L21 – An introduction to neuroanatomy PDF

Preview

Summary

Download L21 – An introduction to neuroanatomy PDF

Description

Yath Prem

L21 – An introduction to neuroanatomy Common terms



A collection of cell bodies within the CNS is called a nucleus. A collection of cell bodies within the PNS is called a ganglion.



In the CNS a collection of axons is called a tract. In the PNS a collection of axons is called a nerve.



Any branch given off from an axon is called a collateral.



Neurotransmitters are released from the presynaptic neuron and excite (glutamate in CNS) or inhibit (GABA in the brain and glycine in spinal c.) the postsynaptic neuron.



Grey matter consists of all unmyelinated neurons. Grey matter is present in the brain, brainstem, cerebellum and throughout the spinal cord.



Ipsilateral means same side and contralateral means opposite side.

Development of the brain This initially starts as the neural plate epithelium. The epithelium folds to form a groove. The lips of the neural groove come together to form the neural tube (that forms the spinal cord and brain). Neural crest - this gives rise to cells of the peripheral nervous system and the glia. If the neural tube fails to seal, you can get anencephaly and spina bifida. Folic acid supplements can counter this, but since formation occurs in the first month, folic acid needs to be taken before pregnancy. Brain vesicles Brain vesicles form at the rostral end of the neural tube – the forebrain, midbrain and hindbrain vesicles.

Yath Prem

Two outgrowths are formed from the forebrain vesicle, and so the forebrain vesicle can now be split up into the telencephalon (the two outgrowths) and the diencephalon (the original forebrain vesicle). The telencephalon forms the two cerebral hemispheres. There is a space in each hemisphere – these are called the lateral ventricles (as they aren’t in the midline), and they are filled with fluid. The lateral ventricles have an anterior horn, an inferior horn, a posterior horn and a body. The anterior horns of the lateral ventricles are separated by the septum pellucidum. The dienceplalon forms anything that has ‘thalamus’ in its name i.e. the thalamus and hypothalamus. The third ventricle is the space in this area that is surrounded by the thalamus and hypothalamus (the diencephalon) on each side. This is a midline structure. The fourth ventricle lies between the cerebellum and the brainstem. The cerebral aqueduct joins the third and fourth ventricles. Structure of the brain Brain stem The brain stem is split into three parts: 

Medulla oblongata



Pons (the swollen bit on top of the medulla oblongata)



Midbrain (above the pons)

The brain stem is important in the origin of the cranial nerves, involuntary movement, heart rate, respiratory rate, sensory innervation to the face and neck etc.

Yath Prem On top of the brain stem sits the diencephalon. The thalamus is above the hypothalamus. The hypothalamus controls the autonomic nervous system as well as other things. The thalamus is a gateway to the cerebral cortex – information from the cerebral cortex passes through the thalamus.

Cerebellum The cerebellum sits posterior-inferior to the cerebral hemispheres and coordinates movement. There are 3 pairs of cerebellar peduncles that anchor the cerebellum to the other parts of the brainstem so that the cerebellum can communicate with the rest of the brain. The 3 pairs of peduncles are the superior cerebellar peduncle (to midbrain), the middle cerebellar peduncle (to pons) and the inferior cerebellar peduncle (to medulla). The peduncles sit on either side of the midline and cannot be seen in a midline sagittal section.

Yath Prem

The midbrain The midbrain is composed of superior and inferior colliculi on the dorsal surface, which are bumps just above the superior cerebellar peduncle. The superior colliculus is involved in directing vision towards objects through head and eye movements. The inferior colliculus is involved in auditory pathways.

The ventral surface contains the crura cerebri. The ventral surface has the peduncles and the occulomotor nerve (III). The dorsal surface has the trochlear nerve (IV). The pons Inferior to the midbrain is the Pons (latin for bridge). It is also superior to the medulla, and ventral to the cerebellum.

On the ventral surface of the pons is the basilar pons. This is where the trigeminal (V), abducens (VI), facial (VII) and vestibulocochlear (VIII) nerves exit. The pontine tegmentum (dorsal pons) contains the middle peduncles and the nuclei for cranial nerves V-VIII.

Yath Prem

The medulla This is the most caudal part of the brainstem and is continuous with the most rostral part of the spinal cord. The anterior surface of the medulla is complicated and contains: 

The ventral (or anterior) median fissure.



Nerve roots for the glossopharyngeal (IX), vagus (X), accessory (XI) and hypoglossal (XII) nerves.



Pyramids containing the corticospinal pathways.



Olives (swellings lateral to the pyramids containing relay nuclei).

The dorsal surface of the medulla contains: 

The inferior cerebellar peduncles.



The dorsal column nuclei.

Yath Prem 

Fourth ventricle (beneath the cerebellum with the medulla forming the floor of the caudal half).

CSF circulation The lateral ventricles have 3 horns – the anterior horn extends into the frontal lobe; the posterior horn extends into the occipital lobe; the inferior horn extends into the temporal lobe. The brain and spinal cord are ‘bathed’ in CSF. 1. CSF is made in the choroid plexus in the ventricles (mostly in the lateral ventricles as these are the biggest). 2. It passes to the third ventricle through the interventricular foramina, and it then circulates. The third ventricle is surrounded by the diencephalon structures. 3. When leaving the third ventricle, it flows through the cerebral aqueduct – a passage that goes through the midbrain. 4. It reaches an area called the fourth ventricle. The fourth ventricle is between the brainstem and the cerebellum, the pons and medulla forming the floor of the fourth ventricle. 5. The CSF leaves the fourth ventricle and then goes through the median aperture of the fourth ventricle and into the subarachnoid space. 6. CSF also leaves by the right and left lateral apertures of the fourth ventricle. When in the subarachnoid space, the CSF goes around the brain and spinal cord, and eventually gets reabsorbed in the arachnoid granulations. If the aqueduct is blocked, then there would be hydrocephalus, and the lateral and third ventricles would be enlarged, raising pressure in the intracranial cavity.

Yath Prem

Spinal cord The spinal cord is covered in 3 layers of meninges. The arachnoid is stuck to the inside surface of the dura. There is CSF in the subarachnoid space, so the spinal cord ‘floats’ in the CSF. Denticulate ligaments go from the pia to the arachnoid. The vertebral artery lies in the subarachnoid space, so when they are cut, bleeding occurs in the CSF.

Yath Prem Sections through the spinal cord In the grey matter, there are mostly cell bodies of neurones. There is a dorsal horn (sensor functions) and a ventral horn (motor functions). The white matter mainly contains axons. This can be split into the ventral, dorsal and lateral columns of white matter. Lots of the axons go up and down the spinal cord, connecting it to the brain.

There is a central canal. There are stem cells here, but we don’t know their use. Radicular arteries supply blood to the spinal arteries. The anterior spinal artery (formed from a branch from each vertebral artery) supply the anterior section of the spinal cord (pattern of distribution is shown in red on this image). There are other contributory arteries, including the artery of Adamkiewcz, which is the largest anterior segmental medullary artery. This area of distribution includes the feeling for pain. Laminae of Rexed

Yath Prem The grey matter can be divided into sections on each side called laminae. There are 10 laminae from the dorsal horn to the ventral horn. Some of the important aspects of the laminae of Rexed: 

I-IX go from the dorsal horn to the ventral horn



X is present in the central horn



I-VI are present in the dorsal horn



I and II are responsible for pain (nociception) (lamina II = substantia gelatinosa)



IX – location of motor neurons (more medial supply trunk, more lateral = extremities)



VII – location of preganglionic sympathetic neurons (but only in thoracic spinal cord)

Injuries to peripheral nerves Lower motor neuron lesions (at anterior horn or below) Lower motor neurone lesions can occur due to nerve injury or poliomyelitis (the virus somehow gets out of the gut and kills motor neurones). Features include flaccid paralysis, rapid muscle wasting, fibrillation and fasciculation.

If the injury is close enough to it, then the cell body may die. Otherwise, the nerve can regenerate along the course of the pre-existing fibre. Axons do not regenerate in the CNS (there are no explanations why). Peripheral nerves can be repaired by suturing, but only 50% of patients report a useful degree of recovery. Neuropathic pain is one consequence of failed functional regeneration.

Yath Prem Upper motor neuron lesions (before anterior horn) In an upper motor neurone lesion, one of the tracts is damaged (see below). However, there is still innervation to the muscle, and so there is only a little muscle wasting. The reflexes are still there, so you still get the muscle stretch reflex, and the muscle can still contract. Babinski’s sign can indicate an upper motor neurone lesion (in adults; note that this is a normal reflex in infants). This is where during plantar flexion, there is a reflex dorsiflexion of the big toe (it should normally have a downward response). Until the corticospinal tracts are myelinated, babies also show a positive Babinski sign.

Tracts in the spinal cord Descending tracts carry instruction from the brain to the motor neurones, whereas ascending tracts carry sensation up to the brain. Sensory and motor tracts both cross the midline. Therefore, the sides of the brain are contralateral to the side of the body it controls. The tracts do not always cross the midline at the same position. Corticospinal tracts These arise from neurons in the cerebral cortex and pass down the brainstem near its ventral surface. The corticospinal tracts begin in the cerebral cortex, from which they receive a range of inputs: •

Primary motor cortex

•

Premotor cortex

•

Supplementary motor area

Yath Prem They also receive nerve fibres from the somatosensory area, which play a role in regulating the activity of the ascending tracts. After originating from the cortex, the neurones converge, and descend through the internal capsule (a white matter pathway, located between the thalamus and the basal ganglia). This is clinically important, as the internal capsule is particularly susceptible to compression from haemorrhagic bleeds, known as a capsular stroke. Such an event could cause a lesion of the descending tracts. After the internal capsule, the neurones pass through the midbrain, the pons and into the medulla. The fibres within the lateral corticospinal tract decussate at the level of the pyramids (medulla). They then descend into the spinal cord, terminating in the ventral horn (at all segmental levels). From the ventral horn, the lower motor neurones go on to supply the muscles of the body. The anterior corticospinal tract remains ipsilateral, descending into the spinal cord. They then decussate and terminate in the ventral horn of the cervical and upper thoracic segmental levels.

Important features through the brainstem Upper midbrain Periaqueductal grey is the grey matter around the aqueduct. There are cerebral peduncles, which are a big mass of axons. This contains the corticospinal tract, as well as other descending tracts that originate from parts of the cerebral cortex. This bundle lies anterior to the substantia nigra at the level of the midbrain.

Yath Prem

Note that the inferior colliculus also lies at the level of the midbrain but below the superior colliculus. Lower pons Here, the corticospinal tract has split up into fibres. The basilar artery goes in front of the pons and supplies the front part of the pons. It gives branches to the descending motor parts of the pons that lie on the front (not the sensory ones that lie at the back). If a branch of the basilar artery that supplies the corticospinal tracts is blocked, then you get lockedin syndrome, where there is a problem with the motor parts. This is often misdiagnosed as a coma (however, in a coma, you also get a loss of sensory function, not just the motor function).

Open medulla This image shows the important landmarks at the level of the open medulla (the nuclei and tracts have been removed). Note the fourth ventricle is on the posterior surface of the open medulla.

Yath Prem At this level, the inferior cerebellar peduncles connect the medulla to the cerebellum. At the open medulla, the corticospinal tracts come together again and form pyramids. The major unique function of corticospinal tracts seems to be fine hand movements. The olives are a landmark in the open medulla. These contain the olivary nuclei. The inferior olivary nucleus sends afferents to the cerebellum.

Closed medulla The part of the medulla that has a central canal is the closed medulla. The pyramids are still here, and they cross over at the lower part of the medulla....