nervous

• Neuron structure and organization reflect function in information transfer
• structure of a Typical Neuron

  
• Introduction to Information Processing

  Sensory Neurons - transmit information about external stimuli

  Interneurons - form the local circuits connecting neurons in the brain or ganglia. Interneurons are responsible for the integration (analysis and interpretation) of sensory input

  Motor Neurons - transmit signals to muscle cells, causing them to contract
• a neuron receives information, transmits it along an extension called an axon, and transmits the information to other cells via specialized junctions called synapses

  
• Most of a neuron’s organelles, including its nucleus, are located in the cell body
• In a typical neuron, the cell body is studded with numerous highly branched extensions called dendrites (from the Greek dendron, tree). Together with the cell body, the dendrites receive signals from other neurons.
• A typical neuron has a single axon, the extension that transmits signals to other cells
• Axons are often much longer than dendrites. The specialized structure of axons allows them to use pulses of electrical current to transmit information, even over long distances.
• The cone-shaped base of an axon, called the axon hillock, is typically where signals that travel down the axon are generated. Near its other end, an axon usually divides into many branches.
• Each branched end of an axon transmits information to another cell at a junction called a synapse
• The part of each axon branch that forms this specialized junction is a synaptic terminal.
• At most synapses, chemical messengers called neurotransmitters pass information from the transmitting neuron to the receiving cell.
• Synaptic terminals on the cell body of a postsynaptic neuron

  
• The cone snail’s siphon acts as a sensor, transferring information to the neuronal circuits in the snail’s head for processing. If prey is detected, these circuits issue motor commands—signals that control muscle activity. In this example, motor commands trigger release of a harpoon-like tooth from the proboscis, spearing the prey.

  
• Introduction to Information Processing

  Information processing by a nervous system occurs in three stages: sensory input, integration, and motor output.
• Sensory input describes the response in a sensory organ (eyes, ears, nose, tongue, and skin) when it receives stimuli.

  ex. the snail surveys its environment with sensors in its tubelike siphon, sampling scents that might reveal a nearby fish
• The integration involves processing of information, and is carried out by the central nervous system (CNS), which consists of brain and spinal cord.

  ex. networks of neurons in the snail brain process this information to determine if a fish is in fact present and, if so, where the fish is located.
• The nervous system activates effector organs such as muscles and glands to cause a response called motor output.

  ex. Motor output from the processing center then initiates attack, activating neurons that trigger release of the harpoon-like tooth toward the prey.
• All neurons transmit electrical signals within the cell in an identical manner. The particular connections made by the active neuron are what distinguish the type of information being transmitted. Interpreting nerve impulses therefore involves sorting neuronal paths and connections.
• the shape of a neuron can vary from simple to quite complex, depending on its role in information processing.
• When grouped together, the axons of neurons form the bundles we call nerves
• In these drawings of neurons, cell bodies and dendrites are black and axons are red.

  
• the neurons that carry out sorting, processing, and integration are organized in a central nervous system (CNS), that includes a brain or simpler clusters called ganglia.
• he neurons that carry information into and out of the CNS constitute the peripheral nervous system (PNS), it consists of the nerves that branch out from the brain and spinal cord.
• Neurons of both the CNS and PNS require supporting cells called glial cells, or glia (from a Greek word meaning “glue”)
• Compare and contrast the structure and function of axons and dendrites.

  Dendrites receive electrochemical impulses from other neurons, and carry them inwards and towards the cell body, while axons carry the impulses away from the cell body. Dendrites are short and heavily branched in appearance, while axons are much longer.
• Describe the basic pathway of information flow through neurons that causes you to turn your head when someone calls your name.

  The ear receptors detect the name's sound signals and transmit them to the brain cells. The interneurons facilitate the processing of the sound signal and interpreting the signal as the individual's name. As a response, the brain commands the neck muscles to contract and turn the head in the sound direction.
• WHAT IF? How might increased branching of an axon help coordinate responses to signals communicated by the nervous system?

  increased branching allow greater number of postsynaptic cells, enhancing coordination of responses to nervous system signals.
• the inside of a cell is negatively charged relative to the outside. This charge difference, or voltage, across the plasma membrane is called the membrane potential
• For a resting neuron—one that is not sending a signal—the membrane potential is called the resting potential and is typically between -60 and -80 millivolts (mV).
• This charge difference, or voltage, across the plasma membrane is called the membrane potential, reflecting the fact that the attraction of opposite charges across the plasma membrane is a source of potential energy.
• When a neuron receives a stimulus, the membrane potential changes.
• Potassium ions (K+) and sodium ions (Na+) play an essential role in the formation of the resting potential. These ions each have a concentration gradient across the plasma membrane of a neuron
• In most neurons, the concentration of K+ is higher inside the cell, while the concentration of Na+ is higher outside.
• The Na+ and K+ concentration gradients are maintained by the sodium-potassium pump. This pump uses the energy of ATP hydrolysis to actively transport Na+ out of the cell and K+ into the cell.
• The sodium-potassium pump transports three Na+ out of the cell for every two K+ that it transports into the cell
• Although this pumping generates a net export of positive charge, the pump acts slowly. The resulting change in the membrane potential is therefore quite small—only a few millivolts.
• Why, then, is there a membrane potential of -60 to -80 mV in a resting neuron? The answer lies in ion movement through ion channels, pores formed by clusters of specialized proteins that span the membrane. Ion channels allow ions to diffuse back and forth across the membrane. As ions diffuse through channels, they carry with them units of electrical charge. Furthermore, ions can move quite rapidly through ion channels. When this occurs, the resulting current—a net movement of positive or negative charge—generates a membrane potential, or voltage across the membrane.
• Summary of active transport by the sodium-potassium pump

  
• Concentration gradients of ions across a plasma membrane represent a chemical form of potential energy that can be harnessed for cellular processes