Spinal Nerves SI - Hargroder PDF

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KIN 2500 – Human Anatomy Spinal Nerves SI Worksheet

SI: Matt Landry [email protected]

Gross Anatomy of the Spinal Cord  The spinal cord can be divided into five parts o Cervical o Thoracic o Lumbar o Sacral o Coccygeal  The tapering inferior end of the spinal cord is called the conus medullaris. This marks the official end of the spinal cord proper.  Inferior to the last feature is a group of axons called the cauda equina, which projects inferiorly from the spinal cord. o The nerve roots are so named because it resembles a horse’s tail.  Within the last feature is the filum terminale which is a thin strand of the pia mater which helps anchor the conus medullaris to the coccyx.  When viewed in a cross section, o A narrow grove, the posterior median sulcus, dips inferiorly on the posterior surface. o A slightly wider groove, the anterior medial fissure, is viewed on the anterior surface. o Notice the posterior side has a sulcus and the anterior side a fissure!  The cervical enlargement, located in the inferior cervical part of the spinal cord, contains the neurons that innervate the upper limbs.  The lumbosacral enlargement extends through the lumbar and sacral parts of the spinal cord and innervates the lower limbs.  Where does the thoracic enlargement innervate? There is no thoracic enlargement. The Spinal Cord and Spinal Nerves  The spinal cord is associated with 31 pairs of spinal nerves that connect the CNS to muscles, receptors, and glands.  Spinal nerves are considered mixed because they contain both motor and sensory axons.  List the number of spinal nerves in each section of the spinal cord. o Cervical (8) o Thoracic (12) o Lumbar (5) o Sacral (5) o Coccygeal (1)  Anteriorly, multiple anterior rootlets arise from the spinal cord and merge to form a single anterior root, which contains only motor axons.  Posteriorly, multiple posterior rootlets arise from the spinal cord and merge to form a single posterior root, which contains only sensory axons. o The cell bodies of these neurons are housed in a posterior root ganglion. (Only present on the posterior side.)  After leaving the intervertebral foramen, a typical spinal nerve splits into branches called rami. o The posterior rami are smaller out of the two main branches.  They innervate muscles of the deep back and the skin of the back o The anterior ramus is the larger of the two main branches  It splits into multiple branches which innervate the anterior lateral parts of the trunk, the upper limbs, and the lower limbs.  Many anterior rami go on to form plexus. Spinal Cord Meninges

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KIN 2500 – Human Anatomy Spinal Nerves SI Worksheet

SI: Matt Landry [email protected]

The spinal cord is protected and encapsulated by spinal cord meninges which are continuous with the cranial meninges.  The epidural space lies between the dura mater and the periosteum covering the inner walls of the vertebra.  Deep to the epidural space is the dura mater, the most external of the meninges which provides stability to the spial cord. o Although the cranial dura mater has an outer periosteal layer and an inner meningeal layer, the spinal dura mater only consists of an meningeal layer.  A narrow subdural space separates the dura mater from the arachnoid mater.  Deep to the arachnoid mater is the subarchanoid space, which is filled with cerebral spinal fluid (CSF).  The pia mater, which is deep to the subarachnoid space is a delicate innermost meningeal layer.  Denticulate ligaments are paired, lateral triangular extensions of the pia mater that attach to the dura mater. They help suspend and anchor the spinal cord laterally to the dura mater. Sectional Anatomy of the Spinal Cord  The gray matter of the spinal cord is dominated by the dendrites and cell bodies of neurons and glial cells and unmyelinated axons.  The white mater is composed of myelinated axons.  Gray Mater o Centrally located on the spinal cord and forms the letter H o It can be subdivided into anterior horns, lateral horns, posterior horns, and the gray commissure o The anterior horns primarily house cell bodies of somatic motor neurons (innervate skeletal muscle) o The lateral horns primarily house cell bodies of autonomic motor neurons (innverate glands, smooth muscle, and cardiac muscle) o The posterior horns primarily house the axons of sensory neurons and the cell bodies of interneurons. o The gray commissure is a horizontal bar that surrounds a narrow central canal.  It contains primarily unmyelinated axons and serves as a route of communication between the right and left gray mater.  White Mater o External to the gray mater. o Partitioned into three regions each called a funiculus  Posterior  Lateral  Anterior o The anterior funiculus is interconnected by the white commissure. Dermatomes  A dermatome is a specific segment of skin supplied by a single spinal nerve.  All spinal nerves except for C1 innervate a segment of skin Nerve Plexus  What is a nerve plexus? A network of interweaving anterior rami of spinal nerves. (The posterior rami are smaller and do not form plexuses.) Intercostal Nerves  Composed of the anterior rami of spinal nerves T1-T11  These nerves travel in the intercostal space between adjacent ribs. 

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KIN 2500 – Human Anatomy Spinal Nerves SI Worksheet

SI: Matt Landry [email protected]

Note about Spinal Nerves: I traditionally only cover the major branches of each plexus. Please refer to your in-class notes for details regarding minor branches of nerves from each plexus. You should be competent in being able to answer possible text questions, and more importantly being able to identify them in a figure.

Cervical Plexus  Branches of the cervical plexus innervate: anterior neck muscles as well as the skin of the neck and portions of the head and shoulders.  An important branch of the cervical plexus is the phrenic nerve, which is formed from the C4 nerve. o It travels through the thoracic cavity and innervates the diaphragm  Be certain to study the cadaver picture from the old textbook to be able to identify additional nerves associated with the cervical plexus. Brachial Plexus  The left and right brachial plexuses are networks of nerves that innervate the upper limbs.  The anterior rami of the brachial plexus are continuations of C5-T1. The five roots combine to form the three trunks. o Superior Trunk o Middle Trunk o Inferior Trunk  The three trunks form three cords o Posterior Cord o Medial Cord o Lateral Cord  Emerging from the three cords are five terminal branches (Also list the motor innervation of each terminal branch): o Axillary - deltoid and teres minor o Median – most anterior forearm muscles o Musculocutaneous – anterior arm muscles o Radial – posterior arm muscles, posterior forearm, brachioradialis o Ulnar – anterior forearm and intrinsic muscles of the hand  Brachial Plexus Drawing  Be certain to study the cadaver picture from the old textbook to be able to identify additional nerves associated with the brachial plexus Lumbar Plexus  The lumbar plexuses are formed from the anterior rami of spinal nerves L1-L4. It is broken into the anterior and posterior division.  The main nerve of the posterior division is the femoral nerve. o This nerve innervates the anterior thigh muscles  The main nerves of the anterior division is the obturator nerve. o This nerve innervates the medial thigh muscles. Sacral Plexus  The right and left sacral plexus are formed from the anterior rami of spinal nerves L4-S4.  The sciatic nerve is the largest and longest nerve in the body. o It is composed of two divisions: tibial division and the common fibular division  The tibial nerve is from the anterior division of the sciatic nerve. o It innervates the posterior thigh muscles, posterior leg, and plantar foot muscles.  The common fibular nerve is from posterior division of the sciatic nerve. o It innervates the short head of the biceps femoris.  The nerves emerging from a sacral plexus innervate the gluteal region, pelvis, perineum, posterior thigh, and almost all of the leg and foot.  The deep fibular nerve travels in the anterior compartment of the leg and terminates 3 between the first and second toes.

KIN 2500 – Human Anatomy Spinal Nerves SI Worksheet

SI: Matt Landry [email protected]

o It innervates the anterior leg and the dorsal foot Smaller nerves of the sacral plexus are the: inferior and superior gluteal nerve (butt), posterior femoral (abductors of the thigh), and pudendal nerve (groin)  Injury to the sciatic nerve produces a condition known as sciatica, which is characterized by extreme pain down the posterior thigh and leg. Reflexes  Rapid, automatic, involuntary reactions of muscles or glands to a stimulus. All reflexes have similar properties: o A stimulus is required to initiate a response to sensory input o A rapid response requires that few neurons o A preprogrammed response occurs the same way every time o An involuntary response requires no intent or pre-awareness of the reflex activity.  Example: When you accidentally touch a hot stove. Instantly and automatically, you remove your hand from the stimulus (the hot stove), even before you are completely aware that your hand was touching something hot. A reflex 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. Components of a Reflex Arc  A reflex arc always begins at a receptor in the peripheral nervous system, communicates with the central nervous system and ends at a peripheral effector, such as a muscle or a gland.  The general five steps involved with a simple reflex arc are: 1. Stimulus activates receptor (Sensory receptors respond to both external and internal stimuli, such as temperature, pressure, or tactile changes) 2. Nerve impulse travels through the sensory neuron to the CNS (Sensory neurons conduct impulses from the receptor into the spinal cord) 3. Information from nerve impulse is processed in the integration center by interneurons. (More complex reflexes may use a number of interneurons within the CNS to integrate and process incoming sensory information and transmit information to a motor neuron. Sensory information is also sent to the brain through interneuron collaterals. The simplest of neurons do not involve interneurons; rather, the sensory neuron synapses directly on a motor neuron in the anterior gray horn of the spinal cord.) 4. Motor neuron transmits nerve impulse to the effector. (An effector is the peripheral target organ that response to the impulse from the motor neuron. The motor neuron transmits a nerve impulse through the anterior root and spinal nerve to the peripheral effector organ. 5. Effector responds to the nerve impulse from motor neurons. (The effector response is intended to counteract or remove the original stimulus.) 

Clinical Correlations  Erb–Duchenne Palsy: Waiter’s Tip. Not able to extend the hands. Stems from injury from C5-C6  Radial Nerve Injury: Wrist Drop  Median Nerve Injury: Numbness in the palm of the hand  Median Nerve Injury: Carpal tunnel syndrome  Ulnar Nerve Injury: Claw Hand  Long Thoracic Nerve Injury: Winged Scapula

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