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.

• 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 :

• 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).

• 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.

• Cholinergic Neurons and Receptors

  • Acetylcholine was first identified in 1914 by Henry Hallett Dale for its actions on heart tissue.
  • Acetylcholine was also the first neurotransmitter to be identified.
  • In the central nervous system, acetylcholine and the associated neurons form the cholinergic system.
  • In the autonomic nervous system, acetylcholine is released in the following sites: all pre- and post-ganglionic parasympathetic neurons, all pre-ganglionic sympathetic neurons, some post-ganglionic sympathetic fibers, and pseudomotor neurons to sweat glands
  • Cholinergic neurons are capable of producing ACh.

• Somatic Sensory Pathways

  • The somatosensory pathway is composed of three neurons located in the dorsal root ganglion, the spinal cord, and the thalamus.
  • A somatosensory pathway will typically have three long neurons: primary, secondary, and tertiary.
  • The first always has its cell body in the dorsal root ganglion of the spinal nerve.
  • For example, slow, thin, unmyelinated neurons conduct pain whereas faster, thicker, myelinated neurons conduct casual touch.
  • Another important target for afferent somatosensory neurons that enter the spinal cord are those neurons involved with local segmental reflexes.

• The Action Potential and Propagation

  • Neurons typically send signals over long distances by generating and propagating action potentials over excitable axonal membrane.
  • When the membrane potential of the axon hillock of a neuron reaches threshold, a rapid change in membrane potential occurs in the form of an action potential.
  • First is depolarization, followed by repolarization and a short period of hyperpolarization.
  • The depolarization, also called the rising phase, is caused when positively charged sodium ions (Na+) suddenly rush through open voltage-gated sodium channels into a neuron.
  • During this period, the neuron cannot respond to another stimulus, no matter how strong.

• Enteric Nervous System

  • The ENS consists of some 100 million neurons, one-thousandth of the number of neurons in the brain, and about one-tenth the number of neurons in the spinal cord.
  • The myenteric plexus is mainly organized as longitudinal chains of neurons.
  • The sensory neurons report on mechanical and chemical conditions.
  • Other neurons control the secretion of enzymes.
  • The parasympathetic nervous system is able to stimulate the enteric nerves in order to increase enteric function.

• Embryonic Development of the Brain

  • This results in a strip of neuronal stem cells that runs along the back of the fetus.
  • Neuronal precursor cells proliferate in the ventricular zone of the developing neocortex.
  • The first postmitotic cells to migrate form the preplate which are destined to become Cajal-Retzius cells and subplate neurons.
  • There is also a method of neuronal migration called multipolar migration.
  • Neurotrophic factors are molecules that promote and regulate neuronal survival in the developing nervous system.