Veterinary Nervous System (Animal)

Nervous System  

Neuron = functional cell of Nervous System:

             — receives excitation (at a synapse or at a receptor);

             — conducts excitation (along an axon);

              transmits excitation (via release of chemical at a       

              synapse).

Most neurons are multipolar  — cell body is located where input

                                                     excitation occurs

Sensory neurons are unipolar — cell body is located along the

                                                     axon 

Definitions:

  Nerve = bundle of axons ensheathed by supporting cells and enveloped by connective tissue,.

  Root = nerve that is adjacent to the CNS and enveloped by meninges

Ganglion = localized site where a nerve is enlarged due to a collection of cell bodies:

        Spinal ganglia — contain unipolar cell bodies (located on

                                      dorsal roots of spinal nn.)

        Autonomic ganglia — contain multipolar cell bodies that

                                             innervate viscera.

Nervous System Divisions:

 Central (CNS): brain  and  spinal cord

  Peripheral (PNS): 12 pairs of cranial nerves (attached to brain);

36 pairs of spinal nerves in the dog & cat (attached to

spinal cord). [8 cervical; 13 thoracic; 7 lumbar; 3 sacral; & 5 caudal]

Spinal Nerve:

The spinal cord and spinal roots are located within the vertebral

canal of the vertebral column. Dorsal and ventral spinal roots unite toform a spinal nerve (bilaterally). Adjacent vertebrae combine to form an intervertebral foramen (dorsal to an intervertebral disc). The spinal nerve is found within theintervertebral foramen, from which it exits the vertebral canal.The spinal nerve is enveloped by connective tissue (epineurium,perineurium, & endoneurium). In contrast, the spinal cord and the dorsal and ventralspinal roots are surrounded by cerebrospinal fluid enclosed within meninges.

Spinal Nerve:  typical pattern

 — short (<1 cm); located at an intervertebral foramen

—   connected to the spinal cord by two roots (each comprised of

      rootlets):

dorsal root — composed of afferent (sensory) axons; the site of a

                        spinal ganglion

ventral root — composed of efferent axons that innervate muscle

                         & gland

                    — divides into four primary branches:

meningeal branch — small; sensory to meninges

ramus communicans — connects to sympathetic trunk & innervates viscera

ventral branch — largest branch; hypaxial mm. & lateral and ventral cutaneous nn.

dorsal branch — medial & lateral branches.; epaxial mm. & dorsal cutaneous nn.

Fiber types:  types of nerve fibers (axons) found in a spinal nerve and its branches

  • Afferent (sensory) — axons associated with receptors and unipolar cell bodies in spinal ganglia

General Somatic Afferent (GSA): receptors in skin & muscles, tendons, joints

General Visceral Afferent GVA): receptors in viscera

  • Efferent (motor) — axons that innervate muscle & gland;

 cell bodies are located in the spinal cord (or in some cases autonomic ganglia)

          Somatic Efferent (SE): innervates skeletal muscle

          Visceral Efferent (VE): innervates cardiac m., smooth m., &         

          gland

Cutaneous innervation:

Neck — series of dorsal and ventral cutaneous nn.

Thorax — series of  dorsal, lateral and ventral cutaneous nn.

Abdomen — series of dorsal and lateral cutaneous nn.

Limbs — individually named cutaneous branches of regional

                nerves thatoriginate from nerve plexuses (brachial or

                lumbosacral) to the limbs.

Face — named cutaneous branches of cranial nerves.

Brachial and Lumbosacral nerve plexuses:

Individual muscles are composed of multiple myotomes that overlap in forming the muscle.In the case of trunk muscles, which are generally broad, multiple dorsal or ventral branches of spinal

nerves can be seen to serially innervate each individual muscle. The innervations overlap within the muscle because of myotome overlap. In the case of limb muscles, each muscle is innervated by the branch of a single regional nerve. Because of multiple myotomes per muscle, the regional nerves must contain axons from ventral branches of multiple spinal nerves. The exchange of axons among ventral branches as they form regional nerves produces a nerve plexus for each limb.