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BIOLOGY 03048: ANATOMY; WEEKS 15 AND 16: NEUROANATOMY FOR LOCALIZATION

W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)

Readings: Moore and Dalley, (aspects of) Ch. 9. Young and Young, (aspects of) Chs. 2-13, 15, 19, 21

van Wynsberghe and Cooley scenarios 2 and 3

possible web sites: http://www.anatomy.wisc.edu/

(multiple neuroanatomy resources a click away from this departmental page)

possible HVCC a/v material VT 3427,"Common Motion Disorders" (portions to be shown in lecture)

OVERVIEW OF NERVOUS SYSTEM:

In the peripheral nervous system, nerves are visible structure consisting of bundles of axons together. Separate stimuli interpreted as touch, pain, and temperature are carried together.

In the central nervous system, the mixed nerve fibers are sorted out into different bundles or tracts. Tracts are found in white matter, surrounding nerve cell bodies in gray matter.

spinal cord:

gray matter with three components:

anterior (ventral) horns: motor

posterior (dorsal) horns: sensory

lateral horns: sympathetic (T1-L2)

Tracts bear compound names of where they begin to where they end, e.g., corticospinal tract.

a) efferent (motor) or descending tracts

b) afferent (sensory) or ascending tracts

brain stem:

medulla oblongata:

rostral (superior) vs. caudal (inferior) portions

Contains nuclei associated with CN VIII - XII, and multiple involuntary centers.

pyramids: large ventral structures of medulla that carry major ("pyramidal") pathways

pons:

contains nuclei associated with CN V - VII, and centers such as those associated with regulation of breathing.

midbrain:

contains nuclei associated with CN III - IV, and centers associated with auditory, visual, and pupillary reflexes. The red nucleus, maintaining connections with the cerebrum and cerebellum, is associated with motor coordination, e.g., the red nucleus facilitates flexor movements in the contralateral upper limb via the rubrospinal tract. This accounts for the difference in decorticate vs. decerebrate posturing in comatose patients:1

decorticate: lower limb in extension and plantar flexion, upper limb in flexion (so lesion not involving red nucleus)

decerebrate: lower limb in extension and plantar flexion, upper limb in extension as well (so that lesion involves red nucleus)

The posterior surface of the midbrain is the tectum, with the superior and inferior colliculi (corpora quadrigemina). The superior colliculi are involved with visual reflexes and the inferior colliculi are involved in auditory relays.

MAJOR SPINAL TRACTS

AFFERENT (SENSORY) PATHWAYS:

Sensation is initiated by sense receptors and an impulse generated by a afferent (sensory) nerve fiber in a peripheral nerve. The afferent (sensory) nerve fiber follows the posterior (dorsal) root. Once in spinal cord, (for the most part) its impulses move along either the:

Ascending tracts (sensory)

name of pathway

sensation(s)

pathway route

spinothalamic tract

(lateral, anterior [ventral])

pain/temperature (crude touch)

crosses over in spinal cord before ascent to thalamus

posterior columns

fasciculus gracilis

fasciculus cuneatus

conscious proprioception, stereognosis, fine touch

ascends ipsilaterally before crossing over at medullary nuclei

spinocerebellar tract

(ventral, dorsal)

unconscious proprioception

ascends ipsilaterally, entering cerebellum via peduncles

At the thalamic level (in the diencephalon), there is the general quality of perception, but fine distinctions are made with a third synapse of sensory neurons carrying impulses into the sensory cortex of the brain.

EFFERENT (MOTOR) OR DESCENDING PATHWAYS:

The body must control what kinds and what amounts of movements. These are a concern of 1) the ventral/anterior horn cells, or what we can call the final common pathway and 2) higher motor pathways that impinge on the anterior horn cells.

Descending (motor) tracts:

name of pathway

action(s)

pathway route

pyramidal tracts

 

 

corticospinal tract

(anterior, lateral)

voluntary control of skeletal muscles

passes through internal capsule before crossing over at pyramids; synapses in anterior horn before leaving spinal cord

corticobulbar tracts

voluntary control of skeletal muscles controlled by cranial nerves

typically, fibers from both sides influence CNs; except LMNs of lower facial mm. only contralaterally supplied1

 

 

 

Extrapyramidal tracts

Note: these are not to memorize, but to acknowledge that such wiring exists

 

rubrospinal tract

involuntary regulation of posture, muscle tone

red nucleus (midbrain), crosses over in brain stem, to spinal cord

vestibulospinal tract

involuntary regulation of posture, balance from inner ear sensations

vestibular nucleus (medulla) to spinal cord without crossing over

reticulospinal tract

involuntary reflex activity regulation and autonomic functions, more involvement for posture, basal nuclei input, etc.

reticular formation (medulla) to anterior/lateral horns, crossed

tectospinal tract

involuntary response of eye, head, neck, arm position in response to visual/auditory stimuli (e.g.,"What was that?")

tectum (midbrain, the superior colliculus) to cervical spinal cord, crossing over in brain stem

 

With regards to the pyramidal tracts (corticospinal and corticobulbar), the following considerations apply:

upper motor neuron (UMN):

any neuron or level of motor control before the synapse in the ventral horn.

lower motor neuron (LMN):

the"common pathway" of the motor neurons whose nuclei are in the ventral horn and whose axons extend peripherally.

 

As a result, two different syndromes of:1

upper motor neuron (UNM) lesion:

"spastic paralysis" of increased muscle tone, increased reflexes

lower motor neuron (LMN) lesion:

"flaccid paralysis" of muscle weakness, atrophy, decreased reflexes

 

Although we're making a distinction between the ascending and descending tracts above, lesions that"cross" both can have mixed motor and sensory deficits. The internal capsule is a region lateral to the thalamus where descending and ascending tracts traverse near to each other. The lateral striate arteries of the middle cerebral artery are small penetrating branches that"permit" the internal capsule to be a common place for a devastating"stroke."

BASAL NUCLEI:

Basal nuclei (ganglia): a general term for structures in forebrain and midbrain that are embedded within the central white matter.

Overall, we think of these involved in the extrapyramidal (striatal) motor system, where they influence stereotyped motor activity of postural/reflexive nature. In effect, the basal ganglia act as"brakes" for motor activity, inappropriate amounts of which happen with lesions in these areas.

striatum (caudate and putamen):

function:

control the cycles of arm and leg movements occurring between the"start" and"stop" of walking.

 

pallidum (globus pallidus):

function:

controls and adjusts muscle tone in appendicular muscles, in preparation for a voluntary movement.

lesions: choreas, or rapid, involuntary"dancing" movements; athetosis, or involuntary writhing

substantia nigra:

normal:

Dark because of melanin deposition. Neurons here contain dopamine. Substantia nigra works with striatum to control movement

lesion:

Parkinson's disease, with bradykinesia, rigidity, tremor

 

subthalamus:

normal:

influence on the pallidum

lesion:

hemiballism (involuntary flinging movement)

 

Overall summary of interactions:1

cerebral cortex à (+) striatum (striatum and substantia nigra interact) à (-) pallidum à (-) thalamus (also (-) from substantia nigra) à (+) cerebral cortex

cerebral cortex à (+) subthalamus, which interacts with pallidum

Note that dopamine can interact with different receptors: D1 receptors are excitatory, D2 receptors are inhibitory.

CEREBELLUM:

Three major functions of the cerebellum:

  1. maintenance of posture and balance
  2. maintenance of muscle tone
  3. coordination of voluntary motor activity

Two major types of cerebellar syndromes:1,

 

anterior lobe syndrome.:posterior lobe syndrome:

ipsilateral, with multiple cerebellar signs, e.g., intention tremor

 

CEREBRUM

three major groupings of axons in the white matter:

commissural fibers linking the two hemispheres: corpus callosum

association fibers interconnecting portions of the cerebral cortex: arcuate fasciculus

projection fibers linking cerebral cortex to other parts of the brain: internal capsule

cerebral cortex:

frontal lobe with:

  1. motor, premotor cortex
  2. Broca's speech area in dominant hemisphere; lesion: inability to express language
  3. prefrontal cortex; lesion: deficits in concentration, orientation, judgement

parietal lobe with:

  1. sensory cortex
  2. superior parietal lobule; lesion: contralateral astereognosis and sensory neglect

temporal lobe with:

  1. primary auditory cortex; lesion: unilateral--slight hearing loss; bilateral--cortical deafness
  2. Wernicke's speech area in dominant hemisphere; lesion: inability to understand language
  3. hippocampal cortex; lesion: if bilateral, loss of ability to form new long-term memory

occipital lobe with:

  1. visual cortex; lesion: cortical blindness patterns

So overall, is the lesion:

cortical?

  1. check for aphasia, cortical sensory or other specialized deficit(s). (e.g., loss of Broca's area and motor aphasia)

subcortical?

at the level of the internal capsule, basal nuclei, thalamus

  1. internal capsule: collection of projection fibers of ascending/descending tracts (face, arms leg equally involved if internal capsule involved from branches of middle cerebral artery)
  2. check for resting tremors or other movements, e.g., if basal nuclear lesion

brainstem?

  1. check for crossed hemiplegia, e.g, right hemiplegia, left-sided brainstem signs (e.g., stroke in L posterior inferior cerebellar artery distribution: L spinothalamic tract disrupted, so R loss of pain/temperature, L cerebellar signs, L cranial nerve deficits1)
  2. check for cerebellar signs, e.g., intention tremors, ataxia

spinal cord?

  1. face, language not involved
  2. level of dermatome involvement with sensation, e.g., biceps reflex C 5-6, abdominal reflexes T8-12, depending where you are compared to umbilicus, knee reflex L2-4, ankle reflex S1)

 

VISUAL PATHWAY

The retina is divided in"½" by the fovea, or central portion of the macula.

optic nerve:

cranial nerve II

optic chiasma:

here, the fibers from each nasal hemiretina cross and the fibers from the temporal hemiretina remain ipsilateral, e.g., the fibers of the L nasal hemiretina and R temporal hemiretina will form the R optic tract

optic tract:

this will synapse in the lateral geniculate body (in the thalamus).

optic (visual) radiations:

to the occipital lobe, with parietal and temporal radiations.

occipital lobe:

of the cerebral cortex

 

OPTIC REFLEXES

The pupillary light reflexes involve a direct pathway to the midbrain, with a direct vs. consensual reflex.

Marcus-Gunn pupil:

direct response less than consensual response; test by"swinging flashlight." Seen in unilateral optic nerve damage.

consensual reflex:

shining light into one eye causes other pupil to constrict. Done via superior collicus to connection in the pretectal area of the midbrain, then to the Edinger-Westphal nucleus (parasympathetic portion of CN 3), so that a lesion around there (as yet not exactly located)1 creates…

Argyll-Robertson pupil:

constricts to accomodation but not to light (syphilis, diabetes)

Accomodation involves the occipital cortex projecting onto the structures mentioned above.

 

In conjugate gaze, typically the eyes work in tandem in looking at an object. This involves the nucleus of CN 3 (midbrain) and the nucleus of CN 6 (pons) being connected by the medial longitudinal fasciculus (MLF). With damage to the MLF (e.g., MS), the eyes can converge, but can't look to the other side past midline. A horizontal gaze center is in the paramedian pontine reticular formation (PPRF), so that lesion here will lead to gaze paralysis toward the ipsilateral side.1

NYSTAGMUS

nystagmus: repetitive, tremor-like oscillation of eyes (term refers to the fast part of the movement). The ocular appearance of nystagmus occurs because of the vestibular tract tie-ins to ocular systems, e.g.,the MLF just mentioned.

horizontal nystagmus: eyes repetitively slowly to one side and then jerk back. This may be created by vestibular problems, by rotation (e.g., spinning), among other things. The fast component (the one for which we name the direction of the nystagmus) is more cortical, and so is lost first with lethargy. Increasing lethargy affects the slow deviation, then with more obtundation, showing no deviation at all.

One can create nystagmus, and so test the level of obtundation, with calorics. This means stimulating the semicircular canals with cool water (nystagmus to the opposite side) or warm water (nystagmus to the same side), or COWS.

AUDITORY PATHWAYS

A reflex pathway through the tectospinal tract: reflex head/eye turning to site of the sound. Fibers through the inferior colliculi (midbrain) to the medial geniculate bodies (thalamus) and then into the primary auditory cortex in the temporal lobe.

tinnitus:

sounds heard without auditory stimulus; often a peripheral or cochlear disease

conduction deafness:

e.g., earwax, otosclerosis, otitis media

sensorineural deafness:

e.g., disease of the cochlea, cochlear nerve, or central auditory connections.

presbycusis:

lessening of hearing in old age, with high frequency loss.

Weber test:

normally bilateral

Rinne test:

normally air conduction > bone conduction

vestibular pathway: Input via vestibulospinal tracts to allow reflex adjustment of the head

vertigo: illusory sensation of turning or spinning, e.g., peripheral from labyrinth lesion, or central from brainstem damage

PERIPHERAL NERVE LESIONS

To sum up our whirlwind tour of neurolocalization, I want to remind you of the dermatome distributions that have been discussed along the way, as well as the sensory/motor distributions of major nerves described, e.g., wrist drop with radial nerve damage, etc. Two other examples of peripheral involvement include:

Peripheral neuropathy:

May present in a dermatome fashion, as well as in a"glove-and-stocking" distribution, especially in diabetes mellitus. Unfortunately, glove-and-stocking distribution is also seen in many nonorganic sensory disorders.

Guillain-Barré syndrome:

A polyneuritis of rapid onset affecting peripheral nerve roots, with motor > sensory deficits. Typically affecting lower extremities first, then ascension to upper extremities and face.
  1. PA Young, PH Young, Basic Clinical Neuroanatomy (Williams & Wilkins, Baltimore, 1997), pp. 67, 77, 93, 107, 111, 124, 162, 243

 

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Please send comments and questions to: cronewil@hvcc.edu

 

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This page updated on December 6, 1999