2. Nerve Cells, Neural Circuitry, and Behaviour Flashcards
The human brain contains a huge number of nerve cells, on the order of 10^11 neurons, that can be classified into at least a thousand different types. What matters more for behaviour than the variety of these neurons?
The complexity of human behaviour depends less on the variety of neurons than on their organisation into anatomical circuits with precise functions. One key organisational principle of the brain, therefore, is that nerve cells with similar properties can produce different actions because of the way they are interconnected.
Because relatively few principles of organization give rise to considerable complexity, it is possible to learn a great deal about how the nervous system pro- duces behavior by focusing on five basic features of the nervous system.
Name each of these
- The structural components of individual nerve cells;
- The mechanisms by which neurons produce
signals within and between nerve cells; - The patterns of connections between nerve cells and between nerve cells and their targets: muscles
and gland effectors; - The relationship of different patterns of interconnection to different types of behaviour; and
- How neurons and their connections are modified
by experience
There are two main classes of cells in the nervous system, what are they?
nerve cells, or neurons, and glial cells, or glia.
A typical neuron has four morphologically defined regions. Name these
(1) The cell body (soma)
(2) Dendrites,
(3) Axon, and
(4) Presynaptic terminals
What is generally contained in the soma?
The cell body or soma is the metabolic centre of the cell. It contains the nucleus, which contains the genes of the cell, and the endoplasmic reticulum, an extension of the nucleus where the cell’s proteins are synthesised.
The cell body usually gives rise to two kinds of processes, what are these?
Several short dendrites and one long, tubular axon. Dendrites branch out in tree-like fashion and are the main apparatus for receiving incoming signals from other nerve cells. The axon typically extends some distance from the cell body and carries signals to other neurons.
What distance range can an axon convey electrical signals?
ranging from 0.1 mm to 2 m
What are these signals called and where are they initiated?
These electrical signals, called action potentials, are initiated at a specialised trigger region near the origin of the axon called the initial segment from which they propagate down the axon without failure or distortion at speeds of 1 to 100 m/s.
To what extent does the amplitude of an action potential vary and what is this range?
The amplitude of an action potential traveling down the axon remains constant at 100 mV because the action potential is an all-or-none impulse that is regenerated at regular intervals along the axon.
What is done to increase the speed at which action potentials are conducted?
To increase the speed by which action potentials are conducted, large axons are wrapped in an insulating sheath of a lipid substance, myelin. The sheath is interrupted at regular intervals by the nodes of Ranvier, uninsulated spots on the axon where the action potential is regenerated.
What is located near the end of the axon?
Near its end the axon divides into fine branches that contact other neurons at specialised zones of communication known as synapses.
What names are given to the cells 1. sending and 2. receiving this transmission?
The nerve cell transmitting a signal is called the presynaptic cell; the cell receiving the signal is the postsynaptic cell.
From where do the presynaptic cells transmit signals from? (broadly)
The presynaptic cell transmits signals from specialised enlarged regions of its axon’s branches, called presynaptic terminals or nerve terminals.
Where do the signals go from the presynaptic terminal?
The presynaptic and postsynaptic cells are separated by a very narrow space, the synaptic cleft. Most presynaptic terminals end on the postsynaptic neuron’s dendrites; but the terminals may also terminate on the cell body or, less often, at the beginning or end of the axon of the receiving cell
The coherent structure of the neuron did not become clear until late in the 19th century (due to the prevailing belief that the cell theory did not apply to the brain, which they thought of as a continuous, web-like reticulum of very thin processes.) What allowed for this?
Ramón y Cajal began to use the silver-staining method introduced by Golgi, still used today. The stain reveals that there is no cytoplasmic continuity between neurons, even at synapses between two cells.
What two advantages does this silver-staining method introduced by Golgi have?
First, in a random manner that is not understood, the silver solution stains only about 1% of the cells in any particular brain region, making it possible to examine a single neuron in isolation from its neighbours. Second, the neurons that do take up the stain are delineated in their entirety, including the cell body, axon, and full dendritic tree.
How did Ramón y Cajal further utilise this method?
Ramón y Cajal applied Golgi’s method to the embryonic nervous systems of many animals as well as humans. By examining the structure of neurons in almost every region of the nervous system, he could describe classes of nerve cells and map the precise connections between many of them.
In this way Ramón y Cajal adduced, in addition to the neuron doctrine, two other principles of neural organisation that would prove particularly valuable in studying communication in the nervous system. Name and describe these two principles
The first of these has come to be known as the principle of dynamic polarisation. It states that electrical signals within a nerve cell flow only in one direction: from the receiving sites of the neuron, usually the dendrites and cell body, to the trigger region at the axon. From there the action potential is propagated along the entire length of the axon to its terminals.
The other principle advanced by Ramón y Cajal is that of connectional specificity, which states that nerve cells do not connect randomly with one another in the formation of networks. Rather each cell makes specific connections—at particular contact points—with certain postsynaptic target cells but not with others.
Ramón y Cajal was also among the first to realise the feature that most distinguishes one type of neuron from another. What feature is this?
Form, specifically the number of the processes arising from the cell body. Neurons are thus classified into three large groups: unipolar, bipolar, and multipolar.
Describe unipolar neurons
Unipolar neurons are the simplest because they have a single primary process, which usually gives rise to many branches. One branch serves as the axon; other branches function as receiving structures.
Where are unipolar neurons often found?
These cells predominate in the nervous systems of invertebrates; in vertebrates they occur in the autonomic nervous system.
Describe the typical structure of a Bipolar cell
Bipolar neurons have an oval soma that gives rise to two distinct processes: a dendritic structure that receives signals from the periphery of the body and an axon that carries information toward the central nervous system
What types of neurons are often bipolar neurons?
Many sensory cells are bipo- lar, including those in the retina and in the olfactory epithelium of the nose.
Describe the receptor neurons that convey touch, pressure, and pain signals to the spinal cord
The receptor neurons that con- vey touch, pressure, and pain signals to the spinal cord, are variants of bipolar cells called pseudo-unipolar cells. These cells develop initially as bipolar cells but the two cell processes fuse into a single continuous structure that emerges from a single point in the cell body.
The axon splits into two branches, one running to the periphery (to sensory receptors in the skin, joints, and muscle) and another to the spinal cord