neuron

Examples of neuron in the following topics:

• Neurons

  • A human brain contains around 86 billion neurons.
  • While some neurons have no dendrites, other types of neurons have multiple dendrites.
  • Neurons usually have one or two axons, but some neurons, like amacrine cells in the retina, do not contain any axons.
  • Dendrites from a single neuron may receive synaptic contact from many other neurons.
  • Multipolar neurons are the most common type of neuron.

• Signal Summation

  • Signal summation occurs when impulses add together to reach the threshold of excitation to fire a neuron.
  • Each neuron connects with numerous other neurons, often receiving multiple impulses from them.
  • One neuron often has input from many presynaptic neurons, whether excitatory or inhibitory; therefore, inhibitory postsynaptic potentials (IPSPs) can cancel out EPSPs and vice versa.
  • If the neuron only receives excitatory impulses, it will also generate an action potential.
  • A single neuron can receive both excitatory and inhibitory inputs from multiple neurons.

• Nerve Impulse Transmission within a Neuron: Action Potential

  • Signals are transmitted from neuron to neuron via an action potential, when the axon membrane rapidly depolarizes and repolarizes.
  • A neuron can receive input from other neurons via a chemical called a neurotransmitter.
  • If this input is strong enough, the neuron will pass the signal to downstream neurons.
  • The action potential travels down the neuron as Na+ channels open.
  • The Na+ channels close, beginning the neuron's refractory period.

• Glia

  • Glia guide developing neurons to their destinations, buffer ions and chemicals that would otherwise harm neurons, and provide myelin sheaths around axons.
  • Astrocytes make contact with both capillaries and neurons in the CNS .
  • Satellite glia provide nutrients and structural support for neurons in the PNS.
  • Glial cells support neurons and maintain their environment.
  • Ependymal cells produce cerebrospinal fluid that cushions the neurons.

• Sensory-Somatic Nervous System

  • Unlike the autonomic nervous system, which has two synapses between the CNS and the target organ, sensory and motor neurons have only one synapse: one ending of the neuron is at the organ and the other directly contacts a CNS neuron.
  • The sensory neuron cell bodies are grouped in structures called dorsal root ganglia .
  • Each sensory neuron has one projection with a sensory receptor ending in skin, muscle, or sensory organs, and another that synapses with a neuron in the dorsal spinal cord.
  • These neurons are usually stimulated by interneurons within the spinal cord, but are sometimes directly stimulated by sensory neurons.
  • The cell bodies of sensory neurons are located in dorsal root ganglia.

• Nerve Impulse Transmission within a Neuron: Resting Potential

  • The resting potential of a neuron is controlled by the difference in total charge between the inside and outside of the cell.
  • For the nervous system to function, neurons must be able to send and receive signals.
  • The lipid bilayer membrane that surrounds a neuron is impermeable to charged molecules or ions.
  • 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.
  • However, the neurons have far more potassium leakage channels than sodium leakage channels.

• Neurons and Glial Cells

  • Neurons and glia coordinate actions and transmit signals in the CNS and PNS.
  • Instead, they have a system of separate-but-connected nerve cells (neurons) called a "nerve net."
  • 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.

• Autonomic Nervous System

  • A preganglionic neuron (originating in the CNS) synapses to a neuron in a ganglion that, in turn, synapses on the target organ .
  • The acetylcholine activates the postganglionic neurons.
  • The axons of the preganglionic neurons release acetylcholine on the postganglionic neurons, which are generally located very near the target organs.
  • In the autonomic nervous system, a preganglionic neuron of the CNS synapses with a postganglionic neuron of the parasympathetic nervous system.
  • The postganglionic neuron, in turn, acts on a target organ.

• Excitation–Contraction Coupling

  • Without the breakdown of acetycholine, motor neurons fire continuously.
  • Each skeletal muscle fiber is controlled by a motor neuron, which conducts signals from the brain or spinal cord to the muscle.
  • The area of the sarcolemma on the muscle fiber that interacts with the neuron is called the motor-end plate.
  • The end of the neuron's axon is called the synaptic terminal; it does not actually contact the motor-end plate.
  • Acetylcholine (ACh) is a neurotransmitter released by motor neurons that binds to receptors in the motor end plate.

• Neurodegenerative Disorders

  • These diseases generally worsen over time as more and more neurons die.
  • The symptoms of a particular neurodegenerative disease are related to where in the nervous system the death of neurons occurs.
  • Spinocerebellar ataxia, for example, leads to neuronal death in the cerebellum.
  • The death of these neurons causes problems in balance and walking.
  • Loss of neurons in the hippocampus is especially severe in advanced Alzheimer's patients .