Nervous System Student Copy PDF

NERVOUS SYSTEM PREPARED BY: JIAN MIDJ A. OCO, PTRP PRESENTED BY: ORIEL M. REPIL, PTRP, MD, OHP  OVERVIEW With a mass of only 2 kg (4.5 lbs), about 3% of total body weight, the nervous system is one of the smallest and yet the most complex of the 11 body systems. This intricate ne...

NERVOUS SYSTEM PREPARED BY: JIAN MIDJ A. OCO, PTRP PRESENTED BY: ORIEL M. REPIL, PTRP, MD, OHP  OVERVIEW With a mass of only 2 kg (4.5 lbs), about 3% of total body weight, the nervous system is one of the smallest and yet the most complex of the 11 body systems. This intricate network of billions of neurons and even more neuroglia is organized into two main subdivisions: central nervous system peripheral nervous system Neurology deals with normal functioning and disorders of the nervous system. Neurologist is a physician who diagnoses and treats disorders of the nervous system ▪ Central nervous system (processing center) ▪ Peripheral nervous system (delivery network) DIVISIONS OF THE NERVOUS SYSTEM CENTRAL NERVOUS SYSTEM ▪ Brain – encased on the skull and vertebral column ▪ Cerebrum ▪ Brainstem ▪ Cerebellum ▪ Spinal Cord - embedded in the muscles and viscera ▪ Cervical cord ▪ Thoracic cord ▪ Lumbar cord ▪ Sacrococcygeal cord PERIPHERAL NERVOUS SYSTEM ▪ Spinal nerves (31 pairs) ▪ Cranial Nerves (12 pairs) ▪ Peripheral nerves ▪ Autonomic nerves STRUCTURAL CLASSIFICATION OF NEURONS Copyright 2009 John Wiley & Sons, Inc. 6 PARTS OF A NEURON ▪ Cell body/ soma ▪ Contains the organelles of a cells including the nucleus ▪ Processes and interprets all the messages that it receives and transmits ▪ They are grouped together in the entire nervous system (Gray matter) ▪ Dendrites ▪ Most often receives stimulus or the message and transmit it towards the cell body ▪ Axon ▪ Often transmit the message away from the body ▪ Axons vary in length and are usually a lot longer than densrites ▪ Covered by a membrane axolemma ▪ Axon terminal ▪ The most distal end the axon ▪ The specific part of the axon that will transfer the impulse or the message to another neuron or an effector organ. PARTS OF A NEURON ▪ Axon Hillock ▪ A cone shaped-portion of the cell body as it connects into the axon ▪ Attached is the initial segment which is the most excitable part of the axon. ▪ Nucleus (nuclei) ▪ Within the brain but outside of the cerebral cortex ▪ Ganglion (ganglia) ▪ Outside of the spinal cord ▪ Autonomic ganglion ▪ A cluster of neuronal cell bodies and their dendrites and is essentially a junction between autonomic nerves originating from the CNS and autonomic nerves innervating their target organs in the periphery. ▪ Dorsal root/ Spinal ganglion ▪ Nosule on a dorsal root of the spine that contains cell bodies of nerve cells that carry signals from sensory organs toward the appropriate integration center. MYELIN SHEATH ▪ The lipid-rich insulating material the axon, arranged in multiple concentric layers ▪ Produced by the Schwann cells in the PNS and Oligodendrocyctes in the CNS NODES OF RANVIER ▪ Points along the axon NOT myelinated ▪ Allows SALTATORY conduction ▪ Impulse jumps from one node to another along the length of the axon for faster conduction TYPES OF NEURON (ACCORDING TO STRUCTURE) ▪ Unipolar ▪ Neuron with single axon ▪ Bipolar ▪ Neuro with 1 dendrite and 1 axon ▪ Multipolar ▪ Neuron with several dendrites and 1 axon TYPES OF NEURON (ACCORDING TO FUNCTION) ▪ Sensory (Afferent) Neurons ▪ Conduct impulses from receptors to CNS ▪ Association or Interneurons ▪ Not specifically motor or sensory neurons ▪ Motor (efferent) Neurons ▪ Convey motor impulses from the CNS to effectors NEUROGLIA/GLIAL CELLS CNS PNS Astrocytes Schwann cells/ Neurolemnocytes Oligodendrocytes Satellite cells Microglia Ependymal cells 12 ASTROCYTES MICROGLIA ▪ A structure and metabolic support ▪ They are the phagocytic cells and ▪ Satellite cells -> specific to PNS macrophages or scavengers of the CNS ▪ Regulators of electrolyte balance ▪ They form part of the nervous ▪ Formation of blood-brain-barrier system's defense against infection (BBB) or injury. ▪ Repair of damage neural tissue EPENDYMAL CELLS ▪ Lines the ventricles of the brain and spinal cord to form the CSF WHITE MATTER VS GRAY MATTER ▪ White matter ▪ Represents the nerve fibers (axons) or processes of neurons ▪ Gray matter ▪ Represents the cell bodies of neurons arranged in a laminar layered or layered manner  COMPOSITION OF THE NEURAL TISSUE ▪ Peripheral nerve ▪ covered by Epineurium ▪ Fascicles ▪ covered by Perineurium ▪ Nerve Fibers ▪ covered by Endoneurium SYNAPSES ▪ Communication between neurons ▪ Usually occurs from the axon terminal of the transmitting neurons to the receptive region of the receiving neuron ▪ TYPES: ▪ Axodendrirtic - connection formed between the axon of one neuron and the dendrite of another ▪ Axosomatic - direct connection between the axon of one neuron to the cell body of another neuron ▪ Axoaxonic - synapses made by one neuron onto the synapse of another neuron  ORGANIZATION OF THE NERVOUS SYSTEM ▪ Central Nervous System ▪ consists of the brain and spinal cord. ▪ The brain is the part of the CNS that is in the skull and contains about 85 billion neurons. ▪ The spinal cord is connected to the brain through the foramen magnum of the occipital bone and is encircled by the bones of the vertebral column. ▪ The spinal cord contains about 100 million neurons. ▪ The CNS processes many kinds of incoming sensory information. It is also the source of thoughts, emotions, and memories. Most signals that stimulate muscles to contract and glands to secrete originate in the CNS  ORGANIZATION OF THE NERVOUS SYSTEM ▪ Peripheral Nervous System ▪ consists of all nervous tissue outside the CNS (nerves and sensory receptors) ▪ A nerve is a bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels that lies outside the brain and spinal cord. ▪ Twelve pairs of cranial nerves emerge from the brain and thirty-one pairs of spinal nerves emerge from the spinal cord. ▪ The term sensory receptor refers to a structure of the nervous system that monitors changes in the external or internal environment. Examples of sensory receptors include touch receptors in the skin, photoreceptors in the eye, and olfactory (smell) receptors in the nose.  PERIPHERAL NERVOUS SYSTEM ▪ Divided into 2 divisions: A. Sensory or afferent division - conveys input into the CNS from sensory receptors in the body. This division provides the CNS with sensory information about the somatic senses (tactile, thermal, pain, and proprioceptive sensations) and special senses (smell, taste, vision, hearing, and equilibrium) B. Motor or efferent division - conveys output from the CNS to effectors (muscles and glands). This division is further subdivided into a somatic nervous system and an autonomic nervous system a. Somatic nervous system (SNS) - conveys output from the CNS to skeletal muscles only. Because its motor responses can be consciously controlled, the action of this part of the PNS is voluntary. b. Autonomic nervous system (ANS) - conveys output from the CNS to smooth muscle, cardiac muscle, and glands. Because its motor responses are not normally under conscious control, the action of the ANS is involuntary.  AUTONOMIC NERVOUS SYSTEM ▪ 2 main branches of ANS: 1. Sympathetic division – helps support exercise or emergency actions—the so-called “fight-or- flight” responses 2. Parasympathetic division - “rest-and-digest” activities With a few exceptions, effectors (motor division) receive innervation from both of these parts, and usually the two divisions have opposing actions. A third branch of ANS: Enteric plexuses - an extensive network of over 100 million neurons confined to the wall of the digestive canal. o The enteric plexuses helps regulate the activity of the smooth muscle and glands of the digestive canal. o Although the enteric plexuses can function independently, they communicates with and are regulated by the other branches of the ANS  FUNCTIONS OF THE NERVOUS SYSTEM 1. Sensory function (INPUT). Sensory receptors detect internal stimuli, such as an increase in blood pressure, or external stimuli (for example, a raindrop landing on your arm). This sensory information is then carried into the brain and spinal cord through cranial and spinal ner ves. 2. Integrative function (PROCESS). The nervous system processes sensory information by analyzing it and making decisions for appropriate responses—an activity known as integration. 3. Motor function (OUTPUT). Once sensory information is integrated, the nervous system may elicit an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves. Stimulation of the effectors causes muscles to contract and glands to secrete BRAIN A. Cerebrum a. Frontal lobe b. Parietal lobe c. Temporal lobe d. Occipital lobe B. Diencephalon a. Thalamus b. Hypothalamus c. Epithalamus C. Cerebellum D. Brainstem a. Midbrain b. Pons c. Medulla Oblongata BRAIN BRAIN ORGANIZATION  PROTECTIVE STRUCTURES 1. First layer of protection for the central nervous system is the hard bony skull and vertebral column. The skull encases the brain and the vertebral column surrounds the spinal cord, providing strong protective defenses against damaging blows or bumps. 2. The second protective layer is the meninges, three membranes that lie between the bony encasement and the nervous tissue in both the brain and spinal cord. 3. Finally, a space between two of the meningeal membranes contains cerebrospinal fluid, a buoyant liquid that suspends the central nervous tissue in a weightless environment while surrounding it with a shock-absorbing, hydraulic cushion  MENINGES The meninges are three distinct protective, connective tissue coverings that encircle the spinal cord and brain. 1. dura mater – most superficial 2. arachnoid mater 3. pia mater - deep The spinal meninges cover the spinal cord and are continuous with the cranial meninges, which cover the brain. The spinal cord is also protected by a cushion of fat and connective tissue located in the epidural space a space between the dura mater and the wall of the vertebral canal  MENINGES 1. Dura mater (= tough mother) ▪ most superficial, a thick strong layer composed of dense irregular connective tissue. ▪ forms a sac from the level of the foramen magnum in the occipital bone, where it is continuous with the meningeal dura mater of the brain, to the second sacral vertebra. ▪ is also continuous with the epineurium, the outer covering of spinal and cranial nerves 2. Arachnoid mater (arachn- = spider; -oid = similar to) ▪ middle layer, is a thin, avascular covering comprised of cells and thin, loosely arranged collagen and elastic fibers. ▪ Is so called because of its spider’s-web arrangement of delicate collagen fibers and some elastic fibers that extend between the arachnoid mater and pia mater. ▪ It is deep to the dura mater and is continuous through the foramen magnum with the arachnoid mater of the brain. Between the dura mater and the arachnoid mater is a thin subdural space, which contains interstitial fluid. MENINGES 3. Pia mater (pia = delicate) ▪ This innermost meninx, is a thin transparent connective tissue layer that adheres to the surface of the spinal cord and brain. ▪ It consists of thin squamous to cuboidal cells within interlacing bundles of collagen fibers and some fine elastic fibers. ▪ Within the pia mater are many blood vessels that supply oxygen and nutrients to the spinal cord. Denticulate ligaments - Triangular-shaped membranous extensions of the pia mater suspend the spinal cord in the middle of its dural sheath. - These extensions are thickenings of the pia mater. - They project laterally and fuse with the arachnoid mater and inner surface of the dura mater between the anterior and posterior nerve roots of spinal nerves on either side - They protect the spinal cord against sudden displacement that could result in shock. Between the arachnoid mater and pia mater is a space, the subarachnoid space, which also contains shock absorbing cerebrospinal fluid. CEREBROSPINAL FLUID ▪ - is a clear, colorless liquid composed primarily of water that protects the brain and spinal cord from chemical and physical injuries. ▪ 4 CSF filled cavities in the brain called VENTRICLES 1. Lateral ventricle in each hemisphere of the cerebrum. (Think of them as ventricles 1 and 2.) 2. Anteriorly, the lateral ventricles are separated by a thin membrane, the septum pellucidum (pellucid = transparent). 3. The third ventricle is a narrow, slit-like cavity along the midline superior to the hypothalamus and between the right and left halves of the thalamus. 4. The fourth ventricle lies between the pons and medulla anteriorly and the cerebellum posteriorly.  FUNCTIONS OF CSF ▪ 1. Mechanical protection. CSF serves as a shock-absorbing medium ▪ 2. Chemical protection. CSF provides an optimal chemical environment for accurate neuronal signaling. ▪ 3. Circulation. CSF is a medium for minor exchange of nutrients and waste products between the blood and adjacent nervous tissue. ▪ CHOROID PLEXUS – production of CSF CIRCULATION OF CSF ▪ Mnemonics: ▪ Come Let’s Meet Tita Sylvia For Lunch Mamaya sa Aristrocrat ▪ Choroid Plexus > Lateral Ventricle > Foramen of Monroe > Third Ventricle> Sylvian Aqueduct > Fourth Ventricle > Foramen of Luschka > Foramen of Magendie > Subarachnoid Space Summary of Functions of principal parts of the Brain CEREBRUM ▪ is the “seat of intelligence.” It provides us with the ability to read, write, and speak; to make calculations and compose music; and to remember the past, plan for the future, and imagine things that have never existed before. ▪ CEREBRAL CORTEX - is a region of gray matter that forms the outer rim of the cerebrum ▪ During embryonic development, when brain size increases rapidly, the gray matter of the cortex enlarges much faster than the deeper white matter. ▪ As a result, the cortical region rolls and folds on itself forming a series of elevated ridges (CEREBRAL GYRI) and depressions called GROOVES. ▪ Cerebral sulci (singular is cerebral sulcus) are grooves that separate neighboring cerebral gyri. ▪ Interlobar sulci are grooves that separate the various lobes of the cerebrum. ▪ Cerebral fissures are grooves that separate parts of the brain. CEREBRUM ▪ LONGITUDINAL CEREBRAL FISSURE - the most prominent cerebral fissure ▪ CEREBRAL HEMISPHERE - separates the cerebrum into right and left halves ▪ FALX CEREBRI - within the longitudinal cerebral fissure between the cerebral hemispheres is the falx ▪ cerebri. ▪ CORPUS CALLOSUM – connects the cerebral hemispheres, a broad band of white matter CEREBRUM LOBES OF THE CEREBRAL HEMISPHERES ▪Frontal ▪Parietal ▪Temporal ▪Occipital FRONTAL LOBE ▪Occupies area anterior to the CSR & superior to Lateral Sulcus ▪Precentral gyrus: Primary motor cortex ▪Functions ▪Behavior, judgement and emotion ▪Intelligence ▪Voluntary movement ▪Brocas area: motor aspect of speech CORTICAL AREAS OF THE FRONTAL LOBE Gyrus Name BA Function Pre- 1o Motor 4 Movement Central Area Sup Pre-Motor 6 Movement Frontal Area Planning Frontal Eye 8 Conjugate Eye Field Deviation CORTICAL AREAS OF THE FRONTAL LOBE Gyrus Name BA Function Inf Broca’s 44, 45 Motor Aspect of Frontal Area Speech Pre- 9 – 12 Judgement, Frontal Behaviors PARIETAL LOBE ▪Occupies area posterior to the CSR & superior to Lateral Sulcus ▪Post Central Gyrus: pimary sensory cortex ▪Functions ▪ Receives tactile, proprioceptive, pain and temperature sensation from the opposite side of the body. CORTICAL AREAS OF THE PARIETAL LOBE Gyrus Name BA Function Post- 1o 3,1, 2 Proprioceptive & Central Somesthetic Tactile Sensation Area Gustatory 43 Taste Area Sup 2o 5, 7 Interprets Parietal Somesthetic Proprioceptive & Lobule Area Tactile Sensation CORTICAL AREAS OF THE PARIETAL LOBE Gyrus Name BA Function Inf Supramargin 40 Parietal al Lobule Angular 39 Initial Processing of Written Language TEMPORAL LOBE ▪Occupies area inferior to Lateral Sulcus ▪Primary Auditory cortex: auditory stimuli ▪Associative: processes auditory stimuli ▪Wernicke’s: language comprehension ▪Functions ▪Speech and language ▪Hearing (Auditory) CORTICAL AREAS OF THE TEMPORAL LOBE Gyrus Name BA Function Sup 1o Auditory/ 41-42 Auditory Input Temporal Heschl’s Gyrus 2o 22 Interprets sound; Auditory/Wer Speech nicke’s Area comprehension OCCIPITAL LOBE ▪Occupies area posterior to Parieto-Occipital Sulcus ▪Primary Visual cortex: receives/ processes visual stimuli ▪Visual Asociation cortex: process visual stimuli ▪Function ▪Vision CORTICAL AREAS OF THE OCCIPITAL LOBE Gyrus Name BA Function Cuneus 1o Visual 17 Visual Input Area Lingual 2o Visual 18-19 Interpretation Area of Visual Input  DIENCEPHALON ▪ THALAMUS ▪ major relay station for most sensory impulses that reach the primary sensory cortex of the cerebrum from the spinal cord and brainstem. ▪ contributes to motor functions by transmitting information from the cerebellum and corpus striatum to the primary motor cortex of the cerebrum. ▪ Relays nerve impulses between different areas of the cerebrum and plays a role in the maintenance of consciousness. ▪ regulating the body through sleep and wake cycles ▪ HYPOTHALAMUS ▪ Inferior to the thalamus that controls many body activities and is one of the major regulators of homeostasis ▪ secretes hormones that control multiple functions, such as body temperature, sleep, and thirst/hunger. ▪ EPITHALAMUS ▪ small region, superior and posterior to the thalamus ▪ contains PINEAL GLAND – secretes melatonin; and HABENULAR NUCLEI - involved in olfaction, especially emotional responses to odors CEREBELLUM o second only to the cerebrum in size, occupies the inferior and posterior aspects of the cranial cavity. o posterior to the medulla and pons and inferior to the posterior portion of the cerebrum ▪ TRANSVERSE CEREBRAL FISSURE – a deep groove ▪ TENTORIUM CEREBELLI - supports the posterior part of the cerebrum, separates the cerebellum from the cerebrum ▪ VERMIS – central constricted area between cerebellar hemisphere ▪ FLOCCULONODULAR LOBES - on the inferior surface contributes to equilibrium and balance. ▪ CEREBELLAR CORTEX – superficial layer of cerebellum that consists of gray matter ▪ FOLIA – slender parallel ridges on the gray matter of cerebellar cortex ▪ ARBOR VITAE – tracts of white matter deep to gray matter ▪ CEREBELLAR NUCLEI – even deeper, within the white matter, give rise to axons carrying impulses from the cerebellum to other brain centers. BRAINSTEM ▪ MEDULLA OBLONGATA Continues with the superior part of the spinal cord, forms inferior of the brainstem, begins at foramen magnum. ▪ PONS Lies directly superior to medulla and anterior to cerebellum ▪ MIDBRAIN Extends from pons to diencephalon LIMBIC SYSTEM ▪ The limbic system is sometimes called the “emotional brain” because it plays a primary role in a range of emotions, including pain, pleasure, docility, affection, and anger. It also is involved in olfaction (smell) and memory. Eye movements: OCULOMOTOR - Medial rectus, superior and inferior rectus, inferior oblique TROCHLEAR - Superior oblique ABDUCENS - Lateral rectus CRANIAL NERVES CRANIAL NERVES CRANIAL NERVE FUNCTIO N I-OLFACTORY SENSOR Y 2-OPTIC SENSOR Y 3-OCULOMOTOR MOTOR 4-TROCHLEAR MOTOR 5-TRIGEMINAL MIXED 6-ABDUCENS MOTOR 7-FACIAL MIXED 8- SENSOR VESTIBULOCOCHLE Y AR 9- MIXED GLOSSOPHARYNGEA L 10-VAGUS MIXED 11-ACCESSORY MOTOR 12-HYPOGLOSSAL MOTOR  THE SPINAL CORD AND SPINAL NERVES The spinal cord and spinal nerves contribute to homeostasis by providing quick, reflexive responses to many stimuli. The spinal cord is the pathway for sensory input to the brain and motor output from the brain White matter is composed primarily of myelinated axons. The whitish color of myelin gives white matter its name. The gray matter of the nervous system contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. It appears grayish, rather than white, because the Nissl bodies impart a gray color and there is little or no myelin in these areas. BRAIN WHITE MATTER – inner part GRAY MATTER – outer part SPINAL CORD WHITE MATTER – outer part GRAY MATTER – inner part  EXTERNAL ANATOMY OF SPINAL CORD ▪ Spinal cord - roughly oval in shape, being flattened slightly anteriorly and posteriorly. In adults, it extends from the medulla oblongata, the inferior part of the brain, to the superior border of the second lumbar vertebra In newborn infants, it extends to the third or fourth lumbar vertebra. During early childhood, both the spinal cord and the vertebral column grow longer as part of overall body growth The length of the adult spinal cord ranges from 42 to 45 cm (16–18 in.). Its maximum diameter is approximately 1.5 cm (0.6 in.) in the lower cervical region and is smaller in the thoracic region and at its inferior tip.  EXTERNAL ANATOMY OF SPINAL CORD ▪ When the spinal cord is viewed externally, two conspicuous enlargements can be seen. ▪ Cervical enlargement ▪ superior enlargement that extends from the fourth cervical vertebra (C4) to the first thoracic vertebra (T1). ▪ nerves to and from the upper limbs arise from the cervical enlargement. ▪ Lumbosacral enlargement ▪ inferior enlargement that extends from the ninth to the twelfth thoracic vertebra. ▪ nerves to and from the lower limbs arise from the lumbar enlargement.  EXTERNAL ANATOMY OF SPINAL CORD ▪ Conus medullaris o A conical structure that is inferior to the lumbar enlargement and where the spinal cord terminates as a tapering, which ends at the level of the intervertebral disc between the first and second lumbar vertebrae (L1–L2) in adults. Spinal Tap – L3-L4, L4-L5 ▪ Filum Terminale o an extension of the pia mater that extends inferiorly, fuses with the arachnoid mater and dura mater, and anchors the spinal cord to the coccyx.  EXTERNAL ANATOMY OF SPINAL CORD ▪ Spinal nerves o are the paths of communication between the spinal cord and specific regions of the body. o 31 pairs of spinal nerves that emerges from the intervertebral foramen 8 pairs of cervical nerves (C1-C8) 12 pairs of thoracic nerves (T1–T12) 5 pairs of lumbar nerves (L1–L5) 5 pairsof sacral nerves (S1–S5) 1 pair of coccygeal nerves (Co1)  EXTERNAL ANATOMY OF SPINAL CORD ▪ ROOTS - two bundles of axons, called roots, ▪ ROOTLETS - connect each spinal nerve to a segment of the cord by even smaller bundles of axons ▪ Posterior root and rootlets - contain only sensory axons, which conduct nerve impulses from sensory receptors in the skin, muscles, and internal organs into the central nervous system. ▪ SPINAL GANGLION (dorsal root) - which contains the cell bodies of sensory neurons. ▪ Anterior root and rootlets - contain axons of motor neurons, which conduct nerve impulses from the CNS to effectors (muscles and glands). ▪ CAUDA EQUINA (horse’s tail) – are roots of the lower spinal nerves angle inferiorly alongside the filum terminale in the vertebral canal like wisps of hair.  INTERNAL ANATOMY OF SPINAL CORD Anterior median fissure is a wide groove on the anterior (ventral) side. Posterior median sulcus is a narrow furrow on the posterior (dorsal) side. The gray matter of the spinal cord is shaped like the letter H or a butterfly; it consists of dendrites and cell bodies of neurons, unmyelinated axons, and neuroglia. Gray commissure forms the crossbar of the H. o In the center of the gray commissure is a small space called the central canal; it extends the entire length of the spinal cord and is filled with cerebrospinal fluid. o At its superior end, the central canal is continuous with the fourth ventricle (a space that contains cerebrospinal fluid) in the medulla oblongata of the brain. Anterior to the gray commissure is the anterior white commissure, which connects the white matter of the right and left sides of the spinal cord.  INTERNAL ANATOMY OF SPINAL CORD ▪ In the gray matter of the spinal cord and brain, clusters of neuronal cell bodies form functional groups called nuclei. ▪ Sensory nuclei receive input from receptors via sensory neurons, and motor nuclei provide output to effector tissues via motor neurons. ▪ The gray matter on each side of the spinal cord is subdivided into regions called horns. ▪ Posterior gray horns contain axons of incoming sensory neurons as well as cell bodies and axons of interneurons. Recall that cell bodies of sensory neurons are located in the spinal ganglion of a spinal nerve. ▪ Anterior gray horns contain somatic motor nuclei, which are clusters of cell bodies of somatic motor neurons that provide nerve impulses for contraction of skeletal muscles. ▪ Between the posterior and anterior gray horns are the lateral gray horns, which are present only in thoracic and upper lumbar and mid-sacral segments of the spinal cord. ▪ The lateral gray horns contain autonomic motor nuclei, which are clusters of cell bodies of autonomic motor neurons that regulate the activity of cardiac muscle, smooth muscle, and glands. PERIPHERAL NERVES: CERVICAL PLEXUS PERIPHERAL NERVES: BRACHIAL PLEXUS PERIPHERAL NERVES: LUMBAR PLEXUS PERIPHERAL NERVES: SACRAL PLEXUS QUESTIONS