The Nervous System PDF

THE NERVOUS SYSTEM With a mass of only 2 kg (4.5 lb), about 3% of the 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: th...

THE NERVOUS SYSTEM With a mass of only 2 kg (4.5 lb), about 3% of the 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: the central nervous system and the peripheral nervous system. A. Central Nervous System 1. Brain 2. Spinal cord B. Peripheral Nervous System ▪ consists of all nervous tissue outside the CNS. Components include nerves (cranial and spinal nerves), ganglia, enteric plexuses, and sensory receptors. ▪ can be divided into two major divisions: 1. Autonomic Nervous System (Involuntary) ▪ consists of (1) sensory neurons ( within nerves) that convey information from autonomic sensory receptors, located primarily in visceral organs such as the stomach and lungs, to the CNS and (2) motor neurons that conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands ▪ The motor part of the ANS consists of two branches: a. Sympathetic Nervous System b. Parasympathetic Nervous System 2. Somatic Nervous System (Vountary)- ▪ consists of (1) sensory neurons that convey information from somatic receptors in the head, body wall, and limbs and from receptors for the special senses of vision, hearing, taste, and smell to the CNS (2) motor neurons that conduct impulses from the CNS to skeletal muscles only. Note: Sensory (Afferent) Division- conducts action potentials (impulses) from sensory receptors to the CNS Motor (Efferent) Division- conducts action potentials (impulses) from the CNS to the effector organs, such as muscles and glands Functions of the Nervous System Receive sensory 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 nerves. Integrate information ▪ The nervous system processes sensory information by analyzing it and making decisions for appropriate responses—an activity known as integration. Motor function (Control muscle and glands) ▪ 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. Maintain homeostasis o can stimulate or inhibit the activities of other systems to help maintain a constant internal environment Establish and maintain mental activity CELLS OF THE NERVOUS SYSTEM (Neurons and Neuroglia) Neurons (nerve cells) ▪ receive stimuli, conduct action potentials, and transmit signals to other neurons or effector organs ▪ do not undergo cell division ▪ responsible for the reception of stimuli and transmission of impulses ▪ Three parts of a neuron: a. Cell body (soma) b. Dendrites- short, often highly branching cytoplasmic extensions that are tapered from their bases at the neuron cell body; usually receive information from other neurons or from sensory receptors and transmit the information TOWARD the neuron cell body  c. Axon- a long, slender projection of a nerve cell, or neuron, that typically conducts electrical impulses AWAY from the neuron's cell body Types of Neurons Types as to number of protoplasmic processes 1. Pseudo-unipolar neuron- with only one protoplasmic process; found in dorsal root ganglion of the spinal cord o This process divides into two processes a short distance from the cell body. One extends from the periphery, and the other extends to the CNS 2. Bipolar neuron- with two protoplasmic processes (axon and dendrites); found in the retina of the eye and in the nasal cavity 3. Multipolar neuron- with several dendrites and one axon; found within the CNS, including nearly all motor neurons Types as neurons as to function 1. Sensory neuron- transmit impulses from the sensory receptors of the sense organ to nerve center (CNS) 2. Motor neuron- transmit impulses from the nerve center to the effectors (muscles or glands) 3. Adjustor or associative neuron (Interneuron)- connects sensory and motor neuron; located in the nerve center A. NEUROGLIA nonneuronal cells of the CNS and PNS supporting cells for the nourishment, protection and insulation of the neurons undergo cell division, thus may cause brain tumor Types of Neuroglia 1. Astrocytes ▪ star-shaped neuroglia located between neurons and capillaries; ▪ major supporting tissue in the CNS ▪ contribute with the blood-brain barrier (BBB) 2. Microglia ▪ acts as immune cells of the CNS (protects against infection) ▪ help protect the brain by removing bacteria and cell debris (phagocytes) 3. Ependymal cells ▪ line the fluid-filled cavities (ventricles) of the Central Nervous System  ▪ some form choroid plexus to produce cerebrospinal fluid (CSF), and others, with cilia on the surface, help move the CSF through the CNS 4. Oligodendrocytes- cell processes form myelin sheaths around the axons or enclosed unmyelinated axons in the CNS 5. Schwann cells- form the myelin sheath around axons or enclose unmyelinated axons in the PNS Difference between Oligodendrocytes and Schwann cells ▪ starting with the obvious; oligodendrocytes myelinate axons in the CNS whereas schwann cells myelinate axons in the PNS ▪ a single oligodendrocyte is able to myelinate multiple axons whereas a schwann cell can only myelinate a single axon. ▪ schwann cell myelination allows for axonal grown and regeneration where as oligodendrocytes inhibit axonal growth and regeneration. Myelin sheath specialized sheath wrapped around the axons of neurons within the CNS and PNS an excellent insulator that prevents almost all electrical current flow through the cell membrane Organization of Nervous Tissue Both the CNS and PNS contain areas of gray matter and white matter o Gray matter- consists of groups of neuron cell bodies and their dendrites ▪ In the CNS, gray matter on the surface of the brain is called cortex ▪ In the PNS, a cluster of neuron cell bodies is called a ganglion o White matter- consists of bundles of parallel axons with their myelin sheaths, which are whitish in color ▪ CNS- white matter forms nerve tracts, or conduction pathways ▪ PNS- bundles of axons and their connective tissue sheaths are called nerves ELECTRICAL SIGNALS AND NEURAL PATHWAYS Resting membrane potential o the uneven charge distribution in an unstimulated (resting) cell membrane o develops for three reasons: ▪ the concentration of K+ inside the cell membrane is higher than outside the cell membrane ▪ the concentration of Na+ outside the cell membrane is higher than the inside the cell membrane  ▪ the cell membrane is more permeable to K+ than it is to Na+ Thus, the resting membrane potential results from differences in the concentration of ions across the membrane and the permeability characteristics of the membrane. The difference in membrane permeability is due to the difference in the number of open ion channels which are of two types: ▪ Leak channels ▪ Gated-ion channels o Chemically gated channels- opened by neurotransmitters or other chemicals o Voltage-gated channels- opened by change in membrane potential ACTION POTENTIAL Muscle and nerve cells are excitable cells, meaning that the resting membrane potential changes in response to stimuli that activate gated ion channels 1. Resting membrane potential Na+ channels (pink) and some, but not all, K+ channels (purple) are closed. A few K+ diffuse down their concentration gradient through the open K+ channels, making the inside of the cell membrane negatively charged compared to the outside 2. Depolarization Na+ channels are open. A few Na+ diffuse down their concentration gradients through the open Na+ channels, making the inside of the cell membrane positively charged compared to the outside. o In a stimulated muscle fiber or nerve cell, the stimulation causes Na+ channels to open quickly o results in local potential 3. Repolarization Na+ channels are closed, Na+ movement into the cell stops. More K+ channels open. K+ movement out of the cell increases, making the inside of the cell membrane negatively charged compared to the outside, once again. the changing back of the resting membrane potential Action potential- the rapid depolarization and repolarization of the cell membrane In summary, the resting membrane potential is set by the activity of the leak channels. On stimulation, chemically gated channels are opened and initiate local potentials. If sufficiently strong, local potentials activate voltage-gated channels and initiate action potential. Action potentials occur in an all-or-none fashion. That is, if threshold is reached, an action potential occurs; of the threshold is not reached, no action potential occurs. Action potentials are conducted slowly in unmyelinated axons and more rapidly in myelinated axons o Unmyelinated axons→ CONTINUOUS CONDUCTION- the action potential is conducted along the entire axon cell membrane o Myelinated axons→ SALTATORY CONDUCTION- the action potentials “jump” from one node of Ranvier to the next along the length of axon THE SYNAPSE A synapse is a junction where the axon of one neuron interacts with another neuron or with an effector organ. Presynaptic terminal- the end of the axon Postsynaptic membrane- the membrane of the dendrite or effector cell Synaptic cleft- the space separating the presynaptic and postsynaptic membranes 1. An action potential arrives at the presynaptic terminal, causing Ca+ channels to open 2. Calcium ions (Ca+) enter the presynaptic terminal and initiate the release of a neurotransmitter, acetylcholine (Ach), from the synaptic vesicles into the presynaptic cleft 3. Diffusion of Ach across the synaptic cleft and binding of Ach to Ach receptors on the postsynaptic muscle fiber membrane opens the Na+ channels 4. Sodium ion (Na+) diffuse down their concentration gradient, which results in depolarization of the muscle fiber membrane; once threshold has been reached, a postsynaptic action potential results. REFLEXES A reflex is an involuntary reaction in response to a stimulus applied to the periphery and transmitted to the CNS. allow a person to react to stimuli more quickly than if conscious thought is involved most reflexes occur in the spinal cord or brainstem rather than in the higher brain centers Reflex arc- the neuronal pathway by which a reflex occurs; has five components o The simpest reflex arc does not have interneuron (association neuron)  THE CENTRAL AND PERIPHERAL NERVOUS SYSTEMS Central Nervous System- brain and spinal cord Peripheral Nervous System- 12 pairs of cranial nerves and 31 pairs of spinal nerves THE SPINAL CORD ▪ extends from the foramen magnum at the base of the skull to the second lumbar vertebra ▪ is the pathway for impulses from the body to the brain, and from the brain to the body. These impulses are different signals our brain sends and receives from our bodies. ▪ with cauda equina- the inferior end of the spinal cord and the spinal nerves Each column of the spinal cord contains ascending and descending tracts, or pathways The ventral and dorsal roots unite just lateral to the spinal cord to form spinal nerves SPINAL NERVES ▪ arise along the spinal cord from the union of dorsal roots and ventral roots ▪ All the spinal nerves contain axons of both sensory and somatic motor neuron, and thus called mixed nerves. The dorsal roots of spinal nerves transmit sensory impulses while the ventral roots transmit motor impulses. Spinal nerves are categorized by the region of the vertebral column from which they emerge→ 31 pairs of spinal nerves ▪ Cervical- C1-C8 ▪ Thoracic- T1-T12 ▪ Lumbar- L1-L5 ▪ Sacral- S1-S5 ▪ Coccygeal- C or Cx Most of the spinal nerves are organized into three major plexuses, where nerves come together and separate: o cervical plexus- originates from spinal nerves C1-C4; one important branch is phrenic nerve o brachial plexus- originates from spinal nerves C5-T1 o lumbosacral plexus- originates from spinal nerves L1-S4 Spinal nerve T2 through T11 do not join a plexus THE BRAIN The major regions of the brain are: o brainstem o diencephalon o cerebrum o cerebellum 1. Brainstem ▪ connects the spinal cord to the remainder of the brain ▪ consist of the medulla oblongata, the pons, and the midbrain ▪ contains several nuclei involved in vital body functions ▪ Damage to small areas of the brainstem can cause death, whereas damage to relatively large areas of the cerebrum or cerebellum often does not. Medulla oblongata ▪ the most inferior portion of the brainstem and is continuous with the spinal cord; extends from the level of the foramen magnum to the pons ▪ contains ascending and descending nerve tracts ▪ contains discrete nuclei with specific functions in the regulation of: o heart rate and blood vessel diameter o breathing--- “respiratory center of the brain” o swallowing  o vomiting o coughing o sneezing o hiccuping o balance o coordination ▪ On the anterior surface, two prominent enlargements called pyramids extend the length of the medulla oblongata. The pyramids consist of descending nerve tracts, which transmit action potentials from the brain to motor neurons of the spinal cord and are involved in the conscious control of skeletal muscles Pons ▪ immediately superior to the medulla oblongata ▪ contains ascending and descending nerve tracts, as well as, several nuclei ▪ functional bridge between cerebrum and cerebellum ▪ several nuclei of the medulla oblongata, extend in the lower part of the pons so functions such as: o breathing; swallowing; balance; chewing; salivation ▪ Within the pons are the: o pneumotaxic center, a nucleus in the pons that regulates the change from inspiration to expiration o Apneustic center- promotes inspiration by stimulation of the neurons in the medulla oblongata providing a constant stimulus. Midbrain ▪ just superior to the pons, the smallest region of the brainstem ▪ the dorsal part consists of four mounds, called colliculi o two superior colliculi- visual reflexes and receive touch and auditory input o two inferior colliculi- major relay centers for the auditory nerve pathways in the CNS ▪ contains nuclei involved in coordinating eye movements and controlling pupil diameter and lens shape ▪ also contains a black nuclear mass, called the substantia nigra, which is part of the basal nuclei, and is involved in regulating general body movements ▪ the rest of the midbrain consists largely of ascending tracts from the spinal cord to the cerebrum and descending tracts from the cerebrum to the spinal cord or cerebellum 2. Cerebellum ▪ “little brain” ▪ attached to the brainstem by several large connections called cerebellar peduncles ▪ internally, consists of gray matter and white nerve tracts ▪ Functions: o maintaining balance and muscle tone o coordinating fine movement o participates with the cerebrum in learning motor skills 3. Diencephalon part of the brain between the brainstem and the cerebellum main components are: thalamus, the epithalamus, and the hypothalamus Thalamus ▪ the largest part of the diencephalon ▪ Most sensory input that ascends through the spinal cord and brainstem projects to the thalamus, where  ascending neurons synapse with thalamic neurons →major relay station of sensory information ▪ influences mood and registers an unlocalized, uncomfortable perception of pain Epithalamus ▪ small area superior and posterior to the thalamus ▪ consists of a few small nuclei, which are involved in the emotional and visceral response to odors, and the pineal gland Hypothalamus ▪ the most inferior part of diencephalon ▪ Functions of hypothalamus ▪ plays a central role in: o control of body temperature o hunger o thirst ▪ sensations such as sexual pleasure, rage, fear, and relaxation after a meal are related to hypothalamic functions ▪ plays a major role in controlling the secretion of hormones from the pituitary gland ▪ mamillary bodies form externally visible swellings on the posterior portion and are involved in emotional responses to odors and in memory 4. Cerebrum the largest part of the brain divided into left and right hemispheres by longitudinal fissures Lobes of the cerebrum 1. Frontal lobe- important in the control of: o voluntary motor functions o motivation o mood o involved is short-term memory 2. Parietal lobe- principal center for receiving and consciously perceiving most sensory information , such as: o touch o pain o temperature (hot and cold) o balance o taste 3. Occipital lobe- functions in receiving and perceiving VISUAL input 4. Temporal lobe- involved in: o olfactory (smell) o auditory sensations o important role in memory o abstract thought and judgment (“psychic cortex”) Sensory and Motor Areas of the Cerebral Cortex Sensory Areas of the Cerebral Cortex Ascending tracts project to specific regions of the cerebral cortex, called primary sensory areas, where sensations are perceived. Cortical areas immediately adjacent to the primary sensory areas, called association areas, are involved in the process of recognition. Motor Areas of the Cerebral Cortex Primary motor cortex Primary motor area- where motor functions are organized before they are actually initiated in the primary motor cortex Basal Nuclei ▪ a group of functionally related nuclei ▪ composed of five nuclei o the corpus striatum- located deep within the cerebrum ▪ putamen ▪ caudate nucleus o globus pallidus o substantia nigra- group of darkly pigmented cells in the midbrain o subthalamic nucleus ▪ important in planning, organizing, and coordinating motor movements OTHER BRAIN FUNCTIONS Left hemisphere of the cerebrum→ more analytical hemisphere emphasizing suck skills as mathematics and speech Right hemisphere of the cerebrum→ spatial perception and musical ability Speech ▪ In most people, the speech area is in the left cerebral cortex ▪ Two major cortical areas are involved in speech: o sensory speech area- in Wernicke’s area, a broad region in the left temporal lobe, interprets the meaning of speech by recognizing spoken words.  o motor speech area- in Broca’s area in the inferior portion of the frontal lobe Note: Damage to these parts of the brain or to associated brain regions may result in APHASIA, absent or defective speech or language comprehension. Limbic System ▪ composed of the olfactory cortex and certain deep cortical regions and nuclei of the cerebrum and the diencephalon ▪ includes: o olfactory cortex o cingulated gyrus (an area of the cerebral cortex) o nuclei such s those of the hypothalamus and thalamus o hippocampus ( a mass of neuronal cell bodies in the temporal lobe) o fornix, a connecting nerve tracts o amygdale (almond shaped mass of nuclei located deep within the temporal lobe) ▪ Functions: o influences long-term declarative memory o emotions o visceral responses to emotions o motivation o mood ▪ Lesions in the limbic system can result in voracious appetite, increased sexual activity, and docility (including loss of normal fear and anger responses MENINGES, VENTRCILES, AND CEREBROSPINAL FLUID Meninges ▪ surround and protect the brain and spinal cord ▪ Three layers of the meninges: 1. dura mater- the most superficial and thickest layer 2. arachnoid mater- “spider-like”; the middle layer which is very thin 3. pia mater- the innermost layer; very tightly bound to the surface of the brain and spinal cord Ventricles ▪ fluid-filled cavities in the CNS ▪ Four main ventricles: 1. Lateral ventricles- relatively large cavity in each of the cerebral hemispheres 2. Third ventricle- located in the center of the diencephalon 3. Fourth ventricle- located at the base of the cerebellum; continuous with the central canal of spinal cord Foramina of Monro- connects lateral ventricles to third ventricle Cerebral aqueduct of Sylvius- connects third ventricle to fourth ventricle Foramen of Magendie and Luschka- connects fourth ventricle to the subarachnoid space/ cisterna magna Cerebrospinal fluid (CSF) ▪ bathes the brain and spinal cord, providing a protective cushion around the CNS ▪ produced by the choroid plexuses, specialized structures made of ependymal cells, which are located in the ventricles ▪ fills the brain ventricles, the central canal of the spinal cord, and the subarachnoid space CSF FLOW Produced in the choroid plexuses of the four ventricles→ lateral ventricles→ Foramin of Monro→ third ventricle→ cerebral aqueduct of Sylvius→ fourth ventricle (small amount enters the central canal)→ Foramen of Magendie and Luschka→ subarachnoid space→ arachnoid granulations in the superior sagittal sinus→ enters the venous circulation Hydrocephalus accumulation of the CSF in the ventricles due to blockage of the opening in the fourth ventricle or the cerebral aqueduct resulting to the enlargement of the brain CRANIAL NERVES ▪ nerves that arise from the brain ▪ 12 pairs of cranial nerves General Number Name of Cranial Nerve Origin Specific Function Function I Olfactory Nerve Sensory Smell Cerebrum II Optic nerve Diencephalon Sensory Vision Innervates levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique→ Elevation of the upper Midbrain Motor III Oculomotor nerve eyelid Parasympathetic: innervates sphincter pupillae→ Pupil constriction Innervates the superior oblique muscle, which depresses, rotates IV Trochlear nerve Midbrain Motor laterally around the optic axis and helps to intort the eyeball Sensory: It receives sensation Trigeminal nerve from the face and teeth Mixed V Pons Ophthalmic nerve Motor: Innervates the muscles of Maxillary nerve mastication (chewing) Mandibular nerve It innervates the lateral rectus, which helps to abduct the eye VI Abducens nerve Pons Motor →moves the eye laterally causing abduction of the eye Sensory: receives the special sense of taste from the anterior 2/3 of the tongue Motor: Innervation to the muscles VII Facial nerve Pons Mixed of facial expression Parasympathetic: Provides secretomotor innervation to the salivary glands (except parotid) and the lacrimal (tear) gland Vestibular branch- carries Vestibulocochlear/ Cerebellopontine angle (at impulses for equilibrium (balance) VIII Acoustic/ Auditory the margin of Sensory Cochlear branch carries impulses nerve the cerebellum and pons) for hearing. Sensory: receives the special IX Glossopharyngeal nerve Medulla oblongata Mixed sense of taste from the posterior 1/3 of the tongue  Motor: provides motor innervation to the stylopharyngeus, a pharyngeal (throat) muscle, a muscle involved in swallowing Parasympathetic: provides secretomotor innervation to the parotid gland Sensory: sensory to pharynx, larynx and viscera Motor: The nerve supplies branchiomotor innervation to palate, most laryngeal, and all the pharyngeal muscles (except the stylopharyngeus) The muscles innervated by X Vagus nerve Medulla oblongata Mixed Vagus nerve are also involved in swallowing. Parasympathetic: thoracic and abdominal viscera *A major function of this nerve is to control muscles for voice and resonance along with the soft palate. Controls sternocleidomastoid and XI Spinal Accessory nerve Cranial and spinal roots Motor trapezius muscles → shrugging of shoulders Provides motor innervation to muscles of the tongue (except for the palatoglossus, which is innervated by the vagus nerve) along with other glossal XII Hypoglossal nerve Medulla oblongata Motor muscles→ Movement of the tongue * An important nerve for swallowing and speech articulation Some Say Money Matters, But My Brother Says Baked Brownies Matter Most." Sensory: I, II, and VIII Motor: III, IV, VI, XI, and XII Mixed: V, VII, IX, and X With parasympathetic: III, VII, IX, and X Eye movement: III, IV, and VI Taste sensation: VII and IX Largest cranial nerve: CN V Longest nerve: CN X AUTONOMIC NERVOUS SYSTEM comprises motor neurons that carry action potentials from the CNS to the periphery innervates the smooth muscle, cardiac muscle , and glands In ANS, two neurons extend from the CNS to effector organs: o Preganglionic neuron o Postganglionic neuron composed of: o Sympathetic Nervous System- o Parasympathetic Nervous System Location of Preganglionic Location of Division General Function Cell Body Postganglionic Cell Body Sympathetic T1-L2 (thoracolumbar) Sympathetic chain ganglia “Fight-or-flight”; prepares or collateral ganglia the body for activity Parasympathetic Cranial nerves III, VII, IX, Terminal ganglia near or Stimulates involuntary and X; S2-S4 spinal nerves embedded in the walls of activities of the body at (craniosacral) target organs rest Sympathetic chain ganglia- form a chain along both sides of the spinal cord Collateral ganglia- located nearer target organs Effects of ANS on Various Tissues