Download THE NERVOUS SYSTEM The individual neuron cannot do much

THE NERVOUS SYSTEM
The individual neuron cannot do much alone, it must be interconnected with others in
order to produce meaningful pattern of neural activity to regulate animal behaviours; this
aggregate is the nervous system.
This is a common term for a hugely rich and varying body of systems. In the main, there
are two distinct groups of nervous system, the vertebrates and the invertebrates.
Invertebrate nervous systems tend to be smaller and simpler than vertebrate ones. They
vary widely in design and extent, from the sponge with no nervous system to the octopus
which has almost vertebrate complexity. Many invertebrate nervous systems are
distributed about the body of the animal with only a small, rudimentary brain in the head,
the more complex the animal the larger the brain. In the vertebrate most of the complex
neural behaviour is located in one place, the head.
The number of neurons in invertebrates is usually limited and in some instances appears
to be constant among individual animals of the same species. Yet between species the
number varies widely.
Neuronal organisation between these two type of animal is distinct also. Most neurons in
the brains of vertebrates are usually bipolar or multipolar, with one or several dendrites
extending from one side of the cell body and an axon from the other. Most invertebrate
are monopolar, i.e. a single protrusion is made from the cell body and both axons and
dendrites extend from this. An invertebrate neuron can have several axons within the
dendritic tree with its own axon hillock. Invertebrates often develop more complex neural
circuitry by making individual neurons more complex, unlike vertebrates who add on more
neurons. There is evidence that the latter method seems to be less limiting than the prior.
BEHAVIOUR
Animal behaviour can be divided up into several broad classes, all of which are governed
by different combinations of processes in the brain/nervous system.
The simplest behaviour is the reflex, a stereotyped response triggered by stimuli in the
environment, e.g. removing a part of the body from a pain inducing stimulus. The
intensity and duration of the response is dependant on the force and duration of the
stimulus. Reflexes are quick, instinct reactions which allow an animal to adjust its
behaviour in response to sudden environmental changes. Taxes, also known as orientation
behaviour, e.g. light following, is where the animal will turn itself towards, or away from
an environmental agent.
Fixed-action patterns are the most complex group of reactive behaviours, the stimulus
involved is usually more complex than that involved in reflex actions, and it leads to an
extended, usually stereotyped response to a sensory stimulus.
There are also behaviours called motivated behaviours, which are governed primarily by
the internal state of the animal. For example, feeding is not just based on the availability
of food, but also on the animal's need for it, i.e. hunger. characterised motivational
behaviour with six points:
1. grouping and sequencing of component behaviour in time
2. goal directedness: the sequence of component behaviours generated can only be
understood by reference to some goal
3. spontaneity: the behaviour can occur in the absence of the eliciting stimuli
4. changes in responsiveness: the modularity effect of the motivational state varies
depending upon its level of arousal or satiation
5. persistence: the behaviour can greatly outlast any initiating stimulus
6. associative learning.
These behaviours are grouped together and combined in a hierarchy which elicit the total
behaviour of an animal. Taxes, reflexes and fixed-action patterns are all found in most
animals, governed by some type of motivational control. Some behaviours overrule others
in the scheme of things, others taking over from each other depend on internal and
external state. The hierarchy is dependant in a lot of cases on environmental context, it is
not fixed and rigid. These combinations will be discussed in more detail in the next two
sections with specific examples.
INVERTEBRATE NERVOUS SYSTEMS
Behaviour Mechanisms
Some behaviours in invertebrates can be attributed to specialised physiological properties
of certain neurons. For example, the sea snail Aplysia produces ink when a prolonged and
strong tactile stimulus is applied, i.e. it tries to blind any predator. However, the ink is
noxious to it as well, so unless the stimulus is prolonged, it does not release the ink. This
delayed response can be linked to special voltage sensitive channels in the motor neurons
involved in the behaviour, which oppose the normal depolarisation of the cell by the
sensory inputs. The depolarisation for activation is therefore much more slow in activating.
However other mechanisms underlie other behaviours. Neuronal circuits, more than one
neuron working in tandem with each other, are responsible for, for example the lateral
eye in Limulus, the horseshoe crab. The eye in Limulus is a compound eye made up of
separate photoreceptive units. Each unit contains about fifteen photoreceptive cells. These
cells are all coupled to one process, the eccentric cell where the action potentials are
generated. Each eccentric cell is networked laterally to adjacent eccentric cell axons to
form inhibitory synapses. An axon inhibited by its neighbour in turn inhibits that one. This
creates a system where axons of eccentric cells along a bright side of a dark-light border
are inhibited weakly by their neighbours in the dimmer light. Thus they generate more
actions potentials than brightly lit cells away from the border which are inhibited strongly
by all their neighbours. Conversely, the axons on the dimmer side of the border are
strongly inhibited and fire less vigorously than other dark axons away from the border.
Therefore, the perception of the border between light and dark is enhanced.
The study of the lateral in the Limulus provides a model of perception based on neuronal
circuits, collections of neurons working in tandem. However, there is also the specialized
neuron model for the inking in Aplysia. These and other studies have lead to a conclusion
that most information processing and behaviours are controlled at both levels, both cell
and circuit. This function has been observed such that some neurons innervate a single
patch of skin or muscle, but others can initiate complex behaviours involving many other
neurons. These initiators are called command neurons.
Rhythmic Motor Behaviour - Example Of Cell and Circuit Control
The ability to move and carry out purposeful acts distinguishes animals from plants.
Motor activity such as walking, swimming, breathing is rhythmic, i.e. it needs a
stereotypical, repetitive sequence of neural outputs and muscle contractions. The neuronal
circuit which controls rhythmic behaviour is termed a central pattern generator CPG.
The Function of A CPG
The Heart of The Leech
The Heart Of The Leech
A good example of rhythmic motor action is the heart beat, and one studied closely is the
heart system of the leech. In the leech, blood is pumped around its body by two heart
tubes running down each side of its body. The left and right tubes contract alternately.
The heart tubes are innervated by motor neurons termed heart-excitatory, HE, neurons.
The arrangement of these cells is an example of circuit level organisation. The leech has
21 ganglia, these motor neurons are found in ganglia 3 to 18 on both sides of each
ganglion. For this reason they are called HE(R) and HE(L) cells, those which innervate the
right and left tubes respectively. In addition, there are seven pairs of heart interneurons
located in the first seven ganglia HN(1- 7) - these cells control and integrate the HE
outputs and make up the CPG. The first four pairs HN cells HN(1-4) regulate the heart
rhythm, they can reset and entrain the whole system. HN(5-7) cannot reset the system,
but co-ordinate motor neuron inter-ganglia activity. All the HE cells are innervated by
HN(3) and HE(5-18) have an extra innervation from HN(4). HN(1) and HN(2) do not
directly innervate any motor neurons, but co-ordinate the activity if HN(3) and HN(4),
which are the principle controllers of the heart rhythm. All synapse between interneurons
and motor neurons are inhibitory, therefore activity in the interneurons is out of phase
with the motor neurons. Without inhibition the motor neurons would fire continuously.
Pairs of interneurons, i.e. the right and left of the same number inhibit each other
reciprocally, so that when one is firing, the other is inhibited, and so the heart tubes
contract alternately.
However, this is not the whole picture, something is needed to terminate the inhibition
after a certain time period, otherwise one neuron would be permanently inhibited and the
other permanently on. This is where the cell level control comes in. Neurons possess a
mechanism to oppose the hyperpolarisation which allows the cell to gradually depolarise.
Therefore the inhibited interneuron will fire again even if the inhibition is constantly
applied. Returning to circuit level control, HN(1) and HN(2) as previously mentioned, do
not interact directly with the motor neurons, instead they are used to reciprocally inhibit
HN(3) and HN(4).
The Circuitry Of The First Four Heart Interneurons
When HN(R3) and HN(R4) are active, HN(L3) and HN(L4) are inhibited, as are HN(R1) and
HN(R2). When HN(L3) and HN(L4) are active, HN(R3), HN(R4), HN(L1) and HN(L2) are all
inhibited. HN(4) is the orchestrator of the transitional activity, i.e. its time period to escape
from inhibition controls the others. Figure 10 shows the cycle of inhibitions and activations
in the leech. States A and C are quite long ~6 seconds, where as states B and D are short.
The Inhibitory Cycle Of HN(1-4)
The heart control of the leech is a prime example of the tandem working of circuit and cell
level control. The inhibitory nature and cross connections of the interneuron synapses
would be useless without the added speciality of the cell membranes to slowly depolarise
and oppose the inhibition to allow the other states to exist.
It is also important to recognise that it is not the nature of the cells in an isolated situation
that produces the behaviour. There is a temporal aspect to the dynamics of the system,
i.e. in the cylce shown above, teh working of the system is ocntrolled very much by the
dischare rate of HN(4) and it is this cell changing in time that controls the oevrall
behaviuor - a different discharge rate and the length and exact performance of the cycle
could be changed.
Neural Circuit Modulation
Neuromodulation can redefine the circuitry of a CPG in a variety of ways: it can switch the
circuit from one configuration to another so that quite different behaviours are mediated
by the circuit; it can also fine tune a circuit gradually narrowing its behaviour to suit the
situation. There are many different types of neuromodulator and a neural circuit can be
affected by a large number of these chemicals and each has a specific affect. One
neuromodulator can have multiple affects on a single neuron, or different effects on other
cells. Neuromodulators can affect at a circuit and cell level. Both chemical and electrical
synapses can be modulated, the neuromodulator can change membrane properties and
synaptic strengths. The different types of neuromodulator give the neural circuitry
adaptability are an important consideration in the modelling of neural networks.
Memory and Learning
Changes take place at synapses when those synapses are repeatedly activated, these
changes can last for weeks, months, longer. These adaptations have been examined in
detail, they will be discussed here using Aplysia as an example. The marine snail will
withdraw its gills when a stimulus is applied. A gentle stimulus applied to the
mantle/siphon causes rapid gill withdrawal. With repeated stimulation, the response
diminishes, this is called habituation. If stimuli is repeated over several training sessions,
habituation can last for weeks. A strong stimulus to the head or tail before a gentle
mantle/siphon stimuli sensitises the reflex, i.e. the response is greater. Again, more
training sessions mean longer lasting 'memory'.
Neuronal Interaction Involved in Memory and Learning
Habituation is localized to synapses between motor and sensory neurons, with repeated
stimulus the sensory neurons release less and less neurotransmitter. Sensitisation is
mediated via interneuronal synapses on sensory neuron terminals and action potentials
from sensory neurons in the head and tail. Either head or tail related stimuli activate
interneurons which release a neuromodulator at synapses with mantle sensory neurons.
These cells then release more neurotransmitter onto the motor neurons than is normally
the case, so the response is more pronounced. Long term habituation and sensitisation
can create structural changes in the sensory neurons themselves.
This kind of study add weight to the supposition that short term memory/learning is a
chemical change in the neuronal circuit which will eventually die off, but with repeated
stimulus, this can become a long term memory/learning as a structural change results
from the neuromodulation.
VERTEBRATE NERVOUS SYSTEMS
The nervous system in the vertebrate consists of two parts, the central and peripheral
nervous systems. The peripheral nervous system is made up of all the nerves and ganglia
which lie outside the brain and spinal cord.
The Central Nervous System
A Diagrammatic View Of The Central Nervous System
The Central nervous system consists of the
spinal cord and the brain, and unlike
invertebrate systems is highly centralised. All
higher
neural
functions,
perception,
movement control, learning and memory are
carried out in the brain. The spinal cord
contains the CPG's for rhythmic motor
behaviour, mediates reflexes and conducts
sensory information from peripherals to the
brain. Sensory neuron axons can do one of
three things on entering the spinal cord.
They can travel up the cord to its higher
levels or the brain, they can connect with
interneurons in the cord itself, or they can
connect directly to motor neurons.
The Brain
The brain consists of three main structures, the hindbrain, midbrain and forebrain.
hindbrain comes from the spinal cord and contains the medulla, cerebellum and pons.
midbrain is a small region above the pons. The forebrain consists of two main units,
containing the thalamus and hypothalamus and the other holding the basal ganglia
cerebral cortex.
The
The
one
and
Medulla and Pons
Nerve tracts run through the medulla connecting the spinal cord and the higher brain
centres. There are also many clusters of neurons which are involved in the control of the
head, face, eyes, tongue and vital body functions. Also to be found distributed throughout
the medulla and pons are groups of clusters of neurons called the reticular formation.
Their axons extend widely throughout the rest of the brain and spinal cord. The formation
regulates the level of activity in parts of the brain and is important in arousal and
awareness levels. Destruction in this part of the brain can lead to coma from which there
is no arousal. These formations also modulate pain pathways.
Hypothalamus
This is a regulatory centre of the brain - it is concerned with regulation of basic drives and
acts, e.g. eating, sexual activity, heart rate. It can also effect emotional behaviour, so
lesions can lead to aggressive behaviour.
The hypothalamus also regulates hormone release - the hypothalamus controls hormone
release from the pituitary gland which in turn controls the release of hormones from other
glands throughout the body, such as thyroid. Some of the hormones released by the
hypothalamus are released directly into circulation to their targets.
Cerebellum
The cerebellum has a highly regular structure and its main function is to co-ordinate and
integrate motor behaviour. The cortex gives the commands for movement, but the
cerebellum co-ordinates them with sensory input from the spinal cord and other sensory
systems to produce a smooth output.
Thalamus
The thalamus consists of clusters of neurons which: convey sensory information to the
cerebral cortex; or relay information about motor activity to the cortex. It is believed that
the inputs to the thalamus modify the sensory and motor information before passing it on.
Basal Ganglia
These are five prominent clusters that are positioned above, below and either side of the
thalamus. They are concerned with movement, its initiation and execution. They receive
most of their information from the cortex and their output returns there via the thalamus.
Cerebral Cortex
This is where the higher central nervous system functions are collected. Skilled
movements originate from here and it is considered the centre of consciousness, memory
and intelligence. It is divided into two hemispheres, the left and the right, and each
hemisphere has four lobes, frontal, parietal, occipital and temporal. The frontal lobes are
concerned with movement and olfaction, the parietal lobes with somatic sensation, the
occipital lobes with vision and the temporal lobes with audition and memory. Within each
lobe are areas dedicated to the initial cortical analysis of specific sensory information or
movements.
Structures and Pathways in the vertebrate brain
The Development of The Nervous System
The nervous system is a remarkable phenomenon in nature, highly complex, consisting of
almost incalculable numbers of nerves, neurons and their connections. How it develops is
of prime concern when designing a model.
The nervous systems cells all derive from neural plate cells Daughter cells produced after
cell division one produced migrate to their final positions, passing through an intermediate
zone on the way. Some neurons divide after migration and later form certain brain
structures. There are two theories to explain cell differentiation in the nervous system: 1 -
the lineage theory states that cells inherit developmental directives; 2 - the other suggests
that cells differentiate as a consequence of environmental clues, chemical signals.
Axon formation is another aspect of the nervous system development. Chemical affinity
suggests that cells have specific chemical markers which allow them to recognise each
other during development and regeneration. There is a certain amount of flexibility in the
formation of connections during these stages. There is also evidence for competition for
space during normal brain development, if axons do not compete successfully, their
terminal arbors are restricted and they end up with fewer synaptic connections.
Axonal formation is all very well, but the guidance of these structures to the correct places
in the brain to make connections is also necessary. Chemical gradients are a likely
mechanism, i.e. the axon growth follows the gradient, another is that certain cells act as
guideposts along the way. the axon grows towards them due to chemical attractors,
makes a connection and then continues, removing these guideposts can lead to stunting
in axonal growth. Axons may also follow pioneer cells, cells which form connections early
in development when distances between brain structures are short. One it has been
developed, the nervous system has to mature in which time the synaptic connections and
neuronal fields are subject to rearrangement and restriction. Also cells die during this
development due to competition for synaptic sites and lack of innervation. An agent
involved in this process, known as nerve growth factor, promotes: the survival of certain
neurons; the growth of processes from specific neurons; guides axonal growth.
Functional unit
• Neuron - a cell
Definitions
o Cell body - soma or perikaryon
o Fiber - neurites, may be afferent or efferent
o
•
Synapse - release of neurotransmitters
Divisions of the nervous system
o Central nervous system (CNS) - brain or spinal cord
o Peripheral nervous system (PNS) - cranial spinal nerves
o Autonomic nervous system (ANS) - hybrid of CNS and PNS
Central Nervous System (CNS)
•
•
•
•
Organization of components
o Cell bodies - nuclei
o Fibers - tracts
Brain
Spinal cord
o Location
o Structure
Meninges - coverings of CNS
o Dura - tough, outer layer - separate in vertebral column, but fused to interior
of skull
o Arachnoid - web-like membrane
o Pia - intimate surface of CNS structures
Peripheral Nervous System (PNS)
• Organization of components
o Cell bodies - ganglia
o Fibers - nerves
o Rule: for any nerve in the PNS, always know its fiber content and location of
the cell bodies
• Cranial nerves
o Definition - attached to brain and pierce skull
•
Spinal nerves
Autonomic Nervous System (ANS)
• Definition - a functional, more than anatomical system, partly central and partly
peripheral
•
Rules
o
o
o
o
•
Efferent only
2 neurons from CNS to target
1st neuron is in CNS - preganglionic or presynaptic
2nd neuron is in PNS - postganglionic or postsynaptic
Divisions
o Thoracolumbar (sympathetic)
 Cell bodies - intermedio-lateral gray of spinal cord segments, T1 - L2
 Fibers - distributed throughout the body
 Pathways
o
Craniosacral (parasympathetic)
 Cell bodies - associated with cranial nerve nuclei in brain, and in
sacral levels of spinal cord
 Fibers - restricted to viscera, reproductive organs, salivary glands
 Pathways