed through individual channels.== 
• The opening of Na+ channels exemplifies positive feedback-meaning that the occurrence of an event makes the same event more likely to recur. 
  • ==Positive feedback is rare in organisms: It cannot be employed as a regulatory mechanism under many circumstances, because often leads to uncon oiled events.== 
• Neurotoxins are poisons that affect neuron function often resulting in convulsions, paralysis, or unconsciousness.
• In the central nervous system, these accessory oligodendrocytes.
• In the peripheral nervous system, they are Schwann cells.
  • Oligodendrocytes and Schwann cells are two examples of glia, which are nervous system cells that support neurons.
• When oligodendrocytes or Schwann cells wrap around the axon，they form a myelin sheath, which acts as a type of electrical insulation. 
• Consequently, the cations moving down the membrane are able to spread until they reach a gap in the myelin sheath, called a node of Ranvier.
• The autoimmune disease multiple sclerosis (MS) develops when the immune system targets oligodendrocytes, destroying myelin in the CNS.

43.3 The Synapse

• In the 1920s, Otto Loewi showed that this indirect mechanism involves neurotransmitters.
• Such channels, called ligand-gated channels, have a binding site for a specific ligand on the part of the channel protein that faces the synaptic cleft. 
• Changes in the membrane potential of a postsynaptic cell that makes the cell more likely to produce an action potential are called excitatory postsynaptic potentials (EPSPs).
• Changes in the membrane potential of a postsynaptic cell that makes the cell less likely to produce an action potential are called inhibitory postsynaptic potentials (IPSPs).
• The additive nature of postsynaptic potentials is termed summation.
• The sodium channels that trigger action potentials in a neuron are typically located near the place where the axon emerges from the cell body, a site called the axon hillock.

43.4 The Vertebrate Nervous System

• Anatomical and functional studies indicate that the PNS consists of two divisions with distinct functions:
  • The afferent division transmits sensory information to the CNS.
  • The efferent division carries commands from the CNS to the rest of the body. 
• The two types often have opposite effects on the same organ.
  • Nerves in the parasympathetic nervous system promote “rest-and-digest” functions that converse or restore energy. 
  • Nerves in the sympathetic nervous system typically prepare organs for stressful “fight-or-flight” situations.
• This enteric nervous system interacts with autonomic nerves but can also function independently.
  • ==The enteric nervous system plays a major role in regulating digestion, but it may also be important in immune function, mental health, cognition, and memory formation in the CNS.==
• Each structure of the human brain has distinct functions:
  • The cerebrum accounts for the bulk of the human brain. 
  • ==It makes up most of the forebrain, is divided into left and right hemispheres, and is the seat of conscious thought and memory.==
  • The diencephalon is also part of the forebrain. 
  • ==It relays sensory information to the cerebrum and functions in maintaining homeostasis.==
  • The brainstem connects the brain to the spinal cord and contains the midbrain and parts of the hindbrain. 
  • ==It is the autonomic center for regulating cardiovascular, digestive, and other involuntary functions.== 
  • The cerebellum is a structure in the hindbrain that coordinates complex motor patterns.
• In 1953, surgeons treated him for life-threatening seizures by removing a small portion of his temporal lobe and about two 1irds of his hippocampus, a structure at the inner edge of the temporal lobe.
• The technique known as optogenetic uses light to control the activity of targeted types of neurons in the brain.
• Memory is the retention of learned information. 
  • Learning and memory are thus closely related and are often studied in tandem. 

A change in the responsiveness or structure of a synapse is termed synaptic plasticity.