Outline Week 5 - The Central Nervous System and Peripheral Nervous System The Central Nervous System (CNS) Brain - structures and functions Spinal cord - structures and functions The Peripheral Nervous System (PNS) Cranial nerves Spinal nerves The Autonomic Nervous System (ANS) Sympathetic division Adrenergic stimulation Parasympathetic division Cholinergic stimulation Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. No figures are included in this lecture note. Central Nervous System (CNS) The Central Nervous System (CNS) Composed of the brain and spinal cord Receives input from sensory neurons and directs activity of motor neurons that innervate muscles and glands Association neurons integrate sensory information and help direct the appropriate response to maintain homeostasis and respond to the environment.
Central Nervous System Composed of gray and white matter Gray matter contains neuron cell bodies and dendrites. White matter contains myelinated axons. Adult brain weighs 1.5 kg Contains100 billion neurons. Brain Brain Development The CNS of an embryo begins as a neural tube around 20 days after conception. By middle of fourth week after conception, three swellings form on neural tube (forebrain, midbrain, and hindbrain) By the 5 th week of development, forebrain and hindbrain further divide into two regions each Cerebrum ( 대뇌 ) Largest portion of the brain 80% of total brain mass Responsible for higher mental functions Consists of a right and left cerebral hemisphere connected by the corpus callosum
Cerebral Cortex The outer region of the cerebrum composed of 2 4 mm gray matter with underlying white matter. Characterized by raised folds called gyri separated by depressed grooves called sulci; together called convolutions Each hemisphere is divided by deep sulci or fissures into 5 lobes - Frontal, Parietal, Temporal, Occipital, Insula Cerebral Cortex: Frontal and Parietal Lobes Separated by the central sulcus The precentral gyrus is located in the frontal lobe and is responsible for motor control; called motor cortex The postcentral gyrus is in the parietal lobe and is responsible for somatesthetic sensation (coming from receptors in the skin, muscles, tendons, and joints); called the somatosensory cortex Cerebral Cortex: Temporal, Occipital, and Insula Lobes Temporal lobe: contains auditory centers for receiving and analyzing the sense of hearing Occipital lobe: combining visual images, visual recognition of objects Insula: interpreting olfactory(smell) information; integrating sensations of pain with visceral responses Cerebral Lateralization Refers to specialization of each hemisphere for certain functions Each side of the precentral gyrus controls movements on the contralateral (opposite) side of the body due to decussation of fibers. Somatesthetic sensation from each side of the body projects to contralateral sides of the postcentral gyrus. Communication between the sides occurs through the corpus callosum
Cerebral Lateralization Some tasks seem to be performed better by one side of the brain than the other. Left hemisphere: Language, speech, writing, calculations, understand music Right hemisphere: Visuospatial tasks (e.g., recognizing faces, composing music, arranging blocks, reading maps) Language Two principle areas associated with the left cerebral hemisphere Broca s Area ( 브로카영역 ) Located in left inferior frontal lobe Controls motor aspects of speech Damage leads to Broca s aphasia Slow, poorly articulated speech (but, there is no impairment in understanding). Wernicke s Area ( 베르니케영역 ) Located in left superior temporal lobe Controls understanding of languages. Damage leads to Wernicke s aphasia Inability to comprehend spoken or written language (i.e., producing rapid speech with no meaning) Limbic System( 변연계 ) Group of brain regions responsible for emotions including aggression, fear, sex drive, and goal-directed behaviors (shown in green) Areas of the cerebrum included: cingulate gyrus( 띠이랑 ), amygdala( 편도체 ), hippocampus( 해마 ), septal nuclei( 중격핵 ), anterior insula The hypothalamus (in the diencephalon) are also part of this system The hippocampus and amygdala are related to memory. Basal Nuclei ( 기저핵, Basal Ganglia) Masses of gray matter located deep inside the cerebrum Function in the control of voluntary movement Have reciprocal excitatory connections with the cerebral cortex that create a motor circuit Maintaining purposeful motor activity while suppressing unwanted activity
Diencephalon ( 간뇌 ) Thalamus and Hypothalamus Thalamus A relay center through which all sensory information (except olfactory, smell) passes to cerebrum Diencephalon: Hypothalamus Very important for maintaining homeostasis and regulating the autonomic system. Contains neural centers for hunger/satiety, thirst, body temperature Regulates sleep and wakefulness, sexual arousal, and emotions of anger, fear, pain and pleasure. Control of pituitary hormone release from anterior pituitary Produce ADH and oxytocin and regulate the release of the hormones from posterior pituitary Brain Stem ( 뇌간 ) (Midbrain and Hindbrain) Midbrain ( 중뇌, mesencephalon) Located between diencephalon and the pons Contains numerous neuron cell bodies(nuclei) Corpora quadrigemina( 사구체 ) Superior colliculi control visual reflexes Inferior colliculi control auditory reflexes Midbrain Brain Stem Substantia nigra( 흑질 ): Regulates activity of basal nuclei (motor activity) involved in motor coordination Substantia nigra dopamine-releasing neurons are degenerated in Parkinson s diseases Mesolimbic dopamine-releasing neurons are involved in emotional reward.
Hindbrain( 후뇌 ) Brain Stem Composed of Metencephalon: pons( 뇌교 ), cerebellum ( 소뇌 ) Myellencephalon: medulla oblongata ( 연수 ) Pons( 뇌교 ) Contains several nuclei of cranial nerves. Two important respiratory control centers for regulation of breathing Pneumotaxic area and Apneustic area Hindbrain Brain Stem Cerebellum( 소뇌 ) The second largest structure in brain Receives input from proprioceptors (in muscles, tendons and joints) Works together with the basal nuclei and motor cortex of the cerebrum to coordinate of body movements Maintaining posture and balance Learning motor skills Hindbrain Brain Stem Medulla oblongata ( 연수 ) Contains all tracts that pass between brain and spinal cord Have many nuclei of cranial nerves Principle site of decussation of information Contains crucial center for breathing and cardiovascular systems Vasomotor center Cardiac center Respiratory center Figures 1. The structure of brain 2. The cerebral cortex: The motor and sensory cortex 3. The lobes of the left cerebral hemisphere
Spinal Cord ( 척수 ) Spinal Cord The spinal cord is composed of white matter surrounding a gray matter core The white matter is composed of ascending and descending fiber tracts. The gray matter contains the cell bodies of motor and association neurons. Spinal Cord Spinal nerves join with spinal cord through spinal nerve roots Dorsal Root ( 후근 ) Carry afferent axons into the spinal cord Dorsal root ganglion Contains afferent cell bodies Ventral Root ( 전근 ) Carry efferent axons out of spinal cord Ascending Spinal Tracts Ascending sensory tracts decussate so that brain hemispheres receive information from the opposite side of the body
Descending Spinal Tracts Pyramidal (or corticospinal) tracts descend from cerebral cortex to spinal cord without synapsing Originate in motor cortex Function in control of fine movements Extrpyramidal tracts descend with many synapses Originate in various brain locations Influence movement indirectly Figures 1. Cross section of the spinal cord 2. Ascending sensory neuron tracts 3. Descending corticospinal(pyramidal) motor tracts Peripheral Nervous System (PNS) The Peripheral Nervous System (PNS) Designed for rapid communication between the CNS and remainder of the body Consists of nerves that exit from brain and spinal cord, and their ganglia (=collection of cell bodies outside the CNS) 12 pairs cranial nerves 31 pairs spinal nerves
Cranial Nerves ( 뇌신경 ) Parts of the PNS 12 pairs of cranial nerves 2 pairs arise from the forebrain 10 pairs arise from the midbrain and hindbrain Most (except three of the cranial nerves) are mixed nerves containing both sensory and motor fibers Spinal Nerves ( 척수신경 ) Parts of the PNS 31 pairs spinal nerves 8 cervical( 경수 ), 12 thoracic( 흉수 ), 5 lumbar( 요수 ), 5 sacral( 천수 ), 1 coccygeal( 미수 ) Spinal Nerves All are mixed nerves that separate near the spinal cord into dorsal roots and ventral roots Dorsal roots( 후근 ) are composed of sensory fibers Ventral roots( 전근 ) are composed of motor fibers Reflex Arc Unconscious motor response to a sensory stimulus Motor neurons are stimulated by association neurons or directly by sensory neurons Does not require input from the brain
Autonomic Motor Neurons The Autonomic Nervous System (ANS) Convey impulses from CNS to smooth muscle, cardiac muscle and glands Regulate and adjust activity of effector tissues Visceral effector organs are somewhat independent of innervation and will not atrophy if a nerve is cut (unlike skeletal muscle). Cardiac muscle and some smooth muscle contract and relax by themselves (they have automatic activity). Autonomic innervation can speed up or slow down intrinsic contractions. Autonomic motor neurons can stimulate or inhibit depending on the organ and the receptors. Autonomic Motor Neurons Two neurons are involved in efferent pathway (connecting CNS to effector) 1 st neuron (=preganglionic neuron) has cell body in CNS. Synapses with 2 nd in the autonomic ganglion 2 nd neuron(=postganglionic neuron) sends signal from autonomic ganglion to the effector organ. NOTE somatic efferent pathways have only a single motor neuron extending from the CNS to the skeletal muscle fibers Divisions of the ANS ANS has sympathetic and parasympathetic divisions. Sympathetic Division Dominates in stressful situations Prepare the body for fight or flight (e.g., increasing heart rate and blood glucose levels, diverting blood to skeletal muscles) Parasympathetic Division Dominates during relaxed situations Is antagonistic to the sympathetic division Allows the body to rest and digest (e.g., slowing heart rate and increasing digestive activities)
Sympathetic Division Preganglionic neurons come from the thoracic and lumbar regions of the spinal cord Also called the thoracolumbar division Preganglionic neurons synapse in sympathetic ganglia that run parallel to the spinal cord. These are called the paravertebral ganglia. These ganglia are connected, forming a sympathetic chain of ganglia. Sympathetic Division Some preganglionic neurons do not synapse in the sympathetic chain of ganglia. Instead, they form splanchnic nerves, which synapse in collateral ganglia. Sympathoadrenal System The adrenal medulla, located in adrenal gland on the top of kidney, appears to be modified collateral ganglion. Some preganglionic sympathetic axons synapse in the adrenal medulla. These axons stimulate the adrenal medulla to secrete its hormone, mostly epinephrine (85%) and norepinephrine (15%) into blood Parasympathetic Division Also called the craniosacral division because its long preganglionic neurons originate in midbrain, pons, medulla, and sacral region of the spinal cord. Preganglionic axons synapse with postganglionic motor neurons at terminal ganglia (located next to or within target organ) Postganglionic neurons have short axons that innervate the target organ.
ANS Neurotransmitters All preganglionic neurons (both sympathetic and parasympathetic) release acetylcholine (ACh). Most parasympathetic postganglionic neurons also release ACh Most sympathetic postganglionic neurons release norepinephrine (noradrenaline) ANS Neurotransmitters Postganglionic neurons have unusual synapses, which have various swellings called varicosities which release neurotransmitter along the length of the axon They form synapses en passant(in passing) Adrenergic Stimulation Most postganglionic sympathetic neurons release norepinephrine onto the effector tissues Effector tissues have different forms of adrenergic receptors (α or β, with different subvariants) Adrenergic effects include stimulation of the heart, vasoconstriction in the viscera and skin, bronchodilatiion and glycogenolysis in the liver. Function through G-protein receptors
Cholinergic Stimulation ACh is used at all motor neuron synapses on skeletal muscle, all preganglionic neurons, and parasympathetic postganglionic neurons. Cholinergic receptors have two subtypes: Nicotinic Receptor: Found in autonomic ganglia and stimulated by release of ACh from preganglionic neurons Serve as ligand-gated ion channels Excitatory Cholinergic Stimulation Cholinergic receptors have two subtypes: Muscarinic Receptor: Found in visceral organs and stimulated by release of ACh from postganglionic neurons. G-protein-linked receptors Can be excitatory or inhibitory. Organs without Dual Innervation Most visceral organs receive dual innervation (supplied by both sympathetic and parasympathetic divisions) The two ANS divisions are usually antagonistic For organs without dual innervation, regulation is achieved by variations in sympathetic nerve activity. E.g., adrenal medulla, sweat glands, and most blood vessels receive only sympathetic innervation. Control of the ANS by Higher Brain Centers The medulla oblongata of the brain stem is the area most directly controls many activities of the ANS. It has centers for control of cardiovascular, pulmonary, urinary, reproductive, and digestive systems. It is influenced by sensory input and by input from the hypothalamus. Higher brain regions regulate the medulla. Hypothalamus: major regulatory center of the ANS body temperature, hunger, thirst, pituitary gland
Table and Figure 1. Overview of the ANS 2. Receptors involved in autonomic regulation