Central Nervous System (CNS)

Comprised of brain and spinal cord

Peripheral Nervous System (PNS)

Comprised of spinal nerves, cranial nerves, and ganglia

Cerebrum

• Seat of consciousness and cognitive functions
• Receives all sensory info that reaches conscious perception
• Makes decisions on the most appropirate responses
• Initiates/directs respnoses as needed

Cerebral Cortex

• Outer layer (gray matter) of cerebrum
• Major function is consciousness

Clinical signs of cerebral cortex disease

• Disturbances of consciousness
• Paresis (weakness) of voluntary mov't
• Disturbances of sensory function perception
• Seizures

Cerebellum

• Adjusts activity in the motor centers of the brain to insure that the rate, range and force of a voluntary movement is appropriate to the intended task
• Does NOT initiate or direct voluntary movement

Clinical signs of cerebellar disease

• Ataxia (inability to coordinate muscle activity during voluntary mov't)
• Spasticity (Increase in muscle tone at rest and during movement)
• Dysmetria (inability to regulate the rate, range, and force of movements)
• Hypermetria (marked increase in range of voluntary mov'ts of limbs causing over measurement of gait
• DOES NOT cause paresis (weakness in mov't)

3 Functions of Spinal Cord

1. Sensory input and Motor output for neck, trunk, limbs, and tail. Receives sensory input from peripheral nerves via dorsal roots, that originates from receptors Motor commands are sent from neurons in the spinal cord to skeletal and smooth mm.\
2. Reflex control of patterned mov't and adjustment of glandular activity in response to local afferent input w/o voluntary control via interneurons
3. Transmission of info to and from brain.

Diencephalon

Most rostral part of the brainstem containing:

• Thalamus: centrally located
• Hypothalamus: below thalamus
• Epithalamus: dorsal midline

Infundibulum

Connects the hypophysis to the tuber cinerium

of the hypothalamus. (Lumen of infundibulum communicates with third ventricle)

Tectum

• Part of midbrain dorsal to the mesencephalic aqueduct
• Composed of the Corpora Quadrigemia, which consist of 4 grps of neuronal cell bodies (the 2 pairs of rostral and caudal colliculi, where the rostral colliculi are associated with the visual system, and caudal colliculi are associated with the auditory system

Mesencephalon

Contains:

• Mesencephalic aqueduct
• Tectum (Dorsal to the aqueduct)
• Cerebral peduncle (aka cerebri; ventral to the aqueduct)

Brachium/Commissure of the caudal colliculus

• Brachium runs rostroventrally from the caudal colliculus to the medial geniculate nucleus of the thalamus
• Commissure runs on the dorsal surface and connects the two caudal colliculi

Lateral Geniculate Nucleus

• lateral eminence on the caudodorsal surface of the thalamus, which then connect with the rostral colliculus of the midbrain via the brachium of the rostral colliculus
• receives fibers of the optic tract and functions in the visual system

Medial geniculate nucleus

• Functions in the auditory system and connects to the caudal colliculus via the brachium of the caudal colliculus.

Cerebral Peduncle

Consists of (from dorsal to ventral):

• Tegmentum (reticular formation)
• Substantia Nigra
• Crus cerebri

Metencephalon

Composed of cerebellum and pons, which are connected via the cerebellar peduncles

Rostral medullary velum

• forms the roof of the fourth ventricle between the caudal colliculi rostrally, and the midventral surface of the cerebellum caudally
• Crossing fibers of CN IV pass through the velum

Myelencephalon

• Medulla: extends from the transverse fibers of the pons to the level of the ventral rootlets of the first cervical spinal nerve
• Contains: Pyramids and Trapezoid body

Trapezoid body

• Rostral part of the medulla with fibers that run parallel to the transverse pontine fibers
• Continuous with the CN VIII and cochlear nuclei laterally and fuctions in auditory system

Pyramids

• Longitudinal fiber bundles coursing the length of the ventral medulla
• Continuation of pontine fibers that didn't terminate in the pontine nuclei
• Separated by the ventral median fissure
• Axons continue into the spinal cord

Decussation of Pyramids

• Where the ventral median fissure ends on the ventral myelencephalon at the level of CN XII

Cerebellar peduncles

Connects the cerebellum to the brainstem:

• Rostral cerebellar peduncles: to mesencephalon
• Middle cerebellar peduncles: to pons
• Caudal cerebellar peduncles: to myelencephalon

Median Sulcus

Groove in the center of the floor of the fourth ventricle

Sulcus limitans

Longitudinal groove on the lateral wall

of the fourth ventricle

Caudal medullary velum

• Roof of the 4th ventricle caudal to the cerebellum
• Thin layer of ependyma lining ventricle
• Attaches to the caudal cerebellar peduncle and the fasciculus gracilis. This attachment at the apex is called the obex
• At this level, 4th ventricle is continuous with the central canal of the spinal cord

Fasciculus gracilis

• Longitudinal bulge flanking the dorsal median sulcus
• Ascends the entire length of the spinal cord and functions primarily in pelvic limb proprioception

Fasciculus cuneatus

• Longitudinal bulge lateral to the fasciculus gracilis
• Ascends the spinal cord from the mid-thoracic region and functions primarily in thoracic limb proprioception

Telencephalon

Cerebrum

Corpus callosum

• commissural pathway for axons crossing between the neopallium of each cerebral hemisphere

Name the 3 commissural connection between hemispheres

• Corpus callosum: connects the neopallial portion of each hemisphere
• Rostral commissure: connects the paleopallial or olfactory components of each hemisphere
• Hippocampal commisure: located just caudal to the junction of the crus of each fornix. This connects the archipallial components of each hemisphere

Internal capsule

• contains projection fibers that course between the telencephalon and the diencephalon and descend from the telencephalon to the brain stem and the spinal cord
• Sensory afferents get relayed in the thalamus to the cerebrum through the internal capsule.

Fornix

• connects the hippocampus with the diencephalon and rostral cerebrum
• begins caudally by the accumulation of fibers on the lateral side of the hippocampus

Septum Pellucidum

Vertical sheet of tissue ventral to the corpus callosum which separates the left and right lateral ventricles,

3 General Functions of the Spinal Cord

1. Sensory input (via dorsal roots) and motor output (ventral roots) for neck, truck, limbs, and tail.
2. Reflex control of patterned mov't and adjustment of glandular activity in response to local afferent input without voluntary control
3. Transmission of info to and from brain. Ascending fibers carry sensory info, and descending fiber carry motor commands.

Spinal nerve

• Formed by a fusion of a dorsal (sensory) and ventral (motor) root. 
• Eash spinal cord segment has assoc. pair of spinal nerves

Nissel Bodies

Ergastoplasm within cell body of a neuron. Clusters of rough ER, free ribosomes, and free polysomes

Axolemma

Cell membrane of the axon

Axoplasm

Cytoplasm within the axon

Telodendrion

Terminal branches of the axon, which

terminates on the effector.

Multipolar neurons

Neurons with many dendrites, which are described according to the shape of the cell body (e.g. Stellate, pyramidal, pyriform).

Stellate cell bodies

Found within the ventral horn of the spinal cord

(and other places too)

Pyramidal cell bodies

Triangular cell bodies found in the cerebral cortex

Pyriform cell bodies

Pear-shaped cell bodies found in the

Purkinje cells of the cerbellar cortex

Bipolar Neurons

Neurons associated with special senses,

and have one dendrite opposite the axon

Psudounipolar Neurons

Sensory neurons that have long axons that conduct information from the periphery to the CNS

Anaxonic neurons

Neurons without axons (amacrine cells of the retina and the chromaffin cells of the adrenal medulla)

Neuropil

Glial cells, axons and cell processes

(everything except neuronal cell bodies)

Glial cells of the CNS

Astrocytes, oligodendrocytes,

microgliocytes,ependymal cells

Glial cells of the PNS

Schwann cells and satellite cells

Oligodendrocytes

• Most numerous, and middle in size
• nucleus is variable in appearance
• Found between neuronal cell bodies and neuronal processes and around capillaries.
• Form the myelin sheath around axons in the white matter of the CNS

Astrocytes

• Largest nuclei of glial cells, where it is round and pale-staining.
• Found in both white and gray matter of the CNS
• Provide structural support in brain and spinal cord
• Repair neuronal damage, are phagocytic, and insulate neuronal receptor surfaces.

Microgliocytes

• Smallest nuclei (similar to endothelial cell nuclei of capillaries)
• Phagocytic in response to injury.
• Smallest of the glial cells

Ependymal cells

• Line the ventricular system of the brain and central canal of spinal cord.
• Many ciliated
• Those within spinal cord are columnar
• Those within ventricles are cuboidal
• Aid in production of CSF, while cilia help move CSF

Schwann cells

• Found in PNS, where they myelinate axons of neurons
• Plasma membrane is called neurolemma
• Cell nucleus is peripheral to the myelin wrapping

Satellite cells

• Specialized schwann cells located around cell body of the neurons in the ganglia.
• Produces a complete shell around the cell body in dorsal root ganglia (sensory)

Neurosecretory cells

• Cell body located within hypothalamus of the brain, and produces neurosecretory materia.
• Axon terminates on capillaries within the pituitary, where secretory products released into the blood

Conducting nuerons

• Secrete NTs only at synapses or neuromuscular junctions
• Typically have dendrites, cell body, and axon.
• Consists of functional regions: dendritic zone, axonic zone, and telodendritic zone.

Dendritic zone

• Includes dendrites and may/may not include cell body
• Subjected to excitatotry/inhibitory stimulation and produces graded response

Axonic zone

• Composed of the axon, and conducts action potentials

Telodendritic zone

Terminal portion of the axon that transfers the info to the effector by releasing a NT

3 Main types of Synapses

• Axoaxonic
• Axodendritic
• Axosomatic

6 Layer of the Cerebral Cortex

• Molecular layer: fibers running horizontally
• Outer granular layer: pyramidal cells
• pyramidal cell layer: bigger pyramidal cells
• inner granular layer: bigger pyramidal cells
• inner pyramidal cell layer: biggest pyramidal cells
• polymorphic cell layer: no pyramidal cells

3 Layers of the Cerebellar cortex

• Molecular layer: light staining (dendrites of Purkinje cells)
• Pyriform cell layer: Purkinje cell bodies (pyriform)
• Granular layer: dark staining (axons that course upward into molecular layer).

White matter of spinal cord

Divided into dorsal, lateral, and ventral funiculi

Gray matterof spinal cord

Dorsal and ventral columns separated by the intermediate gray commissure (lateral column in the thoracolumbar region too for the ANS) Ventral horn neurons biggest.

Sensory ganglia

• Craniospinal ganglia
• Include cranial ganglia of the cranial nerves and the dorsal root ganglia of the spinal nerves
• Neurons are sensory pseudounipolar cells
• Cell bodies are varying sizes and have central nuclei
• Cell bodies covered completely by satellite cells

Motor Ganglia

Sympathetic:

• Neuronal cell body of postganglionic neuron located within the ganglion
• Large cell body with large vesicular, eccentrically placed nucleus
• Satellite cells do not make complete sheeth around cell body because it is multipolar

Parasympathetic: Between the inner circular and outer longitudinal smooth muscle layers (myenteric plexus)

What is the primary excitatory NT in the CNS?

Glutamate

Primary inhibitory NT of the Brain

GABA

Primary inhibitory NT of the spinal cord

Glycine

nAChR

Nicotinic ACh receptor, which is a ligand-gated ion channel with conductance primarily for Na+ and Ca ++ which is excitatory.

m2AChR

M2 muscarinic receptor: G-protein receptor that causes decrease in cAMP in the cell, inhibition of voltaged gated Ca2++ channels, and efflux of K+... generally inhibitory

m3AChR

M3 Muscarinic Receptor: g-protein receptor that activate pathways that lead to an increase in intracellular calcium that mediates its various effects.

Dopamine

Concentrated in brain circuits associated with motor behaviors and mood. G-protein linked, where some are excitatory and some are inhibitory

GABA

Primary inhibitory NT of the brain. GABA_A is the only real important receptor, which is a Cl- channel and hypopolarizes the cell.

Glutamate

Primary excitatory NT of the CNS. AMPA and NMDA receptors are ionotropic receptors. AMPA receptors are ligand-gated Na+ channels. NMDA receptors are Na+ and Ca++ ion channels that only open when the membrane is already depolarized. There are also metabotropic glutamate receptors, that are G-protein linked, though they’re not all that clinically relevant.

Glycine

Primary inhibitory NT of the spinal cord, where the receptors are ligand gated Cl- channels, which cause hyperpolarization.

Norepinephrine

      Released by post-ganglionic sympathetic neurons and in a wide variety of brain circuits associated with mood, attention, and sleep-wake cycles. Alpha1: constriction of smooth muscles esp for vasoconstriction, Alpha2: presynaptic terminals throughout the brain, and inhibitory to ANS effectors, Beta1: excitatory to cardiac cells, Beta 2: smooth muscle relaxation. All types are also found in the brain.

Serotonin

Acts at synapses throughout brain, but many concentrated in a few small midline nuclei of the brainstem. Most are G-protein linked, while one is ligand gated cation channel. Plays role in influencing mood, attention, sleep-wake cycles, cardiovascular function, and others.

CN I

• Olfactory nerve
• Exits skull through cribriform plate
• Sensory innervation for olfaction

CN II

• Optic n.
• Exits skull through optic canal
• Sensory innervation to the eye

CN III

• Oculomotor n.
• Exits through the orbital fissure
• Somatic motor: m. levator palpebrae superioris, ventral, medial, and dorsal rectus mm, and the ventral oblique m.
• Parasympathetic: smooth muscles through the ciliary ganglion for accomodation through the ciliary m, and pupillary sphincter mm for pupil constriction

What cranial nerves exit the orbital fissure?

Oculomotor

Trochlear

Abducent

Ophthalmic branch of Trigeminal

Lesion of Oculomotor will cause what?

• Ventrolateral strabismus
• Eye will not move properly
• Eye will be dilated
• Drooping of upper eyelid
• Can't elicit proper nystagmus
• Proper pupillary light reflex will not occur

Pupillary Light Reflex

Afferent limb: Optic n.

Efferent limb: Oculomotor n.

Menace Reaction

Afferent Limb: Optic n.

Efferent Limb: Facial n.

CN IV

• Trochlear N.
• Motor to the dorsal oblique m.
• Originates in the dorsal side of the brainstem
• Decussates to innervate contralateral m.

CN V

• Trigeminal Nerve
• V1 = Ophthalmic (Sensory to skin of forehead, upper eyelid, and medial canthus, eyeball, cornea, and nasal vestibule). Exits Orbital fissure
• V2 = Maxillary (Sensory to skin of maxillary and frontal area (minus medial canthus), nasal cavity, sinus, palate. Enters skull through round foramen, and exits through rostral alar foramen.
• V3 = Mandibular (sensory to jaw + rostral 2/3 of tongue. motor to muscles of mastication except caudal belly of digastricus). Exits oval foramen

Corneal Reflex

Afferent limb: Ophthalmic division of Trigeminal

Efferent limb: Abducent (for retraction of globe) and facial for blinking.

Palpebral Reflex

Afferent limb: Ophthalmic (medial canthus) + Maxillary (lateral canthus) divisions of trigeminal

Efferent limb: Facial for blink

CN VI

• Abducent n.
• somatic motor for lateral rectus m. and m. retractor bulbi
• Exits skull through Orbital fissure

CN VII

• Facial n.
• Enters skull via the internal acoustic meatus, exits via stylomastoid foramen
• Somatic motor: muscles of facial expression and caudal belly of the digastricus
• Sensory: taste through rostral 2/3 of the tongue
• Parasympathetic to lacrimal, palatine, nasal, sublingual salivary, and mandibular salivary glands

Schirmer Tear Test

Afferent limb: Ophthalmic division of trigeminal

Efferent limb: facial n. (parasympathetic)

CN VIII

• Vestibulocochlear n.
• Sensory to inner ear for hearing and equilibrium
• Enters skull through internal acoustic meatus, but never exits.

Nystagmus

• Involuntary movement of the eye
• Physiologic nystagmus occurs as head moves and eyes snap as it tries to center the object
• Pathologic nystagmus indicates issues with the vestibular system. Named for the fast component, and laterality is the same as the directon of the slow component.

CN  IX

• Glossopharyngeal n.
• Taste and sensation to the caudal 1/3 of tongue (sensation also to palate and pharynx, and baroreceptors/chemoreceptors of the carotid sinus)
• Somatic motor to voluntary mm of pharynx (crico, thyro, hyo mm)
  
• Parasympathetic to parotid + zygomatic salivary gland, and to baroreceptors/chemoreceptors of carotid sinus
• Enters skull at jugular foramen, exits via tympano-occipital fissure

CN X

• Vagus n.
• Somatic motor to pharyngeal mm and intrinsic mm of larynx (cricothyroideus thru cranial laryngeal, and rest through recurrent laryngeal)
• Sensory to pharynx, larynx, and thoracic and abdominal viscera
• Parasympathetic to pharynx, larynx, trachea, esophagus, and thoracic and abdominal organs
• Enters skull via jugular foramen and exits via tympano-occipital fissure

Gag reflex

Affarent limb: glossopharyngeal and vagus nn

Efferent limb: glossopharyngeal, vagus, hypoglossal, mandibular, and accessory nn

CN XI

• Accessory n.
• Somatic motor to mm of nuck and forelimb that are of branchial origin (sternocephalicus, trapezius, omotransversarius, brachiocephalicus)
• Enters skull through the jugular foramen and exits via the tympano-occipital fissure

CN XII

• Hypoglossal n.
• somatic motor to intrinsic and extrinsic mm (styloglossus, hyoglossus, geniohyoideus of the tongue
• Exits skull through the hypoglossal canal

Cerebral arterial circle

• Circle formed by the anastemoses of 2 internal carotid arteries and the basilar artery
• Supplies the cerebral hemispheres, dienchephalon, and part of the cerebellum

Basilar artery

• Formed by the union of the two vertebral arteires
• Supply the mesencephalon, ventral metencephalon, myelencephalon, and a part of the cerebellum

Rostral cerebral arteries

Supplies the medial and a small part of the dorsal apects of the cerebral hemispheres

Middle cerebral arteries

Supplies the majority of the lateral aspects of the cerebrum

Rostral and caudal communicating arteries

Completes the rostral and caudal part of the cerebral arterial circle

Caudal cerebral arteries

supplies the majority of the occipital lobes of the cerebrum

Rostral and caudal cerebellar arteries

Supplies the cerebellum

Labyrinthine arteries

Supplies the vestibulocochlear nerve and the inner ear.

Rete mirabile

Network of arteries that replaces the intracranial parts of the internal carotid artery (feline, ruminants, and swine). Participates in the formation of the cerebral arterial circle.

Stroke

• Neurologic dysfunction produced by sudden loss of blood suppy to nervous tissue often as a result of changes in the walls of the cerebral vasculature
• Very rare in vet med

Fibrocartilaginous emboli

• Ischemic injury to the spinal cord due to occlusion of blood vessels from degenerating intervertebral disks (usually at the central branch of the ventral spinal artery, produing infarction of the ventral grey matter)

Meninges

• Connective tissue membranes surrounding the brain and spinal cord
• Functions:
  • production, circulation, and reabsorption of CSF
  • Conveyance of arterial vasculature into nervous tissue, and return of venous blood via the dural sinuses
  • Suspension of the nervous tissue wihtin the cranial vault and vertebral canal
  • Division of cranium into smaller compartments through infoldings of the dura mater

Pia mater

Deepest meninx, lying in direct contact with the substance of the spinal cord and brain. Highly vascular and branches penetrate the cord to supply the nervous tissue

Denticulate ligament

Thin ligament of the pia mater that attaches at regular intervals to overlying meninges

Arachnoid

Middle layer of meninges

Arachnoid trabeculae

Fibers connecting the arachnoid to the pia mater

Leptomeninges

pia mater and arachnoid together

Subarachnoid space

Space between the pia mater and arachnoid

Dura mater

aka pachymeninx.

Thick outermost meningeal layer

Subdural space

Potential space between the arachnoid and

dura mater, which are firmly attached to

each other via a fibroblast layer

Epidural space

Fat and vessel filled space in the spine between

the dura mater and the periosteum, where

epidural anesthesia is injected.

Conus medullaris

Terminus of the spinal cord

Filum terminale

Fine thread of collagen (continuation of pia mater) and ependymal cells from the central canal that continue caudad beyond the conus medullaris

Caudal ligament

When the arachnoid and dura mater narrow down to close the subarachnoid space and wrap around the filum terminale to form a structure that attaches the spinal cord to the distal part of the vertebral column.

Cisterns

Enlarged regions of the subarachnoid space, where you have lots of CSF

Cerebellomedullary cistern

Big cistern between the caudal portion of the cerebellum and the medulla, which is the preferred site for CSF collection in companion aninmals

Periosteal layer

Layer of cranial dura mater that takes place of the inner cranial periosteum

Meningeal layer

Layer of cranial dura mater that is attached to the arachnoid

Epidural/Subdural hematoma

Blood accumulating between the calvaria and the dura mater = dpidural hematoma. Hemorrhage into potential space between the dura mater and the arachnoid produces a subdural hematoma.

Dural reflection

When the menigeal layer of the dura splits form the periosteal layer, extend into a fissure of the brain, then fold back upon itself to return and reassociate with the periosteal layer

Falx cerebri

Dural reflexion that is lying in the longitudinal cerebral fissure

Tentorium cerebelli

Dural reflection that creats a transverse sheet between the occipital lobes and the cerebellum.

Tentorial notch

Incisure in the tentorium cerebelli that allows it to surround the mesencephalon

Diaphragma sellae

Dural reflection that covers the floor of the cranium

Dural sinus

Space between the meningeal layer and periosteal layer when forming a dural reflection that is lined with endothelium and filled with venous blood.

Dorsal sagittal sinus

Dural sinus associated with the falx cerebri

transverse sinus

dural sinus associated with the tentorium cerebelli

Cavernous sinuses

Dural sinus associated with the diaphragma sellae

Lateral apertures

Two openings projecting laterad from the lumen of the fourth ventricle to communicate with the subarachnoid pace

Functions of the CSF

1. Provision of a protective "water jacket" around the nervous tissue
2. Maintenance of hte nervous tissue's unique extracellular fluid composision
3. Serving as the "lymphatic system" for the CNS
4. Transport of  certain hypothalamic hormones within the brain
5. Influence on brainstem respiratory centers via [CO2]

Differences between serum and CSF

Less protein

Less potassium

Less calcium

More Chloride

More magnesium

Production of CSF is influenced by what?

Not blood pressure, but osmotic pressure of the plasma!

Arachnoid villi

Tiny herniations of the arachnoid through the meningeal dura and into the dural sinus, which are associated especially with the dorsal sagittal sinus.

Arachnoid granulations

Structures that form as an aggregate of arachnoid villi, which act as one-way valves for the flow of CSF back into the blood. Movement of CSF from subarachnoid space into sinus is entirely dep on hydrostatic pressure gradient between the two compartments.

Hydrocephalus

Compression of the nervous tissue associated with dilation of the ventricles.

Causes of hydrocephalus

• Obstruction of CSF outflow at the arachnoid granulations as a result of meningitis
• Mass in midbrain compressing the cerebral aqueduct
• malformation of the mesencephalic aqueduct in toy and brachycephalic breeds.

Blood Brain Barrier

• Capillaires are surrounded completely by astrocytic end feet, which induce the formation and maintain tight junctions between endothelial cells.
• The basement membrane of capillaries are continuous and especially thick.

Blood-ECF Barrier

The separation of the CNS cells from capillaries of the choroid plexus. In this case, there are tight junctions between the ependymal cells of the choroid plexus.

Reflex

Involuntary response to a stimulus applied to the periphery and transmitted to the reflex networks in the spinal cord or brainstem.

Withdrawal reflex

• Sensory n: sciatic n (spinal cord segments L6-S1 (2))
• Efferent nerve: Sciatic n for flexion at hock and stifle (L6-S1 (2)). Femoral nerve for flexion at hip (L4-L6)
• Muscles: Agonist: Flexors, Inhibitory: Extensors