CHAPTER 14: THE BRAIN AND CRANIAL NERVES



INTRODUCTION TO THE BRAINCerebrum: The cerebrum is the largest part of the brain, which is divided into lobes, and is supported by the brain stem and diencephalon.Diencephalon: The diencephalon is located superior to the brain stem and is composed of structures that are located deep within the cerebrum.Brain stem: The brain stem is composed of 3 structures (midbrain, pons, and medulla oblongata) and connects and is continuous with the spinal cord.Cerebellum: The cerebellum is located posterior to the brain stem and inferior to the cerebrum.As many know, Albert Einstein was a scientific genius, who changed the study of physics with his evidence to support the Theory of Relativity. Because of his extreme intelligence, many scientists have been interested in studying his brain to determine the cause of his unique trait. To learn more about Albert Einstein’s brain, please view: The Secrets of Einstein’s Brain.CRANIAL MENINGESCranial meninges: The cranial meninges are three connective-tissue coverings that are continuous with the spinal meninges. They function to enclose and protect the brain, cerebellum, and brain stem.Dura mater: The dura mater is the superficial meninx composed of dense irregular connective tissue with no epidural space above it. It separates into three extensions, which separate specific brain structures.Falx cerebri: This dura mater extension separates the two cerebral hemispheres.Falx cerebelli: This dura mater extension separates the two cerebellar hemispheres.Tentorium cerebelli: This dura mater extension separates the cerebrum from the cerebellum.Arachnoid mater: The arachnoid mater is the middle meninx composed of an extensive arrangement of collagen and elastic fibers. It’s avascular, and subdural space is located above it.Subdural space: The subdural space is a narrow cavity located between the dura mater and arachnoid mater, and it contains interstitial fluid.Pia mater: The pia mater is the deepest meninx composed of thin, delicate, transparent connective tissue, which is highly vascularized. It adheres to the surface of the brain, cerebellum, and brain stem, and the subarachnoid space is located above it.Subarachnoid space: The subarachnoid space is a wide cavity located between the arachnoid mater and pia mater, and it contains cerebrospinal fluid.BRAIN BLOOD FLOW AND BLOOD-BRAIN BARRIERBrain blood flow: When active and at rest, neurons in the brain use about 20% oxygen and glucose in the blood to synthesize ATP via aerobic cellular respiration (oxidative metabolism). When neuronal activity in a specific brain area increases, blood flow to that area will, also, increase to provide the neurons with specific substances needed to maintain activity. If blood flow to the brain is disrupted for 1-2 minutes, neuronal function will be impaired, and lack of oxygen to the brain for 4 minutes causes permanent damage to the nervous tissue. Interestingly, no glucose is stored in the brain, so a constant supply of glucose is necessary to maintain normal brain function. If the brain experiences a lack of glucose, mental confusion, dizziness, and loss of consciousness can result.Blood-brain barrier (BBB): The BBB is formed by a single layer of endothelial cells (simple squamous epithelium), which surrounds brain capillaries causing the capillaries to be highly-selective permeable vessels. The BBB functions to protect neurons and neuroglia from pathogens and harmful substances, like toxins and metabolic wastes, by preventing the direct passage of materials from the blood to nervous tissue. Although proteins and many drugs, like antibiotics, do not cross the BBB, lipid-soluble substances, like oxygen, carbon dioxide, alcohol, and anesthetic drugs, cross the BBB easily due to the polarity of the cell membrane of the endothelial cells and lipid-soluble substances.VENTRICLES AND CEREBROSPINAL FLUIDVentricles: The ventricles are cavities within the brain that contain cerebrospinal fluid (CSF). CSF is produced by the choroid plexuses, which are networks of capillaries covered by ependymal cells located in the walls of the ventricles.Lateral ventricles (2): The brain contains two lateral ventricles, which are very large cavities located in each cerebral hemisphere near the corpus callosum.Third ventricle (1): The third ventricle is a narrow cavity located superior to the hypothalamus and between the right and left halves of the thalamus.Fourth ventricle (1): The fourth ventricle is a large cavity located between the brain stem and cerebellum.Cerebrospinal fluid (CSF): CSF is a clear fluid that’s produced by the choroid plexuses in the ventricles of the brain, and it circulates in the ventricles, in the subarachnoid space around the brain, cerebellum, brain stem, and spinal cord, and in the central canal of the spinal cord. It’s formed via filtration and secretion of the blood plasma by the ependymal cells that cover the capillaries of the choroid plexuses, and it contains water, oxygen, glucose, ions, proteins, metabolic wastes (i.e. urea, uric acid, lactic acid), and leukocytes (white blood cells). Functions of CSF:Mechanical protection: The CSF functions to absorb shock as the nervous tissue of the brain and spinal cord moves in the cranial and vertebral cavities.Chemical protection: The CSF functions to maintain the proper ionic environment necessary to support neuronal signaling (action potential generation).Circulation: The CSF functions to exchange nutrients and waste products between the blood and nervous tissue as it circulates in the CNS.Circulation of CSF: The CSF flows from the choroid plexuses of the lateral ventricles through the Foramen of Monro into the third ventricle, where CSF is added via the choroid plexuses of the third ventricle. Then, it flows from the third ventricle through the cerebral aqueduct to the fourth ventricle, where CSF is added via the choroid plexuses of the fourth ventricle. It, then, flows through the Foramen of Magendie to the central canal of the spinal cord, and it enters the subarachnoid space, where it will circulate around the brain, cerebellum, brain stem, and spinal cord.CEREBRUMCerebral hemispheres: The cerebrum is divided into two halves, which are the right and left cerebral hemispheres. The hemispheres are separated by the falx cerebri (extension of the dura mater).Hemispheric lateralization: Hemispheric lateralization refers to the functional asymmetry of the cerebral hemispheres, where each hemisphere specializes in specific functions.Left cerebral hemisphere: The left cerebral hemisphere specializes in analytical and logical reasoning and integrating and interpreting written and spoken language.Right cerebral hemisphere: The right cerebral hemisphere specializes in recognizing faces, discriminating smells, and interpreting the emotional content of language.Cerebral cortex: The cerebral cortex is the superficial layer of gray matter of the cerebrum. It’s composed of billions of neuronal cell bodies and neuroglia, and the cerebral white matter is located deep to it.Gyri: Gyri are the convolutions (folds) of the cerebral cortex.Precentral gyrus: This gyrus contains the primary motor cortex and is located on the frontal lobe.Postcentral gyrus: This gyrus contains the primary somatosensory cortex and is located on the parietal lobe.Fissures: Fissures are the deep grooves the separate brain structures.Longitudinal fissure: This fissure separates the right and left cerebral hemispheres.Lateral fissure: This fissure separates the temporal lobe from the frontal and parietal lobes.Parieto-occipital fissure: This fissure separates the parietal lobe from the occipital lobe.Transverse fissure: This fissure separates the occipital lobe from the cerebellum.Sulci: Sulci are shallow grooves located between the gyri on the surface of the cerebrum.Central sulcus: This sulcus separates the frontal lobe from the parietal lobe.120650013398500Lobes of cerebrum: The lobes are distinct areas of the cerebrum that exhibit specific functions.Frontal lobe: This lobe is the location of the motor areas of the cerebrum.Parietal lobe: This lobe is the location of the somatosensory areas of the cerebrum.Temporal lobe: This lobe is the location of the auditory, olfactory, and gustatory areas of the cerebrum.Occipital lobe: This lobe is the location of the visual areas of the cerebrum.Corpus callosum: The corpus callosum is a large white matter tract that functions to connect the right and left cerebral hemispheres together allowing them to communicate.Basal ganglia (basal nuclei): The basal ganglia are a series of structures located deep in the cerebral hemispheres near the thalamus; they include the caudate nucleus, putamen, and globus pallidus. The basal ganglia receive inputs from the entire cerebral cortex, thalamus, and midbrain, and it sends inputs to the motor areas of the frontal lobe. It functions to regulate the initiation and termination of skeletal-muscle movements, especially those that are slow and repetitive (i.e. swinging arms when walking).Limbic system: The limbic system is a series of structures located deep in the cerebral hemispheres, superior to the brain stem near the corpus callosum in the frontal, parietal, and temporal lobes, and the limbic structures include the hippocampus, amygdala, cingulate gyrus, and mammillary bodies. The limbic system functions in learning and memory and elicits emotional responses (i.e. pain, pleasure, affection, anger).FUNCTIONAL ORGANIZATION OF CEREBRAL CORTEXSensory areas: The sensory areas of the cerebral cortex, typically, receive sensory information from the posterior areas of each hemisphere via the thalamus or other areas of the cerebrum.Primary somatosensory area: This area is located on the parietal lobe, and it functions to receive somatosensory information (i.e. touch, itch, tickle, vibration, pressure, pain, temperature).Primary visual area: This area is located on the occipital lobe, and it functions to receive and perceive visual information.Primary auditory area: This area is located on the temporal lobe, and it functions to receive and perceive auditory information.Primary gustatory area: This area is located on the temporal lobe, and it functions to receive and perceive gustatory information.Primary olfactory area: This area is located on the temporal lobe, and it functions to receive and perceive olfactory information.Motor areas: The motor areas of the cerebral cortex, typically, send output responses from the anterior areas of each hemisphere to the motor tracts in the spinal cord.Primary motor area: This area is located on the frontal lobe, and it functions to control voluntary skeletal-muscle movements.Broca’s speech area: This area is located on the frontal lobe, and it functions to articulate speech by coordinating muscular contractions of the mouth, pharynx (throat), and larynx (voice box) to allow you to speak.Association areas: The association areas are located on all cerebral lobes and are connected to each other via association tracts.Somatosensory association area: This area is located on the parietal lobe, and it functions to allow you to determine the exact shape and texture of objects without looking at them. It, also, stores memories of past sensory experiences.Prefrontal cortex (frontal association area): This area is located on the frontal lobe, and it functions in cognition (performing complex learning tasks), judgement, analysis and reasoning, planning future events, and to recall information. It’s the location of intelligence, mood, and one’s personality.Visual association area: This area is located on the occipital lobe, and it functions to recognize and evaluate visual stimuli.Auditory association area: This area is located on the temporal lobe, and it functions to recognize the sound of speech, music, and noise.Wernicke’s area: This area is located on the temporal and parietal lobes, and it functions to interpret the meaning of speech by recognizing spoken words. It translates spoken words into mon integrative area: This area functions to integrate ally sensory interpretations from all association areas to form thoughts about sensory inputs.Premotor area: This area is located on the frontal lobe, and it functions to allow complex and sequential motor activities to be learned and remembered.Frontal eye field area: This area is located on the frontal area, and it functions to control the voluntary scanning movements of the eyes.BRAIN WAVE PATTERNSBrain waves: Brain waves are nerve impulses (action potentials) that are generated by millions of neurons in the cerebral cortex at any given moment. These electrical signals are helpful in studying brain function and in diagnosing brain disorders, like epilepsy, trauma, hematomas, degenerative diseases, and metabolic abnormalities.Alpha waves: These waves occur at frequencies of 8-13 Hz, which are present in all normal individuals when awake and resting with their eyes closed, and disappear during sleep.Beta waves: These waves occur at frequencies of 14-30 Hz, which are evident when the nervous system is active (i.e. receiving sensory input and during mental activity).Theta waves: These waves occur at frequencies of 4-7 Hz, which are present in children and adults experiencing emotional stress and occur in many brain disorders.Delta waves: These waves occur at frequencies of 1-5 Hz, which are evident during deep sleep in adults and are normal in awake infants. If present in awake adults, they indicate brain damage.INTEGRATIVE FUNCTIONS OF THE CEREBRUMWakefulness: Wakefulness is state of readiness, where the body is able to react consciously to stimuli and the cerebral cortex is extremely active.Circadian rhythm: Circadian rhythms are the natural 24-hour sleep-wake cycles of the brain, which are regulated by the hypothalamus.Arousal: Arousal is awakening from sleep, which involves increased activity of the RAS. The RAS can be activated by various sensory stimuli (i.e. touch, pressure, pain, movement of the limbs, light, sound).Consciousness: Consciousness is a state of wakefulness that occurs after the RAS has been activated.Sleep: Sleep is a state of altered consciousness or partial unconsciousness, and it may play a role in learning and forming long-term memories.Learning: Learning is the ability to acquire new information through instruction and experience, and it involves strengthening synaptic connections among cortical neurons.Memory: Memory is the process of acquiring learned information and storing it, so it can be retrieved at a later time. When memories form, structural and functional changes occur among the cortical neurons, which strengthen synapses to “encode” the memory. All of the cerebral lobes, hippocampus, amygdala, and diencephalon are involved in memory processes.Immediate memory: Immediate memory is the ability to recall experiences for a few seconds, and it involves the hippocampus.Short-term memory: Short-term memory is the temporary ability to recall a few pieces of information for seconds to minutes, and it involves the hippocampus.Long-term memory: Long-term memory is the permanent ability to recall information from days to years, and it involves all of the cerebral lobes.Memory consolidation: Memory consolidation occurs when a piece of information is frequently retrieved from the brain causing cortical neural circuits (neurons that are communicating with each other) to be constantly and continuously reinforced.DIENCEPHALONThalamus: The thalamus is a structure composed of several nuclei located superior to the midbrain and comprises about 80% of the diencephalon. It functions as the major relay station for most sensory nerve impulses that travel from the spinal cord to the cerebral cortex. It, also, transmits motor information from the cerebellum and basal ganglia to the cerebral cortex, it helps maintain consciousness, and it helps regulate autonomic activities.Hypothalamus: The hypothalamus is located inferior to the thalamus and anterior to the pituitary gland. It functions to maintain homeostasis of the body by regulating body temperature, food intake, water balance, thirst, and circadian rhythms. It, also, controls the autonomic nervous system by regulating specific areas of the brain stem and spinal cord, it produces and secretes hormones and controls glandular secretions from the anterior pituitary, and it’s involved in perceiving and eliciting emotional responses (i.e. pleasure, fear, rage, aggression, sexual desire).Epithalamus: The epithalamus is located superior and posterior to the thalamus, and it includes the pineal gland. The pineal gland functions to induce sleep via secretion of melatonin (hormone), it regulates circadian rhythms, and it’s involved in eliciting emotional responses to olfactory stimuli.Mammillary bodies: The mammillary bodies are paired pea-like nuclei located inferior and posterior to the hypothalamus. They function to relay nerve impulses about olfactory inputs to the cerebral cortex (temporal lobe).Pituitary gland: The pituitary gland is located inferior and anterior to the hypothalamus. It functions to produce and secrete hormones.BRAIN STEMMidbrain: The midbrain is located inferior to the thalamus and superior to the pons. It functions to relay nerve impulses from the cerebrum to the spinal cord, and it serves as the startle-reflex center with the superior and inferior colliculi. The superior colliculi function to control subconscious skeletal-muscle movements of the eyes, head, and neck in response to visual stimuli, and the inferior colliculi function to relay nerve impulses from neurons in the ear to the thalamus and causes a reflexive response by turning your head to an unexpected sound. The midbrain, also, includes the substantia nigra, which functions to control and coordinate subconscious skeletal-muscle contractions, and the red nuclei, which function to coordinate skeletal-muscle movements with the cerebellum.Pons: The pons is located inferior to the midbrain, superior to the medulla, and anterior to the cerebellum. It functions to relay nerve impulses about voluntary skeletal-muscle movements from the cerebral cortex to the cerebellum, and it helps maintain rhythmic breathing.Medulla oblongata (medulla): The medulla is located inferior to the pons and superior to the spinal cord. It’s comprised of many sensory and motor white-matter tracts, which extend between the spinal cord and brain. About 90% of the motor tracts descending from the cerebrum to the spinal cord decussate (cross over) in the medulla, which explains why the right cerebral hemisphere controls motor responses on the left side of the body and the left cerebral hemisphere controls motor responses on the right side of the body. It functions to control and regulate all vital bodily functions, such as regulating heart rate, blood pressure, blood-vessel diameter, and rhythmic breathing patterns and controls coughing, sneezing, hiccupping, and vomiting reflexes. It, also, relays nerve impulses about proprioception to the cerebellum, and it relays nerve impulses about touch, vibration, and conscious proprioception to the thalamus.Reticular formation: The reticular formation is a broad region of gray and white matter that extends through the entire brain stem (midbrain, pons, and medulla), and it’s the location of the reticular activating system (RAS). The RAS is comprised of sensory tracts that project to the cerebral cortex, which function to help maintain consciousness and to awaken you from sleep.CEREBELLUMThe cerebellum is located inferior to the cerebrum and posterior to the pons and medulla. It functions to relay nerve impulses between it and the cerebral cortex and brain-stem structures. It, also, functions to evaluate skeletal-muscle movements initiated by motor areas of the cerebral cortex (frontal lobe) and corrects any discrepancies via feedback signals to the neurons in the motor areas. This action helps coordinate complex skeletal-muscle contractions and helps maintain posture and balance.CRANIAL NERVESCranial nerves: The cranial nerves are part of the PNS, and they emerge from the base of the brain and brain stem. They pass through the foramina of the cranial and facial bones to innervate peripheral structures of the head (i.e. skeletal muscles and skin). There are 12 pairs of cranial nerves, which are named and numbered. The numbers of the nerves indicate their superior to inferior positions on the brain stem, and the names of the nerves indicate their locations and functions.Olfactory nerve (I) – sensory nerveOptic nerve (II) – sensory nerveOculomotor nerve (III) – motor nerveTrochlear nerve (IV) – motor nerveTrigeminal nerve (V) – sensory and motor nerveAbducens nerve (VI) – motor nerveFacial nerve (VII) – sensory and motor nerveVestibulocochlear nerve (VIII) – sensory nerveGlossopharyngeal nerve (IX) – sensory and motor nerveVagus nerve (X) – sensory and motor nerveAccessory nerve (XI) – motor nerveHypoglossal nerve (XII) – motor nerveINTRODUCTION TO THE SPINAL CORDSpinal cord: The spinal cord is a long cylindrical structure that’s slightly flattened on its anterior and posterior sides, and it’s connected to the medulla and extends to the superior border of L2. It’s about 16-18 inches long and does not extend the entire length of the vertebral column.Protective structures of spinal cord:Vertebral column: The vertebral column is composed of vertebrae, and the spinal cord passes through the vertebral foramina of the vertebrae. The vertebrae provide a sturdy, protective covering that surround and protect the spinal cord.Spinal meninges: The spinal meninges are three connective-tissue coverings that are continuous with the cranial meninges. They function to enclose and protect the spinal cord.Dura mater: The dura mater is the superficial meninx composed of dense irregular connective tissue with an epidural space above it.Epidural space: The epidural space is located between the dura mater and wall of the vertebral cavity, and it contains adipose tissue to protect the spinal cord.Arachnoid mater: The arachnoid mater is the middle meninx composed of an extensive arrangement of collagen and elastic fibers. It’s avascular, and subdural space is located above it.Subdural space: The subdural space is a narrow cavity located between the dura mater and arachnoid mater, and it contains interstitial fluid.Pia mater: The pia mater is the deepest meninx composed of thin, delicate, transparent connective tissue, which is highly vascularized. It adheres to the surface of the spinal cord, and the subarachnoid space is located above it.Subarachnoid space: The subarachnoid space is a wide cavity located between the arachnoid mater and pia mater, and it contains cerebrospinal fluid.Denticulate ligaments: The denticulate ligaments are triangular-shaped thickened extensions of the pia mater, which project laterally from the spinal cord and fuse with the arachnoid mater and dura mater. They extend at regular intervals from the spinal cord, and they function to protect the spinal cord from sudden displacement due to movement.External anatomy of the spinal cord:Cervical enlargement: This is the superior enlargement of the spinal cord that extends from C4-T1, and the spinal nerves that innervate the upper limbs arise from this region.Lumbar enlargement: This is the inferior enlargement of the spinal cord that extends from T9-T12, and the spinal nerves that innervate the lower limbs arise from this region.Conus medullaris: This is the tapered, conical, terminal end of the spinal cord located inferior to the lumbar enlargement, which extends to the intervertebral disc located between L1 and L2.Filum terminale: This is an extension of the pia mater that arises from the conus medullaris. It extends inferiorly and functions to anchor the spinal cord to the coccyx.Cauda equina: This structure is referred to as the “horse’s tail” and is composed of roots of spinal nerves from the lumbar, sacral, and coccygeal regions, which exit the spinal cord inferiorly at an angle.Spinal nerves: Spinal nerves are pathways of communication to and from the periphery and spinal cord and are formed from the union of the posterior (dorsal) roots and anterior (ventral) roots that emerge from the spinal cord. There are 31 pairs that emerge at regular intervals from the spinal cord, and they are mixed nerves comprised of axons of sensory and motor neurons.Cervical nerves: 8 pairs; C1-C8Thoracic nerves: 12 pairs; T1-T12Lumbar nerves: 5 pairs; L1-L5Sacral nerves: 5 pairs; S1-S5Coccygeal nerves: 1 pair; Co1Roots (spinal roots): The roots are bundles of axons that connect each spinal nerve to a segment of the spinal cord.Posterior (dorsal) roots: The dorsal roots contain only sensory axons of sensory neurons, which conduct nerve impulses about sensory information from the periphery (i.e. sensory receptors in the skin, muscles, and viscera) to the posterior (dorsal) gray horns of the spinal cord.Posterior (dorsal) root ganglion: The dorsal root ganglion is the swollen area located on each dorsal root that contains the neuronal cell bodies of the unipolar sensory neurons.Anterior (ventral) root: The ventral roots contain only motor axons of motor neurons, which conduct nerve impulses about motor information from the anterior (ventral) gray horns of the spinal cord to effectors (i.e. skin, muscles, joints, glands, viscera) in the periphery.Internal anatomy of the spinal cord:Gray commissure: The gray commissure is gray matter in the spinal cord that surrounds the central canal and functions to connect the gray matter in the spinal cord.Central canal: The central canal is the space in the center of the spinal cord that functions to contain CSF. Nuclei: The nuclei in the CNS are clusters of neuronal cell bodies that function in groups.Sensory nuclei: These nuclei function to receive sensory information from sensory receptors.Motor nuclei: These nuclei function to elicit output responses to effectors.Horns: The horns are regions of gray matter in the spinal cord.Posterior (dorsal) gray horns: These horns contain somatic and autonomic sensory nuclei, which function to integrate sensory inputs from the periphery.Anterior (ventral) gray horns: These horns contain somatic motor nuclei, which function to provide nerve impulses for skeletal muscle contractions.Lateral gray horns: These horns are only present in the thoracic, lumbar, and sacral segments of the spinal cord, and they contain autonomic motor nuclei, which function to regulate activities of smooth and cardiac muscle and glands.Columns: The columns are regions of white matter in the spinal cord that contain tracts, which ascend (sensory tracts) and descend (motor tracts) in the spinal cord.Posterior (dorsal) white columns: These columns contain ascending sensory tracts, which function to relay sensory nerve impulses from the spinal cord to the brain.Anterior (ventral) white columns: These columns contain mainly descending motor tracts, which function to relay motor nerve impulses from the brain to the spinal cord.Lateral white columns: These columns contain ascending sensory tracts and descending motor tracts relaying nerve impulses to and from the brain.Tracts: The tracts are bundles of myelinated axons that exhibit common origins and destinations in the spinal cord and comprise the columns of the spinal cord. They function to conduct nerve impulses of similar information to and from the brain.Sensory (ascending) tracts: These tracts conduct sensory nerve impulses from the spinal cord to the brain.Motor (descending) tracts: These tracts conduct motor nerve impulses from the brain to the spinal cord.SPINAL NERVESSpinal nerve: A spinal nerve is a mixed nerve because it conveys sensory nerve impulses to the spinal cord and motor nerve impulses from the spinal cord. It’s formed from the union of a posterior (dorsal) root and anterior (ventral) root, which serve as a pathway of communication between the periphery and CNS.Distribution of spinal nerves:Rami: Rami are the branches of spinal nerves that innervate specific areas of the body.Posterior (dorsal) rami: These rami innervate deep skeletal muscles and skin of the dorsal surface of the trunk.Anterior (ventral) rami: These rami innervate the skeletal muscles, joints, and skin of the upper and lower limbs and the skin of the lateral and anterior surfaces of the trunk.Plexuses: Plexuses are networks of spinal nerves that extend from the anterior rami.Cervical plexus: This plexus is formed by the anterior rami of spinal nerves C1-C4, which innervate the head, neck, superior shoulder region, and chest.Brachial plexus: This plexus is formed by the anterior rami of spinal nerves C5-C8 and T1, which innervate the shoulders and upper limbs.Lumbar plexus: This plexus is formed by the anterior rami of spinal nerves L1-L4, which innervate the anterolateral abdominal wall, external genitals, and regions of the lower limbs.Sacral plexus: This plexus is formed by the anterior rami of spinal nerves L4-L5 and S1-S4, which innervate the buttocks and lower limbs.Coccygeal plexus: This plexus is formed by the anterior rami of spinal nerves S4-S5 and Co1, which innervate a small area of skin in the coccygeal region.Intercostal (thoracic) nerves: These nerves are formed from the anterior rami of spinal nerves T2-T12. They provide direct innervation to the abdominal muscles, intercostal muscles, and skin of the axilla.Dermatomes: Dermatomes are areas of skin that provide sensory input to the CNS via one pair of spinal nerves. Since spinal nerves function to detect stimuli on very specific areas of the skin, damaged areas of the spinal cord can be located if spinal nerves are not functioning properly.SPINAL CORD PHYSIOLOGYSpinal cord functions:1. Nerve impulse propagation: The spinal cord functions to propagate sensory and motor nerve impulses via the white-matter tracts, which are located in the white columns of the spinal cord. Sensory (ascending) tracts: These tracts conduct sensory nerve impulses from the spinal cord to the brain in order to keep the CNS informed about changes occurring in the internal and external environments of the body.Lateral and anterior spinothalamic tracts: These tracts convey nerve impulses about pain, temperature, itch, tickle, deep pressure, and crude sensations of touch.Right and left posterior columns: These tracts convey nerve impulses for vibration, proprioception, discriminative touch (pin-point touch), and two-point discriminative touch (distinguishing touch on different points on the skin).Motor (descending) tracts: These tracts conduct motor nerve impulses from the brain to the spinal cord to provide appropriate motor responses based on the stimulus that was detected.Direct pathways: These tracts originate in the cerebral cortex and travel down the spinal cord to convey motor nerve impulses to skeletal muscles to produce precise skeletal-muscle movements.Indirect pathways: These tracts originate in the brain stem and other cortical areas and travel down the spinal cord to convey motor nerve impulses to produce automatic skeletal-muscle movements to coordinate the body with visual stimuli.2. Integration of information: The gray matter of the spinal cord functions to receive and integrate sensory nerve impulses in order to elicit appropriate motor nerve impulses to allow effectors to respond.Reflexes and reflex arcs: Reflex: A reflex is a fast, automatic sequence of actions that occurs in response to a stimulus, which can be innate or learned.Spinal reflex: A reflex that involves integration at the level of the spinal cord.Cranial reflex: A reflex that involves integration at the level of the brain stem.Somatic reflex: A reflex that involves contraction of skeletal muscle.Autonomic reflex: A reflex that involves responses of smooth and cardiac muscle and glands.Reflex arc: A reflex arc is a specific pathway that nerve impulses follow to produce a reflex.1. Sensory receptor: The sensory receptors are dendrites of sensory neurons, which are located in the periphery, and they function to detect stimuli. When a strong stimulus is present, they will generate graded potentials to send to the cell bodies of sensory neurons.2. Sensory neuron: The cell bodies of sensory neurons are located in the dorsal root ganglion, and they function to receive and integrate the graded potentials from the sensory receptors. If a strong stimulus is detected, then the cell bodies will fire action potentials, which travel down the length of the axons to the axon terminals that are located in the posterior (dorsal) gray horns of the spinal cord.3. Integrating center: The integrating center contains interneurons located in the gray matter of the spinal cord. The interneurons receive the nerve impulses sent by the sensory neurons, analyze this information, and convey nerve impulses to the cell bodies of motor neurons located in the anterior (ventral) gray horns of the spinal cord.4. Motor neuron: The cell bodies of motor neurons respond to the interneurons by firing action potentials, which travel down the length of the axons to the axon terminals that are located in the periphery.5. Effector: The effectors are the peripheral structures that respond to the nerve impulses elicited by the motor neurons. Effectors include cells, glands, tissues, and organs, and the action of the effector is called a reflex. ................
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