Control of movement is largely determined by incoming (afferent) and outgoing (efferent) neural impulses in the cerebellum.
Motor information input travels from the spinal cord, cerebral cortex and vestibular system via mossy fibers.
Feedback regarding movements returns to the cerebellum via the inferior olivary nucleus in the medulla oblongata. This feedback loop allows the brain to coordinate movement.
All outgoing neural impulses from the cerebellum travel via the deep cerebellar and vestibular nuclei. Proper functioning of the neuronal pathway between mossy fibers, granular cells, parallel fibers, climbing fibers and Purkinje cells are thought to be essential for coordinated muscular movement. Glutamate is a neurotransmitter in the excitatory synapses between climbing fibers and Purkinje cells as well as between granular cells and mossy fibers. Disruptions in this system are thought to be involved in a variety of movement disorders.
The Reticular Formation
It’s a ‘diffuse net’ which is formed by nerve cells and fibers. It extends from the neuroaxis spinal cord through medulla, pons, midbrain, subthalamus, hypothalamus and thalamus (spinal cord is relayed superiorly to the cerebral cortex).
Many afferent and efferent pathways project in and out of the RF from most parts of the CNS. The main pathways through the RF is poorly defined and difficult to trace using silver stains. Reticular formation can be divided into three columns : median, medial and lateral columns.
Functions of the Reticular formation
1. Control of skeletal muscles:
- RF modulates muscle tone and reflex activities (via reticulospinal and reticulo bulbar tracts). It is important in controlling muscles of facial expression when associated with emotions.
2. Control somatic and visceral sensation (influence can be excitatory or inhibitory)
3. Control of autonomic nervous system
4. Control of endocrine nervous system (hypothalamus and the pituitary)
5. Influence on the biological clock (rhythm)
6. The reticular activating system (arousal and level of consciousness are controlled by the RF)
When a person smiles for a joke, the motor control is provided by the RF on both side of the brain. The fibers from RF is separated from corticobulbar pathway (supply for facial muscles). If a patient suffers a stroke that involves corticobulbar fibers, he or she has facial paralysis on the lower part of the face, but is still able to smile symmetrically.
The Limbic System
|Limbic structures||Functions of the limbic system|
||1. Influence the emotional behavior:a. Reaction to fear and angerb. Emotions associated with sexual behavior
2. Hippocampus is involved in converting short term memory to long term memory (If the hippocampus is damaged, patient is unable to store long term memory – Anterograde amnesia)
The Basal Ganglia and their connections
Connections of the Basal Ganglia
Yellow arrow : Pallidofugal fibers
Caudate nucleus and the Putamen: main sites of receiving inputs
Globus pallidus: main site from which output leaves
Afferent and Efferent fibers
|Connections of the caudate nucleus and Putamen||Connections of the Globus pallidus|
|CS: CorticostriateTS: Thalamostriate
BS: Brainstem striatal fibers
SN: Striatonigral fibers
Functions of the Basal Nuclei
Basal Nuclei controls muscular movements by influencing the cerebral cortex (it doesn’t have direct control through descending pathways to the brainstem and spinal cord). It helps to prepare for the movements (enables the trunk and limbs to be placed in appropriate positions before discrete movements of the hands and feet).
Functional connections of the Basal Nuclei and how they influence muscle activities
1. Ben Greenstein, Ph.D, Adam Greenstein, BSc (Hons) Mb, ChB Color Atlas of Neuroscience
2. Allan Siegel Ph.D, Hreday N. Sapru Ph.D Essential Neuroscience, 1st Edition
3. Stanley Jacobson, Elliot M. Marcus Neuroanatomy for the Neuroscientist
4. Patrick f. Chinnery Neuroscience for Neurologists
5. Dale Purves Neuroscience, 3rd Edition
6. Suzan Standring Gray’s Anatomy
7. Keith L. Moore, Arthur F. Dalley, Anne M. R. Agur Clinically Oriented Anatomy
8. Frank H. Netter Atlas of Human Anatomy
9. Walter J. Hendelman, M.D., C.M. Atlas of Functional Neuroanatomy
10. Mark F. Bear, Barry W. Connors, Michael A. Paradiso Neuroscience Exploring the Brain
11. Dale Purves et al. Principles of Cognitive Neuroscience
12. Eric R. Kandel et al. Principles of Neural Science
Descending tracts have three neurons:
1. 1st order neurons (UMN): cell bodies are in the cerebral cortex and other supra spinal areas
2. 2nd order neurons: short and situated in the anterior grey column of the spinal cord
3. 3rd order neuron (LMN): situated in the anterior grey column and innervate the skeletal muscles through anterior roots of the spinal nerves
Corticospinal tract: rapid, skilled and voluntary movements
1st order neuron
Axons arise from the pyramidal cells of the cerebral cortex (situated in the 5th layer), 2/3 from the pre central gyrus and 1/3 from the post central gyrus:
1. 1/3 of fibers arise from the 1stry motor cortex (Area 4)
2. 1/3 of fibers arise from the 2ndry motor cortex (Area 6)
3. 1/3 of fibers arise from the parietal lobe
(Area 1, 2 and 3).
Descending fibers converge in the corona radiata and pass though the posterior limb of the internal capsule; organization of fibers within the internal capsule:
1. close to genu (medial): concerned with the cervical parts of the body
2. away from the genu (lateral): concerned with the lower extremity.
The tract then passes through the middle 3/5 of the basis pedunculi of the midbrain; organization of fibers in the midbrain:
- medially: cervical parts of the body
- laterally: lower limbs.
When the tract enters the pons, it’s broken into many bundles by the transverse pontocerebellar fibers. In the medulla oblongata, the bundles group together to form the pyramids. At the junction of the MO and the spinal cord, most fibers cross the midline at the decussation of the pyramids and enter the lateral white column of the spinal cord to form the lateral corticospinal tract (LCST). LCST descends length of the spinal cord and terminates in the anterior grey column of all the spinal segments.
The fibers which didn’t cross, descend in the anterior white column of the spinal cord as the anterior corticospinal tract (ACST). Fibers of the ACST eventually cross and terminate in the anterior grey column of the spinal cord segments in the cervical and upper thoracic regions.
2nd order neuron:
It’s an internuncial neuron.
3rd order neuron:
It’s a alpha or gamma motor neuron.
To read more click on this link to the full article: Descending Tracts
They are located in the white matter and conduct afferent information (may or may not reach consciousness). There are two types of information:
- Exteroceptive : originates from outside the body (pain, temperature and touch
- Proprioceptive : originates from inside the body (from muscles and joints)
Normally there are three neurons in an ascending pathway:
- 1st order neuron: cell body is in the posterior root ganglion
- 2nd order neuron: decussates (crosses to the opposite side) and ascends to a higher level of the CNS
- 3rd neuron: located in the thalamus and passes to a sensory region of the cortex
Pain and temperature pathway: lateral spinothalmic tract
1st order neuron
Peripheral process extends to skin or other tissues and ends as free nerve endings (receptors). Cell body is situated in the posterior root ganglion. Central process extends into the posterior grey column and synapses with the 2nd order neuron.
2nd order neuron
The axon crosses obliquely to the opposite side in the anterior grey and white commissures within one spinal segment of the cord. It ascends in the contralateral white column as the lateral spinothalamic tract (LSTT).
As the LSTT ascends through the spinal cord new fibers are added to the anteromedial aspect of the tract (sacral fibers are lateral and cervical fibers are medial). The fibers carrying pain are situated anterior to those conducting temperature.
As the LSTT ascends through the medulla oblongata, it’s joined by the anterior spinothalamic tract and the spinotectal tract and forms the spinal lemniscus. Spinal lemniscus ascends through the pons and the mid brain.
Fibers of the LSTT end by synapsing with the 3rd order neurons in the ventral posterolateral nucleus of the thalamus (here crude pain and temperature sensations are appreciated).
3rd order neuron
Axons pass through the posterior limb of the internal capsule and corona radiata to reach the somatosensory area in the post central gyrus of the cerebral cortex. From here information is transmitted to other regions of the cerebral cortex to be used by motor areas. The role of the cerebral cortex is interpreting the quality of the sensory information at the level of the consciousness.
Light (crude) touch and pressure pathway: anterior spinothalamic tract (ASTT)
1st order neuron
It is similar to the pain and temperature pathway.
2nd order neuron
The axon crosses obliquely to the opposite side in the anterior grey and white commissures within several spinal segments. It ascends in the contralateral white column as the anterior spinothalamic tract (ASTT). As the ASTT ascends through the spinal cord new fibers are added to the anteromedial aspect of the tract (sacral fibers are lateral and cervical fibers are medial).
As the ASTT ascends through the medulla oblongata, it’s joined by the lateral spinothalamic tract and the spinotectal tract and forms the spinal lemniscus. Spinal lemniscus ascends through the pons and the midbrain. Fibers of the ASTT end by synapsing with the 3rd order neurons in the ventral posterolateral nucleus of the thalamus (here crude awareness of touch and pressure sensations are appreciated).
3rd order neuron
Axons pass through the posterior limb of the internal capsule and corona radiata to reach the somatosensory area in the post central gyrus of the cerebral cortex. The sensations can be crudely localized. Very little discrimination is possible.
To read more click on this link to the full article: Ascending Tracts (pdf).
Every year, hundreds of thousands of people suffer from paralyzed limbs as a result of peripheral nerve injury. Recently, implantation of artificial nerve grafts has become the method of choice for repairing damaged peripheral nerves. Grafts can lead to some degree of functional recovery when a short segment of nerve is damaged. But they are of little use when it comes to regenerating nerves over distances greater than a few millimeters, and such injuries therefore often lead to permanent paralysis.
Now though, surgeons from Germany have made what could be a significant advance in nerve tissue engineering. They have developed artificial nerve grafts made from hollowed-out pig veins filled with spider silk fibres and, in a series of animal experiments, showed that the grafts can enhance the regeneration of peripheral nerves over distances of up to 6cm. Their findings have just been published in the open access journal PLoS One.
Peripheral nerves have a greater regenerative capacity than those in the central nervous system, but regenerating them properly is challenging. The individual nerve fibres must not only regrow into the damaged area, but also find their proper targets. Furthermore, the regenerated nerve will not function properly unless it is populated by Schwann cells, which produce myelin. This fatty tissue is essential for full recovery, as it wraps itself around the nerve fibres at regular intervals (a process called myelination), facilitating the conductance of nervous impulses along their length.
Conventionally, damaged peripheral nerves are treated either by suturing or by implantation of nerve grafts. The two ends of a severed nerve can be surgically re-attached to each other, as long as the nerve is not stretched in the process. This is not possible for gaps longer than about 5mm, in which case a short length of nerve from elsewhere in the patient’s body can be grafted into the damaged area. But this often causes causes pain in the donor area, and it can be difficult to find a nerve segment that has the same diameter as the damaged nerve. Nerves can be obtained from another person, but they can be rejected by the recipient’s immune system, so drugs that suppress the immune response are usually administered.
An alternative approach, which has emerged in the past ten years or so, is the use of artificial nerve grafts made from silicon or synthetic polymers such as polyethylene. These form scaffolds which bridge the gap in the damaged nerve and serve as conduits through which the nerve fibres can regrow. Artificial grafts can lead to some degree of functional recovery, but they can become toxic with time, or they can constrict the nerve.
These problems can potentially be overcome by using nerve grafts made from biodegradable materials. Five years ago, Peter Vogt and his colleagues in the Department of Plastic, Hand and Reconstructive Surgery at Hannover Medical School reported that Schwann cells readily ensheath spider silk fibres when grown on them, and that nerve grafts made of de-cellularized veins filled with spider silk can be maintained in culture for periods of up to a week. More recently, they showed that spider silk vein grafts can be used to regenerate 20mm gap in the sciatic nerve of rats, either alone or when supplemented with Schwann cells.
In the new study, Vogt’s group dissected 6cm lengths from the small veins in pigs’ legs, washed them and stripped away most of the endothelial cells from their inner walls. They then harvested dragline silk from the golden silk spider Nephila clavipes and pulled the silk through the de-cellularized veins, until it filled about one quarter of their diameter. Using adult sheep, the researchers removed a 6cm length of the tibial nerve in the leg. In one group of animals, the gap was bridged with the spider silk constructs; in another, the section of nerve that had been removed was replaced in reverse orientation.
Defects in the animals’ gait became apparent immediately after the surgery – the hind limb was partially paralyzed and flexed abnormally. But within three weeks there was a significant improvement, with both groups of animals being able to stand properly. By four months, the animals could stand upright on both hind limbs, the hind limbs moved in co-ordination with one another during walking, and there was no obvious difference in strength between the operated and unoperated limbs.
Ten months after surgery, the sheep were killed and their regenerated nerves examined under the microscope. In both groups of animals, the severed nerve fibres had regrown into the nerve grafts to bridge the 6cm gap; Schwann cells had migrated into the grafts and wrapped themselves around the entire length of the regenerated nerves; and the sodium channels required for generating nerve impulses were distributed irregularly along the fibres. This shows that myelination had occurred properly, with the formation of Nodes of Ranvier, the regular gaps in the myelin sheath at which the sodium channels normally cluster. No trace of residual spider silk was detected in the experimental animals, and there was no sign of inflammation at the repair site, indicating that the silk fibres were absorbed subtly without adverse effects.
These findings could have important applications in reconstructive nerve surgery. This is the first time that a large animal model has been used to study nerve regeneration, and the study is the first in which a defect longer than 2cm in length has been successfully repaired. The spider silk constructs enhanced nerve regeneration at least as effectively as the sheeps’ own nerves, and would be advantageous in the clinic, because transplanting large lengths of a patient’s own nerves is unfeasible.
More work will be needed before the technique can be applied to humans. Meanwhile, the regeneration reported here could be further enhanced in a number of ways. The spider silk constructs could, for example, be loaded with substances such as Nerve Growth Factor, or they could be grafted together with Schwann cells, to speed up nerve regrowth. But ultimately, engineering fully functional peripheral nerves will probably require a combination of advanced microsurgery, transplantation of both cells and tissues, advances in materials science and, possibly, gene transfer for the effective delivery of growth factors.
Radtke, C. et al. (2011). Spider Silk Constructs Enhance Axonal Regeneration and Remyelination in Long Nerve Defects in Sheep. PLoS ONE, 6 (2) DOI: 10.1371/journal.pone.0016990.
Arterial supply : Supplied by 3 small arteries + feeder arteries
Segmental spinal arteries
ASA and PSA arteries are reinforced by segmental arteries, which enter the vertebral canal through the inter vertebral foramina. These arteries are branches of arteries outside the vertebral column (deep cervical, intercostal and lumbar arteries). Segmental arteries give rise to anterior and posterior radicular arteries.
Enter the vertebral arteries and anastomose with the ASA & PSA. The most important feeder artery is the Great anterior medullar artery (GAM) of Adamkiewicz. It arises from the aorta at lower thoracic or upper lumbar vertebral levels. This artery is unilateral. It lies in the left side of most people. It represents the major source of blood to the lower 2/3 of the spinal cord.
Veins of the spinal cord
Drain mainly into the veins of the brain and the venous sinuses via 6 tortuous longitudinal channels. Finally, they drain into the internal vertebral venous plexus.