Unit 6 - Mesencephalon (midbrain)

3 parts of the midbrain

tectum, cerebral aqueduct, and cerebral peduncles

posterior "roof" of the midbrain

tectum

location of the tectum

posterior to cerebral aqueduct

composition of the tectum

quadrigeminal plate made up of 4 collicular bodies (superior and inferior pairs)

channel for CSF exchange between 3rd and 4th ventricle

cerebral aqueduct (of sylvius)

what substance surrounds the cerebral aqueduct?

periaqueductal gray substance (central gray )

which important nuclei are found in or near the periaqueductal gray substance?

nuclei for CN III and IV

endorphins and enkephalins (natural painkillers) are present where?

in the periaqueductal gray area

location of the cerebral peduncles

ventral to the cerebral aqueduct

parts of the cerebral peduncles

tegmentum, substantia nigra, and cerebral crus

cranial nerves associated with the midbrain

Oculomotor (III) and Trochlear (IV)

Cranial Nerve IV: Trochlear

smallest of the cranial nerves

what does CN IV innervate?

superior oblique muscle (trochlear muscle) of the extraocular eye musculature

apparent origin of CN IV

mid-dorsal aspect

-just inferior to the inferior colliculus

CN IV fibers

cross over on the opposite trochlear nuclei as they exit

4 unique characteristics of CN IV

1. its the smallest CN

2. its apparent origin is on the posterior aspect of the brainstem

3. only CN where its fibers cross

4. it travels further inside the skull than any other CN

Cranial Nerve III

oculomotor nerve

CN III apparent origin

ventral form the interpeduncular fossa

-the 6 nuclei of origin are located in the periaqueductal gray substance

CN III function

innervates 4/6 extra ocular muscles

-superior, inferior, and medial rectus, and inferior oblique

somatic motor fibers of CN III reach what?

the levator palpebral superioris

-raises the upper eyelids

CN III carries what kind of axons from _________ to _____________ to ____________

carries preganglionic parasympathetic axons from accessory oculomotor nucleus - ciliary ganglion - ciliaris and sphincter pupillae muscles of the eye

how many types of fibers does CN III carry?

2 types

LMN of CN III carry info to where?

extraocular muscles

pre ganglion sympathetic axons carry info to where?

ciliaris and sphincter pupillae

-synapses in a ganglion

the 4 bumps that exist in the mammalian tectum are called what?

the superior and inferior colliculi

superior colliculus deal with what type of reflexes?

visual reflexes

inferior colliculus deal with what type of reflexes?

auditory reflexes

both sets of colliculi use which tract for the auditory and visual reflexes?

tectospinal tract

-allows us to move our head away from loud sounds and objects moving rapidly in our field of vision

tectospinal tract influences reflexes for what?

CN XI in the upper spinal cord

-in turn it contracts the SCM and trapezius muscles to prevent head injury

tectum lesions

don't impair voluntary eye movements, only perception of motion of objects in the field of vision

hearing reflex pathway to cortex

auditory info - lateral lemniscus - medial geniculate body - temporal lobe

-may or may not synapse in inferior colliculi before going to medial geniculate body

hearing reflex pathway through tectospinal tract

auditory info - lateral lemniscus - inferior colliculi - superior colliculi - tectospinal tract (C1-C4)

motion reflex pathway to occipital lobe

vision info - optic tract - lateral geniculate body - occipital lobe

motion reflexes to tectospinal tract

vision info - optic tract - superior colliculus - tectospinal tract (C1-C4)

tegmentum location

area just ventral to the cerebral aqueduct

which CN nuclei of origin are found on the tegmentum?

CN III and IV

reticular formation of the midbrain is located where?

tegmentum

Nucleus ruber location

primarily found in midbrain, the superior part of the red nucleus extends into diencephalon's sub thalamic region

2 divisions of the nucleus ruber

magnocellular area and parvocellular

magnocellular area

inferior large cell area of the red nucleus

parvocellular

vascular superior area of the red nucleus

Most rubrospinal tract fibers originate where?

magnocellular area of the midbrain

input fibers come to the red nucleus from where?

1. cerebral cortex

2. central cerebellar canal (except fastigial)

function of tegmentum

contralateral motor response necessary for postural control and muscle tone control primarily in flexor musculature (rubrospinal tract)

substancia nigra location

between tegmentum and crus

features of the substance nigra

paired, large, and extends to subthalamus of the diencephalon

"black substance" is only found in who?

mammals

What gives the substantia nigra its black appearance?

melanin

afferent and efferent fibers of the substancia nigra are exchanged between what?

the telencephalon's caudate and putamen nuclei

what do the caudate and putamen nuclei make up?

corpus striatum or basal ganglia of the cerebrum

cause of Parkinson's disease

Loss of dopaminergic neurons in the substantia nigra

symptoms of Parkinson's disease

shaking of digits hands and tongue at rest, odd postures, pain, shuffling gates and loss of speech in lateral stages

crus cerebri (basis pedunculi cerebri) location

ventrolateral aspect of the cerebral peduncle

crus cerebri is composed of descending tracts of fibers from the cerebral cortex such as.....

1. corticospinal fibers (to cord)

2. corticopontine fibers (to pons)

3. corticobulbar fibers (to M.O.)

4. corticomesencephalic fibers (terminate on CN nuclei III and IV to initiate voluntary motor movements of eyes)

types of fibers found in crus cerebri

pyramidal, UMN, descending