16_Spinal Cord and Nerves and ANS.pptx

Spinal Cord and Nerves Chapter 12, Human Anatomy (LibreTexts) "Figure showing the base of the brain, Thomas Geminus Wellcome" by Wellcome Collection gallery is licensed under CC BY 4.0 Functions and Protection of the Spinal Cord Functions: • Carries sensory information to the brain (ascending) an...

Spinal Cord and Nerves Chapter 12, Human Anatomy (LibreTexts) "Figure showing the base of the brain, Thomas Geminus Wellcome" by Wellcome Collection gallery is licensed under CC BY 4.0 Functions and Protection of the Spinal Cord Functions: • Carries sensory information to the brain (ascending) and carries motor information to the muscles and glands (descending). • Responsible for reflexes. Protection: • Spinal cord is housed within the vertebral column and covered by spinal meninges that contain also adipose tissue for padding. Spinal Meninges • Meninges cover the spinal cord for protection. From outermost to innermost: dura mater, arachnoid mater and pia mater. • The spinal dura mater consists of a single layer of connective tissue, differing from the cranial dura mater. • The spinal epidural space between the bone and dura mater is a real space around the spinal cord and houses areolar and adipose connective tissues, and blood vessels. Epidural anesthesia is injected in this space through a catheter anywhere along the spinal cord. • Subarachnoid space between arachnoid mater and pia mater is also a real space filled with CSF and is the site of a lumbar puncture (spinal tap) to collect CSF, usually done at the lumbar level. Gross Anatomy of the Spinal Cord Regions • Divided into regions that correspond to the regions of the vertebral column where the nerves of each region are exiting from: • • • • Cervical region Thoracic region Lumbar region Sacral region ”Spinal Cord" by Chiara Mazzasette is a derivative from the original work of Daniel Donnelly and is licensed by CC BY 4.0 Gross Anatomy of the Spinal Cord Ending • The spinal cord ends at the beginning of the lumbar vertebrae, forming a tapered structure known as the conus medullaris, which corresponds to the sacral spinal cord. The spinal cord is shorter than the vertebral canal that houses it. • The long bundle of nerves inferior to the conus medullaris is called cauda equina. • A thin strand of pia mater called filum terminale that helps anchoring the conus medullaris to the coccyx. ”Spinal Cord" by Chiara Mazzasette is a derivative from the original work of Daniel Donnelly and is licensed by CC BY 4.0 Cross section of Spinal Cord External • Anterior and posterior external surface of the spinal cord has two longitudinal depressions. The anterior midline is marked by the anterior median fissure, and the posterior midline is marked by the posterior median sulcus. • In the central region of the spinal cord, a central canal is the continuation of the fourth ventricle of the brain and contains cerebrospinal fluid (CSF). "Spinal Cord Cross Section" by OpenStax is licensed under CC BY 4.0/Micrograph provided by the Regents of University of Michigan Medical School © 2012) Cross section of Spinal Cord Commissures • The spinal cord is partitioned into gray and white matter. Contrary to the brain, the white matter is external and gray matter is internal. • Surrounding the central canal, a horizontal line of gray matter called the gray commissure contains unmyelinated axons for communication between the left and right sides of the spinal cord. • Surrounding the gray commissure, a horizontal line of white matter called the white commissure contains myelinated axons. "Spinal Cord Cross Section" by OpenStax is licensed under CC BY 4.0/Micrograph provided by the Regents of University of Michigan Medical School © 2012) Cross section of Spinal Cord – Gray • Gray matter is subdivided into Horns horns: • Posterior horn: responsible for sensory processing and contains the axons of sensory neurons and the cell bodies of interneurons. • Lateral horn contains cell bodies of autonomic motor neurons (only found in the thoracic, upper lumbar, and sacral regions) • Anterior horn contains the cell bodies of somatic motor neurons "Spinal Cord Cross Section" by OpenStax is licensed under CC BY 4.0/Micrograph provided by the Regents of University of Michigan Medical School © 2012) Cross section of Spinal Cord – White Funiculi • White matter is subdivided into funiculi or columns that contain afferent and efferent axons: • Posterior funiculi/columns • Lateral funiculi/columns • Anterior funiculi/columns (joined by white commissure) • Within funiculi, there are tracts. "Spinal Cord Cross Section" by OpenStax is licensed under CC BY 4.0/Micrograph provided by the Regents of University of Michigan Medical School © 2012) Spinal Cord Diameter • Spinal cord is roughly cylindrical, but its diameter changes along its length, enlarging at the cervical and lumbosacral levels. • The cervical enlargement is located in the inferior cervical part of the spinal cord and innervates the upper limbs; this is the largest enlargement. • The lumbosacral enlargement extends through the lumbar and sacral parts of the spinal cord and innervates the lower limbs; this is the second largest enlargement. Structure of Spinal Nerves – Dorsal and Ventral Roots • Nerves connected to the spinal cord. • All spinal nerves are mixed sensory and motor axons that separate into two nerve roots. • The sensory axons enter the spinal cord as the dorsal (posterior) root. • The motor fibers, both somatic and autonomic, emerge as the ventral (anterior) root. • ”Spinal Cord within Vertebra" by Chiara Mazzasette is a derivative from the original work of Daniel Donnelly and is licensed by CC BY 4.0 Structure of Spinal Nerves – Ganglia and Roots • A dorsal (posterior) root ganglion, is a somatic sensory ganglion which contains unipolar neurons surrounded by satellite cells. It extends from the dorsal root of a spinal nerve. • Axons coming from the posterior root ganglion enter the posterior side through the dorsal (posterior) root. • The axons emerging from the anterior side do so through the ventral (anterior) root. • The posterior and anterior nerve roots fuse together to form the spinal nerves. • ”Spinal Cord within Vertebra" by Chiara Mazzasette is a derivative from the original work of Daniel Donnelly and is licensed by CC BY 4.0 Spinal Cord and Dorsal Root Ganglion "DRG" by OpenStax is licensed under CC BY 4.0/Micrograph provided by the Regents of University of Michigan Medical School 2012 Structure of Spinal Nerves - Rami • After exiting the intervertebral foramen, each spinal nerve splits into two branches called rami: the dorsal ramus innervates the deep muscle and skin of the back, while the ventral ramus splits into multiple branches that innervate the anterior and lateral portions of the trunk, the upper and the lower limbs. • Spinal nerves are also associated with other branches called rami communicantes which connect the spinal nerve to the sympathetic chain ganglia. • ”Spinal Cord within Vertebra" by Chiara Mazzasette is a derivative from the original work of Daniel Donnelly and is licensed by CC BY 4.0 Spinal Nerve Rami communicantes "Spinal Nerve" by Mysid and Tristanb is licensed under CC BY 4.0 Spinal Nerves • Carry both sensory and motor information since ventral and dorsal roots merge to form spinal nerve. • 31 pairs, named for the level of the spinal cord at which each one emerges. • • • • • 8 pairs of cervical nerves (C1 to C8) 12 pairs of thoracic or intercostal nerves (T1 to T12) 5 pairs of lumbar nerves (L1 to L5) 5 pairs of sacral nerves (S1 to S5) 1 pair of coccygeal nerves • Because the spinal cord is shorter than the vertebral canal, roots of lumbar, sacral and coccygeal spinal nerves travel inferiorly to reach their respective openings, forming the cauda equina. • The axons from different ventral rami will come together into a nerve plexus. Nerves Plexuses • Cervical plexus (C1 to C5) innervates the head and neck. • Brachial plexus (C5 to T1) innervate the arms. • Lumbar plexus (L1 to L5) innervate the pelvic region and anterior leg. • Sacral plexus (L4 to S4) innervate posterior leg. • Intercostal nerves (T2 to T11) are not a plexus and innervate the thoracic region. T7 to T12 innervate the abdominal region. "Spinal Nerve Plexuses" by OpenStax is licensed under CC BY 4.0 Intercostal nerves Cervical Plexus and Intercostal Nerves • The cervical plexus is formed by spinal nerves C1 - C4, although C5 does contribute some processes to this network of nerves. The anterior rami of each of these nerves branch and merge with those from the other nerves that form this plexus. The axons that extend from the plexus innervate the anterior neck, as well as portions of the head and neck. • One famous nerve that originate from this cervical plexus (although is not technically part of it) is the phrenic nerve, which innervates the diaphragm and is responsible for its contraction, thus breathing. • The intercostal nerves include spinal nerves T1 - T11. None of these nerves, except T1 form plexuses. These nerves extend between the ribs ("inter" means between, "-costal" means rib). They primarily innervate the muscles and skin of the thoracic walls. Brachial Plexus • Formed by spinal nerves C5 - T1, this plexus innervates the arms (the name serves as a hint). It is divided into different regions, which in order from medial to lateral are the followings: • Roots, coming from the spinal cord • Trunks, in turn divided in superior, middle and inferior trunks • Divisions, in turn divided in anterior and posterior, based on their anatomical location • Cords, in turn divided into posterior (from the posterior division), lateral and medial (from the anterior division), based on their anatomical location • Terminal Branches: the cords then branch into the terminal nerves of the brachial plexus, which transmit information to and from the muscles and skin of the upper limb. Mnemonic: Read That Damn Cadaver Book Brachial Plexus Regions "Brachial plexus 2" by Captain-n00dle & MissMJ is in the Public Domain, CC0 / A derivative from the original work Terminal Nerves of the Brachial Plexus • Axillary nerve: innervates muscles of the shoulder and surrounding skin. • Median nerve: innervates muscles of the forearm, and surrounding skin. • Musculocutaneous nerve: innervates arm flexors and lateral skin extending from elbow to wrist. • Radial nerve: innervates arm and forearm extensors and posterior skin of the upper limb. • Ulnar nerve: innervates many intrinsic muscles of the hand, and skin of the palm. This is commonly called the "funny bone", although it is not a bone but a nerve, and when you hit it, it is not that funny either. Mnemonic: Amazing Medics Mend Red Unicorns Lumbar Plexus • The lumbar plexus is formed by spinal nerves L1 - L4. The two main terminal branches are the: • Femoral nerve: Innervates primarily anterior and lateral thigh muscles • Obturator nerve: Innervates medial thigh muscles • The femoral nerve is larger than the obturator nerve, which passes through the obturator foramen (a hole) of the os coxae (the hip bone) to reach the medial thigh. Sacral Plexus • The sacral plexus is formed by spinal nerves L4 - S4. • The largest nerve in the body extends from this plexus, the sciatic nerve. It branches into the tibial nerve which innervates the posterior thigh and leg, as well as the plantar region of the foot, and the common fibular nerve, which branches to innervate primarily the anterior and lateral leg, as well as the dorsal region of the foot. • As with the radial and ulnar nerve, the tibial and common fibular nerves are positioned along the bones of the same name, with the tibial nerve medial to the common fibular nerve. Dermatomes • The spinal nerves, which contain sensory fibers with endings in the skin, connect with the skin in a topographically organized manner, illustrated as dermatomes. • A dermatome is supplied by a single spinal nerve. • Dermatomes are important in a clinical setting as they can help identify potential damage to spinal nerves. • In referred visceral pain, pain in a dermatome may arise from an organ nowhere near the dermatome but close to the spinal nerve. "Spinal Nerve Plexuses" by OpenStax is licensed under CC BY 4.0/Modification of work by Mikael Häggström Reflexes • A reflex is a rapid, pre-programmed response to a stimulus under involuntary control. The stimulus is often something that can cause the body harm, such as high temperatures, sharp objects, or muscle distortion. • A reflex arc is the neuronal pathway involved in a reflex action: which neurons are involved. • Somatic reflexes are automatic responses of the somatic nervous system that controls skeletal muscles. • Autonomic (or visceral) reflexes are automatic response of the autonomic nervous system that controls cardiac and smooth muscles, as well as glands (e.g. sweating). Steps of a Reflex Arc 1. A stimulus is received by a sensory receptor. 2. Sensory information is transmitted via sensory neuron to the spinal cord. 3. That information is either 3 4 2 • Processed by an interneuron. • Sent directly to a motor neuron. 4. Motor information is transmitted toward an effector via motor neuron. 1 5. The effector responds. 5 "Imgnotraçat arc reflex eng" by MartaAguayo is licensed under CC BY-SA 3.0 Types of Reflexes • A reflex arc may involve one side of the body or the other: • Ipsilateral: Both the receptor and effector organs are on the same side • Contralateral: Impulses initiating from a receptor cross the spinal cord to activate effector organs on the opposite limb • A reflex arc may or may not involve interneurons: • Monosynaptic: "Mono-" means one, because only one synapse is involved, between the sensory and motor neuron. No interneurons function in monosynaptic reflexes. Example: stretch reflex like kneejerk • Polysynaptic: "Poly-" means multiple, because at least two synapses are involved. One or more interneurons receive and process information from the sensory neuron prior to being transmitted to the motor neuron. Example: withdrawal reflex Monosynaptic Stretch Reflex • Stretch in a muscle is monitored by stretch receptors called muscle spindles • When a stimulus stretches a muscle, the muscle reflexively contracts • Knee-jerk reflex: When patellar ligament is struck, quadriceps femoris lengthens quickly, causing reflexive contraction; foot kicks out Stretch Reflex "Stretch Reflex" by Cenveo is licensed under CC BY 3.0 Polysynaptic Flexor Reflex • Nerve impulse of a stimulus is sensed by two or more neurons in the skin of the foot. • The spinal cord receives sensory impulses of the body and, through an interneuron, sends impulses to the leg muscle through spinal nerves. • After receiving the order, the muscle flexor runs the command, in this case to flex the leg and remove it from the stimulus. Clinical Anatomy: Motor Neuron Diseases • A wider group of disorders known as motor neuron diseases are caused by gradual deterioration (degeneration) and death of motor neurons. • Messages from motor neurons in the brain (called upper motor neurons) are transmitted to motor neurons in the spinal cord and to motor nuclei of brain (called lower motor neurons) and from the spinal cord and motor nuclei of brain to a particular muscle or muscles. • The followings are examples of motor neuron diseases. Amyotrophic lateral sclerosis (ALS) and Spinal muscular atrophy (SMA). Clinical Anatomy: ALS • In ALS, both the upper motor neurons and the lower motor neurons degenerate or die, and stop sending messages to the muscles. Unable to function, the muscles gradually weaken, start to twitch (called fasciculations), and waste away (atrophy). Eventually, the brain loses its ability to initiate and control voluntary movements. • Early symptoms of ALS usually include muscle weakness or stiffness. Gradually all muscles under voluntary control are affected, and individuals lose their strength and the ability to speak, eat, move, and even breathe. • Most people with ALS die from respiratory failure, usually within 3 to 5 years from when the symptoms first appear. However, about 10 percent of people with ALS survive for 10 or more years. "Stephen Hawking” by NASA is in the Public Domain Clinical Anatomy: SMA • The most common form of SMA is caused by defects in both copies of the survival motor neuron 1 gene (SMN1). This gene produces the survival motor neuron (SMN) protein which maintains the health and normal function of motor neurons. • Individuals with SMA have insufficient levels of the SMN protein, which leads to loss of motor neurons in the spinal cord, producing weakness and wasting of the skeletal muscles. This weakness is often more severe in the trunk and upper leg and arm muscles than in muscles of the hands and feet. • Survival depends on the type of SMA, ranging from few years after birth to a regular lifespan. Autonomic Nervous Chapter 14,System Human Anatomy (LibreTexts) "Blausen 0838 Sympathetic Innervation" by BruceBlaus is licensed under CC BY 3.0 Somatic vs Autonomic Nervous System • Somatic nervous system (SNS) receives conscious sensory information and sends out voluntary motor orders to skeletal muscles; information is perceived and controlled consciously (you are aware of them). • A single somatic motor neuron extends its axon from the spinal cord to skeletal muscles. • Autonomic nervous system (ANS) is an involuntary motor system (not sensory) that sends out involuntary motor orders to cardiac and smooth muscles, and glands (you are not aware of them). • Two autonomic motor neurons chain: one autonomic motor neuron extends its axon from the spinal cord to an autonomic ganglion and then another autonomic motor neuron in the ganglion sends out its axon to cardiac/smooth muscle and glands. Organization of the ANS • It is composed of a chain of two lower motor neurons: a central preganglionic neuron within the brainstem or spinal cord and a peripheral ganglionic neuron • Preganglionic fibers are small myelinated axons that exit the central nervous system through cranial or spinal nerves to synapse onto ganglionic neurons • Ganglionic neurons extend unmyelinated postganglionic axons towards target effectors Divisions of the ANS • The two divisions of the autonomic nervous system are the sympathetic division and the parasympathetic division. • The sympathetic division is responsible for fight-orflight response: prepares for emergencies by increasing alertness and making nutrients available for use. • The parasympathetic division is responsible to the rest-and-digest response: conserves energy and replenish nutrient stores. Sympathetic Neurons and Ganglia • Preganglionic neurons are located in the lateral horns of thoracic and upper lumbar spinal cord. • Preganglionic neurons can connect to sympathetic chain ganglia (or paravertebral ganglia), all of which run alongside the vertebral column. • The adrenal medulla is a modified sympathetic ganglion: preganglionic axons innervate internal region of adrenal gland and stimulation of adrenal cells causes release of epinephrine and norepinephrine into the blood. "Blausen 0838 Sympathetic Innervation" by BruceBlaus is licensed under CC BY 3.0 Sympathetic Nerves • Preganglionic fibers leave the spinal cord through ventral roots, spinal nerves and white rami communicantes to reach the sympathetic chain ganglia. • Since the sympathetic ganglia are close to the vertebral column, the preganglionic fibers are short. • Ganglionic neurons send their postganglionic fibers to a target effector. Since the sympathetic ganglia are close to the vertebral column, the postganglionic fibers are long. Parasympathetic Neurons and Ganglia • Preganglionic neurons are located in nuclei of brainstem and lateral horn of sacral spinal cord. • Preganglionic neurons can connect to terminal ganglia which are located near the target organ or intramural ganglia located within the target organ. "Blausen 0703 Parasympathetic Innervation" by BruceBlaus is licensed under CC BY 3.0 Parasympathetic Nerves • Nuclei in the brainstem controlling facial organs send their preganglionic fibers through the following cranial nerves: • oculomotor nerve (CN III) • facial nerve (CN VII) • glossopharyngeal nerve (CN IX) • Nuclei in the brainstem controlling thoracic and abdominal organs send their preganglionic fibers through the vagus nerve (CN X) to synapse onto terminal and intramural ganglia. • Ganglionic neurons send their postganglionic fibers to a target effector. Since the parasympathetic ganglia are close to the target effector organ, the preganglionic fibers are long and the postganglionic fibers are short. Autonomic Tone and Dual Innervation • Organ systems are balanced between the input from the sympathetic and parasympathetic divisions. The two divisions of the autonomic system each play a role in effecting change, usually in competing directions. • For each organ system, there may be more of a sympathetic or parasympathetic tendency to the resting state, which is known as the autonomic tone of the system. • Many effector organs of the autonomic nervous system have dual innervation, meaning that they receive competing inputs from the sympathetic and parasympathetic divisions. • In some organs, opposing effects are achieved without dual innervation thanks to the absence of the parasympathetic innervation. • Not always the sympathetic and parasympathetic divisions have opposite effects and, in a few cases, the two systems cooperate. Autonomic Effects – Examples Target Effector Sympathetic Effect Parasympathetic Effect Arrector pili muscles Contraction to cause hair erection None Sweat glands Secretion None Salivary glands Inhibits Stimulates Heart Increases heart rate Decreases heart rate Blood vessels to skeletal muscles Vasodilation None Bronchi of lungs Dilation Constriction Gastrointestinal (GI) tract gland secretion Inhibits Stimulates Gallbladder Inhibits Stimulates Peristalsis (motility) Inhibits Stimulates Sphincters Contraction (close) Relaxation (open) Urinary bladder Relaxation Contraction Penis Stimulates ejaculation Stimulates erection Table credit: Chiara Mazzasette