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composition of white matter

mostly myelinated axons

two general type of myelinated axons


where ascending axons lead

higher brain centers

where descending axons lead

ventral horns of the spinal cord

divisions of white matter in the spinal cord

posterior/dorsal column
lateral column
ventral/anterior column

another term for white matter columns


subdivisions of columns in the spinal cord


characteristics of axons within each tract

have the same function and destination

important characteristic of the pathway of tracts (in both directions)

the decussate (cross contra-laterally) at some point in their pathway

general structure of ascending pathways

they are a chain of 3 neurons

types of neurons in ascending pathway chains

1st order
2nd order
3rd order

location of 1st order neurons

unipolar cell body in the dorsal root ganglia

function of 1st order neurons

carry impulses from sensory receptors (located in the skin, muscles & joint proprioceptors) into the spinal cord & synapse with 2nd order neurons

location of 2nd order neurons

cell bodies in the dorsal horn of the spinal cord or in the nuclei of the MO

function of 2nd order neurons

transmit impulses to thalamus or cerebellum where they synapse

location of 3rd order neurons

cell bodies found in the thalamus

function of 3rd order neurons

relay sensory input to the somato-sensory cerebral cortex

the three main ascending pathways of the spinal cord

dorsal column medial lemniscal pathways
anterolateral pathways
spinocerebellar pathways

location of the three main ascending pathways of the spinal column

paired sets of three on each side of the spinal cord

function of the dorsal column medial lemniscal pathway

transmits sensations that can be localized on the body

examples of localized sensations

discriminative touch & vibration, 2 point discrimination, stereognosis, from joint and muscle proprioceptors

2 point differentiation

the ability to recognize that there are two pints of contact


the ability to differentiate what an object is using at least two points of contact

discriminative touch

the ability to identify exactly where the sensation of touch or vibration is occurring on the body

structures of the dorsal column medial lemniscal pathway

the fasciculus cuneatus and the fasciculus gracilis - paired tracts in the the dorsal matter - and the medial lemniscal tract (or lemnisucus)

pathway of the medial lemniscal tract

arises in the MO and terminates in the thalamus

1st order axons of the dorsal column medial lemniscal pathway

enter the spinal cord and travel up toward the MO and form paired tracts - either the fasciculus gracilis or the fasciculus cuneatus which synapse with 2nd order neurons in the MO

from where the fasciculus cuneatus arises

1st order axons of the neck, upper extremities and upper trunk (above the diaphragm)

from where the fasciculus gracilis arises

1st order axons of the lower extremities and lower trunk

2nd order neurons of the dorsal column medial lemniscal pathway

the nucleus cuneatus or the nucleus gracilus - located in the MO

pathway of the 2nd order axons of the dorsal column medial lemniscal pathway

2nd order axons of the DCMLP form the medial lemniscus and travel to the thalamus where they synapse with 3rd order neurons of the DCMLP

where the medial lemniscus decussates

in the MO

3rd order neurons of the DCMLP

located in the thalamus

where 3rd order axons of the DCMLP terminate

in the somatosensory cortex (in the cerebral hemispheres)

structures of the anterolateral pathways

ventral/anterior spinothalamic tract
lateral spinothalamic tract

where the tracts of the anterolateral pathways decussate

in the spinal cord

function of the lateral spinothalamic tract

pain and temperature

function of the anterior/ventral spinothalamic tract

crude touch - not localized sensation

structures of the spinocerebellar pathways

anterior spinocerebellar tract
posterior spinocerebellar tract

functions of the spinocerebellar tracts

transmit proprioceptive information from the trunk and lower limbs to the cerebellum to coordinate skeletal muscle activity

two types of descending pathways of the spinal cord

direct/pyramidal tracts
indirect/extrapyramidal tracts

two types of neurons of the motor tracts

upper motor neurons
lower motor neurons

what the upper motor neurons form

the pyramidal interneurons of the motor cortex
the subcortical motor neurons that give rise to the indirect (extrapyramidal) tracts

what the lower motor neurons form

arises with the ventral horn motor neurons and innervates the skeletal muscles

gross pathway of the pyramidal pathway

begins in the motor cortex (precentral gyri) and runs to the ventral horns of the white matter of the spinal cord

pathways of the axons of the pyramidal tracts

run from the cell bodies in the precentral gyri through the white matter of the brain and descend as either the lateral or ventral corticospinal tract

where the axons of the lateral corticospinal tract decussate

in the MO - the pyramidal decussation

where the axons of the anterior corticospinal tract decussate

in the spinal cord at the level in which they synapse in the ventral horn motor neurons

function of the corticospinal tracts

upper motor neurons carry impulses from the cerebral cortex to the lower motor neurons of the ventral horns which activate skeletal muscles

where collateral impulses from the corticospinal tracts go

to the basal nuclei (deep in the cerebral white matter) and the cerebellum

pyramidal tract that synapses with the cranial nerve motor nuclei

the corticobulbar tract - located above the head and neck

function of the cranial nerve motor nuclei

govern head and neck motor activity (i.e. chewing, swallowing, etc)

the four extrapyramidal tracts

tectospinal tract
vestibulospinal tract
rubrospinal tract
reticulospinal tract

characteristic of the tracts of the extrapyramidal pathway

anatomically complex and multi-synaptic

function of the tectospinal tract

mediate head movement for visual tracking - originates in the superior colliculi of the midbrain (corpora quadrigemina)

function of the vestibulospinal tract

transmits motor impulses to mediate balance and equilibrium while standing and moving - originate in the vestibular nuclei of the MO

function of the rubrospinal tract

transmits motor impulses to assist with limb flexion - originates in the red nuclei of the midbrain

function of the reticulospinal tract

transmits nerve impulses that influence muscle tone for maintaining posture and balance - originate in the reticular formation located deep in the central column of the brain stem

what forms the PNS

all neural structures outside of the CNS -
sensory receptors
peripheral nerves
efferent motor endings

what forms sensory receptors

specialized dendritic endings

how sensory receptors are classified

2 ways -
stimulus detected
body location


respond to mechanical force


respond to temperature changes


only in the retina - respond to light energy


respond to chemicals (smell, taste)


respond to pain signals (stimulus created by trauma to cells causing electro-chemical response)

substance that mediates pain response

substance P


arise from specific locations for position sense

two types of simple receptors of general senses


muscle spindles

encapsulated nerves endings in the perimysium of skeletal muscle that detect stretch and enact a protective contraction reflex

structure of muscle spindle nerves

modified skeletal muscle fibers called intrafusal fibers enclosed in a CT capsule

golgi tendon organs

proprioceptors located in tendons near the tendenomuscular junction which monitor muscle tension and enact a protective inhibition of contraction of muscle

general structure of a nerve

consistes of many peripheral axons bound into fascicles

CT sheath of peripheral axons


CT sheath of each fascicle


CT sheath that encloses groups of fascicles that form the nerve


other structures contained within the nerve

blood and lymph vessels

term for nerves containing both motor and sensory fibers

mixed nerves

classifications of motor nerve fibers


function of somatic motor nerves

motor nerves of skeletal muscle

function of autonomic motor nerves

motor nerves of glands, smooth muscle, and cardiac muscle

classifications of mixed nerve fibers

somatic afferent - from the body
somatic efferent - to the skeletal muscle
autonomic afferent - from the effector organs
autonomic efferent - to the effector organs

classifications of peripheral nerves

spinal - arise from spinal cord
cranial - arise from brain stem

term for a group of neuronal cell bodies of the nerves of the PNS

ganglia (ganglion, singular)

ganglia of the afferent somatic nerves

dorsal root ganglion

ganglia of the autonomic efferent nerves

autonomic ganglia

when PNS nerve axons may be able to regenerate

if the cell body remains intact and damage is not too extensive

first stage of axon regeneration after trauma (within hours)

Wallerian degeneration - axon distal to the injury site degenerates due to lack of nutrients from the cell body

2nd stage of axon regeneration (within 1 week)

macrophages degrade the degenerated distal axon; the neurilemma remains intact

3rd stage of axon regeneration

Schwann cells undergo mitosis and migrate to the injury site and release chemicals that stimulate the growth of the proximal axon and form a regeneration tube that guide the regenerating axon "sprouts" across the injury site

axon regeneration rate

about 1.5 mm/day

why CNS axons do not regenerate

oligodendrocytes contain growth inhibiting proteins and astrocytes form scar tissue, blocking axonal growth

number of cranial nerves

12 pair

cranial nerves that do not originate from the brainstem

I & II

areas innervated by the Cranial nerves, and the one exception

Except for CN X (Vagus) cranial nerves only innervate head and neck structures

mnemonic for names of the cranial nerves

Oh, Oh, Oh, Tubby Teens And Fresh Virgins Give Very Awful Head

mnemonic for the type of functions of cranial nerves - motor, sensory, or both

Some Say Money Matters, But My Brother Says Big Boobs Matter More

Function and origin of the CN I, the Olfactory Nerve

sensory - axons arise from olfactory receptor cells; function in sense of smell

function and origin of CN II, the Optic nerve

sensory - axons arise from ganglia in the retina; function in vision

Cranial nerves that move the eyeball


functions of CN III, the Oculomotor nerve

Somatic and Autonomic Motor:
- 4 of 6 extrinsic eye muscles - inferior oblique, superior, inferior and medial rectus
- parasympathetic motor to muscles of the iris: sphincter pupillae (constrict pupils) and ciliary muscle (shape of lens for focus)
- levator palpebrae superioris: raises the upper eyelid

functions of CN IV, the Trochlear nerve

somatic motor:
superior oblique muscle of the eye

three division of CN V, the Trigeminal nerve


function of the opthalmic division of the Trigeminal nerve

anterior scalp
upper eyelid, nose, nasal cavity mucosa
lacrimal gland

function of the maxillary division of the Trigeminal nerve

nasal cavity mucosa
upper teeth
skin of cheek
upper lip
lower eyelid

function of the mandibular division of the Trigeminal nerve

anterior tongue (except taste buds)
lower teeth
skin of chin
temporal scalp
Somatic Motor:
muscles of mastication

function of CN VI, the Abducens nerve

Somatic Motor:
lateral rectus muscle of the eye

Five branches of CN VII, the Facial nerve


functions of CN VII, the Facial nerve

Somatic motor:
muscles of facial expression
Parasympathetic Motor:
lacrimal glands
nasal & palatine glands
submandibular and sublingual glands
taste buds on anterior 2/3 of tongue

functions and branches of CN VIII, the Vestibulocochlear nerve

vestibular branch for equilibrium
cochlear branch for sense of hearing

functions of CN IX, the Glossopharyngeal nerve

somatic motor - stylopharyngeus muscle (elevates pharynx during swallowing)
parasympathetic motor - parotid glands
taste, touch, pressure from posterior 1/3 of tongue and pharynx
from carotid bodies monitoring O2 and CO2 levels
baroreceptors (next to carotid bodies) monitoring Blood Pressure

distinctive characteristic of CN X, the Vagus nerve

fibers emerge from the skull and descend through the neck to the thorax and abdomen

functions of CN X, the Vagus nerve

somatic motor - laryngeal and phayngeal muscles of swallowing
parasympathetic motor - to heart, lung, abdominal organs for heart rate, force of contraction, and digestive organ activity
visceral sensory from the thoracic and abdominal viscera
baroreceptors in the aortic arch (blood pressure)
chemo receptors from the carotid and aortic bodies (O2 & CO2 levels)

distinctive characteristic of CN XI, the accessory nerve

it arises from ventral rootlets of C1 to C5 cord levels, enters the foramen magnum and exits the jugular foramen

function of CN XI, the Accessory nerve

Somatic Motor:
trapezius & SCM muscles

function of CN XII, the Hypoglossal nerve

Somatic motor:
- intrinsic and extrinsic tongue muscle for mixing food and chewing
- tongue movement for swallowing and speech

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