Anatomy Chapter 16 Spinal Cord and Spinal Nerves

Gray commissure

A horizontal bar of gray matter that surrounds a narrow central canal. It primarily contains unmyelinated axons and serves as a communication route between the right and left sides of the gray matter.

Nerve Plexus

A network of interweaving anterior rami of spinal nerves. The anterior rami of most spinal nerves form nerve plexuses on both the right and left sides of the body These nerve plexuses then split into multiple "named" nerves that innervate various body structures. The principal plexuses are the cervical plexuses, brachial plexuses, and lumbar plexuses, and sacral plexuses.

Ipsilateral

A reflex arch may be ipsilateral or contralateral. When both the receptor and effector organs of the flex are on the same side of the spinal cord. Ex. left arm contracts to pull your left hand away from a hot object.

Lumbar region

A shorter segment of the spinal cord Contains neurons for the lumbar spinal nerves

Structure of brachial plexus

Anterior rami (also called roots): The continuations of the anterior rami of the spinal nerves C5-T1. These rami emerge from the intervertebral foramina and travel through the neck. Trunks: The five roots unite to form the superior (C5, C6), middle (C7), and inferior trunks (C8 and T1) in the posterior triangle of the neck. Divisions: Portions of each trunk divide inferior to the clavicle into an anterior division and a posterior division, which primarily contains axons that innervate the anterior and posterior parts of the upper limb, respectively.

Reflexes

Are rapid, automatic, involuntary reactions of muscles or glands to a stimulus. They all similar properties: -A stimulus is required to initiate a response to sensory input. -A rapid response requires that a few neurons be involved and synaptic delay be minimal. -An automatic response occurs the same way every time. -An involuntary response requires no intent or pre-awareness of the reflex activity. It is a survival mechanism; it allows us to quickly respond to a stimulus that may be detrimental to our well-being without having to wait for the brain to process the information.

Radial Nerve

C5-T1 Terminal Branch of the brachial plexus Arises from the posterior cord of the brachial plexus. Travels along the posterior side of the arm and then along the radial side of the forearm. Innervates the posterior arm muscles (forearm extensors) and the posterior forearm muscles (extensors of the wrist and digits and the supinator of the forearm). Receives sensory information from the posterior arm and forearm surface and the dorsolateral side of the hand.

Median Nerve

C5-T1 Terminal Branch of the brachial plexus Formed from branches of the medial and lateral cords of the brachial plexus. Travels along the midline of the arm and forearm, and deep to the carpal tunnel in the wrist. Innervates most of the anterior forearm muscles, the thenar muscles, and the lateral two lumbricals. Receives sensory information from the palmar side of the lateral 3-1/2 fingers (thumb, index finger, and the lateral half of the ring finger) and from the dorsal tips if these same fingers.

Ulnar Nerve

C8-T1 Terminal Branch of the brachial plexus Arises from the medial cord of the brachial plexus and descends along the medial side of the arm. It wraps posterior to the medial epicondyle of the humerus and then runs along the ulnar side of the forearm. Innervates some of the anterior forearm muscles (the median region of the flexor digitorium profundus and all of the flexor carpi ulnaris). Innervates most of the intrinsic hand muscles, including the hypothenar muscles, the palmar and dorsal interossei, and the medial two lumbricals. Receives sensations from the skin of the dorsal and palmar aspects of the medial 1-1/2 fingers (the little finger and the medial half of the ring finger).

Gray matter

Centrally located Resembles a letter H or a butterfly Subdivided into anterior horns, lateral horn, posterior horns, and they gray commissure.

Patellar (knee-jerk) Reflex

Clinically important reflexes. Spinal nerve segments tested: L2-L4 Normal Action of Effector: Extends knee when patellar ligament is tapped.

Plantar Reflex

Clinically important reflexes. Spinal nerve segments tested: L5, S1 Normal Action of Effector: Plantar flexes foot; flexes toes when plantar side of foot is briskly stroked.

Anterior funiculus

Composed of tracts of white matter that occupy the space on each anterior side of the cord between the anterior gray horns and the anterior median fissure; the anterior funiculi are interconnected by the white commissure.

Spinal nerves

Connect the CNS to muscles, glands, and receptors. Contains three types of successive CT wrappings: endoneurium, perineurium, and epineurium. Motor axons in a spinal nerve originate from the spinalcord. Contains both motor axons (from the anterior root) and sensory axons (from the posterior root).

Lumbosacral enlargement

Extends through the lumbar and sacral parts of the spinal cord and innervates the lower limbs

Lumbar Plexuses

Formed from the anterior rami of spinal nerves L1-L4 located to the L1-L4 vertebrae and along the psoas major muscle in the posterior abdominal wall. It is subdivided into an anterior division and a posterior division.

Sacral Plexuses

Formed from the anterior rami of spinal nerves L4-S4 and are located immediately inferior to the lumbar plexuses. The lumbar and sacral plexuses are sometimes considered together as the lumbosacral plexus. The nerves emerging from a sacral plexus innervate the gluteal region, pelvis, perineum, posterior thigh, and almost all of the leg and foot. The anterior rami organize themselves into an anterior division and a posterior division. The nerves arising from the anterior division tend to innervate muscles that flex (or plantar flex) parts of the lower limb, while the posterior division nerves tend to innervate muscles that extend (or dorsiflex) part of the lower limb.

Lateral horns

Found in the T1-L2 parts of the spinal cord Contain cell bodies of autonomic motor neurons, which innervate cardiac muscle, smooth muscle, and glands.

Motor nuclei

In the anterior and lateral horns contain motor neuron cell bodies that send nerve impulses to muscles and glands.

Somatic motor nuclei

In the anterior horns innervate skeletal muscle

Autonomic motor nuclei

In the lateral horns innervate smooth muscle, cardiac muscle, and glands.

Cauda equina

Inferior to conus medullaris a group of axons project inferiorly from the spinal cord

Femoral Nerve

L2-L4 Main nerve of the posterior division of the lumbar plexus. Supplies the anterior thigh muscles, such as the quadriceps femoris (knee extensor) and the the sartorius and iliopsoas (hip flexors). Receives sensory information from the anterior and inferomedial thigh as well as the medial aspect of the leg.

Deep Fibular Nerve

L4-S1 Nerve of the sacral plexus Travels in the anterior compartment of the leg and terminates between the first and second toes. Supplies the anterior leg muscles (which dorsiflex the foot and extend the toes) and the muscles on the dorsum of the foot (which extend the toes). This nerve receives sensory innervation from the skin between the first and second toes on the dorsum of the foot.

Common Fibular Nerve

L4-S2 Nerve of the sacral plexus Formed from the posterior division of the sciatic nerve As the common fibular division of the sciatic nerve, it supplies the short head of the biceps femoris muscle. Along the lateral knee, as it wraps around the neck of the fibula, this nerve splits into two main branches: The deep fibular nerve and the superficial fibular nerve.

Tibial Nerve

L4-S3 Nerve of the sacral plexus Formed from the anterior divisions of the sciatic nerve. In the posterior thigh, the tibial division of the sciatic nerve innervates the hamstrings (except the short head of the biceps femoris) and the hamstring part of the adductor magnus. Travels in the posterior compartment of the leg, where it supplies the plantar flexors of the foot and the toe flexors. In the foot, the tibial nerve splits into the lateral and medial plantar nerves, which innervate the plantar muscles of the foot and conduct sensory impulses from the skin covering the sole of the foot.

Sciatic Nerve

L4-S3 Nerve of the sacral plexus. Largest and longest nerve in the body Projects from the pelvis through the greater sciatic notch of the os coxae and extends into the posterior region of the thigh. Composed of two divisions wrapped in a common sheath; the tibial division and the common fibular division. Just superior to the popliteal fossa, the two divisions of the sciatic nerve split into two nerves: the tibial nerve and the common fibular nerve.

Superficial Fibular Nerve

L5-S2 Nerve of the sacral plexus Travels in the lateral compartment of the leg Just proximal to the ankle, this nerve becomes superficial along the anterior part of the ankle and dorsum of the foot. The superficial fibular nerve innervates the lateral compartment muscles of the leg (foot evertors and weak plantar flexors). Conducts sensory information from most of the dorsal surface of the foot and the anterioinferior part of the leg.

Anterior horns

Left and right anterior masses Primarily houses the cell bodies of somatic motor neurons, which innervate skeletal muscle.

Posterior horns

Left and right posterior masses of gray matter. The axons of sensory neurons and the cell bodies of interneurons are located in this region.

Posterior funiculus

Lies between the posterior gray horns on the posterior side of the cord and the posterior median sulcus.

Thoracic region

Lies inferior to the cervical part Contains the neurons for the thoracic spinal nerves

Cervical Plexuses

Located deep on each side of the neck, immediately lateral to cervical vertebrae. C1-C4. They are formed primarily by the anterior rami of spinal nerves C1-C4. The fifth cervical spinal nerve is not considered part of the cervical plexus, although it contributes some axons to one of the plexus branches. Branches of the cervical plexuses innervate anterior neck muscles as well as the skin of the neck and portions of the head and shoulders.

Cervical enlargement

Located in the inferior cervical part of the spinal cord Contains neurons that innervate the upper limbs

Sensory nuclei

Located in the posterior horns within the nuclei, contains interneuron cell bodies

Filum terminale

Located within the cauda equina A thin strand of pia mater that helps anchor the conus medullaris to the coccyx

Brachial Plexuses

Networks of nerves that supply the upper limb. Each is formed by the anterior rami of spinal nerves C5-T1. The component of the brachial plexus extend laterally from the neck, pass superior to the first rib, and then continue into the axilla. Each innervates the pectoral girdle and the entire upper limb of one side.

Phrenic Nerve

One important branch of the cervical plexus is the phrenic nerve, which is formed primarily from the C4 nerve and some contributing axons from C3 and C5. Travels through the thoracic cavity to innervate the diaphragm.

Denticulate Ligaments

Paired, lateral triangular extensions of the spinal pia mater that attach to the dura mater. They help suspend and anchor the spinal cord laterally to the dura mater.

Spinal Cord Meninges

Protects and encapsulates the spinal cord Continuous with the cranial meninges Spaces between some of the meninges have clinical significance. Outermost to innermost: vertebra, epidural space, dura mater, subdural space, arachoid mater, subarachnoid space, and pia mater.

Somatic sensory nuclei

Receive information from sensory receptors, such as pain or pressure receptors in the skin.

Visceral sensory nuclei

Receives information from sensory receptors such as the stretch receptors in the smooth muscle walls of viscera.

Stretch Reflex

Spinal Reflex A monosynaptic reflex that monitors and regulates skeletal muscle length. Stretch in a muscle is monitored by stretch receptors called muscle spindles. When a stimulus results in the stretching of a muscle, that muscle reflexively contracts. The patellar reflex is an example. The stimulus stretches the quadraceps femoris muscle and initiates contraction of the muscle, thereby extending the knee joint.

Withdrawal (Flexor) Reflex

Spinal Reflex A polysynaptic reflex arc that is initiated by a painful stimulus, such as touching something very hot. Stimulation of a receptor organ causes the transmission of sensory information to the spinal cord. Interneurons receive the sensory information and stimulate motor neurons to direct flexor muscles to contract in response. Simultaneously, antagonistic (extensor) muscles are inhibited so that the traumatized body part may be quickly withdrawn from the harmful situation.

Spinal nerves

Spinal nerves are called mixed nerves because they contain both motor and sensory axons. There are 31 pairs of spinal nerves that connect to the CNS to muscles, receptors, and glands. Each side has: 8 cervical nerves (C1-C8) 12 thoracic nerves (T1-T12) 5 lumbar nerves (L1-L5) 5 sacral nerves (S1-S5) 1 coccygeal nerve (Co1)

Cervical region

Supermost region of spinal cord Continuous with the medulla oblongata Contains motor neurons whose axons contribute to the cervical spinal nerves and receives input from sensory neurons through these spinal nerves.

Anatomy of spinal cord

The spinal cord is partitioned into an inner gray matter region and an outer white matter region. The gray matter is dominated by dendrites and cell bodies of neurons and glial cells and unmyelinated axons, whereas the white matter is composed primarily of myelinated axons.

Conus Medullaris

The tapering inferior end of the spinal cord Marks the official "end" of the spinal cord proper (usually at the level of the first lumbar vertebrae)

Funiculus

The white matter of the spinal cord is external to the gray matter. White matter on each side of the cord is partitioned into three regions, called funiculus.

Lateral funiculus

The white matter region on each lateral side of the spinal cord is the lateral funiculus.

Nuclei

Within regions of gray matter are various functional groups of neuron cell bodies.

Posterior median sulcus

located on the exterior surface of the spinal cord Dips internally on the posterior surface

Anterior median fissure

located on the exterior surface of the spinal cord A slightly wider groove located on the anterior surface