Lecture Five - The nervous system (signals and synapses) Flashcards
What are the differences between the endocrine and nervous systems?
Endocrine system:
Chemicals (hormones).
Output to everywhere (through the blood stream).
Input at specific target cells.
Slow to release (minutes to hours) and long lasting (up to days).
Nervous system:
Electrical and chemical impulses (electrical signals within cells, chemical signals between cells).
Specific connections input - output.
Fast acting (miliseconds) and short lasting.
What is the overall three step process of information processing?
Sensory input -> intergration -> motor output.
What are neurons and what are glia?
Neurons - information conducting cells.
Chemical signals.
Electrical signals.
Connections.
Glia - supporting cells.
Structural.
protection.
metabolism.
Insulation.
There are more glia cells than neurons.
E.g. hold the brain structure together.
What are the central nervous system and periferal nervous systems?
CNS = intergration of information - brain and spinal chord.
PNS = sensory neurons (ganglia) responsive to stimuli and motorneurons, which innervate muscles.
What are the three types of neurons?
Sensory neurons - typically ahve a particularly long axon.
Interneurons - typical neuron in the CNS, typically have short dendrites and axons (can sometimes be long as well).
Motor neurons - Have schwann cells around their axons.
What is happening when the tendon in the knee is tapped?
The wuad is streched suppenly when the hammer hits the knee.
The body thinks that the body is being overbalenced, therefore responds by quickly shortinging the quad muscle and causing the kicking movement.
What is the neuron structure?
Cell body - contains the nucleus and most of the outer organelles.
Dendrites - highly branched and recieve messages.
Axon - Convey out going messages to other cells. usually much longer than dendrites. Neurons often have many dendrites, and one axon. Axons can be sheathed in myelin.
Axon hillock - Region where axon joins the cell body. Essential role in transmission and intergration of nerve signals.
Synaptic terminal - Specialized ending which relays signals to target cells by releasing neurotransmitters. Synapse is contact area between synaptic terminal and target cell.
Some are chemical and some are electrical synapses.
What happens in the membranes of neurotransmitters (with regards to electrical signals with ions)?
neurons transfer information in the form of electrical signals (changes in voltage).
Every cell has a voltage across its plasma membrane.
Resting potantial:
Normally -70mV
Electrically polarised.
Unequal distribution of ions.
Potassium ions:
K specific ion channels in membrane.
K is constrantly trying to get out of cell due to difference in concentration inside and outside the cell.
K is also trying to get back into the cell due to a difference in charge.
Therefore the movement of K ions is in equillibrium.
K is the main contributer to these membrane potentials.
Sodium ions:
Na channels exist as well in the membrane, but there are far fewer compared to K channels.
Work in the opposite way to the Na channels.
Higher concentration outside the cell, therefore Na ions move into the cell due to concentration differences.
Differences in charge cause Na ions to move out of the cell.
This causes an equillibrium to occur as well.
Ungated ion channels allow ions to diffuse across plasma membrane. Flow is determined by concentration and electrical charge.
What is the sodium/potassium pump?
The Na/K pump transports these ions against their concentration gradients.
Potassium ion is the principal intracellular cation and the sodium ion si the principal extracellular cation.
Proteins, amino acids and phosphate are the principal intracellular anions, Cl- principal extracellular anion.
What are the three kinds of gated ion channels?
These open or close in reposnse to one of three kinds of stimuli.
Strech-gated ion channels: open when membrane is deformed.
Chemically-gated ion channels: respond to chemical stimulus (found in synapses).
Voltage-gated ion channels: respond to a change in membrane potential (found in axons and dendrites).
What is hyperpolarization and depolarization?
Hyperpolarization:
Stimulus triggers increase in magnitide of membrane potential (more negitive).
Opening of chloride ion channels.
Depolarization:
Stimulus triggers decrease in magnitide of membrane potential (less negitive).
Opening of sodium ion channels.
Magnitide dependant on stimulus strength.
These are both graded potentials.
What are action potentials?
All or nothing.
Threshold membrane voltage (~55mV) - beyond threshold, action potential triggered.
Carry information along axons.
Very rapid (1-2 milliseconds).
Magnitude always the same.
Occur as a result of voltage gated ion channels.
Resting state - Na+ and K+ activation gates closed. Resting potential.
Depolarisation - Stimulus opens some Na+ activation gates. Membrane potential less negitive.
Rising phase - Depolarisation opens most Na+ activation gates. K+ closed. Membrane potential +ve.
Rising phase - Depolarisation opens most Na+ activation gates. K+ gates closed. Membrane potential +ve.
Falling phase - Most Na+ inactivation gates closed. Most K+ gates open. Membrane potential decreases.
Undershoot - Na+ activation and inactivation gates closed. Some K+ gates still open. Eventually some Na+ activation open to balence. Refactory period.
(Inactivatino gates trump activation gates).
How do action potentials effect neighbouring neurons?
Action potentials self propogate along axon length.
Avtion potential in one region depolarizes neighbouring regions.
Triggers new action potential.
Refactory period ensures impulses is unidirectional.
The axon hillock ensures that the signal travels in only one direction.
How is the speed of transmission along the axon effected?
Diameter of the axon - Speed of propogation is proportioal to the axon diameter. This is important only in invertebrates.
Saltatory conduction (vertebrates) - Myelin (insulation) makes the signal travel more quickly.
Action potentials jump between nodes in the myelin sheath. This makes the messages travel more quickly.
What are synapses?
The junction between a neuron and another cell (could be neuron-neuron, motor neuron-muscle cell or neuron-gland cell to name a few).
Neuron-neuron conducts signals from axon (presynaptic cell) to dendrite or cell body (post-synaptic cell).
Electrical synapse (direct contact - very rapid)
Chemical synapse (synaptic gap - is the most common kind, as the connections between neurons can be modified if they are not attached).
Sum up graded potentials.
Many synapses on a single neuron.
Single EPSP too small to cross threshold.
Integration (summation) of many EPSP and IPSP across dendrites and cell body.
Axon hillock is decision point.
