SBM

NEUROLOGY
COLVIN 2023
 LEARNING OUTCOMES

• Analyze the structure, function, and normal

physiology of the nervous system. (B2.02b)

• Differentiate the pathophysiology relating to

various common conditions of the nervous
system in adults. (B2.02c)
 1. Review sensory pathways

2. Review motor pathways

AGENDA
3. Blood supply to the brain

4. Blood brain barrier

 Action potentials FROM brain TO spinal cord travel in
DECENSENDING (efferent) pathways in two routes:
Direct and Indirect.

▪ Direct: Pyramidal– VOLUNTARY

▪ lateral corticospinal
▪ ventral (anterior) corticospinal

MOTOR ▪
▪ Corticobulbar

Indirect: Extrapyramidal– INVOLUNTARY

▪ Tectospinal
▪ Vestibulospinal
▪ Rubrospinal
▪ Lateral reticulospinal
▪ Medial reticulospinal
 Action potentials FROM brain TO spinal cord travel in
DECENSENDING (efferent) pathways in two routes:
Direct and Indirect.

▪ Direct: Pyramidal– VOLUNTARY

▪ lateral corticospinal
▪ ventral (anterior) corticospinal

MOTOR ▪
▪ Corticobulbar

Indirect: Extrapyramidal– INVOLUNTRAY

▪ Tectospinal
▪ Vestibulospinal
▪ Rubrospinal
▪ Lateral reticulospinal
▪ Medial reticulospinal
 MOTOR PATHWAYS
Upper motor neurons
Includes the corticospinal and corticobulbar tracts
Motor pathways completely in the CNS

Primary roles:
Controlling fine motor movement
Influencing/modifying spinal reflex arcs and circuits
“QUOTES…
Two pathways of direct and indirect:
Direct: pyramidal (with pyramidal cells)
Indirect: extrapyramidal (brainstem)
 MOTOR PATHWAYS
Lower motor neurons
Neurons having direct influence
on muscles

Cell bodies originate in the gray

matter of brainstem and anterior
horn of spinal cord, but their
axons extend into the periphery “QUOTES…

Only lower motor neurons

provide output from CNS to
skeletal muscles fibers
MOTOR PATHWAYS

“QUOTES…
 PYRAMIDAL
SYSTEM
Structure of the Pyramidal System
Pyramidal system fibers originate in the precentral gyrus of
the motor cortex they converge at the internal capsule then
descend to form the central third of the cerebral peduncle,
they descend further through the pons, where small fibers
supply cranial nerve motor nuclei along the way they form
pyramids at the medulla, where most of the fibers decussate,
they continue to descend in the lateral column of the white
matter of the spinal cord

Let's get even more specific….

 MOTOR PATHWAYS
Clinically relevant motor pathways:
Lateral corticospinal- decussate in the medulla
90% corticospinal axons
Precise, agile, skilled movements (both upper and lower)

Ventral corticospinal- decussate in spinal cord

chest, back, and trunk control

“QUOTES…
Corticobulbar
Synapse on motor cranial nuclei within the
brainstem that control muscles of the face, head, and
neck
Only trochlear nerve decussates peripherally
 MOTOR PATHWAYS
Efferent pathways primarily relay information from the
motor cortex to the brainstem or spinal cord

Rubrospinal- fine precise movements of the upper limbs

corticorubrospinal pathway for motor control, also showing “QUOTES…

the relation of this pathway to the cerebellum.

corticorubral tract contains neurons that connect the primary

motor areas to the red nucleus.

rubrospinal tract then descends through the spinal cord. 

MOTOR PATHWAYS
▪ Tectospinal- skeletal muscle of head and trunk
▪ (turn suddenly due to visual or auditory input)
▪ Vestibulospinal- After input from vestibulocochlear
nerve action potentials are sent to trunk and proximal
limbs for contraction in order to maintain posture

TRUNK AND PROXIMAL LIMBS

▪ Lateral reticulospinal- inhibits skeletal muscles
▪ Medial reticulospinal- excites skeletal muscles
Work together to maintain posture and muscle tone
during ongoing movement
 MOTOR PATHWAYS- Injury
Upper motor neuron lesions:
Muscle tone is increased, and deep tendon reflexes
are exaggerated.

Lower motor neuron: Ipsilateral

weakness/paralysis, with hypotonia and absent
reflexes.

What about injury to pathway?

 Action potentials from sensory receptors travel FROM
spinal cord TO the brain along two main routes: dorsal
(posterior) columns and spinothalamic tract.

* Dorsal (posterior) columns: touch, pressure,

vibration, and proprioception.

Ascend to the medulla and decussate there

SENSORY
PATHWAY * Spinothalamic: pain, temperature, itch, and tickle.

Decussate in spinal cord

dorsal and ventral spinocerebellar tracts: proprioception

First-order, second-order, third order… to the primary

sensory area of the cerebral cortex
 Process of sensation involves:
1. Stimulation of the sensory receptor (Afferent)

SENSORY 2. Transduction of the stimulus

3. Generation of action potential

4. Integration of sensory input

 Receptors for sensation:

SENSORY
Mechanoreceptors

Thermoreceptors

RECEPTORS Photoreceptors

Chemoreceptors

Nociceptors
 Spinal cord:
Dermatomal sensory deficit on the trunk on one or both
sides at the level of the lesion, and sensory loss from tract
damage below the level of the lesion
Cerebral cortex: Contralateral sensory loss in the face,
limbs, and trunk on the same side as the motor deficits
Spinal Nerve Root: Corresponding dermatomal sensory
deficits

What about injury to pathway?

 Dermatomes
A sensory area of skin that is representative of a single spinal nerve

Clinically useful in localizing the lesion to a nerve root or spinal cord

It is important to know the dermatomes if you are trying to locate a

pathology that is due to a radiculopathy (root compression)
Protective structures

Cranial Vault
Cranial Floor
Meninges
Spaces
CSF and the ventricular system
Spinal Column
 PROTECTIVE STRUCTURES

Cranial Vault
Structure that encloses and protects the brain
and its associated structures
Eight bones
1 ethmoid bone.
1 frontal bone.
1 occipital bone.
2 parietal bones.
1 sphenoid bone.
2 temporal bones.
Protective structures

Cranial Floor
• Divided into 3 fossae (depressions)
• Anterior (frontal lobe), middle (temporal
lobe, diencephalon base), and posterior
fossa (cerebellum)
PROTECTIVE STRUCTURES
Meninges
Protective membranes that surround the brain
and spinal cord
Dura mater
Periosteum (endosteal layer) of the skull
and the inner dura or meningeal layer
Arachnoid mater
a spongy, weblike structure just
underneath the dura mater that loosely
follows the contours of the cerebral
structures
Pia mater
Protective structures

Dura Mater- most superficial layer

Two layers of the dura:

Periosteal layer is the periosteum lining the internal surface

of the skull and is attached to the cranial bones and sutures.

Meningeal layer is composed of dense collagenous

connective tissue that is continuous with the dura mater of
the spinal cord.
Spaces
• Subdural
• Located between the dura and
Protective structures
arachnoid mater
• Disruption of the small bridging veins
results in a subdural hematoma
• Subarachnoid
• Contains CSF and is located between
the arachnoid and pia mater.
• Epidural
• Located between the dura mater and
skull
• A skull fracture can sever one of the
blood vessels supplying the meninges
and produce an epidural hematoma
 PROTECTION:
CEREBRAL SPINAL
FLUID
Cerebral spinal fluid is a clear, colorless and similar to
blood plasma/interstitial fluid.

Protects CNS from chemical and physical injuries

FUNCTIONS:

Shock absorber

Maintains chemical environment for neuronal

signaling

Exchange of nutrients and waste between blood and

nervous tissue
Circulates between 125 to 150 ml in the
ventricles and subarachnoid space
Protective structures: CSF
Contains glucose, protein, albumin,
electrolytes, pH 7.3

Produced by the choroid plexuses in the

lateral, third, and fourth ventricles.
• Approximately 480mL of CSF is
produced daily.
• Same amount reabsorbed daily
• Replaced 3x a day
• Amount of CSF is constant
Is reabsorbed through the arachnoid villi.
CONSTITUENT NORMAL VALUE
Na+ 148 mM

K+ 2.9 mM Protective structures: CSF

Cl− 125 mM

HCO3 - 22.9 mM

Glucose (fasting) 50–75 mg/dL (60% of serum Normal CSF.

glucose)

pH 7.3

Protein 15–45 mg/dL

Albumin 80%

Gamma-globulin 6–10%

Cells
 White (lymphocytes)
 Red (red blood cell [RBC]) 0–6/mm3

0/mm3
 Protective structures: CSF
Production:
Produced from arterial blood by the choroid plexuses in the lateral
ventricles by a combined process of diffusion, pinocytosis and active
transfer (3rd ventricle)
Capillaries in the choroid plexus are covered by ependymal cells.
The choroid plexus consists of tufts of capillaries with thin fenestrated
endothelial cells. These are covered by modified ependymal cells with
bulbous microvilli.
The tight junctions of the choroid blood vessel provide a limiting
barrier between the CSF and blood that functions similarly to the blood-
brain barrier
PROTECTIVE
STRUCTURES
: CSF
Protective structures: CSF
The CSF pressure, measured at lumbar puncture (LP), is 100-
180 mm of H2O (8-15 mm Hg) with the patient lying on the
side and 200-300 mm with the patient sitting up.
FLOW: Beginning in the lateral ventricles, the CSF flows
through the interventricular foramen into the third ventricle and
then passes through the cerebral aqueduct (aqueduct of Sylvius)
into the fourth ventricle. From the fourth ventricle, the CSF
passes into the subarachnoid spaces of the brain and spinal cord.
It is reabsorbed into the venous circulation through a pressure
gradient between the arachnoid villi and the cerebral venous
sinuses.
Protection: Blood brain barrier

Cellular structures inhibit certain substances in the blood from entering the
interstitial spaces of the brain or CSF, allowing neurons to function normally.
Supporting cells include astrocytes, pericytes, and microglia.
The exact nature of this mechanism is controversial
• Permeability is high for water, carbon dioxide, oxygen, and most lipid-
soluble substances, including alcohol
• Moderate for electrolytes, such as sodium, chloride, and potassium
• Almost totally impermeable to plasma proteins and most non–lipid-
soluble large organic molecules

Substantial implications for drug therapy since some drugs show a greater
propensity than others for crossing the blood-brain barrier
1. Structurally BBB contains tight junctions at
the endothelial cells of the brain capillaries
and astrocytes seal the junction
a) ASTROCYTES: press against the Protection: Blood Brain Barrier
capillaries and secrete chemicals to
maintain tightness at the junctions

2. Functionally the BBB contains membrane

transport proteins in the endothelial cell
plasma membrane that can selectively move
water soluble substances across the brain
capillary walls.
a) If water soluble substance does NOT have
transport protein = no crossing BBB
(Water can cross-- small and uncharged)

If trauma/toxins/inflammation occurs BBB

can break down.
Blood brain barrier: Cellular level
 Contains 33 vertebrae
• 7 cervical
• 12 thoracic
• 5 lumbar
• 5 fused sacral
• 4 fused coccygeal
Between each vertebra: intervertebral disks
• functions to absorb shocks, preventing
Protective damage to the vertebrae
structures: Spinal
Column
 BLOOD SUPPLY

Carbon dioxide (CO2) is the primary regulator

for CNS blood flow
Hypercapnia increases cerebral blood flow by
cerebral vasodilation
CO2 is a potent vasodilator in the CNS –
ensures adequate blood supply

Arterial supply comes from

Internal carotid (anterior circulation)
Vertebral arteries (posterior circulation)
Internal carotid supplies a proportionately
greater amount of blood flow
BLOOD SUPPLY
 BLOOD SUPPLY
After entering the skull, the internal carotids divide into the anterior
and middle cerebral arteries
The vertebral arteries originate at the subclavian arteries
They join at the junction of the pons and medulla oblongata to form the
basilar artery.
The basilar artery divides at the level of the midbrain to form paired
posterior cerebral arteries.

The circle of Willis provides an alternative route for blood flow

when one of the contributing arteries is obstructed (collateral blood
flow)
The anterior cerebral, middle cerebral, and posterior cerebral
arteries leave the arterial circle and extend to various brain
structures.
BLOOD SUPPLY
ARTERIAL ORIGIN STRUCTURES SERVED CONDITIONS CAUSED BY OCCLUSION

Anterior cerebral artery Basal ganglia Hemiplegia on contralateral side of body,

Corpus callosum greater in lower extremities than in upper
Medial surface of cerebral hemispheres extremities
Superior surface of frontal and parietal lobes

Middle cerebral artery Frontal lobe Aphasia in dominant hemisphere

Parietal lobe Contralateral hemiplegia
Temporal lobe

Posterior cerebral artery Part of diencephalon and temporal lobe Visual loss
Occipital lobe Sensory loss
Contralateral hemiplegia if cerebral
peduncle affected
 Blood supply: Spinal Cord
The spinal cord derives its blood supply from branches off the
vertebral arteries and the descending aorta

The anterior spinal arteries and the paired posterior spinal

arteries branch off the vertebral artery at the base of the cranium
and descend alongside the spinal cord.

Venous drainage parallels the arterial supply closely and drains

into venous sinuses located between the dura and periosteum
of the vertebrae.
Blood supply: Spinal Cord
Blood supply: drainage
The venous drainage of the brainstem and
cerebellum does parallel the arterial supply of
the structures.
The veins eventually join the internal
jugular veins at the base of the skull.
Adequacy of venous outflow can have a
significant effect on intracranial pressure.