Neurons and Synapses

Your brain is made of approximately 100-billion nerve cells, called neurons. Neurons have the amazing ability to gather and transmit electrochemical signals

Cell body - This main part has all of the necessary components of the cell, such as the nucleus, ER and ribosomes (for building proteins) and mitochondria (for making energy). Axon - This long, cable-like projection of the cell carries the electrochemical message (nerve impulse or action potential) along the length of the cell. Dendrites - These small, branch-like projections of the cell make connections to other cells and allow the neuron to talk with other cells or perceive the environment.

Neurons also vary with respect to their functions: Sensory neurons carry signals from the outer parts of your body (periphery) into the central nervous system. Motor neurons (motoneurons) carry signals from the central nervous system to the outer parts (muscles, skin, glands) of your body. Interneurons connect various neurons within the brain and spinal cord.

Reflex Arc The simplest type of neural pathway is a monosynaptic (single connection) reflex arc, like the knee-jerk reflex. When the doctor taps the right spot on your knee with a rubber hammer, receptors send a signal into the spinal cord through a sensory neuron. The sensory neuron passes the message to a motor neuron that controls your leg muscles. Nerve impulses travel down the motor neuron and stimulate the appropriate leg muscle to contract. The response is a muscular jerk that happens quickly and does not involve your brain.

Nerve Impulse A nerve impulse is an impulse from another nerve or a stimulus from a nerve receptor. A nerve impulse causes: The permeability of the membrane to sodium ions suddenly increases. Sodium ions diffuse rapidly from the outside to the inside of the membrane. This reverses the polarity of the cell membrane (inside positive and outside negative).

Nerve Impulse Continued This reversal occurs in a small area of the membrane and results in a flow of electrical current that affects the permeability of the adjacent areas of the membrane The reversal of polarization is the nerve impulse and it travels the length of the axon. High permeability of the membrane to sodium ions last only a fraction of a second and then returns to normal. The sodium pump and potassium diffusion allow normal distribution of ions to be restored.

Nerve Impulse Continued A brief recovery period occurs during which the nerve cell membrane cannot be stimulated to carry impulses. This refractory period lasts a few thousandths of a second. The rate at which an impulse travels depends on the size of the nerve and whether or not it is myelinated (unmyelinated = 2 m/s and myelinated = 100 m/s). In myelinated fibers the signal jumps from one node of Ranvier to the next. This is saltatory conduction and occurs because the membrane at the node is highly sensitive and this uses less energy due to polarization only at the nodes.

Nerve Impulse Continued For a nerve impulse to be transmitted, the stimulus must be at least a certain minimum strength or must reach a threshold. The impulses transmitted by a given neuron are all alike, a neuron operates on a “all-or-none” basis. The strength of the stimulus is measured by two effects: 1. A stronger stimulus causes more impulses to be transmitted each second. 2. Different neurons have different thresholds. A large number of neurons fire when a stimulus is stronger.

Synapse

Another Synapse

Transmission at the Synapse The transmission of the impulse across the synaptic cleft is a chemical process. Within the synaptic knob, the synaptic vesicles contain neurotransmitters (which are chemicals such as acetylcholine and norepinephrine). When an impulse reaches the synaptic knob, the synaptic vesicles fuse with the membrane of the synaptic knob and release their contents into the synaptic cleft. Special receptor proteins in the membrane of the neighboring dendrite attach to these neurotransmitters.

Transmission Continued When the impulses are arriving at a faster rate (representing a stronger initial stimulus), more neurotransmitter is released into the synaptic cleft and more impulses per second are sent. When the neurotransmitter has done its work, it is removed from the synaptic cleft by an enzyme that breaks down the molecules. The transmission of the impulse across the synaptic cleft is a chemical process.

Neurotransmitters Excitatory neurotransmitters are chemicals that initiate impulses in adjacent neurons. Examples include: acetylcholine, norepinephrine, histamine, and glutamic acid (an amino acid) Inhibitory neurotransmitters are chemicals that inhibit the firing of impulses. Examples include: serotonin, epinephrine, and glycine If the overall results are excitatory, impulses are transmitted down the axon to the next set of synapses. If the results are inhibitory, no impulses are transmitted.

Neuromuscular Junction The passage of impulses from motor neurons to muscles occur at special points of contact called neuromuscular junctions. The motor end plates contain synaptic vesicles which release acetylcholine. The acetylcholine combines with receptor molecules on the muscle cell membrane, thus sending an impulse to the muscle. The acetylcholine causes muscle cell membrane to become more permeable to sodium, causing an impulse to travel the membrane and the muscle cell to contract.

Neuromuscular Junction

Drugs and Synapses Many poisons and drugs affect the activity of chemical neurotransmitters at the synapses. Nerve gas, curare, botulin toxin, and some poisonous insecticides can interfere with the functioning of acetylcholine and cause muscle paralysis (death for respiratory paralysis). Stimulants cause a feeling of well-being, alertness, and excitement such as amphetamines (mimic norepinephrine by binding to receptors) and caffeine (aids in synaptic transmissions). Depressants slow the body activity or cause depression such as barbiturates (block the formation of norepinephrine). Hallucingens such as LSD or mescaline interfere with the effect of the inhibitory transmitter serotonin.

Taken from: 1. Text book (Biology: The Study of Life) 2. John Broida, PhD. http://www.usm.maine.edu/psy/broida/101/neuron.JPG 3. How Stuff Works http://health.howstuffworks.com/brain1.htm 4. Airline Safety.Com http://www.airlinesafety.com/editorials/PilotsAndMemory.htm 5. Neruoscience Glossary http://shp.by.ru/spravka/neurosci/ 5. Muscel Physiology http://fig.cox.miami.edu/~cmallery/150/neuro/neuromuscular-sml.jpg