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Questions and Answers

How many thoracic spinal segments are present in a horse?

18 (correct)

Which animal has the least number of total spinal segments?

Dog

Human (correct)

Cat

What is the arterial supply that is consistently mentioned for the spinal cord?

Dorsal lumbar artery (DLA)

Ventral spinal artery (VSA) (correct)

Thoracic artery (TA)

Caudal artery (CA)

In terms of lumbar spinal segments, how do humans compare to dogs and cats?

Humans have fewer lumbar segments than dogs and cats Signup and view all the answers

How many cervical spinal segments do all listed species have in common?

8 Signup and view all the answers

Which spinal cord region is responsible for innervation of the pelvic limb muscles and skin?

Lumbosacral region Signup and view all the answers

What is the role of the sympathetic innervation found in the cervicothoracic region?

Sympathetic innervation to the head and eye Signup and view all the answers

In which spinal cord region would you find preganglionic sympathetic neurons?

Thoracolumbar region Signup and view all the answers

Which of the following regions supplies the external anal sphincter?

Sacral region Signup and view all the answers

Where does the spinal cord taper in dogs?

L6-L7 Signup and view all the answers

What is the cauda equina?

A bundle of spinal nerves within the spinal canal Signup and view all the answers

What is the clinical significance of the conus medullaris?

It marks the end of the spinal cord to avoid injury during procedures. Signup and view all the answers

What is the primary function of the cranial cervical region in the spinal cord?

Innervation to axial muscles and skin of the cranial cervical region Signup and view all the answers

What structure is found at the terminal end of the spinal cord in dogs?

Conus medullaris Signup and view all the answers

How many pairs of nerve roots are associated with the dog's spinal cord?

36 pairs Signup and view all the answers

Which spinal cord region is involved with innervation to the thoracic limbs?

Thoracolumbar region Signup and view all the answers

What term describes the collection of nerves that is located caudal to the conus medullaris?

Cauda equina Signup and view all the answers

At which lumbar vertebrae level does the spinal cord typically end in dogs?

L6-L7 Signup and view all the answers

What is NOT true about the spinal cord segments?

Nerve roots mirror the vertebrae levels exactly Signup and view all the answers

Which region of the spinal cord corresponds to the innervation of the pelvic limbs?

Lumbar region Signup and view all the answers

What determines the specific segments of the spinal cord affected by a lesion?

Clinical signs observed Signup and view all the answers

Which area does the cervicothoracic intumescence pertain to?

Innervation of thoracic body wall and limbs Signup and view all the answers

Study Notes

Spinal Cord Anatomy

The spinal cord extends from the foramen magnum to the level of the 6th/7th lumbar vertebrae in dogs.
Spinal cord segments are determined by the attachment points of the dorsal and ventral roots of spinal nerves.
There are 36 pairs of nerve roots, one for each vertebra plus an additional pair (C1).
Nerve root pairs do not always correspond exactly to vertebral levels.
As you move caudally in the vertebral canal, spinal nerves extend further caudally to pass through their assigned intervertebral foramina.
The filum terminale is a thin cord of meninges.
The conus medullaris is the terminal tapering of the cord (sacral and caudal segments).
The level of the conus medullaris varies among species.
Nerves caudal to the conus medullaris are collectively called the cauda equina.

Spinal Cord Regions

The spinal cord has six regional divisions based on the structures supplied:
- Cranial cervical (C1-C5): Innervation to axial muscles and skin of the cranial cervical region.
- Cervicothoracic (C6-T2): Cervical intumescence (enlargement) supplies the brachial plexus, hence innervation of the thoracic limb muscles and skin. Innervates regional axial muscles and skin, and sympathetic innervation of the head/eye (T1-T2).
- Thoracolumbar (T3-L3): Innervation of regional muscles and skin. Contains preganglionic sympathetic neurons ("thoracolumbar").
- Lumbosacral (L4-S2): Lumbar intumescence (enlargement) supplies the lumbosacral plexus, hence innervation to the skin and muscles of the pelvic limb. Innervations of regional axial muscles and skin, and partial supply to the bladder and perineum.
- Sacral (S1-S3): Innervation to perineal muscles and skin, including the external anal sphincter and urethral sphincter (pudendal nerve). Parasympathetic innervation to the viscera of the pelvis and urinary bladder (pelvic nerve).
- Caudal (Cd1-5): Innervation of muscles and skin of the tail.

Spinal Cord Segments (Different Species)

Dog: 8 cervical, 13 thoracic, 7 lumbar, 3 sacral, 5 caudal = 36 total
Cat: 8 cervical, 13 thoracic, 7 lumbar, 3 sacral, 5 caudal = 36 total
Horse: 8 cervical, 18 thoracic, 6 lumbar, 5 sacral, 5 caudal = 42 total
Ox: 8 cervical, 13 thoracic, 6 lumbar, 5 sacral, 5 caudal = 37 total
Human: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 caudal = 31 total

Spinal Cord Histology

White matter: Contains myelinated axons (nerves)
Gray matter: Contains cell bodies of neurons, dendrites, and unmyelinated axons.
Dorsal horn: Contains sensory neurons. Receives sensory information.
Ventral horn: Contains motor neurons. Sends motor commands to muscles and glands.
Lateral horn: Contains pre-ganglionic sympathetic neurons. Found in T1-L4 spinal cord.
Central canal: A narrow channel running through the center of the spinal cord, filled with cerebrospinal fluid (CSF).
Dorsal and ventral rootlets: Form the dorsal and ventral roots, which carry sensory and motor signals, respectively.
Dorsal and ventral median fissure/sulcus: grooves separating the left and right halves of the spinal cord
Dorsal and ventral funiculi: Regions of white matter containing ascending and descending tracts.

Spinal Cord Tracts

Ascending tracts (sensory): Carry sensory information from the spinal cord to the brain. Generally located in the dorsal and lateral funiculi.
- Fasciculus gracilis
- Fasciculus cuneatus
- Dorsal spinocerebellar
- Ventral spinocerebellar
- Spinothalamic tracts
Descending tracts (motor): Carry motor commands from the brain to the spinal cord. Generally located in the ventral and lateral funiculi.
- Reticulospinal
- Corticospinal
- Rubrospinal
Vestibulospinal
Tectospinal

Spinal Cord Arterial & Venous Supply

Longitudinal ventral spinal artery (VSA) and one or two dorsal spinal arteries (DSA).
The arrangement of veins supporting the blood supply of the spinal cord closely mirrors the arterial system. Paired sets of ventral and dorsal radicular veins, and large collecting vessels.

Spinal Cord Function

Sensory (afferent): Receives input from all tissues except the head.
Motor (efferent): Sends commands to skeletal muscles, smooth muscles, cardiac muscles, organs and glands.
Reflexive control: Unconscious reaction of a motor unit cause by sensory input from the environment. Usually involves an interneuron.

Branches of Spinal Nerves

Dorsal branch: Epaxial sensory and motor.
Ventral branch: Hypaxial sensory and motor.
Ramus communicans: Connects to the sympathetic trunk (visceral efferent and afferent).

Spinal Cord Lesion Localization

Spinal cord injury is often caused by compression.
Neuroanatomic diagnosis involves complete physical and neurological exams to identify the area of lesion. Imaging and other diagnostic tests are often useful.