Functional Anatomy of nervous system ( Spine )

FUNCTIONAL ANATOMY AND PHYSIOLOGY OF THE NERVOUS

SYSTEM

BY AHMED SHAWKY ALI

UNDER SUPERVISION OF

PROF. DR. HUSSEIN SHAKER

NERVOUS SYSTEM RELATIONS SPACES AND ATTACHMENTS

Adequate space is needed around the neural and connective tissue and there must be enough space at rest and during physiological movements of the spine. The nervous system is attached to surrounding tissues and structures. Attachments need consideration in terms of those attaching neural tissue into connective tissue, such as the denticulate ligaments, and those attaching connective tissue (and thus the neural tissue) onto other structures, such as the dural ligaments.

NERVOUS SYSTEM RELATIONS SPACES AND ATTACHMENTS

N.B Haset al (1983) have shown that the space around neural tissue, both in the spinal canal and the intervertebral foramen, is less in males than in females.

These authors also point out that developmental and degenerative stenosis is more common in the male.

The external connections of the dura

Inside the cranium, the dura mater is loosely adhered to the central portions of the cranial bones and tightly adhered at the suture levels (Murzin & Gonunov 1979).

There is a firm attachment at the foramen magnum and, at the caudal end, to the coccyx by the external filum terminale. A network of dural ligaments (Hoffman ligaments) attaches the anterior theca to the anterior and and anterolateral aspect of the spinal canal .

(Blikiia ,1969) noted that the dural ligaments around L4 were stronger and more numerous than elsewhere — so strong that they could not be displaced with a probe.

The external connections of the dura

Thoracic dural ligaments tend to be filmier and longer, In the cervical spine, they are shorter and thicker (Romanes 1981).

The studies of Tencer et al (1985) have revealed that, in the lumbar spine, dural ligaments, nerve roots and trunks are of equal importance in the distribution of forces.

Internal dural attachments

Inside the dural sac there are 21 pairs of denticulate ligaments .These run from the pia mater to the dura and are orientated to keep the cord central in the dural theca. N.B Tani et al (1987) have shown that the denticulate ligaments, as well as the filum terminale, prevent excessive elongation of the cord during flexion. Thickened denticulate ligaments associated with cervical spondylosis have been implicated in cord degeneration (Bedf ord et al 1952).

Denticulate ligaments

Attachments of the peripheral nervous system

The peripheral nerves are also attached to surrounding tissue. However, they are allowed movement in their nerve beds, less in some areas than in others, such as where blood vessels enter or where nerves branch.

Attachments of the peripheral nervous system

What is unmistakable is that, along the course of a peripheral nerve, there are some areas where the nerve is more attached than others, for example, the common peroneal nerve at the head of the fibula, and the radial nerve to the head of the radius.

Yet in other areas, a remarkble amount of movement of over 1.5 cm occurs (McLelJan & Swash 1976)

THE BASIS OF SYMPTOMS

Knowledge of three processes important to understanding of symptom reproduction related to the nervous System:

• The supply of blood to the nervous system .• The axonal transport systems .• The Innervation of the Connective tissues of the

nervous system .All of these processes will be influenced by

mechanical deformation .

CIRCULATION

The nervous system consumes 20% of the available oxygen in the circulating blood yet consists of 2% of body mass (Dommjsse 1986). Among cells, neurones are especially sensitive to alterations in blood flow.

Importance of blood supply : An uninterrupted vascular supply is imperative for the

metabolic demands of normal neural function. Blood supplies the necessary energy for impulse conduction and also for the intracellular movement of the Cytoplasm of the neurone.

CIRCULATION

There are extrinsic vessels supplying feeder arteries to the nerve. Once inside the nervous system , there is a ‘well dcveloped intrinsic system.

In many parts of the body, blood supply is so assured that if some feeder vessels are compromised the intrinsic system can provide enough blood for normal neural function. With such an assured supply, it may seem that the nervous system Can be relatively independent of its blood supply.

Vasculature of the spinal canal and neuraxis

These structures have a multiple supply :1-The vertbral artery,2- The deep cervical,3- The posterior intercostal and the lumbar arteries supply the

vertbral column.

They also supply, via segmental subdivisions, the spinal canal and contents.

At certain vertebral levels, medullary feeder branches arise and join the longitudinally running anterior and two small posterior spinal arteries.


The anterior spinal artery supplies about 75% of the cord. There are usually around eight medullary feeder arteries (a branch of the cervical part of the vertebral artry).

These arteries are more common in the lumbar and cervical spines.

Most of the arteries enter the cord in the low cervical spine and the lumbar spine.

During spinal movements these plexus areas have limited movement in relation to the spinal canal (Louis 1981)


When the cord is elongated the vessels running longitudinally are stretched while those running transversely are folded. The opposite effect occurs on shortening of the cord (Fig. 1.)


The veins in the spinal canal are valveless and under little pressure (Penn ing & Wilrnink 1981).

This allows flow reversibility and an accommodating mechanism to sudden in-rushes of blood, as may occur from coughing and straining.


Together with the CSF pressure, via the alterations in the venous system, a balance of intraspinal canal pressure is maintained.

N.B A critical vascular zone exists from the T4 to T9 vertebral levels. The spinal canal is at its narrowest and the blood supply is less rich in this area (Dommisse 1974). This may be relevant in syndromes such as the ‘T4 syndrome’ .

Vasculature of the peripheral nervous system

The extrinsic supply of the peripheral nerves is such that it allows leeway for movement; that is, there is slack in the feeder vessels so that a nerve can glide without alteration in the blood supply.

In general, major feeder vessels enter nerves at areas where there is minimal or no nerve movement in relation to surrounding tissue.


Examples of this are at the elbow for the median and radial nerves. However, if part of the extrinsic supply is occluded, the intrinsic supply is also sufficient for the needs of the nerve fibres (Lundborg 1970, 1975).


The intrinsic vascular system is extensive, linking endoneuriurn, perineurium and epineurium.

Under normal conditions, only part of the intranural vascular system is used.

However, if traumatised, many more vessels come into use (Lundborg 1970). Intraneural blood flow is reversible and collateral systems exist.


Intranural blood vessels are sympathetically innervated (Hromada 1963, Lundborg 1970, Appenzeller ci al 1984).

According to Appenzejler et al (1984), the nerve supply to particular blood vessel arises from the nerve trunk that: the blood vessel supplies. This probably allows an adjustable blood supply for functional demands on the nerve.


N.B Stretch and compression will surely affect the circulation, although, the mechanisms are not fully understood.

Strech will lessen the diameter of the longitudinally running vessels, plus raise intrafascicular pressure and perhaps result in squeezing closed the vessels crossing the perineurium.


N.B Arrest of blood flow will begin at approximately 8% elongation (rabbit sciatic tract) and complete arrest will occur at approximately 15% elongation (Lundborg & Rydevik 1973, Ogata & Naito 1986).

The blood nerve-barriers

A slightly positive pressure exists in the intrafascicular environment. This tissue pressure is referred to as the endoneurial fluid pressure (EFP) and is probably maintained by the elasticity of the perineurium.

The barrier function is bi-directional. As well as protection from the exterior, this mechanism means that if the intrafascicular pressure increases, such as from an oedematous reaction (Lundborg & Rydcvik 1973), the barrier may close.

The blood nerve-barriers

A good example of the protective function of the diffusion barrier is where peripheral nerves travel through infected areas without nerve conduction being altered .The perineurial barrier is also resistant to trauma.

AXONAL TRANSPORT SYSTEMS

Within the cytoplasm of all Cells there is movcment of materials and substances.

The cytoplasm of the neurone (axoplasm) is no different.

However , due to the length of the axon and its function, speciaized intracellular movement mechanisms occur.


The volume of material in an axon and teminals may be thousands of times as great as in the cell body (Lundborg 1988).

Mammalian axoplasm is quite viscous, about five times that of water (Haak er al 1976).

Of necessity, the intracellular transport mechanisms arc complex. These mechanisms are referred to as axonal transport systems and are a major direction for research in present day neurological science.


The axon contains smooth endoplasimic reticulum, ribosomes, microtubules and neurofilaments comprised of actin like material — all structures likely to be part of the axoplasmic transport mechanisms.

N.B Human movement plays a role in this intracellular motility.


Within the axon, the flow of substances is constant and controlled.

From the cell body to the target tissues (antegrade flow) there is a fast and a slow transport system.

From the target tissues to the cell body there is a (retrograde flow) of axoplasm (Fig. 1.28).

This bi-directional flow is evident because a nerve will swell both distally and proximally from circumferential pressure (Mackinnon & Dellon 1988).

D deridrite, N nucleus, M mitochondria, SC synaptic cleft, TT targcc tissue

Axoplasmic transport


Antegrade transport Materials produced in the cell body are transported

along the axon at various velocities. Two groups based on the speed of transport, can be

identified.(A) The fast transport moves at approximately 400 mm

per day and the substances carried such as neurotransmitters and transmitter vesicles, are for use in transmission of impulses at the synapse (Droz et al 1975).


This transport depends on an uninterrupted supply of energy from the blood. Various toxic substances and deprivation of blood will slow or block the transport (Ochs 1974).


(B) In the slow antegrade transport (1—6 mm per day), cytoskeletal material such as microtubules and neurofilaments are carried (Levine & Willard 1980 McL.ean et al 1983)

Essentially, the slow transport exists for maintenance of the structure of the axon.

The exact mechanisms of transport are unknown.


Retrograde transport Retrograde transport from target tissues to the cell

body moves rapidly (approx 200 mm per day). The system carries recycled transmitter vesicles and

extracellular materials such as neurite growth promoting factors from the nerve terminal or from damaged segments of nerve.


N.B It also seems very likely that the retrograde flow carries "trophic messages" about the status of the axon, the synapse and the general environment around the synapse, including the target tissues (Kristensson & Olsson 1977, Varon & Adler 1980, Bisby 1982).


If the retrograde flow is altered by physical constriction or from loss of blood flow, nerve cell body reactions are induced (Ochs 1984, Dahlin & McLean 1986, Dahlin et al 1987).

Viruses, such as herpes simplex, can be transported via the retrograde transport to the cell body (Kristensson 1982).


An understanding of the concepts of axonal transport is important for physiotherapists employing mobilisation of the nervous system as a treatment.

As Korr has suggested for some years (1978, 1985) many of the disorders we treat and the responses from treatment may be related to the axonal transport systems.


Knowledge of these systems is also important in order to understand the development of symptoms along the nervous system (ie, double crush, multiple crush synd romes) and the need to treat often more than the local area for optimum results.

INNERVATION OF THE NERVOUS SYSTEM

The connective tissues of the nervous system are innervated.

They are, thus, able to be a source of symptoms. This innervation also means that the Connective

tissues of the nervous system can contribute o altered sensory input in the same way that muscle, joint and other tissue can.

The meninges

Dura mater is innervated by segmental, bilateral, sinuvertebral nerves, first described by Luschka (1850). (Meningeal Branch Of Spinal Nerves)

Each sinuvertebral nerve emerges distal to the dorsal root ganglion, from the union of a somatic root arising from the ventral rami and an autonomic root from the grey rami communicate or a sympathetic ganglion .

The meninges

As well as supply to the dura, branches of the Sinuvertebral nerve innervate

- The posterior longitudinal ligament, -Periosteum, -Blood vessels And the annulus fibrosis (Edgar & Ghadially 1975,

Bogduk 1983).

The meninges N.B The innervation density varies depending on

the spinal segment. It is richer in the superficial dural layers than in

those deeper. Root sleeves at cervical and lumbar levels have a

richer nerve supply than the thoracic root sleeves (Cuauco et al 1988).

The meninges

N.B All recent authors on the subject agree that the

ventral aspect of the dura mater has a far denser innervation than the dorsal aspect .

Towards the midline, the dorsal dura may be completely insensitive (Groen ct al 1988).

The connective tissues of nerve roots

The ventral nerve root connective tissues receive their innervation from fibres Originating in the dorsal root ganglion.

Connective tissues of the anterior nerve roots are innervated by fine branches from the sinuvertebral nerve (Hromada 1963).

The peripheral nervous system

The connective tissues of peripheral nerves, nerve roots and the autonomic nervous system have an intrinsic innervation: the ‘nervi nervorum’ from local axonal branching.

Free nerve endings have been observed in the perineurium, epineurium and endoneurium .

Thomas (1982) believes that the nervi nervorum must be Considered a source of symptoms in diabetic neuropathy and in inflammatory polyneuropathics.

The innervation of the connective tissues of peripheral nerve

The nervi nervorurm

E epineurium, BV blood vessel,

NN nervi nervorum,

NF nerve fiber P perineurium, PVP perivascular plexus


Sunderlarid (1978) considers the pain from local pressure on a nerve to be due to the nervi nervorum.

The innervation of the nervous system cannot be

neglected — it seems very likely that it plays a part in adverse tension syndromes.


Perhaps innervation could be regarded as a protective mechanism for the nervous system, symptom production being a warning that the impulse conducting mechanisms may be in danger from mechanical or chemical compromise.

THANK YOU

Functional Anatomy of nervous system ( Spine )

Health & Medicine