first order neuron

Examples of first order neuron in the following topics:

• Somatic Sensory Pathways to the Cerebellum

  • Both tracts involve two neurons.
  • The ventral tract (under L2/L3) gets its proprioceptive/fine touch/vibration information from a first order neuron, with its cell body in a dorsal ganglion.
  • Axons first cross midline in the spinal cord and run in the ventral border of the lateral funiculi.
  • Proprioceptive information is taken to the spinal cord via central processes of the dorsal root ganglia (where first order neurons reside).
  • These central processes travel through the dorsal horn where they synapse with second order neurons of Clarke's nucleus.

• Perceiving Motion

  • Motion perception happens in two ways that are generally referred to as first-order motion perception and second-order motion perception.
  • First-order motion perception occurs through specialized neurons located in the retina, which track motion through luminance.
  • The motion-sensing neurons detect a change in luminance at one point on the retina and correlate it with a change in luminance at a neighboring point on the retina after a short delay.
  • Due to first-order motion perception, the luminous impulses are seen as a continual movement.
  • The barber pole illusion also demonstrates how motion is perceived through first-order perception, which only sees movement as continual.

• Introducing the Neuron

  • The brain is made up entirely of neurons and glial cells, which are non-neuronal cells that provide structure and support for the neurons.
  • There are three primary types of neuron: sensory neurons, motor neurons, and interneurons.
  • Electrically charged chemicals flow from the first neuron's axon to the second neuron's dendrite, and that signal will then flow from the second neuron's dendrite, down its axon, across a synapse,  into a third neuron's dendrites, and so on.
  • There are three major types of neurons: sensory neurons, motor neurons, and interneurons.
  • This diagram shows the difference between: 1) a unipolar neuron; 2) a bipolar neuron; 3) a multipolar neuron; 4) a pseudounipolar neuron.

• Nerve Impulse Transmission within a Neuron: Resting Potential

  • For the nervous system to function, neurons must be able to send and receive signals.
  • To understand how neurons communicate, one must first understand the basis of charged membranes and the baseline or ‘resting' membrane charge.
  • The lipid bilayer membrane that surrounds a neuron is impermeable to charged molecules or ions.
  • Some ion channels need to be activated in order to open and allow ions to pass into or out of the cell.
  • A neuron at rest is negatively charged because the inside of a cell is approximately 70 millivolts more negative than the outside (−70 mV); this number varies by neuron type and by species.

• Preganglionic Neurons

  • The ANS is unique in that it requires a sequential two-neuron efferent pathway; the preganglionic neuron must first synapse onto a postganglionic neuron before innervating the target organ.
  • The preganglionic, or first neuron will begin at the "outflow" and will synapse at the postganglionic, or second neuron's cell body.
  • These cell bodies are GVE (general visceral efferent) neurons and are the preganglionic neurons.
  • There are several locations upon which preganglionic neurons can synapse with their postganglionic neurons:
  • These are the preganglionic neurons, which synapse with postganglionic neurons in these locations :

• Stages of the Action Potential

  • A neuron affects other neurons by releasing a neurotransmitter that binds to chemical receptors.
  • However, in order for a presynaptic neuron to release a neurotransmitter to the next neuron in the chain, it must go through a series of changes in electric potential.
  • The action potential is a rapid change in polarity that moves along the nerve fiber from neuron to neuron.
  • In order for a neuron to move from resting potential to action potential—a short-term electrical change that allows an electrical signal to be passed from one neuron to another—the neuron must be stimulated by pressure, electricity, chemicals, or another form of stimuli.
  • A neuron must reach a certain threshold in order to begin the depolarization step of reaching the action potential.

• Autonomic Ganglia

  • Autonomic ganglia are clusters of neuronal cell bodies and their dendrites.
  • The axons of dorsal root ganglion neurons are known as afferents.
  • The first neuron in this pathway is referred to as the preganglionic or presynaptic neuron.
  • This second neuron is referred to as the postganglionic or postsynaptic neuron.
  • The pathways of the ciliary ganglion include sympathetic neurons (red), parasympathetic neurons (green), and sensory neurons (blue).

• Neurons and Glial Cells

  • Neurons and glia coordinate actions and transmit signals in the CNS and PNS.
  • Nervous tissue first arose in wormlike organisms approximately 550 to 600 million years ago.
  • It contains a brain, ventral nerve cord, and ganglia (clusters of connected neurons).
  • The nervous system is made up of neurons, specialized cells that can receive and transmit chemical or electrical signals, and glia, cells that provide support functions for the neurons by playing an information processing role that is complementary to neurons.
  • Although glial cells support neurons, recent evidence suggests they also assume some of the signaling functions of neurons.

• Types of Neurotransmitters by Function

  • Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse.
  • Acetylcholine, which acts on the neuromuscular junction, was the first neurotransmitter identified.
  • Acetylcholine-releasing neurons are also found in the central nervous system (CNS).
  • Additionally, some motor neurons of the ANS release catecholamines like NE.
  • Chemical synapses are specialized junctions through which neurons signal to each other and to non-neuronal cells such as those in muscles or glands.

• Membrane Potentials as Signals

  • In neurons, a sufficiently large depolarization can evoke an action potential in which the membrane potential changes rapidly.
  • First, it allows a cell to function as a battery, providing power to operate a variety of "molecular devices" embedded in the membrane.
  • Second, in electrically excitable cells such as neurons and muscle cells, it is used for transmitting signals between different parts of a cell.
  • For neurons, typical values of the resting potential range from –70 to –80 millivolts; that is, the interior of a cell has a negative baseline voltage of a bit less than one tenth of a volt.
  • In excitable cells, a sufficiently large depolarization can evoke an action potential , in which the membrane potential changes rapidly and significantly for a short time (on the order of 1 to 100 milliseconds), often reversing its polarity.