FUNCTIONAL ANATOMY AND PHYSIOLOGY OF THE NERVOUS SYSTEM

FUNCTIONAL ANATOMY AND PHYSIOLOGY

Cells of the nervous system

The nervous system comprises a complex network of

specialised blood vessels,
ependymal cells which line the cerebral ventricles,
neurons and
glial cells, of which there are three types
.A- Astrocytes form the structural framework for neurons. Astrocyte foot processes are intimately associated with blood vessels and form the blood-brain barrier .
B-Oligodendrocytes are responsible for the formation and maintenance of the myelin sheath, which surrounds axons and is essential for the rapid transmission of action potentials . myelin sheaths consisting of the wrapped membranes of Schwann cells

C- Microglial cells are cells of the monocyte/macrophage lineage which play a role in fighting infection and removing damaged cells.the Schwann cells covering the neurons with sheath.
These cells maintain the tissue by
1-supporting and protecting the neurons.
2- provide nutrients to neurons
3- help to keep the tissue free of debris






.

. There are three types of neurons: sensory neuron,

motor neuron, and interneuron.

. An interneuron is always found completely within the CNS and conveys messages between parts of the system (Figure ).

Function of the nervous system rests upon two physiological processes:

the generation of an action potential and its conduction down axons, and
2- the synaptic transmission of impulses between neurons and muscle cells





There are no cell bodies in nerves; cell bodies are found only in the CNS or in the ganglia. Ganglia are collections of cell bodies within the PNS. 12 pair of C.N and 31 pair of P.N

BRAIN

Many of the functions are lateralised and this depends on which of the two hemispheres is 'dominant', i.e. the one in which language function is represented. In right-handed individuals the left hemisphere is almost always dominant, while in left-handers either hemisphere may be dominant with about equal frequency
. The cerebral cortex

grey matter (densely packed neurons for information processing. While the white tissue inside are axons -- (to relay information). The cortex can be broken down into many functional regions,

The spinal cord

The spinal cord contains
1-white matters :afferent and efferent fibres arranged in bundles(3 long tract)dorsal columns,spinothalamic,corticospinal tract
grey matter, collections of cells which are responsible for a-lower-order motor reflexes(anterior horn cells)
b-and the primary processing of sensory information(posterior horn cells), including pain.

The peripheral nervous system

The sensory cell bodies of peripheral nerves are situated in the dorsal root ganglia in the spinal exit foramina, .The motor cell bodies are in the anterior horns of the spinal cord. Motor neurons initiate muscle contraction by the release of acetylcholine across the neuromuscular junction, which results in change in potential in the muscle end plate.

, any peripheral nerve is made up of a combination of large, fast, myelinated axons (which carry information about joint position sense and commands to muscles) and smaller, slower, unmyelinated axons (which carry information about pain and temperature, as well as autonomic function).

1-The autonomic system

The autonomic system plays a key role in regulating the cardiovascular , respiratory systems, the smooth muscle of the gastrointestinal tract, urinary, and reproductive tracts. It also carries messages that help stimulate glands to secrete tears, mucus, and digestive enzymes. ANS innervation is divided into sympathetic nervous system and parasympathetic nervous system divisions. The sympathetic division has thoracolumbar “outflow”, meaning that the neurons begin at the thoracic and lumbar (T1-L2) portions of the spinal cord. The parasympathetic division has craniosacral “outflow”, meaning that the neurons begin at the cranial nerves (CN 3, CN7, CN 9, CN10) and sacral (S2-S4) spinal cord.
.

2-The motor system

Figure 26.6 The motor system. Neurons from the motor cortex descend as the pyramidal tract in the internal capsule and cerebral peduncle to the ventral brain stem, where most cross low in the medulla (A). In the spinal cord the upper motor neurons form the cortico-spinal tract(pyramidal tract) in the lateral column before synapsing with the lower motor neurons in the anterior horns. The activity in the motor cortex is modulated by influences from the basal ganglia and cerebellum. Pathways descending from these structures control posture and balance (B).some fiber from the pyramidal tract(corticobulber tract) innervate nuclear in BRAIN STEM.


.

Lower motor neurons

Lower motor neurons in the anterior horn of the spinal cord innervate a group of muscle fibres termed a 'motor unit'. Loss of function of lower motor neurons causes loss of contraction within this unit, resulting in weakness and reduced muscle tone. Subsequently, denervated muscle fibres atrophy, causing muscle wasting, and depolarise spontaneously, causing 'fibrillations'.

All lower motor neurons are either spinal or cranial nerves. All spinal nerves have a lower motor neuron component as they are mixed nerves. However, not all cranial nerves have lower motor neuron components. Some of the cranial nerves contain only sensory fibers and therefore cannot be classified as lower motor neurons. For example, CN I, the olfactory nerve, CN II the optic nerve, and CN VIII, the auditory nerve, do not have motor components

Upper motor neurons

Upper motor neurons have an inhibitory influence on the function of anterior horn motor neurons. When upper motor neuron lesions occur, motor units have an exaggerated response to stretch. With an increased muscle tone greater in the extensors of the lower limbs and the flexors of the upper limbs (spasticity), brisk tendon reflexes, and extensor plantar responses. Spasticity takes time to develop and may not be present for weeks after the onset of an upper motor neuron lesion. The weakness found in upper motor neuron lesions is more pronounced in the extensors of the upper limbs and the flexors of the lower limbs.

pyramidal tract fibers begin their descent from the cortex as a corona radiata (radiating crown) before forming the internal capsule
The fibers that synapse with cranial nerves form the cortico-bulbar tract. Bulbar refers to the brain stem
The fibers of the pyramidal tract that synapse with spinal nerves sending information about voluntary movement to the skeletal muscles form the cortico-spinal tract
pyramids in the inferior part of the medulla, eighty-five to ninety percent of cortico-spinal fibers decussate, or cross to the other side of the brain(lateral corticospinal tract). The remaining ten to fifteen percent continue to descend ipsilaterally(anterior corticospinal tract).
Almost all of the cranial nerves receive bilateral innervation from the fibers of the pyramidal tract The two exceptions to this pattern are the portion of CN XII that provides innervation for tongue protrusion and the part of CN VII that innervates the muscles of the lower face. These only receive contralateralinnervation from the pyramidal tract

3-sensory pathways.

Fibres from proprioceptive organs 1-(joint position) and those mediating2-well-localised touch and ( 3-vibration enter the spinal cord at the posterior horn and pass without synapsing into the ipsilateral posterior columns. Neural fibres conveying pain and temperature sensory information (nociceptive neurons) synapse with second-order neurons In the posterior ho
rn of the spinal cord which cross the midline in the spinal cord before ascending in the contralateral spinothalamic tract to the brain stem.


The second-order neurons of the dorsal column sensory system cross the midline in the upper medulla to ascend through the brain stem. Here they lie just medial to the (already crossed) spinothalamic pathway. Brain-stem lesions can therefore cause sensory loss affecting all modalities of the contralateral side of the body. Both the dorsal column and spinothalamic tracts end in the thalamus, relaying from there to the parietal cortex.

Figure The pain perception system.

The brain stem

BRAIN STEM

Long tracts are three
Long tracts: the motor and sensory tracts described in the spinal cord are present in the brain stem, but in the brain stem they are all contralateral to the side of the body they serve.
The pyramidal tract (upper motor neuron) start from precentralgyrus(motor cortex) dowon to internal capsule then to brain stem.The pyramids decussate at the junction of the medulla and cervical spinal cord.
The spinothalamic tracts continue in a lateral position throughout the brainstem on their way to the thalamus..
The posterior columns end in the medulla where they synapse in the nuclei cuneatus and gracilis Second order neurons immediately decussate .These sensory pathways both end in the thalamus .
with the exception of the trochelar nucleus (cranial nerve IV), which crosses to innervate the contralateral superior oblique muscle, each of the brainstem cranial nerve nuclei innervate ipsilateral structures. Since the long tracts discussed above are crossed, lesions confined to one side of the brainstem typically present with cranial nerve findings on one side, and motor and sensory findings on the opposite side of the body. This rule is very helpful in localization.
The brain stem

Containing

1- all the sensory and motor pathways entering and leaving the hemispheres,
2-nuclei of the cranial nerves
nuclei projecting to the cerebrum and cerebellum,
other important collections of neurons in the reticular formation
)The cranial nerve nuclei 1-providemotor control to muscles of the head (including the face and eyes) and some in the neck, along with 2-coordinating sensory input from the special sense organs and the face, nose, mouth, larynx and pharynx. They3-also control autonomic functions including pupillary, salivary and lacrimal functions.
The reticular formation is predominantly involved in the 1-control of conjugate eye movements, 2-the maintenance of balance, 3-cardiorespiratory control and4- the maintenance of arousal.


The extrapyramidal system

the extrapyramidal system centers around

the 1-modulation and2- regulation of motor activity (indirect control) of anterior (ventral) horn cells. , ( they modulate motor activity without directly innervating motor neurons
Primarily, the extrapyramidal system is involved in maintaining 1-equilibrium, coordination,2-posture, muscle tone
C-It has projections that carry autonomic motor impulses to voluntary muscles in the body, including the muscles for speech and swallowing
The system, including
the HYPERLINK "http://en.wikipedia.org/wiki/Nigrostriatal_pathway" \o "Nigrostriatal pathway"nigrostriatal pathway,2- the basal ganglia,3- the cerebellum, 4-the vestibular nuclei, and 5-red neuclius,6-tectunm7-reticular formation in pon and medulla.8-olivary neuclius in medulla
. Some structures of the extrapyramidal system do not proceed directly to the spinal motor centers. Others are connected by conducting pathways to the segmental levels of the spinal cord, where they serve as an essential switching station for impulses traveling from the brain to moto-neurons

.Extrapyramidal Projections to Lower Motor Neurons

1-The rubrospinal tract passes through the red nucleus in midbrain . The cerebellum sends messages to the spinal nerves along this.
2-The reticulospinal tract runs from the reticular nuclei of the pons and medulla to the spinal nerves.
3-The tectospinal tract has points of origin throughout the brain stem, but especially in the midbrain area, and ends in the spinal nerves. It is involved in the control of neck muscles.
4-The vestibulospinal tract runs from the vestibular nuclei located in the lower pons and medulla to the spinal nerves. It is involved in balance.
5-olivospinal tract from olivery neucleus in medulla to the anterior horn neucleus

(Note that all of these tracts receive input from

Circuits between the basal ganglia and the motor cortex constitute the extrapyramidal system, which controls muscle tone, body posture and the initiation of movement. Lesions of the extrapyramidal system produce an increase in tone which is not an exaggerated response to stretch but is continuous throughout the range of movement at any speed of stretch ('lead pipe' rigidity). Involuntary movements are also a feature of extrapyramidal lesions , and tremor combined with rigidity produces typical 'cogwheel' rigidity.

speech

.the upper part of the posterior temporal lobe ( comprehension region is referred to as Wernicke's area). The perception of these sounds as meaningful language, occurs predominantly in the lower parts of the anterior parietal lobe (the angular and supramarginalgyri)..

The language information generated in the temporal and parietal lobes passes anteriorly via the arcuate fasciculus to Broca's area in the posterior end of the inferior frontal gyrus on the dominant side. The motor commands generated in Broca's area then pass to the cranial nerve nuclei in the pons and medulla, as well as to the anterior horn cells in the spinal cord. Nerve impulses then travel to the lips, tongue, palate, pharynx, larynx and respiratory muscles via the facial nerve and cranial nerves 9, 10 and 12, and result in the series of ordered sounds known as speech (
The cerebellum also plays an important role in coordinating speech, and lesions of the cerebellum lead to a speech disorder termed dysarthria.