CVM 710 Lecture 16 Nervous System General Concepts

Central Nervous System

• Part of the nervous system that is protected by bony cavities (cranial cavity and vertebral canal) and three layers of membranes known as meninges

• Made up of encephalon (brain) and spinal cord (medulla spinalis)

How many pairs of cranial nerves in PNS of dog?

12 pairs

How many pairs of spinal nerves in PNS of dog?

36

Somas (neuron body) in CNS

• Grey matter

  • Nucleus

  • Cortex

Axons in CNS

• White matter

  • Tract, funiculo, fasciculus, lemniscus, peduncle

Fibers:

• Association

• Projection

• Commissural

Combination of somas and axons in CNS

Reticular formation

Somas (neuron body) in PNS

Ganglion

Axons in PNS

• Nerve

  • Root, trunk, branch

Combination of somas and axons in PNS?

No mixture

What does ectoderm develop into? (embryonic development of nerve system)

Neural tissue and epidermis

What does mesoderm develop into? (embryonic development of nerve system)

Develops into somites that differentiate into skeletal and muscle tissues, the notochord, blood vessels, dermis, and connective tissues

What does endoderm develop into? (embryonic development of nerve system)

Epithelium of the digestive and respiratory systems, organs associated with the digestive system (such as the liver and pancreas)

Where is the neural plate formed?

Ectoderm

What do the cells in the neural plate turn into?

Neural groove

What do the cells in the borders of the neural groove turn into?

Neural crests

What do the cells of the neural crests form?

Most of the peripheral nerve system, including ganglia and spinal nerves

What do the rest of the cells in the neural groove (aka not in the borders) do?

Close and form into neural tube with a neural canal in the middle

What does the neural tube form?

Central nervous system, including encephalon and spinal cord

Prosencephalon

Forebrain

Mesencephalon

Midbrain

Rombencephalon

Hindbrain

Pyramidal decussation

• Limit between spinal cord and brainstem

• Place where pyramidal fiber (upper motor neurons) cross over to the opposite end of the spinal cord

Medullary cone (conus medullaris)

Caudal end of spinal cord

Where is medullary cone in cats? (Where does spinal cord end in cats?)

Between S2 and S3 (reminder: 23 and me-ow)

Where is medullary cone in dogs? (Where does spinal cord end in dogs?)

Between L6 and L7

Intumescentiae

Segments that give origin to the nerves supplying the thoracic and pelvic limbs

Axons (white substance/matter) in spinal cord

• Surrounds somas (gray matter) in spinal cord (swapped in brain)

  • Dorsal funiculus

  • Lateral funiculus

  • Ventral funiculus

Funiculi

Composed of ascending and descending axon fibers, of which many are grouped within bundles (fasciculi or tracts) of common origin, destination, and function

What is the shape of the central canal

H-shaped or butterfly-shaped

Somas (body of neuron) (gray substance) in spinal cord

• Dorsal horn

• Lateral horn (thoracic and lumbar segments)

  • Ventral horn

Sensory/Afferent

From the body toward the CNS (dorsal horn)

Motor/Efferent

Toward a muscle or gland - away from CNS (ventral horn)

What direction do signals in the roots go?

Unidirectional

What directions do nerves and branches go?

Bidirectional

What type of neurons are afferent neurons?

Pseudounipolar

Ganglion

Groups of neuronal somas in the peripheral nervous system

Intervetebral foramen

Point where roots within spinal canal merge into spinal nerve (outside of canal)

How many pairs of cervical spinal nerves are in dogs?

8 pairs

How many pairs of thoracic spinal nerves are in dogs?

13 pairs

How many pairs of lumbar spinal nerves are in dogs?

7 pairs

How many pairs of sacral spinal nerves are in dogs?

3 pairs

How many pairs of caudal spinal nerves are in dogs?

5 pairs

Cervical (region of spinal cord)

C1-C5 (neck)

Cervical intumescence (region of spinal cord)

C6-T2 (thoracic limbs)

Thoracolumbar (region of spinal cord)

T3-L3 (trunk)

Lumbosacral intumescence (region of spinal cord)

L4-S3 (pelvic limbs and pelvic viscera)

Caudal segments (region of spinal cord)

Cd1-Cd5 (tail)

What does General stand for? (neurons)

Similar pathway for skeletal muscles in the body

What does Somatic stand for (neurons)

Body as general

What does Efferent stand for (neurons)

Motor impulse to a muscle that will stimulate the contraction of the muscle cells

GSE Pathway

Soma of motor neuron in ventral horn > Motor neuron in ventral root > Motor neuron in spinal nerve > Motor neuron in dorsal branch OR ventral branch of spinal nerve

Upper motor neuron

Confined to the central nervous system (brain and spinal cord), you may think of it as a “central motor neuron”

Lower motor neuron

Cell body (soma) in the CNS, but the axon is in the periphery, you may think of it as a “peripheral motor neuron”

What does Afferent stand for (neurons)

Transmit sensory impulse (proprioception, tactile, or nociception) toward central nervous system

Proprioception

• Length changes

• Receptors in tendons, ligaments, muscles, joints

Tactile and noniception

Receptors in skin, cornea, mucosa, hair/feather follicle, connective tissue

GSA pathway

Sensory neuron in dorsal branch OR ventral branch of spinal nerve > Spinal neuron in spinal nerve > Spinal ganglion outside of CNS (pseudounipolar) > Sensory neuron in dorsal root > Dorsal horn

Three parts of reflex:

• Sensory input

• Central connection

• Output

Damage to any one of these components will result in a reduction or loss of the reflex

Sensory input (part of reflex)

Peripheral receptor (afferent neuron)

Central connection (part of reflex)

In the brain or spinal cord (interneuron)

Output (part of reflex)

Lower (peripheral) motor/efferent neuron that connects to the muscle via a neuromuscular junction

Myotatic reflex arc

Gravity stretches the extensor muscle > stimulates its contraction via reflex arc > flexor muscle is inhibited by a collateral fiber and an inhibiting interneuron

Damage to Upper Motor Neurons

Reflexes will still be present, they may even be increased due to loss of inhibitory UMN influence

Damage to Lower Motor Neurons

Results in reduced/lost reflexes