Neuroanatomy lecture 3: Spinal cord sensory pathways & asynchronous Flashcards
Central nervous system
Peripheral nervous system
Nerves
Ganglia
Anatomy of the spinal cord:
Posterior: ascending sensory signals
Anterolateral: ascending sensory and descending motor signals
Lateral: autonomic signals
Anatomy of the PNS:
1) 31 pairs of spinal nerves
2) 12 pairs of cranial nerves
3) autonomic nervous system
Dorsal root:
provides pathway for afferent/incoming information to flow from spinal nerve into the spinal cord.
(Sensory information may synapse in dorsal grey horn OR ascend in white columns)
Ventral root:
provides pathway for efferent/outgoing signals to leave spinal cord.
(Motor information may synapse in ventral grey horn or descend in white columns)
Posterior/dorsal root:
carries sensory/afferent information into CNS so it can ascend int he cord to reach the cortex.
Ventral/anterior root:
carries information out of the CNS that has descended from the cortex to the cord.
Ventral and dorsal roots come together to form:
spinal nerve in the intervertebral foramen
Somatic:
“of the body” what you are aware (conscious of)
sensory information from surface of the skin/innervation and control of the skeletal muscles.
Autonomic:
what you are not aware of (unconscious)
feedback information from the internal organs, glands, smooth, cardiac muscle and innervation/control of autonomic muscles (smooth and cardiac)
GSE:
General somatic efferent:
Conscious/voluntary motor (body movement)
GSA:
General somatic afferent: general sensation (sensation of skin)
GVE:
General visceral efferent:
Involuntary/subconscious motor to organs/viscera (beating of heart)
GVA:
General visceral afferent:
Visceral sensation (stretch of vessels)
Axon Relay:
communication to and from the brain involves chains of neurons (like a relay race) and they are numbered according to position.
Decussate:
when signals cross over from one side to another (left to right; or right to left) Occurs within the CNS.
Integration between PNS and CNS:
(peripheral nerves signals to the cortex)
-Receptors for sensory information are located at the end of peripheral nerves within the layers of the skin.
-Gather information about the surrounding environment and send information about sensation back to CNS for processing.
-Afferent/sensory information comes into the CNS via the dorsal root of the spinal nerve
-Ascends int he spinal cord via posterior, lateral and anterior columns
-Majority of signals synapse in Thalamus
Integration between PNS and CNS:
(Motor innervations to peripheral nerves)
-Efferent/motor signals originate in the pre-central gyrus of the frontal lobe (primary motor cortex)
-Descend in the spinal cord via anterior or lateral columns
-Exits the CNS via ventral root of the spinal nerve
-Target for motor signals are known as effectors
Sensory tracts of the spinal cord:
Three types of neurons:
-first order neuron
-second order neuron
-third order neuron
First-order neuron:
resides in the dorsal root ganglion of the spinal nerve.
-conducts impulses from sensory receptors to the spinal cord or medulla oblongata
Second- order neuron:
located either in the dorsal horn of the spinal cord or the medulla oblongata.
-relays impulses to the thalamus
Third-order neuron:
residing in the thalamus conveys impulses to the general sensory area (postcentral gyrus)
-Parietal lobe of the cerebral cortex
Major ascending, sensory tracts:
1) Spinothalamic: pain & temperature
2) Spinocerebellar: proprioception
3) Posterior/Dorsal column: discriminative touch
4) Spinotreticular tract
5) Spinotectal tract
6) Spinomesecephalic tract
7) spino-olivary tract
(all of these enter the spinal cord using the dorsal root of the spinal nerve)
Posterior/Dorsal columns PNS pathway:
1st order neuron:
-Originates in the peripheral receptor
-travels towards CNS via spinal nerve
-Enters CNS via dorsal root of spinal nerve and does NOT synapse
-Axon continues to ascend in dorsal columns of spinal cord
-Synapses in nuclei of medulla
2nd order neuron:
-Cell body originates in nucleus of medulla
-Axon decussates in medulla oblongata
-Ascends to thalamic nuclei via medial lemniscus pathway
-Synapses in thalamus
3rd order neuron:
-Cell body originates in thalamus
-Axon travels from thalamus to post-central gyrus of parietal lobe
-Synapses in primary somatosensory cortex
-Sensation is interpreted
Dorsal columns can be further subdivided in order to categorize tough information:
Upper limb: fasciculus Cuneatus
Lower limb: fasciculus Gracilis
Pathway for these posterior/dorsal columns in the PNS:
1st order neuron:
-originates in perioheral receptor in Upper limb or Lower limb
-travels towards spinal cord via spinal nerve
-Enters CNS via dorsal root and ascends in dorsal columns (FC or FG) of spinal cord
-Synapses in one of the nuclei in medulla
FC (lateral columns): fasciculus Cuneatus; transmitting touch information from the upper limbs
-Information traveling in this column will synapse in the “nucleus cuneatus” (NC) of the medulla
FG (medial columns): fasciculus Gracilis; transmitting touch information from lower limbs
-Information travelling in this column will synapse in the “nucleus gracilis” (NG) of the medulla
Dorsal Column (Medial lemniscus) Tract:
conducts nerve impulses for conscious proprioception, and discriminative (fine) touch & vibration.
Conscious proprioception in the dorsal column (medial lemniscus):
collected from the joints and muscles except for the temporomandibular joint.
Discriminative (fine) touch & vibration in the dorsal column (medial lemniscus) tract:
from the entire body (except most for the head region) to the cerebral cortex.
Dorsal Column (medial lemniscus) tract:
first-order neuron: located in the dorsal root ganglion of the spinal nerve.
The axon to this neuron ascends in the dorsal column of the spinal cord. (fibers from upper limbs ascend through fasciculus cuneatus) (fibers from lower body ascend through fasciculus gracilis)
-Axons from the first order neuron synapse to the second order neuron in the nucleus gracilis & cuneatus w/in the medulla oblongata
2nd order neuron: decussates (crosses the midline) and ascends through the brainstem as the medial lemniscus.
Medial lemniscus synapses with the 3rd order neuron in the thalamus.
Axon of 3rd order neuron passes through the internal capsule and projects to the somatosensory area of the cerebral cortex (postcentral gyrus) for conscious sensation.
Spinocerebellar tract pathway:
1st order neuron:
-Originates in peripheral receptor
-Travels towards CNS via nerve
-Axons enters dorsal root and synapses in dorsal grey horn upon entry
2nd order neuron: Cell body originates in dorsal grey horn
-Remains ipsilateral
-Ascends in anterolateral white column of spinal cord
-Axon ascends to the level of the cerebellum
-Synapses in cerebellar hemisphere
(injury will always be considered ipsilateral)
Dorsal spinocerebellar tract:
-Posterior spinocerebellar fibers receive joint muscle information from the muscle spindles, tendon organs, and joint receptors of the trunk & lower limbs
-Cerebellum uses this information concerning the tension of muscle tendons and the movements of muscles and joints to coordinate limb movements & maintain posture
-1st order neuron, located in the dorsal root ganglion of the spinal nerve. Synapses with the 2nd order neuron that resides in the dorsal horn of the spinal cord. (known collectively as the NUCLEUS DORSALIS (Clarke’s column)
-Axons of the 2nd order neurons from dorsal spinocerebellar tract travels the spinal cord’s lateral column (funiculus). Tract ascends through the medulla oblongata and ends in the ipsilateral cerebellar cortex after passing through the inferior cerebellar peduncle.
Clarkes column (nucleus dorsalis):
extends only from the C8 spinal cord segment caudally to the L3 spinal cord segment, axons entering the spinal cord from the posterior roots of the lower lumbar and sacral segments ascend in the fasciculus gracilis in the posterior white column until they reach the L3 segment, where they enter the nucleus dorsalis.
Cuneocerebellar tract:
unconscious proprioception from the upper limb to the cerebellum
Spinothalamic tract pathway:
1st order neuron:
-Originates in peripheral receptor
-Travels towards CNS via nerve
-Axon enters dorsal root and synapses in dorsal grey horn upon entry
2nd order neuron:
-Cell body originates in dorsal grey horn
-Axon decussates at the level of entry
-Ascends in antero-lateral white columns of spinal cord
-Synapses in thalamus
3rd order neuron:
-Cell body originates in thalamus
-Axon travels from thalamus to post central gyrus
-Synapses in primary somatosensory cortex of parietal lobe
Lateral spinothalamic tract:
conveys impulses concerned with pain & temperature collected from the entire body (except the head region).
1st order neuron: located in the dorsal root ganglion of the spinal nerve. The axon of this neuron synapses with the 2nd order neuron in the dorsal horn of the spinal cord.
Neurotransmitter used here is: Substance P.
2nd order neuron: axon decussates and ascends through the lateral column of the spinal cord as the lateral spinothalamic tract and synapses with 3rd order neuron.
3rd order neuron: projects to the somatosensory area of the cerebral cortex, where the information is processed into conscious sensation.
The anterior spinothalamic tract:
another ascending tract with similar neuronal design as lateral spinothalamic tract, origin, and termination, but conveys the impulses of non-discriminative (crude) touch.
Somatosensory map of post central gyrus:
-proportional to the number of sensory receptors
-Proportional to the sensitivity of each part of the body
Can be modified with learning:
-Learn to read braille & will have larger area representing fingertips
Spinal cord “gate” synapse:
Spinoreticular pathway:
Levels of consciousness:
1st order neuron:
-Originates in peripheral receptor
-Travels towards CNS via nerve
-Axon enters dorsal root and synapses in dorsal grey horn upon entry
2nd order neuron:
-Cell body originates in dorsal grey horn
-Axon remains ipsilateral
-Ascends in antero-lateral white columns of spinal cord
-Synapses in medulla and midbrain
Spinoreticular tract:
Axons enter the spinal cord from the posterior root ganglion and terminate on unknown 2nd order neurons in the grey matter.
Axons from 2nd order neurons ascend the spinal cord as the spinoreticular tract in the lateral white column mixed with the lateral spinothalamic tract.
Most fibers are uncrossed and terminate by synapsing w/ neurons of the reticular formation in the medulla oblongata, pons, and midbrain. The spinoreticular tract provides an afferent pathway for reticular formation.
This plays an important role in influencing levels of consciousness.
Spinotectal pathway:
coordinates movements of eyes and head:
1st order neuron:
-Originates in peripheral receptor
-Travels towards CNS via nerve
-Axon enters dorsal root and synapses in dorsal grey horn upon entry
2nd order neuron:
-Cell body originates in dorsal grey horn
-Axon cross over midline
-Ascends in antero-lateral white columns of spinal cord
-Synapses in superior colliculus of midbrain
“reflexive turning of head and eyes towards a point of cutaneous stimulation”
Spinotectal tract:
-xons enter the spinal cord from the posterior root ganglion & travel to the grey matter & synapse to 2nd order neuron
-axons of 2nd order neuron cross the median plane and ascend as the spinotectal tract in the anterolateral white column lying close to the lateral spinothalamic tract.
-After passing through the medulla oblongata and pons, they terminate by synapsing w/ neurons in the superior colliculus of the mid brain.
-This pathway provides afferent information for spinovisual reflexes and brings about movements of the eyes and head toward the source of the stimulation.
-Sometimes considered the subdivision of the spinothalamic tract
Spinomesencephalic tract:
Similar to spinoreticular tract, except it terminates in the midbrains periaqueductal gray matter (PAG) (the area surrounding the cerebral aquedect).
Connections to limbic system, likely responsible for: fear & aversion reactions associated w/ pain.
Also activates the descending analgesia system that arises from the PAG.
Spino-olivary pathway:
1st order neuron:
-Originates in peripheral receptor
-Travels towards CNS via nerve
-Axon enters dorsal root and synapses in dorsal grey horn upon entry
2nd order neuron:
-Cell body originates in dorsal grey horn
-Axon cross over midline
-Ascends in antero-lateral white columns of spinal cord
-Synapses in inferior olivary nucleus of medulla
3rd order neuron:
-Cell body originates in nucleus
-Axon cross the midline again
-Enter the cerebellum
-Synapses in cerebellum
Role in controlling movements of the body and limbs
Spino-olivary tract:
-axons enter from spinal root ganglion & terminate on unknown 2nd order neurons in posterior grey column.
-Axons from 2nd order neurons cross midline and ascend as the spino-olivary tract in white matter at the junction of the anterior and lateral columns.
-Synapse onto 3rd order neurons in inferior olivary nuclei in the medulla oblongata. 3rd order neurons cross midline and enter the cerebellum through the inferior cerebellar peduncle.
Conveys information to the cerebellum from cutaneous and proprioceptive organs.
Spinal cord:
Location: Vertebral Canal
Starts: medulla oblongata (foramen magnum)
End: L1/2 intervertebral disk (adult)
L3 (infants)
-elongation of the spinal cord ends at 4-5, but the growth of the vertebral column continues.
External features of the spinal cord:
Cervical enlargement: gives rise to the nerves innervating the upper limbs
Lumbar (lumbosacral) enlargement: supply the lower limbs.
Spinal cord tapers down inferior to the lumbar enlargement forming the conus medullaris.
On the surface of the spinal cord, there are two main longitudinal grooves:
Ventral (anterior) median fissure & dorsal (posterior) median sulcus
Ventral (anterior) median fissure:
ventral (motor) rootlets of the spinal nerves emerge and join to form the ventral (motor) root of the spinal nerves.
Dorsal (posterior) median sulcus:
dorsal (sensory) rootlets of the spinal cord emerge and join to form the dorsal (sensory) root of the spinal cord.
Each dorsal root of the spin is associated with a ganglion:
dorsal root ganglion: housing the cell bodies of the sensory neurons
Formed by the union of the ventral and dorsal roots:
Spinal nerve
The ventral and dorsal roots, of the lumber, sacral, & coccygeal spinal nerves descend to the respective foramina before emerging from the vertebral canal This bundle of roots is called:
Cauda Equina
Somatic nerve plexuses:
Internal features of the spinal cord:
Consists of grey matter and white matter.
Ventral (anterior) horn & Dorsal (posterior) horn on each side.
Grey commissure: narrow strip connecting right and left halves of the grey matter (contains the central canal).
Lateral horn is located:
T1-L2 & S2-S4
Lateral horn in T1-L2:
Carry the cell bodies of the sympathetic nervous system.
Lateral horn in S2-S4:
carry cell bodies of the parasympathetic nervous system.
Ventral horns contain:
cell bodies of the motor neurons who axons form the motor(ventral) roots of the spinal cord.
Dorsal horns contain:
cell bodies of the sensory neurons. These receive information from the sensory neurons residing in the dorsal root ganglia, whose axons form the dorsal root of the spinal nerves.
The white matter of the spinal cord:
Ventral, lateral, and dorsal columns (funiculi) on each half of the spinal cord.
Each contain bundles of myelinated nerve fibres (tracts) that travel along the length of the spinal cord.
Two main groups of tracts in the spinal cord:
Ascending (sensory) tracts & Descending (motor) tracts
Ascending (sensory) tracts:
Convey the messages from the periphery to the upper centers of the nervous system.
ex) dorsal (posterior) spinocerebellar tract, dorsal column (medial lemniscus) tract, and lateral spinothalamic tract.
Descending (motor) tracts:
convey messages from the upper centers if the nervous system to the periphery.
ex) lateral and ventral corticospinal tracts
