The Nervous System PDF

The Nervous System Ryan R. Williams, M.D., Ph.D. Biology 122 California State University Dominguez Hills Overview of the Nervous System Nervous system Receives and processes sensory information from both external and the internal environments Regulates both soma...

The Nervous System Ryan R. Williams, M.D., Ph.D. Biology 122 California State University Dominguez Hills Overview of the Nervous System Nervous system Receives and processes sensory information from both external and the internal environments Regulates both somatic (conscious) and autonomic motor control Has two major divisions: Central nervous system (CNS)—consists of the brain and spinal cord Peripheral nervous system (PNS)—consists of nerves (bundles of axons), and neuron cell bodies which lie outside the CNS Overview of the Nervous System The nervous system has three functions: Sensory—sensory receptors respond to stimuli by generating nerve signals that travel via the PNS to the CNS (afferent information) Integration—the CNS sums up the input it receives from all over the body, stores memories, and creates motor responses Motor—generates motor output, which travels from the CNS via the PNS to muscles, glands, and organs (efferent information)  brain cranial nerves spinal cord spinal nerves Nervous System Tissue Contains two types of cells: neurons and glia. Neurons—transmit nerve impulses Glia—support and nourish neurons Greatly outnumber neurons In the CNS: Microglia—phagocytic cells that remove bacteria and debris Astrocytes—provide metabolic and structural support directly to the neurons Oligodendrocytes—form myelin sheaths in CNS Ependymal cells—line the ventricles and make cerebrospinal fluid (CSF) In the PNS: Schwann cells—form myelin sheaths Satellite cells—support neuron cell bodies Nervous System Tissue Three types of neurons, based on function: sensory neurons, interneurons, motor neurons Sensory neuron—carries nerve signals from a sensory receptor to the CNS (afferent) Sensory receptors—detect changes in the environment. Interneuron—found only within the CNS Receive input from sensory neurons and other interneurons Sum up signals received from other neurons, then communicate with motor neurons Motor neuron—carries nerve impulses away from the CNS (efferent) to an effector (muscle fiber, organ, or gland), which carries out the response to the environmental change Nervous System Tissue Neurons have three structures: cell body, dendrites, and an axon Cell body—contains the nucleus, other organelles Dendrites—short extensions off the cell body that receive signals from other neurons Axon—the portion of a neuron that conducts nerve impulses Individual axons are termed nerve fibers, and collectively they form a nerve The axon hillock is the beginning of the axon Where the electrical impulse (action potential) starts  a. Sensory sensory neuron receptor cell body axon myelin direction sheath of conduction b. Interneuron axon dendrite c. Motor neuron cell body axon node of Ranvier axon terminal direction of conduction Nervous System Tissue Myelin sheath Covers some axons, not all axons Helps conduct the electrical impulse (action potential) Formed when Schwann cells (PNS) or oligodendrocytes (CNS) wrap around an axon many times Node of Ranvier—space between myelin sheaths. Long axons usually have a myelin sheath, short axons usually don’t Gray matter of the CNS is gray because it contains cells bodies, and no myelin White matter is white because it contains myelin. Multiple sclerosis (MS) Oligodendrocytes die, and myelin breaks down Neurons can’t transmit information Peripheral Nerve Regeneration Schwann cells Secrete factors that help axons grow Create channels together with the myelin sheath for severed axons to grow into and along Basic Neurophysiology Cells have a net negative charge Inside the cell relative to the outside Referred to as the membrane potential Typically -70 mV Due to intracellular proteins, which are negatively charged at physiological pH (7.4) The “sodium-potassium pump” Pumps 3 Na+ (sodium ions) out and 2 K+ in This creates both an electrical (charge) and chemical (concentration) gradient across the cell membrane  What is membrane potential? Basic Neurophysiology Ion channels Cells have ion specific channels in their cell membrane Allow ions to flow (diffuse) into or out of the cell Down their concentration and charge gradients Na+, K+, Ca2+, and Cl- “Excitable” cells (like neurons) have channels that can open and close Voltage-gated channels open when the charge inside the cell changes The charge needs to reach threshold (-50 mV) Ligand-gated channels open when a neurotransmitter binds “Leak” channels are always open The opening and closing of channels changes the permeability of the membrane to specific ions Basic Neurophysiology Resting potential—the membrane potential of a neuron at rest “Excitable” cells are capable of changing their membrane potential Becoming more negative or more positive Due to the opening and closing of ion channels A stimulus is anything that moves the membrane potential away from rest (-70 mV) Causes ion channels to open or close Electrical nerve signals, are changes in the membrane potential that convey information within the nervous system These changes are referred to as either action potentials or graded potentials Basic Neurophysiology Graded potentials Local changes in the membrane potential Cause by the opening of ligand-gated ion channels When a neurotransmitter binds at a synapse Usually in the dendrites or cell body Can be summated, like waves in the ocean Hyperpolarization When the inside of the neuron becomes more negative Moves away from threshold Decreases the probability that an action potential will occur Depolarization When the inside of the neuron becomes more positive Moves towards threshold, and increases the probability that action potential will occur Graded Potentials Graded Potentials Excitatory postsynaptic potential (EPSP) Inhibitory postsynaptic potential (IPSP)  Neurons have so much potential… Stimuli received by a neuron Membrane potential recorded at the axon hillock Information Flow in the Neuron Graded Potentials Action Potentials in dendrites and at axon hillock and cell body along axon  Hodgkin & Huxley Discovery of the action potential Basic Neurophysiology If the sum of the graded potentials reaches threshold, then an action potential will occur Action potentials Occur in axons Begins with voltage-gated Na+ channels opening Allows Na+ to diffuse into the axon, causing a depolarization Then, voltage-gated K+ channels open Allows K+ to diffuse out the axon, causing a repolarization “all-or-nothing” Once threshold is reached, the action potential happens completely Doesn’t vary in size Increasing the strength of an excitatory stimulus Does not change the size of an action potential Increases the number (frequency) of action potentials Basic Neurophysiology Propagation of an action potential In unmyelinated axons, action potentials stimulate adjacent parts of the axon membrane to produce an action potential Conduction can be slow (1 m/s) because each section of the axon must be stimulated In myelinated fibers, action potentials only occur at nodes of Ranvier This is called saltatory conduction and is much faster (100 m/s) Regardless of myelination, action potentials are self- propagating along the entire length of the axon The Synapse Axons branch into fine endings called axon terminals Each terminal lies very close to either the dendrite or the cell body of another neuron This region of close proximity is called a synapse Synaptic cleft—a small gap that separates the sending neuron from the receiving neuron Neurotransmitter—transmits information across a synapse Stored in synaptic vesicles in the axon terminals The Synapse The events at a synapse: Action potentials travel along an axon and reach an axon terminal (pre-synaptic terminal) Upon arriving at the terminal, voltage-gated calcium channels open, and calcium ions enter the terminal Calcium stimulates synaptic vesicles to fuse with the pre-synaptic membrane and exocytose their contents (neurotransmitters) into the synaptic cleft Neurotransmitter diffuses across to the cleft to the post-synaptic membrane Neurotransmitters bind to receptor proteins on the post-synaptic membrane, which mediate their action Could cause a graded potential or some metabolic event  arriving action potential 1. After an action potential arrives Sending neuron at an axon terminal (arrow), Ca2+ enters, and synaptic vesicles fuse axon of Ca2+ with the plasma sending membrane of neuron the sending neuron. axon terminal Synaptic vesicles enclose neurotransmitters. Synapse receiving neuron Receiving neuron Synaptic cleft 2. Neurotransmitter molecules are released and bind to receptors on the membrane of the receiving neuron. Receiving neurotransmitter neuron 3. When an excitatory neuro- neurotransmitter transmitter binds to a receptor, Na+ diffuses receptor into the ion receiving Na channel neuron, and + an action Receiving potential neuron begins. The Synapse Depending on the type of neurotransmitter, the response of the receiving neuron can be excitation or inhibition Excitation occurs if the neurotransmitter causes a graded depolarization (i.e. sodium channels open) Inhibition occurs if a neurotransmitter causes a graded hyperpolarization (i.e. potassium channels open, allowing it to flow out) The Synapse Removal of a neurotransmitter After neurotransmitter has initiated a response, it can diffuse away from the synaptic cleft Some synapses have enzymes that inactivate the neurotransmitter. That is, the enzyme acetylcholinesterase (AChE) breaks down acetylcholine In other synapses, the pre-synaptic membrane reabsorbs the neurotransmitter with transport proteins The short existence of neurotransmitters at a synapse prevents continuous stimulation of receiving membranes Neurotransmitters There are more than 30 different neurotransmitters They include, acetylcholine, norepinephrine, dopamine, serotonin, glutamate, and GABA Nerve–muscle, nerve–organ, and nerve–gland synapses all use neurotransmitters Acetylcholine (ACh) and norepinephrine are active in both the CNS and PNS In the PNS ACh excites skeletal muscle but inhibits cardiac muscle Norepinephrine excites smooth muscle Common CNS Neurotransmitters Acetylcholine—essential for memory circuits Norepinephrine—important for dreaming, waking, and mood Dopamine—regulates mood, addiction, helps coordinate movements Serotonin—thermoregulation, sleeping, emotions, and perception GABA—inhibitory in the CNS Neuromodulators—block the release of or modify a neuron’s response to a neurotransmitter That is, substance P is released by sensory neurons when pain is present That is, endorphins block the release of substance P; serve as natural painkillers Synaptic Integration Given the relatively few number of different neurotransmitters, most of their information is conveyed by a huge diversity of receptors There are many different types of receptors for each neurotransmitter Each synapse may have a different receptor Some are excitatory and some are inhibitory Integration—summing up of multiple incoming excitatory and inhibitory signals If a neuron receives enough excitatory signals to outweigh the inhibitory ones Then threshold is reached and an action potential fires On the other hand, if a neuron receives more inhibitory than excitatory signals Then threshold is not reached and no action potential fires  cell body of the axon terminals 700× receiving neuron dendrite axon branches of sending neurons inhibitory synapse excitatory synapse cell body The Central Nervous System CNS—the spinal cord and the brain Receives sensory information and initiates motor control Both the spinal cord and the brain are protected by bone (vertebrae and skull) Also, both the spinal cord and the brain are wrapped in membranes known as meninges Meningitis—infection of the meninges; may be caused by bacteria or viruses The Central Nervous System Cerebrospinal fluid (CSF)—found in between the meninges; cushions and protects the CNS In a spinal tap (lumbar puncture), some is withdrawn for testing CSF is also in the four ventricles of the brain and in the central canal of the spinal cord Ventricles are interconnected chambers that produce cerebrospinal fluid Normally, excess cerebrospinal fluid drains away into the cardiovascular system However, blockages can occur and cause CSF to accumulate, resulting in a condition called hydrocephalus The brain cannot enlarge; it is pushed against the skull Can cause severe brain damage and can be fatal The Central Nervous System The CNS is composed of two types of nervous tissue—gray matter and white matter Gray matter contains cell bodies and short, nonmyelinated axons White matter contains myelinated axons that run together in bundles called tracts Spinal cord—extends from the base of the brain through the foramen magnum Travels in the vertebral canal Contains both gray and white matter, easily seen by cross-section The Spinal Cord White matter (axons) surrounds the gray matter (cell bodies) The spinal nerves project from the cord The dorsal root of a spinal nerve contains sensory fibers entering the gray matter (sensory neuron cell bodies live in ganglia) The ventral root of a spinal nerve contains motor fibers exiting the gray matter The dorsal and ventral roots join, forming a mixed nerve spinal nerve that is part of the PNS  white matter gray matter central canal 10 × spinal cord gray matter vertebra white matter dorsal root dorsal root ganglion spinal nerve ventral vertebra root The Spinal Cord The center for thousands of reflex arcs Steps of a reflex arc: Stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord Interneurons integrate the incoming data and relay signals to motor neurons Motor axons cause skeletal muscles to contract Can be for the somatic or autonomic branches of the nervous system  Sensory-Motor Function: A Simple Circuit Sensory neuron Interneuron Skin Motor neuron receptors Effector  pin dorsal root ganglion central canal white matter sensory dendrites Dorsal gray matter receptor (in skin) dorsal horn cell body of sensory neuron dendrite of sensory neuron interneuron dendrites axon of motor neuron cell body of motor neuron effector (muscle) ventral root ventral horn Ventral The Brain Made up of the cerebrum, diencephalon, cerebellum, and brain stem Cerebrum The largest part of the human brain The last center to receive sensory input and carry out integration before commanding voluntary motor responses Communicates with and coordinates the activities of other parts of the brain Cerebral cortex Thin, highly convoluted outer layer of gray matter; covers the cerebral hemispheres Responsible for sensation, voluntary movement, thought processes and consciousness The Brain Cerebral hemispheres Longitudinal fissure—deep groove that divides the left and right cerebral hemispheres Left and right sides communicate via the corpus callosum, an extensive bridge of white matter Gyri (singular, gyrus)—thick folds separated by shallow grooves called sulci (singular, sulcus) Divided into lobes: Frontal lobe—the most anterior Parietal lobe—posterior to the frontal lobe Occipital lobe—posterior to the parietal lobe Temporal lobe—inferior to the frontal and parietal lobes  lateral third pineal ventricle ventricle gland Cerebrum skull meninges corpus callosum Diencephalon thalamus hypothalamus pituitary gland Brain stem midbrain Cerebellum pons medulla fourth ventricle oblongata spinal cord Parts of the brain Cerebral hemispheres The Brain Primary motor area—in the frontal lobe just before the central sulcus (pre-central gyrus) Voluntary signals to skeletal muscles begin here Primary somatosensory area—just behind the central sulcus in the parietal lobe (post-central gyrus) Sensory information from the skin and skeletal muscles arrives here Special sense areas of the cerebral cortex: Primary visual cortex—in the occipital lobe Primary auditory cortex—in the temporal lobe Primary taste cortex—in the parietal lobe Primary olfactory cortex—in the frontal lobe The Brain Processing centers Prefrontal area—in the frontal lobe Reasoning, planning actions, critical thinking Wernicke’s area—in the posterior part of the left temporal lobe Understanding of both written and spoken words Broca’s area—in the left frontal lobe Directs the primary motor area to stimulate muscles for speaking and writing  Motor cortex Somatosensory Parietal lobe Brain regions Frontal lobe cortex Occipital lobe perform different functions Wernicke’s area Visual Cortex Temporal Broca’s area lobe Auditory Brain stem Cerebellum cortex  neck trunk arm trunk pelvis arm pelvis forearm forearm thigh thumb, fingers, hand,fingers, thigh and thumb and hand leg upper leg face foot and facial toes expression foot and lips genitals toes salivation teeth and vocalization a. Primary mastication longitudinal gums b. Primary longitudinal motor area fissure fissure Somatosensory swallowing tongue and pharynx area  central sulcus Frontal lobe Parietal lobe primary motor area primary somatosensory area somatosensory premotor area leg association area motor speech trunk primary taste area (Broca’s) area arm prefrontal hand general interpretation area area face anterior posterior ventral tongue dorsal Occipital lobe primary primary olfactory visual area area lateral sulcus visual association Temporal lobe area auditory association area primary auditory area sensory speech (Wernicke’s) area The Brain Basal nuclei Masses of gray matter deep within the white matter Integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited Ensures that movements are coordinated and smooth Parkinson disease—caused by degeneration of dopaminergic neurons in the substantia nigra of basal nuclei The Brain Limbic System Integrates emotions with higher mental functions (reasoning, memory) Includes the amygdala and the hippocampus Amygdala—creates the sensation of fear Hippocampus—plays a crucial role in learning and memory Long-term potentiation A type of “plasticity” After synapses have been used intensively, they release more neurotransmitters than before Involved in memory storage The Brain Diencephalon—includes the hypothalamus, thalamus and pineal gland Hypothalamus—integrating center Regulates hunger, sleep, thirst, body temperature, and water balance Controls the pituitary gland; serves as a link between the nervous and endocrine systems Thalamus—two masses of gray matter Receives all sensory input except the sense of smell Sends it on to the appropriate areas of the cerebrum Pineal gland—secretes the hormone melatonin, which helps regulate daily rhythms  corpus callosum thalamus hypothalamus hippocampus olfactory bulb amygdala olfactory tract The Brain Cerebellum—under the occipital lobe Separated from the brain stem by the fourth ventricle Primarily composed of white matter in a treelike pattern called arbor vitae Overlying the white matter is a thin layer of gray matter that forms complex folds Maintains posture and balance Produces smooth, coordinated, voluntary movements The Brain The brain stem Midbrain—relay station between the cerebrum and the spinal cord or cerebellum Has reflex centers for visual and auditory stimuli Pons—communicates between the cerebellum and the rest of the CNS. With the medulla oblongata, regulates breathing rate Reflex centers coordinate head movements in response to visual and auditory stimuli Medulla oblongata Contains reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure) Contains reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing Above the spinal cord; contains tracts that ascend or descend between the spinal cord and higher brain centers The Peripheral Nervous System Ganglia (singular, ganglion)—collections of nerve cell bodies outside the CNS Cranial nerves—the 12 pairs of nerves attached to the brain Referred to by Roman numerals and name Some cranial nerves contain only sensory fibers, some only motor fibers; others are mixed Concerned with the head, neck, and facial regions One, the vagus nerve (X), has branches to the neck and internal organs and is the major parasympathetic nerve Peripheral Nervous System Spinal nerves—emerge from either side of the spinal cord There are 31 pairs of spinal nerves The dorsal root contains sensory fibers Cell bodies of sensory neurons are in the dorsal root ganglia The ventral root contains motor fibers The two roots join to form a spinal nerve; all spinal nerves are mixed nerves  cranial nerves PNS Spinal or spinal Cranial Nerve nerves artery and vein single axon bundle of axons LM Autonomic Nervous System Involuntary control of cardiac and smooth muscles, organs, and glands. Divided into the sympathetic and parasympathetic divisions. These two systems create antagonistic responses. Innervate all internal organs. Use two neurons and one ganglion for each motor output Autonomic Nervous System Sympathetic division Active during emergency situations when you might be required to fight or take flight Increases the heartbeat and dilates the airways for a ready supply of glucose and oxygen Inhibits the digestive and urinary organs The neurotransmitter released by the postganglionic axon is norepinephrine (NE) The structure of NE is like that of epinephrine (adrenaline) Autonomic Nervous System Parasympathetic division Promotes responses associated with a relaxed state That is, promotes digestion of food, slows heart rate Can be called the “rest-and-digest” system The neurotransmitter used by the parasympathetic division is acetylcholine