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.