function Flashcards
sensory neurons
coupled to receptors specialised to detect and
respond to different attributes of the internal and external
environment.
motor neurons
control the activity of muscles, are
responsible for all forms of behaviour including speech.
interneurons
mediate simple reflexes as well
as being responsible for the highest functions of
the brain
glial cells
make
an important contribution to the development of the
nervous system and to its function in the adult brain.
While much more numerous, they do not transmit
information in the way that neurons do.
axons
one of the two ‘processes’ of the neuron. their job is
to transmit information from the neuron on to others to
which it is connected. axons and dendrites both participate in the specialised contacts called synapses.
dendrites
one of the two ‘processes’ of the neuron. their job is to receive the information being transmitted by
the axons of other neurons. axons and dendrites both participate in the specialised contacts called synapses.
spinal cord
has two
functions: it is the seat of simple reflexes such as the knee
jerk and the rapid withdrawal of a limb from a hot object or a
pinprick, as well as more complex reflexes, and it forms a
highway between the body and the brain for information
travelling in both directions.
hind-brain
contains networks of
neurons that constitute centres for the control of vital
functions such as breathing and blood pressure. Within
these are networks of neurons whose activity controls these
functions
cerebellum
plays an absolutely central role in the
control and timing of movements
mid-brain
contains groups of neurons, each of which seem
to use predominantly a particular type of chemical
messenger, but all of which project up to cerebral
hemispheres. It is thought that these can modulate the
activity of neurons in the higher centres of the brain to mediate such functions as sleep, attention or reward.
thalamus
The thalamus
relays impulses from all sensory systems to the cerebral
cortex, which in turn sends messages back to the thalamus.
This back-and-forward aspect of connectivity in the brain is
intriguing - information doesn’t just travel one way.
hypothalamus
controls functions such as eating and
drinking, and it also regulates the release of hormones
involved in sexual functions.
basal ganglia
play a central role in the initiation and
control of movement.
cerebral cortex
The cortical tissue is divided into a large number of discrete
areas, each distinguishable in terms of its layers and
connections. The functions of many of these areas are
known - such as the visual, auditory, and olfactory areas, the
sensory areas receiving from the skin (called the
somaesthetic areas) and various motor areas. The cerebral cortex is required for voluntary actions,
language, speech and higher functions such as thinking and
remembering.
polarization
Roughly speaking, the dendrite receives, the cell-body
integrates and the axons transmit - a concept called
polarization because the information they process
supposedly goes in only one direction.
dendritic spines (and proteins)
These are where incoming axons make
most of their connections. Proteins transported to the
spines are important for creating and maintaining neuronal
connectivity. These proteins are constantly turning over,
being replaced by new ones when they’ve done their job.
growth factors
The
end-points of the axons also respond to molecules called
growth factors. These factors are taken up inside and then
transported to the cell body where they influence the
expression of neuronal genes and hence the manufacture of
new proteins. These enable the neuron to grow longer
dendrites or make yet other dynamic changes to its shape
or function.
synaptic transmission
Most of the synapses on cells in the cerebral cortex are located on the dendritic spines that
stick out like little microphones searching for faint signals.
Communication between nerve cells at these contact points
is referred to as synaptic transmission.
patch-clamping
Nowadays, a modern electrical
recording technique called patch-clamping is enabling
neuroscientists to study the movement of ions through
individual ion-channels in all sorts of neurons, and so make
very accurate measurements of these currents in brains
much more like our own.
refractory period
The marvellous feature of
nerve fibres is that after a very brief period of silence (the
refractory period) the spent membrane recovers its
explosive capability, readying the axon membrane for the next
action potential.
myelin sheath
New research is telling us about the proteins that make up
this myelin sheath. This blanket prevents the ionic currents
from leaking out in the wrong place but, every so often the
glial cells helpfully leave a little gap. Here the axon
concentrates its Na+ and K+ ion channels. These clusters of
ion channels function as amplifiers that boost and maintain
the action potential as it literally skips along the nerve
all-or-nothing
Action potentials have the distinctive characteristic of being
all-or-nothing: they don’t vary in size, only in how often they
occur. Thus, the only way that the strength or duration of a
stimulus can be encoded in a single cell is by variation of the
frequency of action potentials.
neurotransmitters
The electrical currents responsible for the
propagation of the action potential along axons cannot
bridge the synaptic gap. Transmission across this gap is
accomplished by chemical messengers called
neurotransmitters.
calcium
The arrival of an action potential leads
to the opening of ion-channels that let in calcium (Ca++).
This activates enzymes that act on a range of presynaptic
proteins given exotic names like “snare”, “tagmin” and “brevin”. Neuroscientists have only just discovered
that these presynaptic proteins race around tagging and
trapping others, causing the releasable synaptic vesicles to
fuse with the membrane, burst open, and release the
chemical messenger out of the nerve ending.
ionotropic receptors
The attachment of the
transmitter (the key) to the receptors (the lock) generally
causes the opening of an ion channel; these receptors are called ionotropic receptors. If the ion channel
allows positive ions to enter, the inflow of
positive current leads to excitation. This produces a swing in the membrane potential called an excitatory post-synaptic potential (epsp).
inhibition
The great precision of nervous activity requires
that excitation of some neurons is accompanied by
suppression of activity in other neurons. This is brought
about by inhibition.
inhibitory synapses
At inhibitory synapses, activation of
receptors leads to the opening of ion channels that allow the
inflow of negatively charged ions giving rise to a change in
membrane potential called an inhibitory post-synaptic
potential (ipsp) (see Figure). This opposes membrane
depolarisation and therefore the initiation of an action
potential at the cell body of the receiving neuron.
metabotropic receptors
These receptors
don’t contain ion channels, are not always localised in the
region of the synapse and, most importantly, do not lead to
the initiation of action potentials. We now think of these
receptors as adjusting or modulating the vast array of
chemical processes going on inside neurons, and thus the
action of metabotropic receptors is called neuromodulation.
