Purkinje cells

Examples of Purkinje cells in the following topics:

• Functions of the Cerebellum in Integrating Movements

  • In the human cerebellum, information from 200 million mossy fiber inputs is expanded to 40 billion granule cells, whose parallel fiber outputs then converge onto 15 million Purkinje cells.
  • Because of the way that they are lined up longitudinally, the 1,000 or so Purkinje cells belonging to a microzone may receive input from as many as 100 million parallel fibers and focus their own output down to a group of less than 50 deep nuclear cells.
  • A module (a multizonal microcompartment in the terminology of Apps and Garwicz) consists of a small cluster of neurons in the inferior olivary nucleus, a set of long narrow strips of Purkinje cells in the cerebellar cortex (microzones), and a small cluster of neurons in one of the deep cerebellar nuclei.
  • The synapses between parallel fibers and Purkinje cells, and the synapses between mossy fibers and deep nuclear cells, are both susceptible to modification of their strength.
  • As the illustration on the right shows, Purkinje cell dendritic trees are flattened in a way that aligns with the microzone length, and parallel fibers cross the microzones at right angles.

• Modulation of Movement by the Cerebellum

  • Within this thin layer are several types of neurons with a highly regular arrangement, the most important being Purkinje cells and granule cells.
  • Most of them derive from early models formulated by David Marr and James Albus, which were motivated by the observation that each cerebellar Purkinje cell receives two dramatically different types of input.
  • However, each cerebellar Purkinje cell also gets input from one single climbing fiber, which is so strong that a single climbing fiber action potential will reliably cause a target Purkinje cell to fire a burst of action potentials.
  • Divergence and convergence: The 1000 or so Purkinje cells belonging to a microzone may receive input from as many as 100 million parallel fibers, and focus their own output down to a group of less than 50 deep nuclear cells.
  • Plasticity: The synapses between parallel fibers and Purkinje cells, and the synapses between mossy fibers and deep nuclear cells, are both susceptible to modification of their strength.

• Functions of the Cerebellum

  • This neural divergence is followed by parallel fiber outputs that converge onto 15 million Purkinje cells.
  • Due to their longitudinal alignment, the approximately 1000 Purkinje cells belonging to a microzone may receive input via neural convergence from as many as 100 million parallel fibers.
  • The cells then focus their own output down to a group of less than 50 deep nuclear cells.
  • A module consists of a small cluster of neurons in the inferior olivary nucleus, a set of long narrow strips of Purkinje cells in the cerebellar cortex (microzones), and a small cluster of neurons in one of the deep cerebellar nuclei.
  • Plasticity: The synapses between parallel fibers and Purkinje cells and between mossy fibers and deep nuclear cells are both susceptible to modification of their strength.

• Neurons

  • Myelin is produced by glial cells.
  • For example, dendrites from a Purkinje cell in the cerebellum are thought to receive contact from as many as 200,000 other neurons.
  • The Purkinje cell, a multipolar neuron in the cerebellum, has many branching dendrites, but only one axon.
  • Pseudounipolar cells share characteristics with both unipolar and bipolar cells.
  • Examples include (a) a pyramidal cell from the cerebral cortex, (b) a Purkinje cell from the cerebellar cortex, and (c) olfactory cells from the olfactory epithelium and olfactory bulb.

• Electric Activity in the Heart

  • In the atria the electrical signal moves from cell to cell (see section on nerve conduction and the electrocardiogram) while in the ventricles the signal is carried by specialized tissue called the Purkinje fibers which then transmit the electric charge to the myocardium. shows the isolated heart conduction system.
  • These cells are specialized cardiomycetes (cardiac muscle cells).
  • If the AV node also fails, Purkinje fibers are capable of acting as the pacemaker.
  • The reason Purkinje cells do not normally control the heart rate is that they generate action potentials at a lower frequency than the AV or SA nodes.
  • The Purkinje fibers are located in the inner ventricular walls of the heart.

• Parts of the Cerebellum

  • This thin layer contains several types of neurons with a highly regular arrangement, most importantly Purkinje cells and granule cells .
  • Due to its large number of tiny granule cells, the cerebellum contains more neurons than the rest of the brain put together, but comprises only 10% of total brain volume.
  • Transverse section of a cerebellar folium, showing principal cell types and connections.

• Electrical Events

  • Cardiac contraction is initiated in the excitable cells of the sinoatrial (SA) node by both spontaneous depolarization and sympathetic activity.
  • The SA node is a bundle of nerve cells located on the outer layer of the right atria.
  • The SA node nerve impulses travel through the atria and cause direct muscle cell depolarization and contraction of the atria.
  • The AV node receives action potentials from the SA node, and transmits them through the bundle of His, left and right bundle branches, and Purkinje fibers, which cause depolarization of ventricular muscle cells leading to ventricular contraction.
  • The system of nerves that work together to set the heart rate and stimulate muscle cell depolarization within the heart.

• Layers of the Heart Walls

  • It is composed of cardiac muscle cells, or cardiomyocytes.
  • Cardiomyocytes are specialized muscle cells that contract like other muscle cells, but differ in shape.
  • Compared to skeletal muscle cells, cardiac muscle cells are shorter and have fewer nuclei.
  • The inner layer of the heart wall is the endocardium, composed of endothelial cells that provide a smooth, elastic, non-adherent surface for blood collection and pumping.
  • The Purkinje fibers are located just beneath the endocardium and send nervous impulses from the SA and AV nodes outside of the heart into the myocardial tissues.

• Electrocardiogram and Correlation of ECG Waves with Systole

  • The P wave is the first wave on the ECG because the action potential for the heart is generated in the sinoatrial (SA) node, located on the atria, which sends action potentials directly through Bachmann's bundle to depolarize the atrial muscle cells.
  • The QRS complex represents action potentials moving from the AV node, through the bundle of His and left and right branches and Purkinje fibers into the ventricular muscle tissue.
  • Following the T wave is the U wave, which represents repolarization of the Purkinje fibers.

• Arrhythmia

  • The conduction system: Automaticity refers to a cardiac muscle cell firing off an impulse on its own.
  • All of the cells in the heart have the ability to initiate an action potential; however, only some of these cells are designed to routinely trigger heart beats.
  • These cells are found in the conduction system of the heart and include the SA node, AV node, Bundle of His, and Purkinje fibers.