Human Anatomy. Spinal Cord and Spinal Nerves

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1 Human Anatomy Spinal Cord and Spinal Nerves 1

2 The Spinal Cord Link between the brain and the body. Exhibits some functional independence from the brain. The spinal cord and spinal nerves serve two functions: 1. pathway for sensory and motor impulses 2. responsible for reflexes 16-2

3 Structure of the Spinal Cord Typical adult spinal cord ranges between 42 and 45 centimeters (cm) (16 to 18 inches) in length. In cross section roughly cylindrical External surface has two longitudinal depressions: the posterior (dorsal) median sulcus the anterior (ventral) median fissure 16-3

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5 31 Regions of the Spinal Cord The cervical region continuous with the medulla oblongata contains neurons whose axons form the cervical spinal nerves (8) The thoracic region attached to this region are the thoracic spinal nerves (12) The lumbar region contains the neurons for the lumbar spinal nerves (5) The sacral region contains the neurons for the sacral spinal nerves (5) The coccygeal region one pair of coccygeal spinal nerves arises from this region 16-5

6 31Regions of the Spinal Cord each is identified by a number and letter designation. For example C3 is the third cervical segment. 6

7 Structure of the Spinal Cord The spinal cord is shorter than the vertebral canal that houses it. Conus medullaris: tapered inferior end of the spinal cord marks the official end of the spinal cord proper. Cauda equina Inferior to conus medularis nerve roots (groups of axons) that project inferiorly from the spinal cord. Filum terminale Within the cauda equina thin strand of pia mater helps anchor the conus medullaris to the coccyx. 16-7

8 Inferior End of Spinal Cord

9 Structure of the Spinal Cord The spinal cord is associated with 31 pairs of spinal nerves Each side of the spinal cord contains: 8 cervical nerves (called C1 C8) 12 thoracic nerves (T1 T12) 5 lumbar nerves (L1 L5) 5 sacral nerves (S1 S5) 1 coccygeal nerve (Co1) 16-9

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11 Arrangement and Functions of the Spinal Meninges Are continuous with the cranial meninges. Structures that encircle the spinal cord, listed from superficial to deep are: vertebra epidural space dura mater subdural space arachnoid subarachnoid space pia mater 16-11

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14 Nucleus: A group of neuron cell bodies in the central nervous system Ganglia: A group of neuron cell bodies in the Peripheral nervous system Tract: A bundle of axons within the central nervous system that share a common function, origin and destination Column: a group of tracts found within a specific region of spinal tracts. Nerve: A bundle of axons within the Peripheral nervous system. 14

15 Location and Distribution of Gray Matter In the spinal cord, it is centrally located. Its shape resembles a letter H or a butterfly. Gray Matter is dominated by cell bodies of neurons and glial cells. The Gray Matter surrounds a central canal. The projections of the Gray Matter are called horns. The gray matter may be subdivided into the following components: 1. anterior horns 2. lateral horns 3. posterior horns 4. the gray commissure 16-15

16 The Gray Matter is the area where many types of neurons synapse. In dorsal horns (posterior horns) many types of incoming sensory neurons synapse with interneurons. Some interneurons use the gray commissure to cross to the other side of the spinal cord. In ventral horns (anterior horns) the descending upper level motor neuron (and other types of neurons ) synapse with outgoing motor neuron. In lateral horn descending autonomic pathway neurons synapse with outgoing sympathetic motor neuron

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18 Location and Distribution of White Matter The white matter of the spinal cord is external to the gray matter. Three regions (referred to as funiculi or columns) : Composed of Ascending and Descending tracts 1- Posterior funiculus/column: 2- Lateral funiculus/column: 3- Anterior f funiculus/column: 1- A posterior funiculus: lies between the posterior gray horns and the posterior median sulcus. 2- Lateral funiculus. lies between the posterior funiculi and the anterior funiculi 3- Anterior funiculus between the anterior gray horns and the anterior median fissure

19 The white commissure is located anterior to the gray commissure. A specific tract convey either sensory or motor information Axons in sensory tracts carry information up to spinal cord and brain Axons in motor tracts carry information from the brain down to spinal cord

20 Two pairs of nerve roots extend from each segment of spinal cord: The ventral root allows motor neurons to exit the spinal cord. The dorsal root allows sensory neurons to enter the spinal cord. Along each root is a small bulge called dorsal root ganglia that contains the cell body of sensory neuron The ventral root and dorsal root fuse to form the spinal nerve, thus called Mixed spinal nerve 16-20

21 Opening between the vertebrae called intervertebral foramine allow the spinal nerve to enter and exit the spinal cord

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23 Spinal Nerves 31 pairs connect the CNS to: receptors effectors (muscle and glands) Each spinal nerve is mixed: thousands of motor and sensory axons. Sensory axons originate from receptors Motor axons originate from the spinal cord. Spinal nerve is associated with the vertebra of the same number

24 Rami of Spinal Nerves Each spinal nerve forms though the fusion of Anterior root and posterior root as these roots pass through an intervertebral foramen. Then the spinal nerve divides into two branches a dorsal ramus and a ventral rami ramus. Posterior (or Dorsal) ramus Innervates muscles and skin of the neck and the back Anterior (or ventral rami ) Ramus Largest branch Forms plexuses Innervates anterior and lateral trunk, upper and lower limbs. For spinal nerves T1 to L2 there are 4 branches: a white ramus and gray ramus collectively known as Rami communicantes, Posterior and ramus Anterior ramus. Rami communicantes carry visceral motor fibers to and from a near by Autonomic ganglion associated with sympathetic division of Autonomic nervous system (sympathetic) 16-24

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28 Nerve Plexuses A network of interweaving anterior rami of spinal nerves. nerve plexuses on both the right and left sides of the body. Nerve plexuses then split into multiple named nerves that innervate various body structures. Principal plexuses 1. cervical plexuses 2. brachial plexuses 3. lumbar plexuses 4. sacral plexuses

29 4 Principal Plexuses 1-Cervical Plexus Cervical Plexus Consists of cutaneous and muscular branches in the ventral rami of spinal nerves C1-C4 and some of C5 Phrenic nerve - innervates diaphragm Lesions of one or more nerve roots result in neurologic defects (muscle weakness, abnormal sensation, changes in reflexes)

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31 Brachial Plexus Musculocutaneous nerve - innervates biceps and brachialis muscles Median nerve - Ulnar nerve - Radial nerve Axillary nerve study in lab!!

32 Brachial Plexus is formed from ventral rami of spinal nerves 32 C5 to T1

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34 1. lumbar plexuses 2. sacral plexuses Collectively called lumbosacral plexuses The ventral rami supply the pelvis and lower limbs. lumbar plexuses is formed from ventral rami of spinal nerves T12 tol4 The major nerves of lumbar plexuses are Genifemoral nerve Lateral femoral cutaneous nerve Femoral nerve. 34

35 :lumbar plexuses is formed from ventral rami of spinal nerves T12 tol4 lumbar plexuses 35

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37 Sacral plexuses: is formed from ventral rami of spinal nerves L4 to S4 Sciatic nerve pass to biceps femoris muscle Pudendal nerve 37

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43 Reflexes A reflex is a response: Rapid, automatic involuntary reactions of effectors to a stimulus. Properties. a stimulus required to initiate a response to sensory input a rapid response requires that few neurons be involved 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. Reflexes usually can not be suppressed. Awareness of the stimulus occurs after the reflex action in time to correct or avoid a potentially dangerous situation

44 Components of a Reflex Arc The neural wiring of a single reflex. Always begins at a receptor in the PNS Sensory afferent Communicates with the CNS. May involve interneurons Ends at a peripheral effector (muscle or gland) Motor efferent 16-44

45 Reflex arc is a pathway involving: receptor, sensory neuron, motor neuron, one or more association neurons, and an effecter (muscle or gland).

46 Ipsilateral and Contralateral Reflex Arcs Ipsilateral: both the receptor and effector organs of the reflex are on the same side of the spinal cord. Contralateral the sensory impulses from a receptor organ cross over through the spinal cord to activate effector organs in the opposite side 16-46

47 Monosynaptic Reflexes The simplest of all reflexes. No interneurons. The patellar (knee-jerk) reflex is a monosynaptic reflex physicians use to assess the functioning of the spinal cord. tap the patellar ligament with a reflex hammer muscle spindles in the quadriceps muscles are stretched. Produces a noticeable kick of the leg

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49 Polysynaptic Reflexes Have more complex neural pathways exhibit a number of synapses involve interneurons within the reflex arc. Has more components more prolonged delay between stimulus and response

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52 Stretch Reflexes Monosynaptic reflex that monitors and regulates skeletal muscle length. When a stimulus results in the stretching of a muscle, that muscle reflexively contracts. The patellar (knee-jerk) reflex is an example of a stretch reflex. The stimulus (the tap on the patellar tendon) initiates contraction of the quadriceps femoris muscle and extension of the knee joint

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54 Golgi Tendon Reflex Prevents skeletal muscles from tensing excessively. Golgi tendon organs are nerve endings located within tendons near a muscle tendon junction. activation of the Golgi tendon organ signal interneurons in the spinal cord, which in turn inhibit the actions of the motor neurons The associated muscle is allowed to relax, thus protecting the muscle and tendon from excessive tension damage

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57 Reflex Testing in a Clinical Setting Reflexes can be used to test specific muscle groups and specific spinal nerves or segments of the spinal cord. Consistently abnormal reflex response may indicate damage to the nervous system or muscles. A reflex response may be normal, hypoactive, or hyperactive

58 Spinal Cord Development The central nervous system forms from the embryonic neural tube. Cranial and spinal nerves form from neural crest cells that have split off from the developing neural tube. The cranial (superior) part of the neural tube expands and develops into the brain. The caudal (inferior) part of the neural tube forms the spinal cord

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