What does the spinal cord develop from?
the caudal portion of the embryonic neural tube
What happens on the sixth week of spinal development?
the spinal cord has 2 recognizable clusters of neuroblasts the migrated outward from original neural tube; a dorsal alar plate and ventral basal plate
What do alar plate neuroblasts become?
What do basal plate neuroblasts develop into?
motor neurons and sprout axons that grow out to the effector organs
What do axons that emerge from alar plate cells form?
the white matter of the cord by growing along the length of the cord
Dorsal Root Ganglia
formed by neural crest cells that come to lie along the cord; contain sensory neuron cell bodies; these sensory neurons send their axons into dorsal aspect of the cord
about 43 cm (17inch) long and 1.8 cm (3/4 inch) thick; provides a two way conduction pathway to and from the brain;
How far does the spinal cord extend?
extends from the foramen magnum of the skull to the level of the first or second lumbar vertebra
How are the brain and spinal cord similar?
they are both protected by bone, meninges, and cerebrospinal fluid
Spinal Dura Mater
single layered; not attached to bony walls of vertebral column
between the bony vertebrae and spinal dura mater; filled with soft padding of fat and a network of veins
What fills the subarachnoid space between the arachnoid and pia mater meninges?
Where do the dural and arachnoid membranes extend to?
the level of S2, well beyond the end of the spinal cord
Whats an ideal spot for removing CFS for testing
the subarachnoid space within the meningeal sac; called lumbar puncture
where the spinal cord terminates; cone shaped structure
fibrous extension of the conus covered by the pia mater; extends inferiorly from the conus medullaris to the coccyx, where is anchors spinal cord in place so its nots jostled by movements
How is the spinal cord secured to the tough dura mater meninx throughout its length?
by saw tooth shelves of pia mater called denticulate ligaments
How many spinal nerves are in human?
31 pairs of spinal nerves
Where does the spinal cord enlarge?
in the cervical and lumbar regions where the nerves serving the upper and lower limbs arise
collection of nerve roots at the inferior and of the vertbral canal; resembles horses tail
What grooves run length of the spinal cord and partially divide it into right and left halves?
the ventral (anterior) median fissure, and shallower dorsal (posterior) median sulcus
In the spinal cord what are the mirror image lateral gray masses connected by?
by a crossbar of gray matter called the gray commisure which encloses the central canal
Dorsal Posterior Horns
two dorsal projections of the gray matter area
Ventral Anterior Horns
ventral pair projections of gray matter area
additional pair of gray matter columns; present in thoracic and superior lumbar segments of the cord
What are neurons whose cell bodies are in the spinal cord gray matter?
What do the dorsal horns consist of?
entirely of interneurons
What do the ventral horns have?
some interneurons but mainly house cell bodies of somatic motor neurons
where ventral rootlets fuse together; motor neurons send their axons out to skeletal muscles via the vetral rootlets
What reflects the amount of skeletal muscle innervated at a given level?
the amount of ventral gray matter present at a given level of the spinal cord
What type of neurons are in the lateral horns?
the lateral horn neurons are autonomic (sympathetic division) motor neurons that serve visceral organs; their axons leave cord via ventral root along with those of the somatic motor neurons
What forms the dorsal roots of the spinal cord?
afferent fibers carrying impulses from peripheral sensory receptors; dorsal roots fan out as the dorsal rootlets before they enter the spinal cord
Dorsal Root Ganglion
this is where cell bodies of the associated sensory neurons are found; an enlarged region of the dorsal root
where dorsal and ventral roots fuse laterally; part of PNS
Spinal Gray Matter can be divided further according to its neurons relative involvement in the innervation of the somatic and visceral regions of body, such as...
somatic sensory (ss), visceral (autonomic) sensory (vs), visceral motor (vm), somatic motor (sm)
What is the white matter of the spinal cord composed of?
myelinated and unmyelinated nerve fibers that allow communication between different parts of the spinal cord and between the cord and brain
What 3 directions do white matter fibers run in?
(1)ascending- up to higher centers, (2)descending- down to the cord from the brain or within the cord to lower levels, (3)transversely- across from one side of the cord (commissural fibers)
white matter columns on each side of the cord; 3 on each side
What are Funiculi named according to?
their position as dorsal (posterior), lateral, and ventral (anterior) funiculi
What does each funiculi contain?
each contains several fibers tracts, and each tract is made up of axons with similar destinations and functions
What are all major spinal tracts part of?
of the multineuron pathways that connect the brain to the body periphery
Tract Generalization: Decussation
most pathways cross from one side of the CNS to the other (decussate) at some point along their journey
Tract Generalization: Relay
most pathways consists of a chain of two or three neurons (a relay) that contributr to successive tracts of pathway
Tract Generalization: Somatotopy
a precise spatial relationship among the tract fibers that reflects the orderly mapping of the body
Tract Gernalization: Symmetry
all pathways and tracts are paired symmetrically (right and left), with a member of the pair present on each side of the spinal cord or brain
How do ascending pathways conduct sensory impulses upward?
through chains of 3 sucessive neurons (1st, second and third order neurons) to various areas of the brain
cell bodies reside in a ganglion (dorsal root or cranial), conduct impulses from the cutaneous receptors of the skin and from proprioreceptors to the spinal cord or brain stem, where they synapse with second order neurons; impulses from facial area are transmitted by cranial nerves, and spinal nerves conduct somatic sensory impulses from the rest of the body to CNS
their cell bodies reside in the dorsal horn of the spinal cord or in meduallry nuclei, and they transmit impulses to the thalamas or to the cerebellum where they synapse
have cell bodies in the thalamus; they relay impulses to the somatosensory cortex of the cerebrum
On what three main pathways are somatosensory information conveyed along on each side of the spinal cord?
Dorsal Column - Medial Lemniscal, Anterolateral Pathways and Spinocerebellar Tracts
Dorsal Column - Medial Lemniscal Pathways
mediate precise, straight through transmission of inputs from a single type of sensory receptor that be localized precisely on the body surface (such as discriminative touch and vibrations); formed by paired tracts of dorsal white column of spinal cord - fasciculus cuneatus and fasciculus gracilis - and medial lemniscus
Where does the medial lemniscus arise in?
in the medulla and terminates in the thalamus; from thalamus, impulses are forwarded to specific areas of the somatosensory cortex
recieve input from many different types of sensory receptors and make multiple synapses in the brain stem; these pathways are largley formed by lateral and ventral (anterior) spinothalamic tracts;
What do most fibers in Anterolateral Pathways transmit?
pain, temperature, and course touch impulses, sensations that we are aware of but have difficulty localizing precisely on the body surface
ventral (anterior) and dorsal (posterior) tracts convey information about muscle or tendon stretch to cerebellum, which uses this information to coordinate skeletal muscle activity; do not contribute to conscious sensation; these fibers do not cross over twice (decussate)
The descending tracts that deliver efferent impulses from the brain to the spinal cord are divided into what two groups?
the direct pathways and the indirect pathways
What neurons are referred to as the upper and lower motor neurons?
the motor pathways that involve two neurons
Upper Motor Neurons
the pyramidal cells of motor cortex as well as other neurons in subcortical motor nuclei that give rise to other descending motor pathways
Lower Motor Neurons
the ventral horn motor neurons, which actually innervate the skeletal muscle (their effectors)
Direct (Pyramidal) System
originate mainly with pyramidal neurons located in precentral gyri; these neurons send impulses through brain stem via large pyramidal (corticospinal) tracts
Why are the Direct Pathways called so?
because their axons descend without synapsing from the pyramidal neurons to the spinal cord; their they synapse with interneurons or with ventral horn motor neurons
What does the stimulation of ventral horn neurons activate?
the skeletal muscles with which they are asscociated with
What do direct pathways primarily regulate?
fast and fine (or skilled) movements such as going needle worj and writing
Indirect (Extrapyramidal) System
includes brain stem motor nuclei and all motor pathways except pyramidal pathways; these tracts are formerly lumped together as extrapyramidal sytem because their nuclei of origin were presumed to be independent of the pyramidal tracts
What are pyramidal tract neurons known for?
known to project to and influence activity of most "extrapyramidal" nuclei (indirect, multinueral, pathways)
What are the indirect multineuronal pathways involved in?
regulating the axial muscle that maintain balance and posture, the muscles controlling coarse limb movements, and head, nech and eye movements that follow objects in the visual field
Reticulospinal and Vestibulospinal tracts
maintain balance by varying the tone of postural muscles
control flexor muscles
Tectosinal Tracts and Superior Colliculi
mediate head movements in response to visual stimuli
injury to ventral horn neurons or ventral roots
injury to upper motor neurons in brain
dorsal roots or sensory tracts are damadged
from inflammationand destruction of the ventral horn neurons by poliovirus
Amyotrophic Lateral Sclerosis (ALS)
results from destruction of the ventral horn neurons and pyramidal tracts; victim eventually loses ability to swallow, speak and breathe
What are the diagnostic procedures to asses neurological condition?
range from routine reflex testing to sophisticated techniques such as cerebral angiography, CT scans, MRI scan and PET scans
What factors impair embryonic brain development?
maternal and enviornmental factors; oxygen deprivation destroys brain cells; severe congential brain disesases include cerebral palsy, anencephaly, hydrocephalus, and spina bifidia
What is the last area of brain to develop?
the hypothalamus; premature babies have trouble regulating body temperature
When does brain growth end?
in young adulthood; neurons die throughout life and most are not replaced; brain weight and volume also decline with age