lp 8 neurology 2008
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Neurology
VTT 235/245 Anatomy & Pathology
The Nervous System
A complex communication and control system.
It monitors the animal’s internal and external environments and directs activities to maintain the well-being of the body.
2 Main Divisions
Central Nervous System- Composed of the brain and spinal cord.
Peripheral Nervous System- Consists of cord-like nerves that link
the CNS with the rest of the body.
Functions
Sensory Functions- The nervous system senses changes
from inside/outside the body and conveys this information to the spinal cord or brain.
Integrating Functions- In the brain and spinal cord, the sensory
information is received, analyzed, stored, and integrated to produce a response.
Functions…
Motor Response Functions- Instructs the body to do something,
such as contract a muscle or cause a gland to secrete its product(s).
Neurons & Supporting Cells
Neuroglia
“Glial cells”, glia- greek for glue! Structurally and functionally supports
and protects neurons. Outnumbers neurons 10:1. Not directly involved in the
transmission of information or impulses.
Neurons
Nerve cells. The basic
functional units of the nervous system.
Has a high requirement for oxygen.
Neuron Structure
Central cell body- “stoma” Dendrites-
Receive stimuli, or impulses, from other neurons and conduct the stimulation to the cell body.
May also be modified into sensory receptors that receive, or sense, stimuli such as heat, cold, touch, pressure…
Short, numerous, multi-branched projections extending from the cell body.
Neuron Structure…
Axons- Conduct nerve impulses away from the
cell body toward another neuron or effecter cell. A cell that does something when stimulated.
A single process can be very long. Often covered by a fatty substance
called myelin (white matter).
Neuron Structure…
Myelin sheath- Actually cell membranes of specialized
glial cells called oligodendrocytes in the brain & spinal cord, and Schwann cells in nerves outside of the brain and spinal cord.
These special glial cells are wrapped around the axon.
Neuron Structure…
Nodes of Ranvier- Small gaps in the
myelin sheath between adjacent glial cells.
Works with the myelin sheath to enhance the speed of conduction of nerve impulses along the axon.
Organization of the Nervous System
CNS vs. PNS
CNS- anatomically composed of the brain and spinal cord.
PNS- made up of components of the nervous system that extend away from the CNS, towards the periphery of the body.
Cranial nerves- nerves of the PNS that originate directly from the brain.
Spinal Nerves- nerves of the PNS that originate from the spinal cord.
Afferent vs. Efferent
Afferent nerves- Conduct nerve impulses towards the
CNS. Conducts sensations from the sensory
receptors in the skin and other locations in the body to the CNS.
Also called sensory nerves.
Afferent vs. Efferent…
Efferent nerves- Conduct impulses from the CNS out
toward muscles and other organs. They cause skeletal muscle contractions
and movement.
Neuron Function
Resting State
When a neuron is not being stimulated, it is in a resting state.
The cell membranes of neurons are electronically polarized when at rest (like tiny, charged batteries).
Specially charged molecules located in the neuron’s cell membrane pump sodium (Na+) ions from inside the neuron to the outside.
They also pump potassium (K+) ions from the outside to the inside.
Resting State…
This specialized molecule is called the sodium-potassium pump.
The action of the sodium-potassium pump causes a higher concentration of Na+ to accumulate outside the cell.
The pump’s actions and the negative charges inside the cell cause a higher concentration of K+ to accumulate inside the cell.
Resting State…
This keeps the cellular membrane between Na+ and K+ polarized.
The distribution of positive and negative charges creates a difference in electrical charge across the membrane. Inside the cell- negative
This electrical difference in charges is called the resting membrane potential.
Depolarization
When an impulse from an adjoining neuron stimulates another neuron, a set of specific steps occurs, resulting in the nerve “firing” or depolarizing.
The Na+ channel opens and allows only Na+ ions to pass through it by passive diffusion into the cell.
Depolarization refers to this opening of the Na+ channels and the sudden influx of many Na+ ions.
Depolarization…
The inside of the neuron goes from a negative charge to a positive charge.
The significant change in electrical charge is also referred to as the action potential.
Repolarization
Within a fraction of a second, the Na+ channels snap shut, halting the influx.
At the same time, potassium channels open and only allow K+ ions to pass through them.
K+ ions passively diffuse out of the cell by a concentration gradient and positive cell charge.
Repolarization…
The outflow of K+ ions continues until the channels close a split-second after they have opened.
This causes the charge inside the cell to swing back in the negative direction.
The change of the cell’s charge back to negative is called repolarization.
All-or-Nothing Principle
Not every depolarization stimulus results in a complete depolarization-repolarization cycle.
The initial stimulus must be sufficient enough to make the neuron respond.
When the stimulus is strong enough to cause complete depolarization, it is said to have reached the threshold.
All-or-Nothing Principle…
Regardless of how strong the initial stimulus was, if it was sufficient enough to achieve threshold, the action potential would be generated and conducted along the entire length of the neuron with a uniform strength.
All-or-Nothing Principle…
This phenomenon is called the all-or-nothing principle, because either the neuron completes a depolarization to the maximum strength, or it does not depolarize at all.
Refractory Period
If a second threshold stimulus arrives at the dendrites while the Na+ channels are open or while K+ molecules are moving through their open channels, the stimulus is incapable of a second depolarization.
The Synapse
How Neurons Communicate
The Synapse…
Once the action potential has been successfully conducted to the end of the axon, the nerve impulse must be transmitted to the next neuron or to the cells of the target organ or tissue.
Because two neurons do not physically touch, the depolarization wave is unable to continue to the next neuron.
Instead, the neuron must release a chemical that stimulates the next neuron or cell.
The Synapse…
This perpetuation is called a synaptic transmission.
The synapse is the junction between two neurons or a neuron and a target cell.
The synapse consists of a physical gap between the two cells called the synaptic cleft.
The Synapse…
Presynaptic neuron- the neuron bringing the depolarization wave to the synapse and releasing the chemical to stimulate the next cell.
Neurotransmitter- the chemical released from the presynaptic neuron.
Postsynaptic neuron- the neuron that contains the receptors that receive the neurotransmitter.
The Synapse…
On the postsynaptic membrane are specialized proteins called receptors.
The neurotransmitter binds with these receptors and trigger a change in the postsynaptic cell.
The postsynaptic membrane receptors are very specific about which neurotransmitters they will allow to bind.
Neurotransmitters
Can be classified into 2 categories- Excitory- usually
causes an influx of Na+ so that the postsynaptic membrane moves toward the threshold.
Inhibitory- moves the charge away from the threshold.
Neurotransmitters…
Acetylcholine- Excitory or inhibitory, depending on its
location in the body. Excitory- at the junction between somatic
motor neurons that stimulates muscle fibers to contract.
Inhibitory at the site where nerves synapse with the heart and slows the heart rate.
Neurotransmitters…
Catacholamines- Norepinepherine- associated with the “fight-
or-flight” reactions of the sympathetic nervous system.
Epinepherine- released primarily from the adrenal medulla to participate in the “fight-or-flight” response.
Dopamine- Found in the brain. Involved in autonomic functions and muscle control. Low dopamine= Parkinson’s disease.
Neurotransmitters…
GABA & Glycine- GABA- gamma-aminobutyric acid. GABA is found in the brain, glycine is
found in the spinal cord.
Neurotransmitters…
One postsynaptic membrane may have multiple types of presynaptic neurons across the synaptic cleft.
By having both, the nervous system can selectively increase or decrease the activity of specific parts of the brain or spinal cord.
Stopping & Recycling Neurotransmitters
The body needs a way to stop the neurotransmitter or the excitory or inhibitory effect would continue.
In the case of acetylcholine, it is broken down into the enzyme acetylcholinesterase. “-ase”- enzyme
Stopping Neurotransmitters
If nothing breaks down neurotransmitters, the effect would continue indefinitely.
Ex.- This is what happens with organophosphate toxicity: The insecticide combines with
acetylcholinesterase and inactivates it. Overstimulation of acetylcholine
receptors results in vomiting, diarrhea, difficulty breathing, and constricted pupils.
THE BRAIN
CerebrumCerebellum
DiencephalonBrainstem
The Brain
Cerebrum
Made up of gray and white matter fibers.
The largest part of the brain. Responsible for higher-order
behaviors: Learning Intelligence
Cerebrum…
Receives and interprets sensory information- Initiates nerve impulses to skeletal
muscles. Integrates neuron activity normally
associated with: Communication Emotional expression Memory
Cerebrum…
The wrinkled appearance is due to folds called gyri, separated by deep grooves called fissures and more shallow grooves called sulci.
The most prominent groove is the longitudinal fissure which divides the cerebrum into the right and left cerebral hemispheres.
Each hemisphere is divided by sulci into lobes.
Different lobes specialize in certain functions.
Cerebellum
Located caudal to the cerebrum. The second largest component of the
brain. Allows the body to have coordinated
movement: Balance Posture Complex reflexes
Diencephalon
Serves as a nervous system passageway between the primitive brainstem and the cerebrum.
Pituitary- the endocrine “master gland” that regulates hormone production and release.
Thalamus- acts as a relay station for regulating sensory inputs to the cerebrum.
Diencephalon…
Hypothalamus- and interface between the nervous and endocrine systems. Plays a major role in:
Temperature regulation Hunger Thirst
Brainstem
The connection between the brain and spinal cord.
The most primitive part of the brain. Composed of: the medulla oblongata,
pons, and midbrain. Maintains basic body support
functions.
Brainstem…
Heavily involved in autonomic control functions related to: The heart Respiration- including coughing,
sneezing, and hiccuping. Blood vessel diameter Swallowing Vomiting
Meninges
A set of connective tissue layers that surround the brain and spinal cord.
They contain a rich network of blood vessels that supply oxygen and nutrients to the superficial tissues of the brain and spinal cord.
Cerebrospinal Fluid
The brain and spinal cord are bathed and protected from the hard inner surfaces of the skull and spinal column by CSF.
It circulates between layers of meninges and through cavities inside the brain and spinal cord.
The chemical composition may be involved in the regulation of certain autonomic function, such as respiration and vomiting.
Blood-Brain Barrier
A functional barrier separating the capillaries in the brain from the nervous tissue itself.
The composition results in a cellular barrier that prevents many drugs, proteins, ions, and other molecules from readily passing from the blood into the brain.
In this way, the BBB protects the brain from many poisons circulating in the bloodstream. Ex.- Ivermectin Parasites and insects don’t have a BBB so the drug kills
them by allowing it to reach target receptors in the brain.
Cranial Nerves
A special set of 12 nerve pairs in the peripheral nervous system that originate directly from the brain.
Cranial NervesColville p. 155
Nerve Type Function
I Olefactory Sensory Smell
II Optic Sensory Vision
III Occulomotor MotorEye movement,
pupil size, focusing lens
IV Trochlear Motor Eye movement
V Trigeminal MixedS: eye & faceM: chewing
VI Abducens Motor Eye movement
VII Facial Mixed
Face and scalp movement, salivation,
tears, & taste
VIIIVestibulocochlea
rSensory Balance/hearing
IX Glossopharyngeal Mixed
S: 1/3 caudal tongue taste
M: swallowing & salivation
X Vagus Mixed
S: GI tract, resp., M: larynx, pharynx,
parasympathetic motor to the
abdominal viscera & thoracic organs
XI Spinal Accessory MotorSkeletal muscles of the
neck and shoulderAccessory with vagus
XII Hypoglossal MotorSkeletal muscles of the
tongue
I
II
III
IVV
VI
VIIVIII
IXXXI
XII
Pyramid
Pons
Cerebralpeduncle
Pituitary gland
Optic chiasma
Olfactory bulb
Cranialnerves
Spinal Cord
The caudal continuation of the brain stem outside the skull.
It conducts sensory information and motor instructions between the brain and the periphery of the body.
Autonomic Nervous System
Autonomic Nervous System
Controls many functions of the body on a subconscious level.
These autonomic functions are performed by two divisions: Sympathetic nervous system Parasympathetic nervous system
These two systems generally have opposite effects on eachother.
IV. Pathology
By: Dr. Tuya Buchanan, DVM
Cerebral Pathology
Cerebral Trauma/ Hemorrhage
Severe bruising of the brain causes capillary rupture & bleeding of the brain resulting in increased ICP (intracranial pressure)
Cerebellar Hypoplasia
The cerebellum does not grow properly
Due to in utero viral infections or injury, or just bad genetics Panleukopenia in cats Herpesvirus infections in dogs Vaccinating pregnant animals w/ MLV
Problems are first noticed when the young animal starts to ambulate
Cerebellar Hypoplasia (cont)
Signs include: Ataxia
(incoordination) Hypermetria
(over reaching when walking)
Intention tremors
Broad-based stance
Hydrocephalus
Commonly referred to as “water on the brain”
Characterized by a dome-shaped head
Either too much CSF is produced or there is inadequate drainage
Cerebral Hypoxia
Lack of oxygen to the brain
Numerous causes- clots (“stroke”), heart disease, renal disease, hyperthyroidism in cats, parasites, etc.
Brain Tumors
Can arise from any type of neuro cells
Clinical signs depend on the location, size, and degree of pressure they are putting on the brain
Glial tumors include astrocytomas & oligodendrogliomas
Meningiomas are another type of tumor
Brain Tumor in a Boxer
Strychnine Poisoning
Strychnine blocks an inhibitory neurotransmitter (glycine) in the medulla & spinal cord
So you end up with excitation of neurons which results in muscle rigidity & seizures
Animals die from lack of oxygen to vital body parts and exhaustion
Some Infectious Agents which cause Cerebral Pathology
Rabies
Rhabdovirus Furious Rabies- aggressive, snarling,
seizuring, drooling animal Dumb Rabies- depression, dementia,
hind-end weakness, & drooling Some animals will show obsessive
licking/self mutilation of an old wound No ante-mortem tests- need the brain
Rabies (cont)
Rabies (cont)
Bad Raccoon! A Negri Body inside a neuron. When seen this viral inclusion body can mean only Rabies!
Distemper
Caused by a Paramyxovirus The virus depletes the immune
system so the puppy is very prone to secondary infections
Starts out respiratory, then GI signs, then neurological signs
“hard pads” usually occur end-stage
Distemper (cont)
Difficult to diagnose ante-mortem
Serology can be very unrewarding
Can do immunoflourescent assays for the virus in conjunctival scrapings (or wait for necropsy)
Equine Encephalitis
Caused by a virus
3 forms: Eastern, Western, & Venezuelan
Horses have a fever, appear sleepy, and may show incoordination of the rear-end; it can progress to full paralysis and death
Seizures
A seizure is an episode of abnormal electrical activity in the brain resulting in: loss or altered consciousness increased muscle tone involuntary urination & defecation
Most seizures are grand mal but some may be petit mal where there is not a complete loss of consciousness and may only involve a limb shaking or small body tremors
Seizures (cont)
Seizures have either an intracranial or an extracranial cause
Intracranial seizures are caused by primary CNS disease (problem is inside the brain itself)
Extracranial seizures are caused by organ dysfunctions or toxins which have secondary effects on the brain (problem is outside the brain)
Intracranial Causes of Seizures
Brain tumors- primary or those that metastasize to the brain
Bacteria (abscesses) Viruses (distemper) Protozoa (toxoplasmosis) Fungal Hydrocephalus Idiopathic epilepsy
Extracranial Causes of Seizures
Hypoglycemia Liver disease Renal disease Hypocalcemia Toxins- strychnine, lead,
organophosphates
Spinal Cord Pathology
Wobbler Syndrome
In horses, known as Cervical Stenotic Myelopathy
In dogs, known as Caudal Cervical Spondylomyelopthy
Either way, it’s a malformation of the cervical spine causing cord compression
Wobbler (cont)
Breeds of dogs include Dobermans, Great Danes, & Basset Hounds
Affected dogs are mostly young adults
Signs range from hind-end weakness to tetraplegia, neck pain, & the neck is flexed ventrally
Wobbler (cont)
Seen in young, rapidly growing horses, especially Thoroughbreds
Over nutrition is a big contributing factor
Signs consist of limb weakness and incoordination
Vertebral Fractures
Usually secondary to trauma (HBC) or bone pathology (osteomyelitis)
Spinous process fractures do not usually cause problems
Problems arise when the spinal cord becomes compressed or is also fractured
Fractures (cont)
Spinal Cord Concussion
Usually secondary to trauma, such as HBC
Due to severe bruising of the cord, the motor nerves do not function properly
These dogs can look completely paralyzed but radiographs +/- CT scan will show no abnormalities
With time and steroids, there will be improvement
Intervertebral Disc Disease (IVDD)
Affects dogs, rarely cats Disease can occur anywhere in the
spinal tract but the lumbosacral and cervical regions are most common
Can have partial or complete herniation of the disc up into the spinal cord
Typically a degenerative process Dachshunds, Beagles, Pekingese, etc.
IVDD (cont)
Clinical signs can be acute or chronic Severity of signs depends on the degree
of spinal cord compression Signs progress from ataxia/incoordination
& loss of conscious proprioception >> paresis (muscle weakness) >> paralysis >> loss of deep pain sensation
Animals with acutely compressed cords can be very painful while there may be little to no pain with chronic compression
IVDD (cont)
IVDD (cont)
Degenerative Myelopathy
Most common in German Shepherds & Welsh Corgis
A progressive degeneration of the axons & myelin of the white matter of the SC
Dogs are usually > 5 yrs old and develop a gradual onset of non painful ataxia & weakness in the pelvic limbs
Poor prognosis; most are euthanized within 1-3 yrs
Spinal Neoplasia
Relatively common in dogs & cats Tumors are classified according to their
relationship with the spinal cord & meninges Extradural: outside the dura mater; compress
SC; most common SC tumor in dogs/cats Intradural-extramedullary: in the
subarachnoid space; compress SC Intramedullary: inside the SC itself; least
common type
SC Neoplasia (cont)
Laboratory
CSF Analysis
Evaluation
Evaluation is important to patients with neurologic disease.
Helpful in evaluating patients with unexplained fever.
Collection Sites
Atlantooccipital Lumbosacral
Analysis
Examination of CSF should consist of: Physical characteristics-
Color Turbidity
Total nucleated cell count (TNCC) Erythrocyte count Protein concentration Cytologic examination
Analysis…
Analysis of CSF should be done as quickly as possible. The low protein concentration of CSF
cause the cells to degenerate rapidly.
Dissection