SENSATION AND PERCEPTION (II)

York University Psychology 3270

Definitions

This page provides some definitions that for whatever reason seem to be missing from the course kit. They are words that I have talked about in the course and are therefore things that you need to know and might be on the exams. There are no words here that have NOT been talked about in class at some point. Each word can be reached by the link from the 3270 KEYWORDS page.

cytoplasm: the material inside cells, usually greyish in colour. The cytoplasm contains a higher concentration of potassium and a lower concentration of sodium than is found in the extra-cellular fluid.


depolarization: see polarization

electrochemical equilibrium potentials: Some ions are found at a greater concentration in some areas than in others. For example potassium is found at a greater concentration inside cells than outside them and sodium is found at a greater concentration outside than in. These ions will therefore tend to move across any separating boundary (eg. the cell membrane) so as to tend to make the concentration the same on each side. That is, the ions will tend to move down their concentration gradients. When a charged particle moves across a boundary it will take its charge with it. Therefore, if it is a positive ion (such as potassium or sodium), it will tend to make the far side more positive than it was. It is hard to move a positive charged particle into a positively charged region and so there will be a balance point in which the concentration gradient going one way will be balanced by the electric charge resisting the flow of more ions. This balance point is called the electrochemical equilibrium point and the charge at which balance is acheived is the electrochemical equilibrium potential. Since potassium has a higher concentration INSIDE, its balance point is with more positives outside. It's balance point is about -75mv (millivolts). For soium, since it has a higher concentration OUTSIDE, its balance point is about +20mv. These balance points can only be achieved, however, if the membrane is open to allowing that particular ion to travel freely across it. That is if the relevant ion channels (see membrane) are open.


excitatory and inhibitory post synaptic potentials (EPSPs and IPSPs): Neurotransmitters have their effect on the far side of the synaptic cleft (see saltatory propagation). On this side of the synapse, there are receptors which are able to chemically combine with the neurotransmitter that was released into the cleft as a result of an action potential arriving at the pre-synaptic side of the synaptic cleft. When receptors combine with their neurotransmitter, one of two electric phenomena can result: the membrane can either hyperpolarize, tending to take the cell away from its neuronal threshold  (IPSPs) or it can depolarize, tending to take the cell towards its neuronal threshold (EPSPs). EPSPs and IPSPs are very tiny blips which last for only a few milliseconds. They are not in themselves capable of making a cell produce an action potential. However, a given cell has many thousands of inputs and the inputs will be causing many EPSPs or IPSPs at any one time. A cell combines or integrates the electrical activity over its entire surface and if the total exceeds the neuronal threshold then an action potential will result.

glial cells: the nervous contains two types of specialized cells: neurones and support cells, called glial cells which are not actually neurones. One type of glial cell wraps itself around neuronal axons. They are called Schwann cells. They contain insulating fat called myelin and speed up the speed at which action potentials can travel along axons (see saltatory propagation of action potentials).

homunculus: literally 'a little man'. A term sometimes used to describe the representation of the body over the surface of the somatosensory cortex. The distortions of this map look like this:


hyperpolarization: see polarization

inhibitory post synaptic potentials: see excitatory post synaptic potentials

integration: combining together (see EPSPs)

ions: charged particles. Relevant ions include Potassium (K+) and Sodium (Na+)

membrane: outer layer of cells (see cytoplasm). The membrane has channels or pores embedded in it that only allow certain ions in, and only a certain times (see voltage-dependent sodium channels). 

negative feedback: a control process in which when something is detected (eg. a hormone in the blood), the control system shuts off production of that something. This sort of control system is very common and is an effective way to regulate many things in both engineering (eg. temperature in a room) and nature (eg. light levels in the eye by pupil control). cf. positive feedback

neuronal threshold or neural threshold: the electrical level at which an action potential is produced. Usually around -50mv. If a neurone is depolarized to this level, an action potential will result since the voltage-dependent sodium channels will open at this point and allow sodium into the cell. As sodium enters, it brings in positive charge which therefore tends to depolarize the cell still further and open those voltage-dependent channels even more. This is an example of positive feedback which will drive the cell maximally in the positive direction (ie. create an action potential).

polarization, depolarization,hyperpolarization: polarization literally means having poles. Because a cell is negative inside (as a result of being dominated by the electrochemical equilibrium potential of potassium) it is said to be polarized. Increasing that polarization (ie. making is more negative inside) is a hyperpolarization and decreasing it (ie. making it less negative inside) is a depolarization.

positive feedback: a very unstable control system in which when something is detected, the control system will make that something happen even more. cf. negative feedback. Positive feedback control systems are unusual in nature as they are extremely unstable. Examples include the action potential and childbirth.


saltatory propagation of action potentials: Action potentials do not travel down an axon like electricity in a wire. Rather the action potential is regenerated at every point along the axon's membrane. The existance of an action potential at one point will cause the neighbouring part of the axon to depolarize, cross the neuronal threshold and thus create a new action potential at that site. This process then recurs all the way along the axon. This type of propagation is slow and quite energy intensive.

A faster way is, instead of generating action potentials all along the membrane, just to do it at discrete sites, separated along the axon. These sites need to be close enough that an action potential at one site will depolarize the membrane sufficiently at the next site so that neuronal threshold is reached, but far enough apart to reduce the number of times the action potential needs to be generated and thus to move it along quickly and with minimal energy cost.

The glial cells wrapped around the axons create exactly the conditions for this to happen. A single axon is wrapped by several glial cells in sequence with gaps in between each one. The gaps are called the Nodes of Ranvier. The action potentials are generated only at these nodes.

This type of action potential propagation in which the action potential seems to jump from one point to another is called saltatory propagation (literally jumping reproduction).

second messengers: When a receptor in a cell receives what it is looking for (eg. a neurotransmitter or a specific chemical in the extracellular space) it will release a chemical (A) into the cell. This chemical can speed up (catalyse) another reaction in the cell which produces another chemical (B). This mechanism allows a small amount of chemical A to produce a large amount of chemical B. Chemical B can then have an effect on channels in the membrane causing an electrical event. Chemical B in this chain of events is known as a second messenger.

standard stimulus: Stevens found that it was possible to judge the magnitude of a sensory stimulus if a subject was first presented with a standard stimulus. The standard stimulus was assigned a value of 100 and subjects rated other stimuli numerically to indicate their perceived magnitude relative to the standard stimulus.

two alternative forced choice: this is a psychophysical technique in which a subject is presented with two alternatives: either a stimulus is present in the first interval (or in one area of the display) or it is present in the second interval (or in another area). The subject is not allowed a third alternative of 'neither': subjects are FORCED to choose, even if they have no idea in which interval the stimulus occurred. If they CAN detect the stimulus they will be above chance, if not they will be at chance (50% in the case of a choice of 2 possibilities). This technique removes subject bias since they are not likely to choose one interval any more than the other.

voltage-dependent sodium channels: The membrane of cells have special pores or channels that only let certain things through. These channels are not passive, but instead can change their properties under various conditions. Sodium channels are channels in the cell membrane that will only let through sodium. The type of sodium channels that are found in neurone's cell membranes have an additional very important property: the amount they open depends on the voltage across the membrane. The more +ve the charge, the more they open. Under normal circumstances the voltage across the membrane is -70mv, dominated by the electrochemical equilibrium potential of potassium. Under these conditions, the channels are shut. When the cell is depolarized to the neuronal threshold level or around -50mv, the channels start to open. As they do so, sodium passes in and the cell becomes more depolarized. This tends to open the channels more so that more sodium enters, etc... This is a very unstable situation in which the cell is driven by positive feedback, maximally towards the electrochemical equilibrium potential of sodium (around +20mv).