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Summary Adaptive Brain Chapter 1
This document contains a summary of the course The Adaptive Brain chapter 1 in Purves.
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The Nervous SystemThe nervous system is the product of gene expression that begins Astrocytes
at the outset of embryogenesis. Altered genes can sometimes These are restricted to the central nervous system (brain and
underlie neurological and psychiatric disorders. Genes that disrupt spinal cord). Their function is to maintain an appropriate chemical
brain development can cause neurodegenerative disorders of the environment for neuronal signalling. They also secrete substances
adult brain, such as Huntington’s and Parkinson’s diseases. Using that influence the construction of new synaptic connections.
genetics and genomics to understand diseases of the developing
and adult nervous system permits deeper insight into the Oligodendrocytes
pathology, and raises hope for gene-based therapies. These are also restricted to the central nervous system. They
wrap myelin around the axons, which can speed up the
The nervous system transmission of electrical signals. In the peripheral nervous
The nervous system can be divided into two categories, neurons system, the cells that provide myelin are called Schwann cells.
and supporting glial cells.
Microglial cells
Neurons These are derived from the haematopoietic precursor cells, and
Neurons consist of axons and dendrites that arise from the share many properties them. macrophages. They remove cellular
neuronal cell body in the form of dendritic branches. Most neurons debris from sites of injury or normal cell turnover. Like
have only one axon that extends for a long distance. Dendrites are macrophages, microglial cells also secrete cytokines that can
the targets for synaptic input from the axon terminals of other modulate inflammation.
neurons and have a high content of ribosomes and cytoskeletal
proteins. Some neurons lack dendrites while others have a lot. The Glial stem cells
number of inputs a neuron receives depends on the complexity of These are also found in the adult brain. These cells retain the
the dendrites. Neurons that lack dendrites are innervated by the capacity to proliferate and regenerate/differentiate glia and in
axons of just one or a few neurons, which limits their capacity to some cases even neurons. Glial stem cells can be divided into two
integrate information from diverse sources, thus leading to a more categories: a subset of astrocytes found primarily near the
or less faithful relay of the electrical activity generated by the ventricles in a region called the subventricular zone (SVZ) or
synapses. Neurons with a lot of dendritic branches are innervated adjacent to ventricular zone blood vessels, and oligodendrocyte
by a larger number of neurons, which allows for greater precursors scattered throughout the white matter and sometimes
integration of information. The number of inputs a single neuron referred to as polydendrocytes. SVZ astrocytes can give rise to
gets reflects the degree of convergence or divergence. Neurons more stem cells, neurons, and mature astrocytes and
communicate via the secretion of neurotransmitters from the oligodendrocytes. Oligodendrocyte precursors are more limited in
presynaptic terminal in the synaptic cleft. The neurotransmitters their potential. they give rise primarily to mature
bind to receptors in the postsynaptic cell. Axons convey electrical oligodendrocytes as well as to some astrocytes.
signals over a long distance by a self-regenerating wave of
electrical activity called an action potential. Action potentials are Neural circuits
all-or-nothing changes in the electrical potential (voltage) across Neurons are organized into ensembles called neural circuits that
the nerve cell membrane that conveys information from one place process specific kinds of information. The synaptic connections
to another in the nervous system. The process of action potentials that underlie neural circuits are typically made in a dense tangle
is passed on by synaptic transmission. Presynaptic terminals are of dendrites, axon terminals, and glial cells that together is
typically chemical synapses. The other type of synapse is an called neuropil. The neuropil constitutes the regions between
electrical synapse, these are relatively rare in the mature nervous nerve cell bodies where most synaptic connectivity occurs.
system but play a role in the development of the central nervous Preeminent is the direction of information flow in a circuit. Nerve
system. They are involved in the synchronization of local networks cells that carry information from the periphery toward the brain
of neurons. The secretory organelles in the presynaptic terminal of or spinal cord are called afferent neurons, nerve cells that carry
chemical synapses are called synaptic vesicles and are spherical information away from the brain and spinal cord are efferent
structures filled with neurotransmitters. neurons. Interneurons participate in local aspects of a circuit
based on the short distances over which their axons extend.
Glial cells
Glial cells support the signalling functions of nerve cells rather
than generating electrical signals. They are also involved in the
development and repair of the nervous system. They act as stem
cells in some brain areas where they promote the regrowth of
damaged neurons. In other regions where uncontrolled regrowth
might do more harm than good, they prevent regeneration.
Filament proteins (scaffolding proteins, tubules) characterize glial
cells and contribute to their function, such as migration of the
nerve cells, trafficking of membrane components, exocytosis, and
synaptic communication. There are three types of glial cells,
astrocytes, oligodendrocytes and microglial cells.
, Myotatic reflex Intracellularly recorded responses underlying the myotatic reflex
A simple example of a neural circuit is the myotatic reflex (knee-
jerk reflex). The afferent neurons that control the reflex are
sensory neurons whose cell bodies lie in the dorsal root ganglia
and send axons peripherally that terminate in sensory endings in
the skeletal muscles. The central axons of these sensory neurons
enter the spinal cord where they terminate a variety of central
neurons concerned with the motor neurons (efferent neurons).
One group of these neurons projects to the flexor muscles in the
limb and the other to the extensor muscles. The spinal cord
muscles are the third element of the circuit and receive synaptic
contacts from sensory afferent neurons and make synapses on
the efferent neuron motor neurons that project to the flexor
muscles and modulate the input-output linkage. The excitatory
synaptic connections between the sensory afferents and the
extensor efferent motor neurons cause the extensor muscles to
contract at the same time. Interneurons activated by the
afferents are inhibitory, and their activation diminishes electrical Calcium imaging
activity in flexor-efferent motor neurons and causes the muscles Calcium imaging is another way to study neural circuits. This
to become less active. The result is a complementary activation technique records the transient changes in intracellular
and inactivation of the synergistic and antagonistic muscles that concentration of calcium ions that are associated with action
control the position of the leg. potential firing. They do this because calcium channels establish
currents that lead to voltage changes in neurons and because
calcium is an important second messenger. A related approach
uses voltage-sensitive fluorescent dyes that are inserted into the
neuronal plasma membrane and report on the transmembrane
potential, thereby imaging the consequences of action potentials
and other electrical signals in many neurons at once. Calcium
indicators or voltage-sensitive dyes can be introduced directly into
neurons in living slices or into primary cultured neurons based on
their osmotic properties in solution. In addition, viral vectors can
be used to transfect subpopulations of cells in living tissue or in
living animals. Finally, genes that encode calcium or voltage-
sensitive proteins can be introduced into transgenic animals for
more precise control of where and when the proteins are
The neural circuit can be measured by using electrophysiological available for measuring the activity in the living animal.
recording, which measures the electrical activity of a nerve cell.
There are two approaches to this method, extracellular recording
where an electrode is placed near the nerve cell of interest to
detect its activity, and intracellular recording where the
electrode is placed inside the cell of interest. An extracellular
recording is particularly useful for detecting temporal patterns
of action potential activity and relating those patterns to
stimulation by other inputs or specific behavioural events.
Intracellular recording can detect the smaller graded changes in
electrical potential that trigger action potentials, and allows a
more detailed analysis of communication among neurons within a
circuit. These graded triggering potentials can arise at either
sensory receptors or synapses and are called receptor potentials
or synaptic potentials. With electrodes placed near but still
outside individual cells, the pattern of action potential activity
can be recorded extracellularly for each element of the circuit
before, during, and after a stimulus. by comparing the onset,
duration, and frequency of action potential activity in each cell, a
functional picture of the circuit emerges. Using intracellular
recording makes it possible to observe direct changes in the
membrane potential of each element of the myotatic reflex
circuit.
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