Lesson 1 Chapter 14 – The Spinal Cord and Spinal Nerves

Lesson 1 Chapter 14 – The Spinal Cord and Spinal Nerves Unit 6 Study Guide 1 Excluded: Nerve plexuses – pages # 376 - 385 Spinal Cord is part of the Central Nervous System (CNS); which consists of the spinal cord and the brain. Although most info is relayed to the brain, the spinal cord integrates and processes data on its own. Spinal cord can be divided into 31 segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. - Extends from the foramen magnum of the skull to the inferior border of the first lumbar vertebra (L1). The adult spinal cord has a posterior median sulcus (shallow) and an anterior median fissure (wide). The amount of gray matter is increased substantially in segments concerned with both sensory and motor innervation functions. They contain interneurons responsible for the control of complex muscles of the limbs. These areas are known as enlargements (cervical and lumbar): - Cervical enlargement supplies nerves to the pectoral girdle and upper limbs Lumbosacral enlargement provides innervation to structures of the pelvis and lower limbs The conus medullaris is the pointed end of the spinal cord and is located at L1 - L2 vertebral level of the vertebral column. The cauda equina (horse’s tail) is made up of the dorsal and ventral rami of spinal nerves and the filum terminale. The spinal nerves are traveling toward the sacral foraminae. The filum terminale is a continuation of the pia mater from the terminal part of the spinal cord. It fuses with sacral vertebrae. Every spinal segment is associated with a pair of dorsal root ganglia that contain the cell bodies of sensory neurons. These sensory ganglia lie between the pedicles of adjacent vertebrae. On the other side of the spinal cord, a typical dorsal root contains axons of the sensory neurons in the dorsal root ganglion. Anteriorly Lesson 1 Chapter 14 – The Spinal Cord and Spinal Nerves of the dorsal root is a ventral root leaving the spinal cord. The ventral root contains the axons of somatic motor neurons and, at some levels, visceral motor neurons that control peripheral effectors. The dorsal and ventral roots of each segment enter and leave the vertebral canal between adjacent vertebrae at the intervertebral foramina. The dorsal roots are usually thicker than the ventral roots. Because the spinal cord terminates at L1 - L2, the subarachnoid space can be entered here and not damage the spinal cord. This is where a spinal tap is performed. A needle is pushed into this area and the CSF (cerebrospinal fluid) can be removed for examination. The pressure of the CSF can also be tested here. Trauma can produce bleeding into the above mentioned spaces and may be dangerous. An extradural hemorrhage is one that fills the space around the dura and can be reached relatively easily. A subdural hemorrhage is one in which blood forces itself between the dura and arachnoid. A subarachnoid hemorrhage is one found within the subarachnoid space and tints the cerebrospinal fluid. To check for this, a spinal tap can be performed and the fluid examined for red blood cells.          spinal cord (1) arachnoid (2) dorsal rootlets of spinal nerve (4) spinal nerve (5) posterior surface of body of vertebra (6) conus medullaris (7) cauda equina (8) filum terminale (9) subarachnoid space (10) 2 Lesson 1 Chapter 14 – The Spinal Cord and Spinal Nerves 3 Spinal Meninges They provide protection, physical stability, and shock absorption. They cover the spinal cord and surround the spinal nerve roots. [At the foramen magnum, the spinal meninges are continuous with the cranial meninges that surround the brain; cranial meninges have the same three layers] There are three meningeal layers: o Dura mater o Arachnoid mater o Pia mater subarachnoid space space between arachnoid and pia mater subdural space space between dura mater and arachnoid layers Dura mater [simple squamous epithelium] The outermost, toughest, and most fibrous of the three membranes (meninges) covering the brain and spinal cord; caudally it forms the coccygeal ligament woth the filum terminale. The outer epithelium is not bound to the bony walls of the vertebral canal, and the intervening epidural space contains areolar tissue, blood vessels, and adipose tissue. Arachnoid mater [simple squamous epithelium] Internal to the inner surface of the dura mater is the subdural space. When present it separates the dura mater from the middle meningeal layer, the arachnoid mater. Internal to the arachnoid mater is the subarachnoid space, which has networks of collagen and elastic fibers, the arachnoid trabeculae. This space also contains cerebrospinal fluid (CSF) which acts as a shock absorber and diffusion for gases, nutrients, chemical messengers, and waste products. Pia mater The pia mater is the innermost meningeal layer. It is bound firmly to the underlying neural tissue. Paired denticulate ligaments are supporting fibers extending laterally from the spinal cord surface, binding the spinal pia mater and arachnoid mater to the dura mater to prevent either side-to-side or inferior movement of the spinal cord. Conforms to brain’s bulges and fissures. Lesson 1 Chapter 14 – The Spinal Cord and Spinal Nerves 4 Sectional Anatomy of the Spinal Cord The central gray matter surrounds the central canal and contains cell bodies of neurons and glial cells. The gray matter projections toward the outer surface of the spinal cord are called horns. The peripheral white matter contains myelinated and unmyelinated axons in tracts and columns. > Organization of Gray Matter Shaped like the letter H. Neuron cell bodies in the spinal cord gray matter are organized into groups, termed nuclei. The posterior gray horns (dorsal) contain somatic and visceral sensory control. The anterior gray horns (ventral) are involved with somatic motor control. The lateral gray horns contain visceral motor neurons. The gray commissures are posterior and anterior to the central canal. They contain the axons of interneurons that cross from one side of the cord to the other. > Organization of White Matter The white matter can be divided into six columns (funiculi), each of which contains tracts (fasciculi). 2 grooves divide the spinal cord into right and left sides: Anterior median fissure, posterior median sulcus. Ascending tracts relay information from the spinal cord to the brain. (Carry sensory info from peripheral body to the brain.) Descending tracts carry information from the brain to the spinal cord. (Transmit motor information from brain/brainstem to muscles of glands) - Posterior white columns are sandwiched between the posterior gray horns and the posterior median sulcus. Anterior white columns lie between the anterior gray horns and the anterior median fissure; they are interconnected by the anterior white commissure. The white matter on either side between the anterior and posterior columns represents the lateral white columns. Lesson 1 Chapter 14 – The Spinal Cord and Spinal Nerves Spinal Nerves As mentioned above, there are 31 pairs of spinal nerves; each is identified through its association with an adjacent vertebra (cervical, thoracic, lumbar, and sacral.) Each spinal nerve is ensheathed by a series of connective tissue layers. - The outermost layer, the epineurium, is a dense network of collagen fibers The middle layer, the perineurium, partitions the nerve into a series of bundles (fasicicles) and forms the blood-nerve barrier The inner layer, endoeurium, is composed of delicate connective tissue fibers that surround individual axons. Peripheral Distribution of Spinal Nerves The first branch of each spinal nerve in the thoracic and upper lumbar regions is the white ramus, which contains myelinated axons going to an autonomic ganglion. Two groups of unmyelinated fibers exit this ganglion: a gray ramus, carrying axons that innervate glands and smooth muscles in the body wall or limbs back to the spinal nerve, and an autonomic nerve 5 Lesson 1 Chapter 14 – The Spinal Cord and Spinal Nerves 6 carrying fibers to internal organs. Collectively, the white and gray rami are termed the rami communicantes. Each spinal nerve has both a dorsal ramus (provides sensory/motor innervation of the skin and muscles of the back) and a ventral ramus (supplies ventrolateral body surface, body wall structures, and limbs). Each pair of spinal nerves monitors a region of the body surface, an area called a dermatome. Pathways of the Nervous System The CNS communicates with body structures via pathways. Used for sensory/motor info and processing/integration. - - - - They travel through the white matter of the spinal cord. o It consists of a tract and a nucleus o Tracts are groups or bundles of axons o Each tract may work with multiple nuclei groups in the CNS o A nucleus is a collection of neuron cell bodies located within the CNS These tracts function as o A highway for sensory & motor information o Sensory tracts ascend o Motor tracts descend Naming of tracts o Indicate the position & direction of the signal o Eg: anterior spinothalamic tract (impulses travel from spinal cord towards thalamus, found in anterior part of spinal cord) All pathways are composed of paired tracts. A pathway on the left side of the CNS has a matching tract on the right side of the CNS. Pathways are composed of a series of two or three neurons that work together. Sensory pathways: have primary neurons, secondary neurons, and tertiary neurons. Motor pathways: upper motor neuron and a lower motor neuron. Lesson 1 Chapter 14 – The Spinal Cord and Spinal Nerves Mixed nerves = both sensory and motor axons. Nerve Plexus A complex interwoven network of nerves is called a nerve plexus. The four major plexuses are the cervical plexus, the brachial plexus, the lumbar plexus, and the sacral plexus. Reflexes A neural reflex is a rapid, automatic, involuntary motor response to stimuli. Reflexes help preserve hemostasis by rapidly adjusting the functions of organs or organ systems. 7 Lesson 1 Chapter 14 – The Spinal Cord and Spinal Nerves 8 A reflex arc is the neural “wiring” of a single reflex. A receptor is a specialized cell that monitors conditions in the body or external environment. Each receptor has a characteristic range of sensitivity. There are five steps involved in a neural reflex: 1. 2. 3. 4. 5. Arrival of a stimulus and activation of a receptor Relay of information to the CNS Information processing Activation of motor neuron Response by a peripheral effector Classification of Reflex Reflexes are classified by their: 1. 2. 3. 4. Their development (innate, acquired) Where info is processed (spinal, cranial) Motor response (somatic, visceral [autonomic]) Complexity of the neural circuit [monosynaptic, polysynaptic] Innate reflexes are genetically determined. Acquired reflexes are learned following repeated exposure to a stimulus. Cranial reflexes are processes in the brain. In a spinal reflex the important interconnections Somatic reflexes Visceral reflexes and processing occur inside the spinal cord. control skeletal muscle contractions. (autonomic) control the activities of smooth and cardiac muscles and glands A monosynaptic reflex is the simplest reflex art. A sensory neuron synapses directly on a motor neuron that acts as the processing center. Polysynaptic reflexes have at least one interneuron placed between the sensory afferent and the motor efferent. Thus they have a longer delay between stimulus and response.