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The Neurological System
Development:


The entire nervous system is derived from from the ectodermal germ layer
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Neural crests form from the neuroectoderm and form the peripheral nervous
system while the neural tube forms the central nervous system
...

The brain begins as 3 primary vesicles: the prosencephalon (forebrain), the
mesencephalon (midbrain) and the rhombencephalon (hindbrain)
...


Classification:

The nervous system can be classified by anatomy, segmentation or function
...


Suprasegmental brain (recent)
⇒ Cerebrum
⇒ Midbrain
⇒ Cerebellum

Segmental brain (primitive brain)
⇒ Pons
⇒ Medulla oblongata


Functional Classification:

Somatic Nervous System (innervates structures with voluntary control e
...
skeletal muscle
tissue)

Afferent/sensory – efferent/motor

Visceral Nervous System (innervates structures without voluntary control e
...
the heart)

Afferent/sensory – efferent/motor


Autonomic Nervous System


Traditionally, the autonomic nervous system was split into two categories:
sympathetic and parasympathetic
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The central control of the CNS comes from the brain, particularly the hypothalamus,
amygdala, hippocampus, olfactory cortex, nucleus of the solitary tract and the reticular
formation
...
This classification refers to the
location of the preganglionic cell bodies of each nervous system
...
It is composed of the submucosal plexus and the myenteric plexus in the
submucosa and smooth muscle layers of the gut wall
...
The two hemispheres are composed of superficial grey matter and central
white matter and are connected by a longitudinal cerebral fissure
...
From this extends caudally
an olfactory peduncle which divides into the lateral and medial olfactory tracts
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The cerebral cortex is concerned with nervous reactions resulting in consciousness
which is the highest type of nervous correlation
...


White matter contains various types of myelinated nerve fibres:
Association fibres – Connect different parts of the cortex
...
The largest bundle forms the corpus
callosum
Projection fibres – Connect the cerebral cortex with other parts of the brain and the spinal
chord
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Cerebellum:


The cerebellum lies caudal to the cerebrum, with a transverse cerebral fissure
separating the two
...
Grey matter can be found within the
cerebellar cortex and also within the central (basal) nuclei
...










The pattern of white matter within the cerebellum is
known as the arbor vitae due to its resemblance to a tree
...
It contains a number of nuclei which control
functions such as the cardiovascular and respiratory
systems, digestive processes, eye movements and
posture via reflexes
...
The brainstem is
connected to the cerebellum by the cerebellar
peduncles
...

It is composed of paired groups of nuclei,
separated by the central third ventricle
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Midbrain:


Immediately caudal to the diencephalon, the midbrain gives rise to the oculomotor
(III) and trochlear (IV) cranial nerves and has a central canal called the mesencephalic
aqueduct
...
The rostral colliculus is a visual
reflex centre and the caudal colliculus is an auditory reflex centre
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Pons:


This contains the rostral end of the fourth ventricle and gives rise to the trigeminal
(V) cranial nerve
...


Medulla Oblongata:


Extending from the transverse fibres of the pons to the ventral rootlets of the first
cervical spinal nerve, the medulla oblongata can be distinguished by its bilateral longitudinal
bands of white matter
...
The medulla
oblongata and pons contain many ascending and descending pathways and also several
reflex centres associated with the regulation of visceral functions
...
It forms four ventricles which are connected to each other with a
central canal which leaves through the brainstem (mesencephalic aqueduct)
...
CSF has the same composition as interstitial fluid in the central nervous system
and drains into the venus sinus
...
All blood is pooled
into the Circle of Willis before being branched off into the brain
...


In the dog and horse, the internal carotid and basilar arteries supply the Circle of
Willis and most of the brain while the vertebral artery supplies the rest of the brain
...
The vertebral artery supplies the medulla oblongata
...
There is an anastomosis between the maxillary and vertebral arteries which connects
them to the distal third of the internal carotid
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Meninges:


The brain and spinal chord are enclosed by three layers of connective tissue known
as the meninges:
Dura mater – Thick, tough, fibrous outer membrane
...

Pia mater – Innermost layer firmly bound to the underlying nervous tissue
...

It contains a fine network of connective tissue fibres that originate from the arachnoid
...
It has a cervical intumescence and a
lumbar intumescence (enlargements) and a conus medullaris at the end where it becomes
thin and elongated
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The spinal chord is split into segments (36 in the dog), each of which give rise to a
pair of spinal nerves leaving through the intervertebral foramina
...
The dorsal branches innervate the epaxial (dorsal)
tissues while the ventral branches innervate the hypaxial (ventral) tissues including the limb
...

Projection neurons – Send axons into white matter to form cranial projecting pathways
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Spinal Nerves:


The spinal nerves are numbered according to the number of the spinal vertebrae
cranial to them e
...
C6 à spinal nerve C5
...


The first cervical spinal nerve leaves through the lateral vertebra foramina and spinal
nerves 2-7 leave through foramina cranial to vertebrae of the same number
...


At the thoracolumbar junction, spinal nerves are positioned within their
corresponding vertebrae
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Cauda equine – Caudally streaming spinal roots
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Grey Matter:


Divided into dorsal, lateral and ventral horns in the centre of the spinal chord, the
grey matter consists of neuron cell bodies, dendrites and axons and a rich blood supply
...



White Matter:


Has both myelinated and non-myelinated axons divided into dorsal, lateral and
ventral funiculi (columns)
...


Tract – A bundle of functionally related axons in the central nervous system
...
Tracts that carry sensory information are ascending while ones that carry motor
commands are descending
...
There are five main motor
tracts found in white matter: corticospinal tract, vestibulospinal tract, reticulospinal tract,
tectospinal tract and rubrospinal tract
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Skeletal muscles do not contract until they are commanded to
by the alpha lower motor neuron
...
In here, it moves through the marginal
nucleus and the nucleus proprius in the grey matter and enters the spinothalamic tract

where the signal is carried to the thalamus
...

2nd order neurons extend across the midline and join other axons in the spinothalamic tract
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Spinocervicothalamic pathway:

1st order neurons exist between the receptor and the spinal ganglion
...

3rd order neurons from the lateral cervical nucleus cross the midline and ascend to the
contralateral thalamus
...



Spinal Reflexes:

A reflex is an inherent, subconscious, relatively constant response to a particular stimulus
...

Some examples of spinal reflexes are:
Myotatic reflex – muscle stretch






Withdrawal reflex






Panniculus reflex






Perineal reflex – anal constriction

Patellar reflex:


A stretch stimulus on the patellar tendon travels to the spinal chord and along an
efferent neuron to the quadriceps muscle, causing it to contract
...


Withdrawal reflex:


This reflex tests primarily for the sciatic nerve (L6, L7, S1) and is elicited by applying a
noxious stimulus to the distal part of the limb causing the entire limb to withdraw
...
A myelinated neuron is one
surrounded by a fatty electrical insulator known as the myelin sheath, produced by the
expanded plasma membrane of a neighbouring Schwann cell
...
It is in the nodes of Ranvier that the voltage gated sodium channels are
confined to and depolarisation at one node will generally reach the threshold of the next
causing the action potential to jump between nodes and resulting in faster transmission
...

G-protein coupled receptors – Activation triggers activation of a G protein which either
directly modifies ion channel function or triggers the production of a chemical second
messenger which modifies ion channels for slower transmission
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The sodium
channels open causing sodium to move into the cell causing membrane depolarisation
which spreads across the neuron
...
001 seconds after the first, another action
potential will not generate as the neuron is in its refractory period
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When an action potential reaches and depolarises the axon terminal, voltage gated,
pre-synaptic calcium channels open causing calcium ions to enter the cell
...
The neurotransmitters diffuse across the synaptic cleft
and activate receptors on the post synaptic membrane and, occasionally, pre-synaptic
receptors (feedback)
...
They can be classified by chemical structure or functional role (excitatory/inhibitory)
...
For EPSP,
neurotransmitter binding causes
increased post synaptic permeability to
sodium ions by opening sodium channels
to cause a small depolarisation
...




A single EPSP alone is not enough to bring a post synaptic membrane to action
potential threshold
...


Temporal summation – High frequency activation of a single presynaptic terminal can cause
summation if the interval between pulses is less than 15 milliseconds
...
For this reason,
synapses which terminate on the low resistance soma have more effect than those on the
dendrites and so multiple of these can generate an action potential
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Colds and ischaemia reduce blood flow to nerve tissue and so
can also have an inhibitory effect
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It is a chemical synapse using
the neurotransmitter acetylcholine, separated
by a synaptic cleft
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The resting
membrane potential of a muscle cell is
-80mV which is reduced by influx of sodium
...



Acetylcholinesterase is an enzyme concentrated on the external surface of the post
synaptic membrane which breaks down acetyl choline, causing the sodium channels to close
to allow the arrival of a new action potential
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It can occur in companion animals due to
tainted/improperly canned food, presenting with muscle weakness, respiratory distress and
difficulty swallowing
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Myasthenia gravis:


A disease commonly seen in dogs and cats which can be either congenital or
acquired
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Tetanus:


Caused by the clostridium tetani bacterium, the tetanus toxin blocks the
transmission of inhibitory nerve signals leading to over excitation in muscles
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Pain:

Pain – An unpleasant sensory and emotion experience typically evoked from tissue damage
...
e
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Acute pain is experienced with real or potential tissue damage as an appropriate protective
mechanism; it can, however, become pathological and often chronic
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Transmission:


There are various sensory pathways which carry pain information to the brain via
pain fibres
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C fibres are polymodal as they respond to chemical, mechanical and
thermal stimuli leading to a slow burning pain
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This pain pathway has the ability to discriminate the location of pain
due to its high degree of somatotophy
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Spinoreticular:






Thought to be bilateral fibres ascending, some
decussate and ascend the contralateral chord to
reach the reticular formation in the brainstem
before projecting to the thalamus and cortex
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It can be
clinically tested by pinching hard
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The gate control theory is that somatic non-painful signals can inhibit the
transmission of pain signals in the spinal chord
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These act as gates that modulate pain
impulse traffic on route to the thalamus in response to a non-nociceptive input
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Paresthesia – pins and needles
Dysesthesia – burning sensation

Hyperalgesia – lower pain threshold or exaggerated response to noxious stimulus
Allodynia – pain in response to a non-noxious stimulus
Deafferentation pain – peripheral nerve lesions such as phantom limb pain
Psychogenic pain – occurring with or without an organic cause or disproportionate to the
organic cause


Excitotoxicity:


For normal activity to occur, there must be a balance of inhibition and excitation in
the neurological system
...
Seizures occur when there is desynchronisation of
neuronal firing due to abnormal and excessive excitation of a population of neurons
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Glutamate transporters remove excess glutamate
to avoid over-excitation
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It acts on
GABAA (Cl- ionotrophic) and GABAB (K+ metabotrophic) receptors to bring the membrane
potential further from the action potential threshold
...
They can be caused by excitation due to too much glutamate causing
inward current of sodium and calcium ions or by inhibition caused by too little GABA causing
inward chloride current and outward potassium current
...


Tonic action – Sustained contraction
Clonic action – Rhythmic contraction
Simple – Patient is conscious
Complex – Patient is unconscious


Excitotoxicity in seizures:



Caused by the prolonged opening of glutamate receptors causing continuous
depolarisation causing the cell to swell and rupture due to increased sodium and water
entry
...



Excitotoxicity in strokes:


A thrombus in a blood vessel in the brain starves the cells of oxygen causing them to
depolarise, rapidly fire and release lots of glutamate
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Excitotoxicity in spinal chord injuries:




Same as seizure but begins with injury to the spinal chord
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The ureters
enter the bladder at an oblique angle to create a
valve-like entry to reduce urinary reflux
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The bladder is controlled by a complex
relationship between the somatic and

autonomic nervous systems (involuntary and voluntary)
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The sympathetic nervous system
innervates the bladder from the sympathetic chain through L2 and hypogastric nerves
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For new born and untrained animals, urination is largely a spinal reflex that can,
however, be trained in some species
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Urination can be induced
by various means; stress, trans-abdominal compression or, in large animals, whistling or
titillation of the vulva
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When the injury is spinal,
the resulting effect on the bladder depends on the location of then injury
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Lumbar region – If sympathetic chain is intact then there will still be control of the internal
sphincter so the bladder will distend until it reaches its threshold before leaking
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Ureterorenal Reflex:


When the bladder fills, pain impulses from the ureters induce sympathetic reflex
vasoconstriction in the renal arterioles restricting the amount of blood that is being filtered
...
This is important in preventing excessive
flow into the ureters when they are blocked
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When this vesicoureteric junction becomes incompetent, often in the case of
urinary tract infections, there can be a reflux of urine which causes renal damage

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