Nervous System (2)

What types of neurons are there? The nervous system comprises three different neuron types:

Sensory neurons (or afferent neurons) Motor neurons (or efferent neurons) Interneurons Sensory neurons transmit action potentials (often called “spikes”) from the receptor cells of the sensory organs to the CNS (central nervous system).

Motor neurons send axons from the nervous system to the muscle cells and, thus, transmit spikes from the CNS to the effectors.

Interneurons connect different parts of the CNS with each other. They do not leave the CNS.

What about nerves and nerve tracts? A nerve is a bundle of nerve fibers, which are the axons of the neurons. A nerve connects the CNS with the periphery. Nerves either come from or go to the brain or spinal cord. A nerve is surrounded by connective tissue.

We classify three types of nerves:

Motor nerves are made of motor neuron axons. They carry excitations from the central nervous system to the muscle cells. They are also called “efferent nerves”, because they leave (latin “ex” for out) the CNS. Their somata are located within the CNS while their axons project into the periphery. The opposite of a motor nerve is a sensory nerve. Sensory nerves carry information from the sense organs to the CNS. Therefore, they are also called afferent nerves, because they enter the CNS (latin “ad” for towards). As an example, the optic nerves (Nervus opticus) carry visual sensations from the retinas to the brain. Mixed nerves consist of motor and sensory neuron axons and carry excitations towards the muscles and towards the CNS at the same time. An example of a mixed nerve is the facial nerve (Nervus facialis). It controls the muscles of the face and transmits taste information from the tongue and pain information from the teeth to the brain. The sciatic nerves in the legs are other examples of mixed nerves. What is an action potential? Neurons are electrically active cells. In other words, they are excitable cells. They generate a voltage over their membrane. This is due to an asymmetric distribution of Na+ ions (high extracellular concentration, low intracellular concentration), and K+ ions (low extracellular, high intracellular). This ionic imbalance makes the neuron negatively charged compared to the extracellular fluid. If no stimulus is acting on the cell, it is in the resting state with a resting potential (voltage) of -70 mV. Information is encoded in a neuron by action potentials. Action potentials are brief pulses of electrical activity that can run along the axon. The membrane becomes very briefly permeable for Na+ ions, which flows into the cell. This makes the cells temporarily positive (+20 mV). Thereafter, the membrane will be open for K+ ions, which will flow out of the cell. This brings the membrane potential back to the resting potential. The action potentials last only a few milliseconds. Action potentials are “all-or-nothing“ events: either a full action potential is fired or none is fired at all. An individual neuron is capable of transmitting tens to hundreds of action potentials each second.

The action potentials of a neuron are only propagated in one direction along the axon. This is called unidirectional transmission and it is very rapidly. The insulating myelin sheath increases the velocity of impulses, so that action potentials only occur at the nodes of Ranvier. This process is called saltatory conduction . Many of our neurons are capable of transmitting impulses at a speed of many meters per second. Imaging an adult who accidently stubs a toe; sensory impulses travel from the toe to the brain and the pain is perceived in a split second.

How about synapses? Information is transmitted from one neuron to the next. For this, the electrical information of the action potential is translated into chemical information. This happens at the contact zone between neurons, which is called a synapse. Here, the action potential arrives at the end of the axon. It depolarizes the membrane and this in turn leads to the release (via exocytosis) of a chemical messenger, a neurotransmitter into the cleft between the neurons (synaptic cleft). Examples for neurotransmitters are acetylcholine, glutamate or GABA. The neurotransmitter diffuses to the next cell (called the postsynaptic neuron) and binds to its specific receptor. This leads to a depolarisation of the postsynaptic neuron, which fires an action potential.

BIBLIOGRAPHY 1. Greenberg DA, Aminoff MJ, Simon RP: Clinical Neurology, 8th edition. Mc Graw Hill Lange 2012. 2. http://dict.leo.org. 3. www.dict.cc. 4. www.webster-dictionary.org.

EXERCISE: 1. A neuron is a a. subdivision of the nervous system that brings information to the CNS b. a nerve cell; the basic building block of the nervous system c. serves the internal organs of the body

2. When a neuron is depolarized, the gated channels of the membran open to allow __________ to enter the neuron. A neuron becomes hyperpolarized when the gated channels of the membrane open, thus allowing _____________ to exit the neuron. a. Na+; K+ b. Na+; protein c. K+; Na+ d. K+; Ca+

3. Examples of neurotransmitters are ______________, ______________ or ________. a. Acetylcholine b. Glutamate c. GABA d. Potassium

You’ll find the answer on: http://tinyurl.com/weiterbildung-englisch

Die Autorin: Carmen Lobitz Carmen Lobitz ist freie Trainerin, Lehrerin und MTA. Sie gründete 1998 in Berlin lobitz seminare, die unter anderem Englischseminare für Health Professionals anbieten. Kontakt: mail@lobitz-seminare.de, www.lobitz-seminare.de

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Lösung für Exercise:

Answer: 1. b Answer: 2. a Answer: 3. a, b, c

Nervous System (2)

What types of neurons are there?

What about nerves and nerve tracts?

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Nervous System (2)

What types of neurons are there?

What about nerves and nerve tracts?

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