Spinal Cord, Reflexes And Descending Pathways

cerebral cortex

spontaneous,novel and adaptive behaviour

hollow fluid filled neural tube

in the dorsal root ganglia

in the basal plate

comprise a system of ten layers of grey matter, to label portions of the grey columns of the spinal cord

make up thedorsal horn, laminae VII and X are mid-region of the cord including the areaaround the fluid-filled central canal.

correspond tothe anterior horn.

thoseinnervating proximal (axial and limb girdle) muscles are found close to themidline

motor neurones supplying the distal limb muscles are placedlaterally

Neuronessupplying flexor groups are nearer the back of the cord

extensors aremore towards the front

(C3, C4, C5)

which supplies the diaphragm via the phrenic nerve and involved in breathing

'C345 keep the diaphragm alive'

(C5, C6)

Supplies trapezius and sternocleidomastoid

S2, S3, S4

Importantfor continence as it supplies the external urethral and anal sphincters

posterior

lateral (clear line)

anterior columns (arbitrary)

containdescending pathways from the brain (e.g. those carrying instructions related tomovements)

contain numerous ascending pathways (e.g. tracts mediating sensationssuch as touch, vibration, pain and temperature).

vibration,proprioception and discriminative touch

bringing painand temperature info to the brain.

conveys information to the cerebellum about length and tension of muscle fibers (i.e., unconscious proprioceptive sensatio

gathers a vast amount of sensory andother information and uses this to “advise” the motor areas of the frontallobe, helping to ensure that actions are performed in a smooth, precise andcoordinated manner

For this reason, cerebellar damage or disease often leadsto clumsiness and poor coordination (termed cerebellar ataxia).

is a disorder that occurs when the cerebellum becomes inflamed or damaged.

The cerebellum is the area of the brain responsible for controlling gait and muscle coordination

a descending tract of the spinal cord which contains bundles of axons which originate in the cerebral cortex and descend to synapse within the brainstem or spinal cord.

The lateralcorticospinal tract occupies the lateral column of the cord

the anteriorcorticospinal tract and occupies the anterior column of the cord.

Lateralcorticospinal tract deals with distal flexors mainly.

Lateralcorticospinal tract contains about 90% of the fibres and these are the onesthat crossed over at the level of the foramen magnum.

Ant spinaltract deals with proximal extensors

Anterior spinaltract is the direct continuation of the pyramids of the medulla. This is why itis situated either side of the midline at the front and doesn’t cross

reflexive

postural

rhythmic

voluntary

vary incomplexity and the degree to which they can be considered voluntary vsautomatic

leastautomatic action goal orientatedvoluntary acts that require participation of the cerebral cortex.

originatefrom the frontal lobe

Involve directconnections from cortex to spinal cord, the lateral corticospinal tract.

semi-automaticmovements that are mainly rhythmic patterns of activity

initiatedand maintained by subcortical structures such as brain stem and spinal cord

coordinated bynetworks of neurones called central pattern generators.

littleor no contribution from the cerebral hemispheres or cortex

Reflexesare rapid and automatic responses to particular sensory stimuli and as such arestereotyped varying only in speed, magnitude or duration

mostbasic form consists of only two nerve cells (one sensory, one motor) plus anintervening synapse eg the stretch reflex

They aretriggered by proprioceptors. These are reflex arcs.

can be a lot more complex involvingnumerous nerve cells and synapses extending over several spinal cord segmentsor brain stem levels and in some cases both sides of the body (e.g. limbwithdrawal reflex).eas(0��z�

muscle spindles are stretch detectors, and found scattered throughout skeletal muscles

Each spindlecontains a few striated fibres described as “intrafusal”. Musclespindles are in parallel with the extrafusal fibres so whenever the msucle belly is put under tension causing the muscle spindles to respond.

spindles are excited triggering reflex contraction of the homonymous (same) muscle group, resisting change inmuscle length.

achieved by asimple spinal cord reflex with a sensory limb, a motor limb and a single synapse,this is the stretch reflex.

Antagonistmuscles are inhibited at the same time (reciprocal inhibition) by inhibitoryinterneurones, activated by collaterals of the afferent.

The stretchreflex is normally dampened down by descending influences from the brain stemto ensure that muscle tone is not excessive. Thismoderating influence tends to be lost in patients with upper motor neuronelesions

With the normaldampening down influence lost, the stretch reflex becomes abnormally strong(hyperreflexia) so excessive muscle tone(hypertonia) which manifests as firm resistance to manipulation of joints. Boththe agonist and antagonist muscles start having a tug-of-war trying to contractmaking the muscle stiff, this is spasticity.

theperipheral nerve supply to the muscle is interrupted

Loss of motornerve supply to the muscle results in flaccid (floppy) paralysis of theaffected muscles accompanied by a complete absence of normal tone (atonia) andloss of deep tendon reflexes (areflexia).

so we get gradual wasting(atrophy) of the muscle due to lack of use (disuse atrophy)

caused by increased receptor concentration on muscles to compensate for lack of innervation

flexor/withdrawalreflex

crossed extensor reflex.

If you’rewalking along and step on a pin you immediately pull your foot away and you maythink you did this voluntarily but actually it was an automatic reflex.At the sametime you straighten your other leg so you support your body and don’t fallover.

The pulling theleg away is the withdrawal reflex, the straightening of the leg is the crossedextensor reflex. These two reflexes coordinate automatic limb withdrawal from anoxious stimulus

The withdrawalreflex is polysynaptic and triggered by nociceptors and is a cutaneous reflex(starts in skin).

networks of neurones within the spinal grey matter or brain stem involved in coordinating rhythmic motor sequences such as walking, chewing, breathing .

canbe selected and recruited by descending projections from the brain (‘commandneurons’

operaterelatively independently and autonomously

in the spinal cord.

command neurones send signals up into the brain

The connections between the motorcortex and the basal ganglia are arranged as a set of “loops” which each arise ina particular region of the frontal lobe and pass through the basal ganglia beforeprojecting back to the cortical region of origin (via a relay in the thalamus)

involved in

(i) the initiation ofvoluntary actions

(ii) selection of a particular action among a range ofpossible or potential actions

(iii) the learning and performance of variousrepetitive, semi-automatic behaviours (or ‘habits’)

dopamine

associatedwith a movement disorder, the best example of which is Parkinson’s disease

Thebasal ganglia are also involved in cognition, behaviour and emotion due to thepresence of non-motor loops that arise and terminate in other parts of thecerebral cortex.