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Neuron

A neuron consists of a cell body or soma and neuronal processes. The neuronal processes that emerge from the cell body are called dendrites (usually multiple) and axons (single).

Nerve Cell Body Or Soma

The nerve cell body has a typical pale staining euchromatic nucleus with a prominent dark nucleolus. The cytoplasm of a neuron contains prominent basophilic material called Nissl bodies.

The cell body also contains the Golgi apparatus, mitochondria, lysosomes, and smooth endoplasmic reticulum.

Normally, neurons after birth are unable to replicate their DNA and thus do not undergo cell division.

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Nerve Cell Processes

The elongated cytoplasmic processes take origin from the cell body. These processes may travel long distances from the neuron. They are of two types:

Dendrites and Axon. Usually, a neuron consists of a single axon and multiple dendrites.

Dendrites

Neurons usually have short, multiple dendrites, and each of these may branch extensively to form a ‘dendritic tree’. Dendrites are involved in receiving information from other cells.

The cytoplasm of dendrites contains Nissl bodies, microtubules, microfilaments, and other organelles.

The cytoskeleton of a neuron is formed by microtubules, neurofilaments, and microfilaments.

Axons

• Axons are the nerve cell processes that send information in the form of electrical signals away from the nerve cell body to other cells.
• Usually, there is a single, extremely long axon (in some neurons, the length may reach up to 1 m) for each neuron.
• It originates at a conical region of the cell body known as the axon hillock.

3 Types Of Neurons And Functions

Classification Of Neurons

Neurons are classified based on their structure, size, and function.

Classification Based On Structure of Neurons

The structure and shape of a cell body are dependent on the number and orientation of cell processes arising from it.

Depending on the number of processes emerging from the cell body, the neurons can be classified as follows:

• Multipolar
• Bipolar
• Unipolar or pseudo-unipolar

Multipolar neurons: Multipolar neurons have a single axon and many dendrites extending from the cell body. Most of the neurons present in the CNS are multipolar.

Bipolar neurons: Bipolar neurons have two processes emerging from the cell body (e.g. bipolar neurons of the retina and neurons of olfactory neuroepithelium).

3 Types Of Neurons And Functions

Unipolar or pseudo-unipolar neurons: Unipolar or pseudo-unipolar neurons are found in the dorsal root ganglion of spinal nerves (sensory ganglion) and sensory root ganglia of cranial nerves.

Such a neuron has a single process that extends from the cell body; this process then bifurcates to form a T-shaped process.

According to some authors, true unipolar neurons have only a single process extending from the cell body, usually the dendrite.

These kinds of neurons are present in the mesencephalic nucleus of the trigeminal nerve.

Classification Based On Size of Neurons

The neurons can also be classified based on their size.

According to this classification, the neurons are as follows:

• Golgi type1 neurons
• Golgi type2 neurons

Golgi typeI neurons: Golgi type1 neurons have a single, long axon, sometimes more than a meter in length.

The axons of these neurons (e.g. pyramidal cells, Purkinje cells) extend long distances within the CNS (as part of the tract) or in the PNS (as part of peripheral nerves, e.g. sciatic nerve;

Golgi type 2 neurons: Golgi type 2 neurons have short axons and these terminate close to the cell body (e.g. stellate and granule cells of the cerebellar cortex).

3 Types Of Neurons And Functions

Classification Based On Function of Neurons

Functionally, neurons are classified into sensory”, motor, and interneurons.

Sensory or afferent neurons: Sensory neurons are specialized cells capable of detecting various kinds of stimuli, i.e. pain, touch, temperature, light, pressure, and chemicals.

Motor (efferent) neurons: Motor neurons carry efferent to the sensory stimuli.

Motor neurons are of two types:

Somatic and visceral. These neurons send motor impulses to the muscles, which results in their contraction.

Somatic motor neurons send impulses to the skeletal muscles while visceral motor neurons stimulate the smooth and cardiac muscles.

The visceral motor neurons also send signals (secretomotor) to exocrine glands, which results in secretion from these glands.

Interneurons: Interneurons are located in between sensory and motor neurons. These neurons interconnect sensory and motor neurons. These neurons carry out the integrative function.

3 Types Of Neurons And Functions

Neuroglia

Besides neurons, the nervous tissue also consists of supporting cells, known as neuroglia (glial cells). These cells and their processes fill up the space between neurons.

Neuroglia provide insulation to neurons and their processes by completely enveloping them.

The size of neuroglial cells is much smaller than that of neurons. However, in the CNS, their number is 5-50 times more than that of neurons.

The neuroglial cells are capable of multiplying in mature nervous tissue. Brain tumors may have their origin in glial cells and are called gliomas.

In contrast to the nerve cells, neuroglial cells are unable to enerate or transmit the impulse.

Neuroglial cells are classified as follows:

1. Neuroglial cells of the CNS

• Ependymal cells
• Astrocytes
• Oligodendrocytes
• Microglia

2. Neuroglial cells of the PNS

• Schwann cells
• Satellite cells

Neuroglial Cells Of The CNS

Ependymal Cells

The ependymal cells are arranged in a single layer. They are cuboidal or columnar. The microvilli/cilia are present on their apical surface. Ependymal cells line the ventricles of the brain and the central canal of the spinal cord.

Function. Ependymal cells help in the formation and circulation of CSF.

Astrocytes

Astrocytes are so-called because they are star-shaped They have many star-like radiating processes.

Astrocytes are of two types: Protoplasmic astrocytes and Fibrous astrocytes.

Protoplasmic astrocytes have thick processes with abundant granular cytoplasm.

They are mainly found in grey matter. Some of these are attached to the neighboring blood capillaries by bulbous ‘end feet’ or ‘vascular feet’.

Fibrous astrocytes have long, thin, and straight processes These cells are present only in the white matter.

Astrocytes Functions. Astrocytes completely cover the brain surface and nonsynaptic regions of the neurons. The blood-brain barrier is formed by astrocytes.

Oligodendrocytes

• Oligodendrocytes are small, round, or oval cells with only a few cytoplasmic processes.
• These cells are only present in the CNS. Embryologically, oligodendrocytes develop from the neural tube.
• Functions Oligodendrocytes produce myelin sheaths around the axons of multiple neurons in the CNS.

Microglia

Microglial cells are involved in phagocytic activities within the CNS. These cells are small with few tortuous processes.

Neuroglial Cells Of The PNS

Schwann Cells

Schwann cells are flattened cells with a flattened nucleus that is surrounded by abundant cytoplasm. These cells are present in the PNS only.

Function. Schwann cells produce a myelin sheath along the axon of a neuron, in the PNS. They also participate in the regeneration of the PNS axon.

Satellite Cells

Satellite cells surround the nerve cells of the ganglia (spinal and with prominent nuclei They are present only in the PNS.

Function. Satellite cells insulate and support neurons of the ganglia (in the PNS).

The functions of glial cells.

Nervous Tissue Summary

• The nervous tissue is composed of neurons, nerve cell processes (axons and dendrites), and neuroglia.
• Neurons are highly specialized cells and carry electrical signals from one cell to another.
• These cells have a typical pale staining achromatic nucleus, dark nucleolus, and Nissl granules in the cytoplasm.
• The elongated cytoplasmic processes take origin from the nerve cell body. They are of two different types:
• Axon and Dendrites.
• Neurons usually have short, multiple, branching dendrites. They receive information from other cells and usually have only one extremely long axon.
• Axons send information away from the nerve cell body to other cells.
• The proteins (neurotransmitters) produced in the cell are transported to the distal region of the axon and back. This is known as axonal transport.
• The classification of neurons is based on their structure (multipolar, bipolar, etc.), their size (Golgi types 1 and 2), and their functions (sensory neurons, motor neurons, and interneurons).
• Neuroglia (glial cells) are supporting cells of nervous tissue. They also provide insulation to neurons (satellite cells)
  and their processes (oligodendrocytes and Schwann cells).
• Ependymal cells help in the formation and circulation of the cerebrospinal fluid. The blood-brain barrier is formed by astrocytes while microglia are involved in phagocytic activities within the CNS.

Multiple Choice Questions

Question 1. Which of the following statements is false?

• The neuron is specialized to carry information in the form of electrical signals
• Dendrites are multiple, short processes while an axon is a single, long process
• The cytoplasm of neurons contains prominent eosinophilic material called Nissl bodies
• The nucleus of a neuron is euchromatic

Answer: 3. Cytoplasm of neurons contains prominent eosinophilic material called Nissl bodies

Question 2. Which of the following statements about axons is false?

1. Axon begins at Axon Hillock
2. The cytoplasm of the axon (axoplasm) contains microfilaments and microtubules
3. Axon sends electrical signals (impulses) in one direction only, i.e. away from the nerve cell body
4. Axonal cytoplasm (axoplasm) carries proteins from the cell body towards the distal end of the axon, i.e. in one direction only.

Answer: 4. Axonal cytoplasm (axoplasm) carries proteins from the cell body towards the distal end of the axon, i.e. in one direction only.

Question 3. Which ofthe following statements about Nissl substance is false?

1. It has an acidic component
2. Nissl bodies are aggregations of rough endoplasmic reticulum
3. Nissl bodies are also the aggregation of ribosomes
4. Axons are free ofNissl granules but dendrites contain Nissl granules
5. None of the above

Answer: 5. None of the above

Question 4. Which of the following statements about the transport of tetanus toxin is true?

1. By retrograde axonal transport
2. By anterograde axonal transport
3. By transport through endoneurial space
4. None of the above

Answer: 3. By transport through endoneurial space

Question 5. Which of the following statements about the location of bipolar neurons is false?

1. Neurons in the inferior ganglion of the vagus
2. Neurons in retina
3. Neurons ofvestibular and spinal ganglion
4. Neurons olfactory neuroepithelium

Answer: 1. Neurons in the inferior ganglion of the vagus

Question 6. Which kind of neurons is found in the dorsal root ganglion of spinal nerves?

1. Multipolar
2. Bipolar
3. Unipolar or pseudo-unipolar
4. Anaxonic

Answer: 3. Unipolar or pseudo-unipolar

Question 7. Which of the following cells is unable to propagate nerve impulses?

1. Sensory neurons
2. Motor neurons
3. Neuroglia
4. Interneurons
5. Golgi type2 neurons

Answer: 3. Neuroglia

Question 8. Which of the following neuroglial cells is not present in the CNS?

1. Ependymal cell
2. Fibrous astrocytes
3. Oligodendrocytes
4. Satellite cells
5. Microglial cells

Answer: 4. Microglial cells

Question 9. Which of the following statements about astrocytes is false?

They secrete growth stimulants

1. They cover the brain surface and the surface of neurons in the ganglia
2. They are metabolic neurotransmitters
3. They help in the formation of the blood-brain barrier
4. They form scar tissue after brain damage

Answer: 2. They cover the brain surface and the surface of neurons in the ganglia

Question 10. Which of the following statements about Schwann cells are true?

• They are present in the peripheral nerve system only
• Embryologically, they are derived from neural crest cells
• They form a neurilemmal sheath around axons in the PNS
• They help in the regeneration of nerve fibers
• All of the above

Answer: 5. All of the above

Spinal Cord

The spinal cord is a long, cylindrical structure present in the upper two-thirds of the vertebral canal.

Within the vertebral canal, the spinal cord is well protected by bony vertebrae, meninges, and cerebrospinal fluid (CSF).

External Structure Of The Spinal Cord

The spinal cord extends from the level of the upper border of the first cervical vertebra up to the lower border of the first lumbar vertebra.

The length of the cord varies from 42 to 45 cm and its weight is about 30 grains. The diameter of the spinal cord is minimum in the thoracic region, about 2 cm.

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The gross anatomical features that are examined externally are as follows:

1. Enlargements
2. Conus medullaris
3. Filum terminal
4. Fissure and sulci
5. Spinal nerves
6. Spinal cord segments

Enlargements

The spinal cord shows the fusiform enlargement in the cervical and lumbosacral regions. These enlargements are due to the collection of a large number of spinal neurons for the nerve supply of the upper and lower limbs.

Therefore, these enlargements are also sometimes called limb enlargements, i.e. cervical and lumbosacral enlargements.

The cervical enlargement extends from C4 to T2 spinal cord segments.

The ventral rami of C5 to T1 spinal nerves form the brachial plexuses, which supply the upper limb.

The lumbosacral enlargement extends front LI to S3 spinal cord segments.

The ventral rami of the 1.2 to S3 spinal cord segment forms the lumbosacral plexus to supply the lower limb.

Conus Medullaris

Just below the lumbosacral enlargement, the lower end of the spinal cord is tapering and is known as conus medullaris.

Filum Terminale

The Filum terminal is a slender filament, which extends from the tip of the conus medullaris to the dorsum of the first coccygeal vertebra.

Filum terminale lies in the middle of cauda equina. The cauda equine is a bunch of spinal nerves around the filum terminale. Cauda equine consists of lumbar, sacral, and coccygeal spinal nerves.

Fissure and Sulci

On the anterior aspect of the spinal cord, there is an anterior median fissure, which extends throughout its length. On either side of this fissure, there is the presence of shallow anterolateral sulci.

Similarly, on the posterior aspect of the cord, a single median posteromedian sulcus and two posterolateral sulcus, one lying on each side of the posteromedian sulcus, are present. This fissure and sulci can be better appreciated in the transverse section of the spinal cord.

Spinal Nerves

Thirty-one pairs of spinal nerves are attached to the spinal cord. These are grouped into 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.

Spinal Cord Segments

Based on the origin of the 31 pairs of spinal nerves, the spinal cord is also divided into 31 spinal segments.

The spinal segment is the portion of the spinal cord to which a pair of dorsal and ventral spinal roots is attached.

The spinal cord is divided into the following segments; 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

Spinal Cord Segments and Vertebral Levels

Students should note that the spinal cord segments do not
correspond to the vertebral column segments in adults.

However, in early fetal age, there is a perfect correlation between spinal cord segments and vertebral segments. However, after the early fetal age, the vertebral column grows at a much faster rate as compared to the growth of the spinal cord.

Therefore, the lengthening of spinal nerve roots between the spinal cord and the intervertebral foramen occurs. This lengthening of spinal nerves is more marked in lumbar and sacral spinal roots.

The spinal nerves at the lower level (lower thoracic and lumbar) have to pass obliquely downwards for a considerable distance to get out of their respective foramina.

Thus, the nerve roots from the lumbosacral region (conus medullaris) emerge in the form of a bunch.

These nerve roots surround the filum terminale and are known as cauda equina (resembling the tail of the horse).

As there are seven cervical vertebrae and eight pairs of cervical spinal nerves, spinal nerves from Cl to C7 come out above the corresponding vertebrae while spinal nerve C8 comes out between C7 and T1 vertebrae.

The T1 spinal nerve and all others below this level come out below their corresponding vertebrae.

Internal Structure Of The Spinal Cord

By studying the transverse section of the spinal cord, one can learn the basics of its internal structure. The transverse section of the spinal cord shows that it is composed of grey and white matter.

Grey matter is placed centrally and white matter is on the periphery. Grey matter is composed of cell bodies (neurons) and their processes while white matter is composed of myelinated and nonmyelinated nerve fibers.

Grey Matter

The grey matter of the spinal cord, on each side, is subdivided into anterior (ventral), posterior (dorsal), and lateral grey columns (horns).

The anterior, posterior, and lateral grey columns (horns) lie anterior, posterior, and lateral to the transverse band of grey commissure.

Anterior (Ventral) Grey Columns (Horns)

• The anterior grey columns contain cell bodies of motor neurons (alpha and gamma) and Renshaw cells. The Renshaw cell is a type of interneuron.
• The cells of this column give origin to axons, which form the ventral root of the spinal nerve. These fibers carry nerve impulses for the contraction of skeletal muscles.

Spinal Cord Function And Structure

Posterior (Dorsal) Grey Columns

The posterior grey columns are directed backward and laterally toward the surface. This column contains somatic and autonomic sensory neurons and receives the central processes of pseudo-unipolar cells of the dorsal root ganglion.

The posterior grey column gives origin to the axons, which form ascending (sensory) tracts of the white matter. Conventionally, the dorsal horn is divided dorsoventrally into the apex, body, and base.

Lateral Grey Columns

The lateral grey columns are present between the anterior and the posterior grey columns on each side.

These columns extend from Tl to L2 and S2 to S4 spinal segments.

They contain the preganglionic autonomic motor neurons that carry nerve impulses for the regulation of the activities of the smooth muscle, cardiac muscle, and glands (see in the following text).

Nuclei in the Grey Matter

The H-shaped grey matter of the spinal cord consists of neurons of various sizes and shapes. These nerve cells are organized in different groups and each group extends longitudinally within the grey matter of the spinal cord.

It is due to this reason that each nerve cell group is called a column or nucleus.

Nuclei in the anterior grey column (ventral horn): The nerve cell groups in the anterior column or ventral horn are motor nuclei.

These are arranged in three groups:

1. Medial,
2. Lateral and
3. Central.

Medial group: This group is present throughout the length of the spinal cord.

Spinal Cord Function And Structure

It is subdivided into two groups:

Ventromedial and Dorsomedial. The ventromedial group supplies the muscles of the back while the dorsomedial group supplies anterolaterally situated trunk muscles.

Central group: This group is present in the cervical and lumbosacral segments of the spinal cord.

It consists of three nuclei:

• Phrenic (between C3 and C5 spinal segments),
• Spinal accessory (between Cl and C6 segments) and
• Lumbosacral nuclei (between L2 and SI spinal segments).

Lateral group: This group is present only in cervical and lumbar enlargements of the cord as it supplies the muscles of the upper and lower limbs.

It is subdivided into three subnuclei:

1. Ventrolateral,
2. Dorsolateral and
3. Retrodorsolateral

Nuclei in the lateral grey column: The lateral grey 3. column, or the intermediate nucleus, is present between the anterior and posterior grey columns on either side of the central canal.

• A distinct group of nerve cells is present in the lateral grey column between the T1 and L2 spinal segments.
• The axons of these cells give rise to preganglionic sympathetic motor fibers, which travel in the ventral spinal root.
• A similar less distinct group is also seen between S2 and S4 spinal segments. The axons of these cells give origin to preganglionic parasympathetic motor fibers.

Spinal Cord Function And Structure

Nuclei in the posterior grey column (dorsal horn): This group consists of sensory nerve cells.

The sensory neurons of the dorsal grey column are much smaller in size as compared to the motor neurons of the anterior grey column.

A brief description of these groups is arranged in the dorsoventral direction.

Marginal nucleus: It is a thin layer of small neurons situated dorsal to substantia gelatinosa at the apex of the dorsal grey column.

Substantia gelatinosa: Substantia gelatinosa cells are present near the apex of the posterior grey column.

This group extends throughout the spinal cord. Substantia gelatinosa is continuous above (in the medulla and lower pons) with the nucleus of the spinal tract of the trigeminal nerve.

Nucleus proprius: It (magnocellular nucleus) is a large nucleus of the posterior grey column. This nucleus extends throughout the spinal cord.

Dorsal nucleus (Clarke’s column): It (thoracic nucleus or Clarke’s column) extends from C8 to L4 spinal segments. It is present at the base of the posterior grey column.

Intermediomedial nucleus: It (visceral afferent) is situated ventral to the dorsal nucleus at the base of the dorsal grey column lateral to the central canal.

It extends throughout the entire spinal cord. Probably, it receives sensory impulses from the viscera.

Laminar Architecture of the Spinal Grey Matter

• The grey matter of the spinal cord may be divided into 10 laminae (layers) based on the cytoarchitectonic organization (i.e. shape, size, and packing density of neurons).
• Though Rexed studied these laminae in the spinal cord of cats, however, it is believed that a similar lamination exists in all mammals including man.
• These laminae are designated by Roman numerals I to X. Laminae I to IX are designated starting from the tip of the posterior horn and moving towards the ventral horn.
• Laminae I to 4 occupy the dorsal grey horn, lamina VII occupies the intermediate grey matter (between dorsal and ventral grey horns) while laminae 8 and 9 are present in the ventral grey horn. Lamina X is the grey matter surrounding the central canal.
• The nuclei of the ventral, dorsal, and lateral grey horns are summarised.

White Matter

The white matter of the spinal cord is organized into three broad areas called funiculi (or columns).

These are anterior (ventral) funiculus, lateral funiculus and dorsal (posterior) funiculus.

Each funiculus consists of distinct tracts (a tract is defined as bundles of nerve axons having a common origin, termination, and carrying similar impulses).

• In the white matter of the spinal cord, tracts are mainly of three types. These are sensory or ascending tracts, motor or descending tracts, and intersegmental tracts.
• Sensory (ascending) tracts carry sensory impulses from the spinal cord to the brain, for example spinothalamic tract.
• The descending (motor) tracts carry motor impulses from the brain to the spinal cord, for example, pyramidal and extrapyramidal tracts.

The intersegmental tracts run for short distances; that is, they remain confined within a few segments of the spinal cord.

They connect the neurons within segments or of different segmental levels.

Dorsal Funiculus

• The dorsal (posterior) funiculus is bounded medially by the posteromedian septum and laterally by the dorsal grey column. It consists of medially placed gracile and laterally placed cuneate fasciculi.
• The gracile fasciculus extends throughout the length of the spinal cord but the cuneate fasciculus is present only above the level of the midthoracic region. Both the gracile and the cuneate asciculi are sensory.

Lateral Funiculus

The lateral funiculus is present between dorsal and ventral grey columns (between ventral spinal roots and posterolateral sulcus). It consists of both motor (descending) and sensory (ascending) tracts.

Ventral Funiculus

The ventral (anterior) funiculus is present between the anterior median fissure and the lateral-most fibers of the ventral roots.

Axons decussate in the ventral white commissure. It also consists of both motor (descending) and sensory (ascending) tracts. Enumerate the ascending and descending tracts in various funiculi of the spinal cord.

Regional Differences In The Arrangement Of White And Grey Matter

• Some regional differences are observed when transverse sections of the spinal cord are examined in different regions.
• The amount of white matter gradually increases from the sacral to the cervical region (below upwards) of the spinal cord. The white matter is the maximum in the cervical part of the spinal cord.
• The amount of grey matter is much higher in the cervical and lumbosacral regions (enlargements). The neurons of these enlargements are responsible for the innervation of limbs.
• The grey matter is much less in the thoracic region than at each segmental level. It mainly innervates the intercostal space only.

Spinal Cord Function And Structure

Formation Of A Typical Spinal Nerve

A typical spinal nerve arises from the spinal cord in the form of two roots: Ventral and Dorsal.

Ventral Root

The ventral root (anterior root) contains bundles of efferent (motor) fibers, which arise from the motor neurons situated in the anterior grey column. These fibers supply the skeletal muscles.

Dorsal Root

• The dorsal root (posterior root) contains bundles of afferent (sensory) fibers.
• These fibers arise from the nerve cells present in the dorsal root ganglion. The dorsal root ganglion cells are pseudo-unipolar.
• ‘The cell body of these neurons gives a single process, which divides into central and peripheral processes.
• Hie peripheral process goes peripherally through the spinal nerve to supply the tissues while the central process passes through the dorsal root to the dorsal grey column of the spinal cord.

The trunk of the Spinal Nerve

The dorsal and ventral nerve roots unite to form the trunk of the spinal nerve. Within the trunk of the spinal nerve, both sensory and motor fibers are present.

Branches

The trunk of the spinal nerve immediately divides into two rami: dorsal and ventral. These rami soon come out of the intervertebral foramen. Both the rami contain mixed fibers (motor and sensory).

Spinal Cord Function And Structure

Ventral Ramus

• The ventral ramus and its branches supply motor and sensory nerve fibers to the anterior and lateral regions of the trunk and the upper and lower limbs. The ventral rami is larger than dorsal rami.
• They supply the limbs and anterolateral aspect of the trunk (thorax and abdomen). In the thoracic region, ventral rami are known as intercostal nerves.
• They run independently and retain their segmental innervations. However, the ventral rami of the cervical, lumbar, and sacral regions form nerve plexuses by uniting with each other. In the cervical region, they form cervical and brachial plexuses.
• The brachial plexus supplies the muscles of the upper limb. The lumbar and sacral ventral rami form lumbosacral plexuses which supply the muscles of the lower limb.

Dorsal Ramus

The dorsal ramus and its branches carry motor fibers to the deep muscles of the back and sensory fibers to the overlying skin. The dorsal rami of spinal nerves supply the dorsal aspect of the body.

They are usually smaller than the ventral rami. Similar to thoracic ventral rami (intercostal nerves), dorsal rami also have segmental distribution.

Spinal Cord Function And Structure

The dorsal rami do not form nerve plexuses. A dorsal ramus usually divides into medial and lateral branches and supplies muscles and skin of the posterior region of the neck and trunk.

Spinal Cord Summary

• The spinal cord is a long cylindrical structure present in the upper two-thirds of the vertebral canal. The vertebral canal is well protected by bony vertebrae, meninges (dura, arachnoid, and pia), and a cushion of cerebrospinal fluid.
• The upper end of the spinal cord lies at the level of the upper border of the first cervical vertebra while the lower end lies at the lower border of the LI vertebra. The length of the cord varies from 42 to 45 cm.
• On the external surface of the spinal cord, we get the following fissures and sulci: one anterior median fissure, two anterolateral sulci, one posteromedian sulcus, and two posterolateral sulci.
• The spinal cord shows the fusiform enlargement in the cervical and lumbosacral regions. Just below the lumbosacral enlargement, the lower end of the spinal cord is tapering and is known as conus medullaris.
• Filum terminate is a slender filament, which extends between the tip of the conus medullaris and the dorsum of the first coccygeal vertebra.
• Thirty-one pairs of spinal nerves are attached to the spinal cord, i.e. 8 cervical, 1 2 thoracics, 5 lumbar, 5 sacral, and 1 coccygeal.
• The transverse section of the spinal cord shows that it is composed of grey and white matter. The grey matter is placed centrally and the white matter is on the periphery.
• The grey matter of the spinal cord, on each side, is subdivided into anterior (ventral), posterior (dorsal), and lateral grey columns (horns).
• Various nerve cell nuclei in the spinal grey columns (horns) are presented,
• The white matter of the spinal cord is organized into three broad areas called funiculi (or columns). These are anterior (ventral) funiculus, lateral funiculus and dorsal (posterior) funiculus.
• Each funiculus consists of distinct tracts, i.e. sensory or ascending tract, motor or descending tract, and intersegmental tract.

Spinal Cord Multiple Choice Questions

Question 1. The spinal cord extends from the upper border of the atlas vertebra to

1. The lower border of S2
2. Lower border of
3. Lower border ofL3
4. Lower border ofL2

Answer: 2. Lower border of

Question 2. At birth, the spinal cord usually lies at the level of

1. The lower border of S2
2. The lower border of SI
3. The lower border of LI
4. The lower border of L3

Answer: 4. Lower border of L3

Question 3. Which ofthe following statements about the spinal cord is false?

1. The anteromedian sulcus is present in the midline anteriorly
2. Anterolateral sulci are present on either side of the midline
3. Posterolateral sulci are present on either side of the posteromedian sulcus
4. Posteromedian sulcus is present in the midline posteriorly

Answer: 1. Anteromedian sulcus is present in the midline anteriorly

Question 4. Which ofthe following statements about filum terminale is false?

1. It extends from the tip of the conus medullaris to the first coccygeal vertebra
2. It consists of pia mater surrounding the neuroglial element
3. It is a vestigial structure
4. It lies above Cauda equine

Answer: 4. It lies above cauda equine

Question 5. Which ofthe following statements about spinal nerves is false?

1. There are 33 pairs of spinal nerves
2. There are 7 pairs of cervical spinal nerves
3. There are 3 pairs of coccygeal nerves
4. All of the above

Answer: 4. All of the above

Question 6. All of the following statements are correct regarding the grey matter of the spinal cord except

1. The anterior grey horn contains cell bodies of motor neurons
2. The posterior grey horn contains cell bodies of sensory neurons
3. The lateral grey horn contains postganglionic autonomic motor neurons
4. The grey matter of the spinal cord contains interneuron

Answer: 3. Lateral grey horn contains postganglionic autonomic motor neurons

Question 7. Which nerve cell group is not present in a large part of the posterior grey column?

1. Marginal nucleus
2. Substantia gelatinosa
3. Nucleus properties
4. Clarke’s column
5. Interomediolateral nucleus

Answer: 5. Interomediolateral nucleus

Question 8. Which is not the major ascending tract of the spinal cord?

1. Ventral spinothalamic tract
2. Pyramidal tract
3. Fasciculus gracilis
4. Lateral spinothalamic

Answer: 2. Pyramidal tract

Question 9. Sensation of touch is carried by

1. Lateral spinothalamic tract
2. Anterior spinothalamic tract
3. Posterior column tracts
4. Anterior spinothalamic and posterior column tract

Answer: 4. Anterior spinothalamic and posterior column tract.

Central Nervous System

The central nervous system (CNS) consists of the brain and the spinal cord, which are made up of nervous tissues (nerve cell bodies, axons and dendrites, neuroglial cells, and blood vessels.

The surface of the brain is covered with a thin layer of grey matter.

This grey matter is called the cortex. However, many areas of grey matter are also present deep within the white matter of the brain. These areas are called ‘nuclei’.

In the spinal cord, grey matter is situated centrally and is ‘H’ shaped. The grey matter is surrounded by the white matter.

The grey matter shows two ventral (anterior) grey columns (horns) that contain motor neurons.

The dorsal grey columns (horns) contain sensory neurons. In the thoracic and upper lumbar regions, the grey matter also exhibits a lateral horn containing preganglionic autonomic neurons.

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The white matter of the spinal cord is the collection of nerve fiber bundles, which are known as tracts. There are some ascending and descending tracts.

Central Nervous System Function

Both the brain and the spinal cord are surrounded by three membranes called meninges, namely the pia mater, arachnoid mater, and dura mater. Pia mater is

A thin delicate membrane, which intimately covers the outer surface of the brain and the spinal cord.

The space between the pia and the arachnoid mater is filled with cerebrospinal fluid (CSF). Blood vessels also run in subarachnoid space.

Central Nervous System Function

The dura mater is the outermost membrane that is very tough. It is fused with the bones of the cranial cavity (brain box).

Brain

The brain lies in the cranial cavity and is continuous with the spinal cord just below the level of the foramen magnum.

The brain, in the ascending order, is divided into three principal parts:

1. Hindbrain
2. Midbrain
3. Forebrain

The medulla, pons, and midbrain are called brainstems. The midbrain is a small part, which connects the forebrain to the hindbrain. Posterior to the brainstem lies the cerebellum. The forebrain lies superior to the brainstem.

Hindbrain

The hindbrain consists of the medulla oblongata, pons, and cerebellum.

Medulla Oblongata

• Medulla oblongata is also known as medulla. It is conical in shape and continuous below the spinal cord. (just below the foramen magnum) and above with the pons.
• On the ventral aspect, there is the presence of an anterior median fissure, which divides it into right and left halves.
• On either side of the fissure is a ridge called a pyramid. Posterolateral to the pyramid is an oval swelling, known as the olive.
• Posterolateral to olives are inferior cerebellar peduncles, which contain the white fibers and connect the medulla to the cerebellum.
• The lower part of the medulla contains the central canal, which is continuous below the central canal of the spinal cord.
• The central canal opens above in the fourth ventricle, which is situated posterior to the upper part of the medulla and pons.

Pons

• Pons lies above the medulla and below the midbrain. On the ventral aspect, it forms a bulge that is convex from side to side and above downwards.
• On each side, the pons are continuous with the middle cerebral peduncle, which extends posteriorly into the cerebellum.

Central Nervous System Function

Cerebellum

• The cerebellum overlaps the posterior surfaces of the brainstem, i.e. midbrain, pons, and medulla.
• It lies in posterior cranial fossa inferior to a horizontal fold of dura matter (tentorium cerebelli).
• The surface of the cerebellum is covered by grey matter which shows a large number of closely set transverse fissures.

Midbrain

• The midbrain is the short segment of the brainstem situated above the pons.
• It joins the forebrain to the hindbrain. On the ventral aspect, it exhibits two broad white bundles of nerve fibers, the crura cerebri.
• These crura cerebri form the posterolateral boundary of a fossa called the interpeduncular fossa containing a posterior perforated substance.
• The posterior surface of the midbrain is deeply situated and overlapped by the cerebellum. It consists of four small swellings, called the colliculi (two superior and two inferior
  colliculi).
• The midbrain is traversed by a narrow canal, the cerebral aqueduct, which is connected superiorly with the third ventricle and below with the fourth ventricle.

Forebrain

The forebrain consists of the diencephalon and cerebrum.

Diencephalon (Thalamus and Hypothalamus)

• The diencephalon is deeply situated in the forebrain. Only the inferior surface of the hypothalamus is visible in the roof of the interpeduncular fossa.
• The inferior surface of the hypothalamus is formed by the tuber cinereum and mammillary bodies.
• The thalamus and hypothalamus are seen on the lateral surface of the third ventricle in the median sagittal section of the brain.
• The thalamus is situated dorsal to the hypothalamus. The third ventricular cavity is situated between the right and the left diencephalons.

Cerebrum

• The cerebrum is the largest part of the brain. The cerebrum consists of two cerebral hemispheres which are separated from each other by a midline fissure, called the longitudinal fissure.
• At the bottom of this fissure, there is the presence of the corpus callosum. The corpus callosum is a thick band of nerve fibers that connects the corresponding areas of two cerebral hemispheres (right and left).
• Each cerebral hemisphere is covered by grey matter (cortex). The surface of each hemisphere shows extensive folding to increase the surface area of the cortex.
• Thus, the cortex presents a large number of grooves (sulci) and ridges (gyri).
• Deep to the grey matter of the cortex, each hemisphere has a central core of the white matter.
• Many large collections of grey matter are present within the subcortical white matter and are referred to as basal nuclei (e.g. caudate and lentiform nuclei.
• Each cerebral hemisphere contains a large cavity, called the lateral ventricle. Each cerebral hemisphere has three surfaces: superolateral, inferior, and medial.

Ventricular System of the Brain

• The brain has large cavity-like spaces that are referred to as ventricles. The ventricles are named lateral ventricles, third ventricles, and fourth ventricles.
• The lateral ventricle is a large cavity situated in each cerebral hemisphere.
• Both the lateral ventricular cavities communicate with a median narrow cavity (third ventricle), which is situated between the right and the left diencephalons.
• The third ventricle communicates below with the fourth ventricle through the cerebral aqueduct of the midbrain.
• The fourth ventricle is continuous below with the central canal situated in the lower medulla and spinal cord.
• The ventricular system is lined with epithelium known as ependyma and at places it contains choroid plexuses which secrete CSF.

Spinal Cord

• The spinal cord is about a 45-cm long, cylindrical structure situated within the vertebral canal of the vertebral column. In adults, it extends from the medulla to the superior border of the second lumbar vertebrae.
• Within the vertebral canal, the cord is well protected by three meninges (pia, arachnoid, and dura) and CSF.
• Meninges covering the spinal cord are continuous above with the meninges covering the brain.
• The spinal cord shows two enlargements—cervical and lumbar—which give origin to cervical and lumbosacral.

Peripheral Nervous System

• The peripheral nervous system (PNS) is that part of the nervous system that is present outside the CNS (brain and spinal cord).
• This part of the nervous system is responsible for carrying the impulses to and away from the CNS.
• The PNS consists of nerve fibers and cell bodies located outside the CNS.
• The collection of cell bodies outside the CNS is called a ganglion, for example, a collection of neurons in the dorsal root ganglion, ganglion of cranial nerve, and sympathetic and parasympathetic ganglia.

Peripheral nerves take their origin from the CNS (brain and spinal cord) and are classified into cranial and spinal nerves. Ganglia are associated with cranial and spinal nerves.

1. Cranial nerves
2. Spinal nerves
3. Ganglia

A typical spinal nerve is attached to the spinal cord through ventral nerve roots (containing motor axons or fibers) and dorsal nerve roots (containing sensory fibers).

The dorsal root contains a ganglion (dorsal root ganglion) in which cell bodies of sensory neurons are situated.

Cranial Nerves

• The 12 pairs of nerves that arise from the brain are called cranial nerves.
• All the cranial nerves come out from the cranial cavity through the foramina present at the base of the skull (cranium).
• Cranial nerves innervate the head and neck and to a lesser extent the thoracic and abdominal viscera.
• Out of 12 cranial nerves, the first two (olfactory and optic) are attached to the forebrain while the remaining 10 are attached to the brainstem.
• Some cranial nerves also carry autonomic (parasympathetic) fibers.

Ganglia

Ganglia are a collection of neurons outside the CNS and are a part of the PNS as they are present in association with cranial and spinal nerves. They are of two types: Sensory and Autonomic.

Some Important Terms

The following terms will be frequently used during discussion in the book.

Grey and White Matter

A section of the fresh brain or spinal cord shows some areas as white and others as grey.

• White matter is the aggregation of myelinated processes (fibers). As the myelin is white, the collection of fibers also looks white.
• On the other hand, grey matter of the nervous system is the collection of nerve cell bodies, dendrites, unmyelinated axons starting from or ending on the nerve cells (forming a synaptic junction), and neuroglial cells.
• The grey matter looks greyish as there is no myelin in these areas. Blood vessels are present in both grey and white matter.
• The grey and white matter of the brain and spinal cord.

Cortex and Nucleus

• The thin layer of grey matter on the surface of the cerebrum and cerebellum is called the cortex. Deep in the cortex is the presence of white matter.
• A nucleus is a mass of grey matter embedded within the white matter inside the CNS (brain and spinal cord).
• The nucleus consists of an aggregation of nerve cell bodies within the CNS, which share a common function, for example, the thalamus and caudate nucleus.

Fibers in a Peripheral Nerve

A peripheral nerve is a mixed nerve containing both afferent (sensory) and efferent (motor) fibers.

Afferent Fibres

The fibers that carry sensory impulses from various parts of the body (skin and deeper structures) to the CNS are called afferent fibers (sensory nerve fibers).

Efferent Fibres

The fibers which carry impulses from the CNS to various parts of the body are called efferent fibers. Many of these fibers are connected to muscles and are called motor nerve fibers.

Summary

• The CNS consists of the brain and the spinal cord.
• The brain consists of the forebrain, midbrain, and hindbrain.
• Both the brain and spinal cord are made up of grey and white matter.

The CNS is surrounded by three meninges:

1. Pia,
2. Arachnoid and
3. Dura mater.

• The hindbrain consists of the medulla, pons, and cerebellum. The forebrain consists of the diencephalon (thalamus and hypothalamus) and cerebrum. The midbrain joins the forebrain with the hindbrain.
• Each cerebral hemisphere presents three surfaces: Superolateral,
• Medial and
• Inferior.
• Each surface is covered by cortex which shows a large number of sulci and gyri.
• Each hemisphere has a central core of white matter. Many large collections of grey matter are present within the white matter and are referred to as nuclei.
• The brain contains many large cavities called ventricles, i.e. lateral ventricles, third ventricles, and fourth ventricles.
• The PNS consists of 1 2 pairs of cranial and 31 pairs of spinal nerves.
• Sensory ganglia are a collection of nerves in association with cranial and spinal nerves.
• Autonomic ganglia are classified into sympathetic and parasympathetic.
• Sympathetic ganglia are located in the sympathetic trunk while parasympathetic ganglia are located close to the organ they supply.

 Central And Peripheral Nervous Systems Multiple Choice Questions

Question 1. Which of the following statements about the spinal cord is true?

1. The ventral grey horn contains motor neurons
2. The dorsal grey horn contains sensory neurons
3. Lateral grey horn contains preganglionic autonomic
   neurons
4. All of the above

Answer: 4. All of the above

Question 2. Which of the following statements about the grey matter of the nervous system is true?

1. It contains nerve cell bodies
2. It contains neuroglial cells
3. There is no myelin in grey matter
4. It contains blood vessels
5. All of the above

Answer: 5. All of the above

Question 3. Which of the following ganglia is a collection of nerve cell bodies?

1. Spinal root ganglion
2. Sympathetic ganglion
3. Parasympathetic ganglion
4. All of the above

Answer: 5. All of the above

Question 4. Which of the following statements is false?

1. Nucleus is the mass of grey matter in the spinal cord
2. The nucleus is the mass of grey matter in the brain
3. The nucleus is a thin layer of grey matter on the surface of the cerebellum
4. The thalamus is an example of nucleus

Answer: 3. Nucleus is a thin layer of grey matter on the surface of the cerebellum

Question 5. The collection of white matter in the CNS is known as

1. Tract
2. Funiculus
3. Lemniscus
4. All of the above

Answer: 4. All of the above

Question 6. Which are not the afferent fibers?

1. They carry the sensory impulse
2. They carry the motor impulse
3. They carry both motor and sensory impulses
4. They carry impulses from the brain to various parts of the body

Answer: 2. They carry the motor impulse

Question 7. Hindbrain consists of

1. Medulla, pons, and cerebellum
2. Midbrain, pons, and medulla
3. The midbrain, pons, medulla, and cerebellum
4. Midbrain, pons, and cerebellum

Answer: 1. Medulla, pons, and cerebellum

Question 8. Which of the following statements is true?

1. Pons is connected to the cerebellum through middle cerebellar peduncles
2. The midbrain is connected to the cerebellum through superior cerebellar peduncles
3. The medulla is connected to the cerebellum through inferior cerebellar peduncles
4. All of the above

Answer: 4. All of the above

Question 9. Diencephalon consists of

1. Telencephalon
2. Thalamus
3. Hypothalamus
4. Corpus striatum

Answer: 2. Thalamus

Question 10. The peripheral nerve consists of

1. Cranial nerves
2. Spinal nerves
3. Nerve fibers and cell bodies outside the CNS
4. All of the above

Answer: 4. All of the above

Question 11. The ganglia are ofthe following type:

1. Sensory ganglia of cranial nerves
2. Autonomic ganglia
3. Dorsal root ganglia of spinal nerves
4. Ventral root ganglia of spinal nerves

Answer: 1. Sensory ganglia of cranial nerves.