Chapter 16

1. a pathway for sensory and motor impulses 2. responsible for reflexes, which are the quickest reactions to a stimulus

The diameter of the spinal cord changes along its length because the amount of gray matter and white matter and the function of the cord vary in different regions. • The cervical enlargement is located in the inferior cervical part of the spinal cord and innervates the upper limbs. • The lumbosacral enlargement extends through the lumbar and sacral parts of the spinal cord and innervates the lower limbs. • The spinal cord is shorter than the vertebral canal that houses it. • The tapering inferior end of the spinal cord is called the conus medullaris and is the official "end" of the spinal cord proper (usually at the level of the first lumbar vertebra). • Inferior to the conus medullaris, groups of axons called the cauda equina project from the spinal cord. • Within the cauda equina is the filum terminale, which is a thin strand of pia mater that helps anchor the conus medullaris to the coccyx.

• The spinal cord is protected and encapsulated by spinal cord meninges, which are continuous with the cranial meninges. • Some of the spaces between some of the meninges have clinical significance.

• lies between the dura mater and periosteum covering the inner walls of the vertebra • houses areolar connective tissue, blood vessels, and adipose connective tissue

• most external of the meninges • fuses with the connective layers that surround the spinal nerves • Narrow subdural space separates dura mater from arachnoid; a potential space • Arachnoid mater is deep to the dura mater and the subdural space • Subarachnoid space is a real space filled with cerebral spinal fluid

• innermost meningeal layer that adheres directly to the spinal cord • delicate layer composed of elastic and collagen fibers and supports some of the blood vessels supplying the spinal cord • has paired, lateral triangular extensions called denticulate ligaments, which suspend and anchor the spinal cord laterally to the dura mater

The spinal cord is partitioned into an inner gray matter region and an outer white matter region: • Gray matter—dendrites and cell bodies of neurons, unmyelinated axons, and glial cells • White matter—myelinated axons

• Centrally located in spinal cord • Sectioned shape resembles butterfly Subdivided into: • Anterior Horns • Lateral Horns • Posterior Horns • Gray Commissure • Anterior horns house the cell bodies of somatic motor neurons, which innervate skeletal muscle • Lateral horns: • found in the T1-L2 parts of the spinal cord only • contain cell bodies of autonomic motor neurons, which innervate cardiac muscle, smooth muscle, and glands • The posterior horns contain axons of sensory neurons and cell bodies of interneurons. • The gray commissure contains unmyelinated axons and serves as a communication route between the right and left side. • The gray commissure houses a narrow central canal. • Within the gray matter are functional groups of neuron cell bodies called nuclei: - Sensory nuclei in the posterior horns contain interneuron cell bodies of: - somatic sensory nuclei - visceral sensory nuclei - Motor nuclei in the anterior horns contain somatic motor nuclei - Autonomic motor nuclei are in the lateral horns

The white matter of the spinal cord is external to the gray matter and is partitioned into three regions, each called a funiculus: • posterior funiculus • lateral funiculus • anterior funiculus - interconnected by the white commissure The axons within each funiculus are organized into tracts.

• 31 pairs • Made up of motor and sensory axons • Contain connective tissue wrappings called endoneurium, perineurium, and epineurium • Multiple anterior rootlets arise from the spinal cord and merge to form a single anterior root. • Anterior roots contain motor axons only. • The cell bodies of the motor axons arise from cell bodies in the anterior and lateral horns of the spinal cord. • Multiple posterior rootlets are derived from a single posterior root. • Posterior roots contain sensory axons only. • The cell bodies of the sensory axons arise from cell bodies in the posterior root ganglion, which is attached to the posterior root. • Each anterior root and its corresponding posterior root unite within the intervertebral foramen to become a spinal nerve. • A spinal nerve contains both motor and sensory axons. • Spinal nerves are numbered according to the location of the intervertebral canal. • In the cervical region the first seven pairs of spinal nerves (C1-C7) exit the intervertebral foramen above the vertebra of the same number. • The eighth pair of cervical spinal nerves (C8) exit above the first thoracic vertebra. • The remaining pairs of spinal nerves exit below the vertebra of the same number. • Because the spinal cord is shorter than the vertebral canal, the roots of the lumbar and sacral spinal nerves travel inferiorly to reach their respective intervertebral foramen. Spinal Nerve Distribution • After leaving the intervertebral foramen, a typical spinal nerve splits into branches termed rami. • The posterior ramus is the smaller of the two main branches and innervates the deep muscles of the back and the skin of the back. • The anterior ramus is the larger of the two main branches and innervates the anterior and lateral portions of the trunk and the upper and lower limbs. • The anterior ramus splits into multiple other branches. • Many of the anterior rami go on to form nerve plexuses. • Additional rami, the rami communicantes, extend between the spinal nerve and the sympathetic trunk ganglion.

• A dermatome is a specific segment of skin supplied by a single spinal nerve. • All spinal nerves except C1 innervate a segment of skin. • The dermatome map follows a segmental pattern along the body. • The dermatome map can be important because anesthesia (numbness) in one or more of the segments could indicate potential spinal nerve damage. • Dermatomes are also involved in referred visceral pain, where a pain in a dermatome may arise from an organ nowhere near the dermatome.

A nerve plexus is a network of interweaving anterior rami of spinal nerves. • The anterior rami of most spinal nerves form nerve plexuses on both sides of the body. • The plexuses split into multiple named nerves that innervate body structures. • The principle plexuses are the: cervical plexuses, brachial plexuses, lumbar plexuses, and sacral plexuses.

• The anterior rami of spinal nerves T1-T11 are called intercostal nerves because they travel in the intercostal spaces between adjacent ribs. • Spinal nerve T12 is called a subcostal nerve, because it arises below the ribs. • With the exception of spinal nerve T1, the intercostal nerves do not form plexuses. • T1 forms part of the brachial plexus. • T2 innervates the intercostal muscles of the second intercostal space and is sensory for the axilla and medial surface of the arm. • T3-T6 innervate the intercostal muscles and are sensory for the anterior chest wall. • T7-T12 innervate the intercostal muscles, the abdominal muscles, and the overlying skin.

• Formed by anterior rami of spinal nerves C1-C4 • Branches of the cervical plexus innervate anterior neck muscles and the skin of the neck and head and shoulders. • The phrenic nerve originated primarily from C4 and some contributing axons of C3 and C5. • travels through the thoracic cavity to innervate the diaphragm

• The left and right brachial plexuses are networks of nerves that supply the upper limbs. • Each plexus is formed by the anterior rami of spinal nerves C5-T1. • Each brachial plexus innervates the pectoral girdle and the entire upper limb of one side. • The anterior rami of C5-T1 form the roots of the brachial plexus • The roots unite to form the: superior trunk—nerves C5 and C6 middle trunk—nerve C7 inferior trunk—nerves C8 and T1 • Portions of each trunk divide into an anterior division and a posterior division • The anterior and posterior divisions converge to form three cords: posterior cord medial cord lateral cord Five major terminal branches emerge from the three cords: 1. axillary nerve 2. median nerve 3. musculocutaneous nerve 4. radial nerve 5. ulnar nerve

• The left and right lumbar plexuses are formed from the anterior rami of spinal nerves L1-L4. • The lumbar plexus is subdivided into an anterior division and a posterior division. • The main nerve of the posterior division is the femoral nerve. • The main nerve of the anterior division is the obturator nerve.

• The left and right sacral plexuses are formed from the anterior rami of spinal nerves L4-S4. • The lumbar and sacral plexuses are sometimes considered together as the lumbosacral plexus. • The anterior rami are organized into an anterior division and a posterior division. • The sciatic nerve is the largest and longest nerve in the sacral plexus and in the body. • The sciatic nerve is composed of two divisions wrapped in a common sheath: 1. the tibial division 2. the common fibular division. The main branches of the sciatic nerve are the: • tibial nerve • common fibular nerve • deep fibular nerve • superficial fibular nerve

Reflexes are rapid, automatic, involuntary reactions of muscles or glands to a stimulus. • A stimulus is required to initiate a response to sensory input. • A rapid response requires that 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. A reflex arc may be: • Ipsilateral—both the receptor and effector organs are on the same side • Contralateral—the sensory impulses from a receptor organ cross over through the spinal cord to activate effector organs on the opposite limb Reflexes may be: • Monosynaptic—sensory axons synapse directly on motor neurons, whose axons project to the effector • Polysynaptic—more complex pathways that exhibit a number of synapses involving interneurons within the reflex arc

• Withdrawal reflex—polysynaptic reflex arc • Painful stimulus causes transmission of sensory information to the spinal cord. • Interneurons receive the sensory information and stimulate the 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 stimulation. • Stretch reflex—monosynaptic reflex arc • Stretch in a muscle is monitored by a stretch receptor called the muscle spindle. • When a stimulus results in the stretching of a muscle, the muscle reflexively contracts. • Golgi tendon reflex: • Golgi tendon organs are nerve endings located within tendons near a muscle-tendon junction. • As a muscle contracts, force is exerted on its tendon, resulting in increased tension in the tendon and activation of the Golgi tendon organ. • Nerve impulses signal interneurons in the spinal cord, which in turn inhibit the actions of the motor neurons.