Spinal cord/ Spinal Nerves: LN Flashcards
Where do afferent sensory fibres enter?
Enter the spinal cord through the dorsal roots of spinal nerves
Where do efferent motor fibres leave?
Leave by way of the ventral roots (the Bell-magendie law)
Where do signal that initiate reflex come from? and where does it go? and what happens after that?
-sensory ending within the spinal cord and these information are relayed to the brain stem and cerebellum to contribute to circuits that influence motor performance and other functions
-Sensory signals are also sent rostrally to the thalamus and cerebral cortex, where they enter conscious experience and may elicit immediate or delayed behavioural responses.
-Motor neurons in the spinal cord are excited or inhibited by impulses originating at various levels of the brain, from the medulla to the cerebral cortex.
What protection does the spinal cord have?
- the vertebrae
- attaching ligaments
- the meninges
- cerebrospinal fluid (cushioning)
What makes up the meninges?
-Pia mater
- arachnoid mater
-dura mater
What is the pia mater?
innermost meningeal layer. it is thin and it adheres to the surface of the spinal cord
What is the arachnoid mater?
the middle layer of meninges the lies against the inner surface of the dura. it forms the outer boundary of the fluid-filled subarachnoid space
What is the dura mater?
outermost layer. thick, forms a tube extending from the level of he second sacral vertebra to the foramen magnum at the base of the skull, where it is continuous with the dura around the brain
What is the spinal cord suspended in?
Dural sheath by a denticulate ligament on each side
What is the denticulate ligament made up of? and what is it in the form of?
-pia-arachnoid tissue (also called pachymeninges)
-form of a ribbon attached to the cord midway between the dorsal and ventral roots
Describe the lateral edge of denticulate ligaments
serrated (jagged) and is attached to 21 points to the dural sheath at internal between the foramen magnum and the level at which the dura is pierced by the roots of the first lumbar spinal nerve
What is the epidural space?
-space that exists between the dura mater and wall of the spinal canal
- filled with fatty tissue (protection)
-contains venous plexus
-Epidural space caudal to second sacral vertebra contains the roots of most caudal spinal nerves
How many external segments does the spinal cord have? and how is it demonstrated?
31 segments, demonstrated by presence of 31 pairs of spinal nerves
What is each dorsal root broken up into?
series of rootlets that are attached to spinal cord along the corresponding segment
How many nerves and vertebrae at level of cervical?
8 nerves and 7 vertebrae
How many nerves and vertebrae at level of thoracic?
12 nerves and 12 vertebrae
How many nerves and vertebrae at level of lumbar?
5 nerves and 5 vertebrae
How many nerves and vertebrae at level of sacral?
5 nerves and 5 vertebraes
How many nerves and vertebrae at level of coccygeal?
1 nerve and 4 vertebrae
Describe the length and obliqueness of the roots
-increase progressively in a rostro-caudal direction because of the increasing distance between cord segments and the corresponding vertebral segments
- the lumbosacral roots are, therefore, the longest and constitute the cauda equina
Where does the spinal cord end?
ends at the conus medullaris
Describe the structure of conus medullaris
tapers into slender filament called the filum terminale
What does the caudal 3cm of the spinal cord contain?
contains most of the segments that communicate with the lower limb and perineum
What is below the conus medullaris?
All the segmental nerve roots below L1
What is the cervical enlargement?
includes segments of C4 to T1, with the most corresponding spinal nerves forming the brachial plexuses for the nerve supply of the upper limbs
What is the lumbosacral enlagement?
includes the segments L2 to S3 and the corresponding nerves constitute most of the lumbosacral plexuses for the innervation of the lower limbs
How many regional enlargements exist and what are they adapted for?
2, enlargements are adapted for the innervation of the upper and lower limbs
What is the surface of the spinal cord marked by?
2 longitudinal furrows
What are the 2 longitudinal furrows at the surface of the spinal cord?
-The deep ventral (anterior) median fissure
-contains:
-connective tissue of the pia mater
-the anterior spinal artery and its branches
-The dorsal (posterior) median sulcus (is a shallow midline furrow)
Describe the gray matter in spinal cord
-Has a roughly H-shaped (or butterfly) outline
-the small central canal is lined by ependymal tissue
-it consists (on each side) of:
*dorsal horn
*ventral horn (anterior)
*Lateral horn (only in the thoracic and upper lumbar segments)
What does the white matter of the spinal cord consist of?
Consists of partially overlapping bundles (tracts or fasciculi) of fibres. The ascending and descending tracts are arranged in 3 identifiable funiculi or “columns”
-The dorsal funiculus
-the lateral funiculus
-the ventral funiculus
What does the dorsal funiculus contain?
-Dorsal columns/medial lemniscus system
What are the 2 halves of lateral funiculus?
-Dorsolateral fasciculus
-Ventrolateral fasciculus
What does the dorsolateral fasciculus contain?
-lateral corticospinal tract
-reticulospinal tract
-unmyelinated hypothalamospinal fibres
-dorsal spinocerebellar tract
What does the ventrolateral fasciculus contain?
-lateral spinothalamic tract
-ventral spinocerebellar tract
-the spinotectal tract (or spinomesencephalic tract)
-spinoreticular tract
-others
What does the ventral funiculus contain?
-The ventral corticospinal tract
-The vestibulospinal tract
-Reticulospinal tracts
-anterior spinothalamic tracts
-small tracts (remaining tracts)
What is the Lateral corticospinal tract and its function ?
-is a descending tract (motor)
-is the most conspicuous tract in the dorsal half of the lateral funiculus
-it consists of axons of neurons in the cortex of the front and parietal lobes of the contralateral cerebral hemispheres
- the function of this tract is concerned mainly with skilled volitional (voluntary) movements
What is the pathway of the lateral corticospinal tract?
-pass through the internal capsule, the basis pedunculi of the midbrain, the pons, and the medullary pyramid before they decussate and enter the lateral funiculus of the cord
-corticospinal fibres from the frontal cortex terminate mainly in the intermediate grey matter and the ventral horn
-those from the parietal lobe end in the dorsal horn
-the somatotopic lamination of the lateral corticospinal tract is such that fibres destined for the lowest levels of the spinal cord are the most laterally placed
Describe the ventral corticospinal tract
-descending white matter
-comprises of a small proportion of the corticospinal fibres (those that did not cross the midline in the lower part of the medulla)
-pathway:
-most ventral corticospinal fibres decussate at spinal segmental levels and terminate next to those of the larger lateral corticospinal tract
-they form synapses at the anterior horn with the lower motor neuron which then synapses with the target muscle at the motor end plate
-function:
-it innervates muscles involved in fine and conscious control of the limbs
What is the pathway of vestibulospinal tract?
-uncrossed (laterally located)
-arises from lateral vestibular nucleus in medulla, descends in ventrolateral and ventral white matter of spinal cord, close to surface
-in upper cervical cord, its fibres located in most medial part of lateral funiculus
-Then move medially so in lower cervical segments, close to the margin of ventral median fissure
-in thoracic cord. tract moves into more lateral location in ventral funiculus, among axons that form the ventral rootlets, maintains this position at more caudal levels
-most vestibulospinal axons terminate in medial part of ventral horn
-they form synapses at the anterior horn with lower motor neuron which then synapses with the target muscle at the motor end plate
-most ventral corticospinal fibres decussate at spinal segmental levels and terminate next to those of the larger lateral corticospinal tract
What is the function of the vestibular tract?
mediate equilibratory reflexes, which are triggered by the activity of the vestibular apparatus of the internal ear and put into effect chiefly by the axial musculature (muscles of postural control) and the extensor muscle of the limbs
Describe the reticulospinal tracts
-descending white matter
-present throughout the ventral funiculus and the ventral half of the lateral funiculus
-pathway:
-originate in several nuclei of the reticular formation of the midbrain, pons, and medulla
-most end by contacting interneurons in the ventral horn at all levels (most abundant in the cervical segments)
-the majority of the fibres are from the same side of the brain stem, and some of these axons cross the midline ventral to the central canal. Lower down the tracts, the fibres shift from the ventral into the lateral funiculus
function:
-the third most prominent descending pathway, thought to control movements that don’t require dexterity/volition or postural control
describe the Raphe spinal tract
-descending white matter
-Pathway:
-Follows reticospinal pathway
-arises in the nucleus raphes magnus in the reticular formation of the medulla and terminates in laminae I, II, and III
- they are made up of unmyelinated fibres which form the raphe spinal tract in the most dorsal part of the lateral funiculus
- they contain histochemicallly demonstrable quantities of serotonin
-function:
-The raphe spinal tract modifies the transmission from the dorsal horn of impulses initiated by noxious stimuli. Therefore, they inhibit the perception of stimuli that would be painful
describe the unmyelinated hypothalamospinal fibres
-descending white matter
-pathway:
-arise from the paraventricular nucleus of the hypothalamus
-they end among the preganglionic autonomic neurons in segments T1 to L3 and S2 to S4
-function
-some hypothalamospinal axons contain the peptide oxytocin necessary for uterine contraction during birth, and myoepithelial cells contraction during milk-let-down reflex
Describe the Dorsal columns/medial lemniscus system
-ascending white matter tracts (sensory)
-Consists of a large body of ascending axons derived from neurons located in the dorsal root ganglia
-function:
-adapted for the purpose for discriminative qualities of sensation (fine touch, two-point discrimination, vibration)
-including the ability to recognise changes in the positions of tactile stimuli applied to the skin and
-conscious awareness of movement and of the positions of joints
Describe the spinothalamic tract
-it consists of the ascending axons of neurons located in the gray matter of the opposite half of the cord
-its fibres are also somatotopically arranged
-pathway:
-the cells of origin are mostly in the nucleus proprius of the dorsal horn (laminae IV and V-VI)
-The axons cross the midline in the ventral white matter commissure close to the central canal and then traverse the ventral horn to enter the ventrolateral and ventral funiculi
-the fibres of the spinothalamic tract end in thalamic nuclei
-as they pass through the brainstem, some of these axons give off collateral branches to the reticular formation in the medulla and pons and to the peri-aqueductal grey matter in the midbrain
-function:
-The spinothalamic tract conducts impulses concerned with tactile, thermal, and painful sensations
Describe the dorsal spinocerebellar tract
-is the largest body of ascending fibres in the dorsal part of the lateral funiculus
-it is superficially located, present only above level L3
-pathway:
-its axons arise from the cells of the nucleus thoracicus (Clarke’s column) in the same side of the spinal cord
-They terminate ipsilaterally in the cortex of the cerebellum, which they enter through the inferior cerebellar peduncle
-function:
- this tract conveys proprioceptive information from the proprioceptors in the skeletal muscles and joints to the cerebellum
Describe the ventral spinocerebellar tract
-is located superficially in the ventrolateral funiculus
-it consists largely of cross fibres
-pathway:
-it arises from the base of the dorsal horn and from the spinal border cells of the ventral horn of the lumbosacral segments
-The tract ascends as far as midbrain and then makes a sharp turn caudally into the superior cerebellar peduncle
-the fibres cross the midline for a second time within the cerebellum before ending in the cerebellar cortex
-function:
-It conveys sensory information (mainly proprioceptive) from one lower limb to the same side of the cerebellum (similar function with dorsal spinocerebellar tract)
Describe the spinotectal tract
-ascending white matter tract
-is also called spinomesencephalic tract
-it consists of axons which originate in the same parts of the grey matter as the spinothalamic fibres (in the nucleus proprius of the dorsal horn, lamina IV and V-VI)
-pathway:
-The axons cross the midline, and then project rostrally to the periaqueductal gray matter, then superior colliculus, and various nuclei in the reticular formation of the midbrain
-function:
-This tract conveys pain, crude touch and temperature to the superior colliculus (midbrain)
(not to be confused with the tectospinal tract which is responsible for spinovisual reflex)
Describe the spinoreticular tract
-ascending white matter tract
-pathway:
-is traditionally describe as including cross fibres that terminate in the pontine reticular formation and uncross fibres that end in the medullary reticular formation
-a significant proportion of spinothalamic fibres have collateral branches that synapse with neurons of the reticular formation
-these projections from the spinal cord to the brain stem form part of the ascending reticular activating system
-function:
-involved in the perception of pain and of various sensations that originate in internal organs
What is the stretch reflex?
-describe as a two-neuron or monosynaptic reflex arc
-first, slight stretching of a muscle stimulate the sensory endings in neuromuscular spindles
-this results in an excitation which reaches that spinal cord through primary sensory neurons that have large (group A) axons
- the proximal branches of these axons in the dorsal funiculus give off collateral branches that excite alpha motor neurons, causing the stretched muscle to contract
What is the importance of the stretch reflex? And what activates it?
-The stretch reflex is an important postural reflex, and through the neuromuscular spindles (which are delicate monitor of change in the length of a muscle), the stretch reflex alters tension in such a way as to maintain a constant length of the muscle
-Tension on a muscle monitored by Golgi tendon organs
-when the tension reaches a certain level, a distinct increase takes place in the discharge from Golgi tendon organs
-the resulting action potentials reach interneurons in the spinal grey matter, which in turn inhibit alpha motor neurons
-relaxation of the muscle follows
-this reflex relaxation helps to prevent excessive tension on teh muscle and tendon
What is the application of stretch reflex?
When a muscle is abnormally contracting (spasm or spasticity), passive stretching can induce relaxation by stimulating the golgi tendon organs
What is the flexor reflex?
-protective
-consists of the withdrawal of a limb in response to a painful stimulus
-at least 3 neurons are involved, so this is polysynaptic
-the cutaneous receptors (free nerve endings) respond to potentially injurious stimuli, and the proximal branches of the afferent fibres synapse in the dorsal horn with interneurons
-these end on alpha motor cells in several segments because a withdrawal response require the action of groups of flexor muscles
-The axons of some neurons in the dorsal horn decussate and contact neurons in the contralateral ventral horn to stimulate the extension of the contralateral limb (this results in the crossed extensor reflex)
