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BIOL 2304 Nervous Tissue Nervous System Overall function is control and communication, achieved through: Sensory - receptors monitor changes (stimuli) and gathers information inside and outside the body Integration - prrocesses and interprets sensory input, makes decisions Motor - dictates a response by activating effector organs Organization of the Nervous System Central nervous system (CNS) Brain and spinal cord The integration and command center Peripheral nervous system (PNS) Carries messages to and from the CNS Paired cranial nerves extending from brain Paired spinal nerves extending from spinal cord Peripheral nerves link all regions of the body to the CNS Ganglia are clusters of neuronal cell bodies 1 Organization of the Nervous System Nervous Tissue Cells are densely packed and intertwined Two main cell types: 1. Neurons Excitable – transmit electrical signals 2. Glial cells – support cells Also called neuroglia or simply glia Non-excitable – do not transmit electrical signals 2 The Neuron Basic structural unit of the nervous system Human body contains billions of neurons Neurons conduct electrical impulses along the plasma membrane Nerve impulses are called action potentials Other special characteristics Longevity – can live and function for a lifetime Do not divide – fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception High metabolic rate – require abundant oxygen and glucose, neurons die after 5 minutes without oxygen The Cell Body (Soma) Size of cell body varies from 5–140 µm Contains Nucleus and Perikaryon (“around nucleus”) Contains usual organelles plus other structures: Chromatophilic bodies (Nissl bodies) are clusters of rough ER and free ribosomes Function to renew cell membrane proteins (channels) Named chromatophilic because they stain darkly Neurofibrils – bundles of intermediate filaments, form a network between chromatophilic bodies 3 The Cell Body (Soma) Most neuronal cell bodies are located within the CNS in cluster called nuclei, protected by bones of the skull and vertebral column Clusters of cell bodies outside CNS are called ganglia which lie along nerves in the PNS Structure of a Typical Large Neuron 4 Neuron Processes – Dendrites Dendrites Extensively branching from the cell body Transmit electrical signals toward the cell body Function as receptive sites for receiving signals from other neurons Neuron Processes – Axons Axons Neuron has only one Impulse generator and conductor Transmits impulses away from the cell body No protein synthesis in axon Neuron Processes Consist of neurofilaments, actin microfilaments, and microtubules Provide strength along length of axon Aid in the axonal transport of substances to and from the cell body Axon collaterals - Infrequent branches along length of axon, prior to axon terminal Multiple branches at end of axon Terminal branches (telodendria) End in knobs called axon terminals (also called end bulbs or boutons) 5 Supporting Cells Six types of supporting cells Four in the CNS Oligodendrocytes Microglia Astrocytes Ependymal cells Two in the PNS Satellite cells Schwann cells Provide supportive functions for neurons Cover nonsynaptic regions of the neurons 6 Neuroglia Neuroglia in the CNS Neuroglia Glial cells have branching processes and a central cell body Outnumber neurons 10 to 1 Make up half the mass of the brain Can divide throughout life Neuroglia in the CNS Astrocytes - the most abundant glial cell type Sense when neurons release glutamate Extract blood sugar from capillaries for energy Take up and release ions in order to control environment around neurons Involved in synapse formation in developing neural tissue Produce molecules necessary for neuronal growth 7 Neuroglia in the CNS Microglia – smallest and least abundant glial cell Phagocytes – the macrophages of the CNS Engulf invading microorganisms and dead neurons Derive from blood cells called monocytes Neuroglia in the CNS Ependymal cells Line the central cavity of the spinal cord and brain Bear cilia – help circulate the cerebrospinal fluid Neuroglia in the CNS Oligodendrocytes – have few branches , wrap their cell processes around axons in CNS, produce myelin sheaths Analogous to Schwann cells of the PNS 8 Neuroglia in the PNS Satellite cells – surround neuron cell bodies within ganglia Schwann cells (neurolemmocytes) – surround axons in the PNS Form myelin sheath around axons of the PNS Structural Classes of Neurons Unipolar Possess one short, single process Dendrite, axon continuous Afferent neurons Multipolar Many dendrites, one axon Most common class of neuron Bipolar One dendrite, one axon Very rare Found in some special sensory organs Neurons Classified by Structure 9 Neuron Classification Functional: Sensory (afferent) — transmit impulses toward the CNS Motor (efferent) — carry impulses away from the CNS Interneurons (association neurons) — shuttle signals through CNS pathways 10 Functional Classification of Neurons Sensory (afferent) neurons Transmit impulses toward the CNS Virtually all are unipolar neurons Cell bodies in ganglia outside the CNS Short, single process divides into The central process – runs centrally into the CNS The peripheral process – extends peripherally to the receptors Motor (efferent) neurons Carry impulses away from the CNS to effector organs Most motor neurons are multipolar Cell bodies are within the CNS Form junctions with effector cells Interneurons (association neurons) Most are multipolar Lie between motor and sensory neurons Confined to the CNS Neurons Classified by Function 11 Myelin Sheaths Segmented structures composed of the lipoprotein myelin Surround thicker axons Form an insulating layer Prevent leakage of electrical current Increase the speed of impulse conduction Nodes of Ranvier – gaps along axon Thick axons are myelinated Thin axons are unmyelinated, conduct impulses more slowly Myelin Sheaths in the PNS Myalin sheaths formed by Schwann cells (neurolemmacytes) Develop during fetal period and in the first year of postnatal life Schwann cells wrap in concentric layers around the axon, cover the axon in a tightly packed coil of membranes Neurilemma - material external to myelin layers Myelin Sheaths in the PNS 12 Myelin Sheaths in the CNS Oligodendrocytes form the myelin sheaths in the CNS Have multiple processes Coil around several different axons Synapses Site at which neurons communicate Signals pass across synapse in one direction Presynaptic neuron - conducts signal toward a synapse Postsynaptic neuron - transmits electrical activity away from a synapse 13 Types of Synaptic Connections Axodendritic - between axon terminals of one neuron and dendrites of another, most common type of synapse Axosomatic - between axons and neuronal cell bodies Axoaxonic, dendrodendritic, and dendrosomatic -uncommon types of synapses Synapses Two types of Synapses: Electrical synapse Neurons connected by gap junctions Electrical activity in pre-synaptic neuron is passed directly to post-synaptic neuron Chemical synapse (Neurotransmitters) Neurons connected by physical space Electrical activity in pre-synaptic neuron produces chemical exocytosis of neurotransmitters that acts on receptors on post-synaptic neuron Electrical Synapse – Gap Junctions Electrical synapse Neurons connected by gap junctions Electrical activity in pre-synaptic neuron is passed directly to post-synaptic neuron 14 Chemical Synapse – Neurotransmitters Chemical synapse Neurons connected by physical space Electrical activity in pre-synaptic neuron produces chemical exocytosis of neurotransmitters that acts on receptors on post-synaptic neuron Features of Chemical Synapse Axodendritic synapses – representative type Synaptic vesicles on presynaptic side 15 Membrane-bound sacs containing neurotransmitters Mitochondria abundant in axon terminals Synaptic cleft - separates the plasma membrane of the two neurons Synaptic Cleft: Information Transfer Nerve impulses (AP) reach the axon terminal of the presynaptic neuron and open Ca2+ channels Neurotransmitter is released into the synaptic cleft via exocytosis Neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic neuron Postsynaptic membrane permeability changes due to opening of ion channels, causing an excitatory or inhibitory effect Central Nervous System: Brain 16 Spinal cord The Brain Performs the most complex neural functions Intelligence Consciousness Memory Sensory-motor integration Involved in innervation of the head Organization of CNS Centrally located gray matter – neuron cell bodies, interneurons, unmyelinated fibers Externally located white matter – myelinated fibers Additional layer of gray matter external to white matter is the Cortex Formed from neuronal cell bodies migrating externally Located in cerebrum and cerebellum Basic Parts and Organization of the Brain Divided into four regions: Cerebral hemispheres Account for 83% of brain mass Diencephalon – includes thalamus and hypothalamus Brain stem - includes midbrain, pons, and medulla Cerebellum – “little brain” The Cerebral Hemispheres Frontal section through forebrain Cerebral cortex 17 Cerebral white matter Deep gray matter of the cerebrum (basal ganglia) Corpus Callosum – commissural fibers (white matter) which connects the two hemispheres The Cerebral Hemispheres Fissures – deep grooves, which separate major regions of the brain Transverse fissure – separates cerebrum and cerebellum Longitudinal fissure – separates cerebral hemispheres Sulci - grooves on the surface of the cerebral hemispheres Gyri - twisted ridges between sulci Prominent gyri and sulci are similar in all people 18 Lobes, sulci, and fissures of the cerebral hemispheres The Cerebral Hemispheres Central sulcus separates frontal and parietal lobes Bordered by two gyri: Precentral gyrus Postcentral gyrus Parieto-occipital sulcus - separates the occipital from the parietal lobe Lateral sulcus - separates temporal lobe from parietal and frontal lobes Deeper sulci divide cerebrum into lobes The Cerebral Hemispheres Lobes are named for the skull bones overlying them: Frontal lobe Parietal lobe Temporal lobe Occipital lobe 19 The Cerebral Cortex Home of our conscious mind Composed of gray matter - neuronal cell bodies, dendrites, and short axons Folds in cortex – triples its size Approximately 40% of brain’s mass The Cerebral Cortex - Functional Areas Three general functional areas: Sensory areas Association areas Motor areas Each of the major senses has a specific brain region called a primary sensory cortex There are also multimodal association areas to process information 20 Functional Areas Of The Cerebral Cortex Primary Somatosensory Cortex Located along the postcentral gyrus Involved with conscious awareness of general somatic senses Primary Visual Cortex On medial part of the occipital lobe Largest of all sensory areas Receives visual information that originates on the retina First of a series of areas processing visual input 21 Primary Auditory Cortex Located at superior edge of the temporal lobe Conscious awareness of sound Impulses transmitted to primary auditory cortex Olfactory Cortex Olfactory nerves transmit impulses to the olfactory cortex Provides conscious awareness of smells Lies on the medial aspect of the temporal lobe Gustatory Cortex Involved in the conscious awareness of taste stimuli Located on the “roof” of the lateral sulcus 22 Motor Areas – Primary Motor Cortex Controls motor functions Located in precentral gyrus The Diencephalon Forms the center core of the forebrain, primarily composed of gray matter Surrounded by the cerebral hemispheres Composed of three paired structures: thalamus, hypothalamus, epithalamus Border the third ventricle 23 24 The Thalamus Makes up 80% of the diencephalon Contains approximately a dozen major nuclei Acts as the relay station for incoming sensory information Every part of brain communicating with cerebral cortex relays signals through thalamic nuclei Is the “gateway” to the cerebral cortex The Hypothalamus Lies between the optic chiasm and the mammillary bodies Pituitary gland projects inferiorly Contains approximately a dozen nuclei Main visceral control center of the body The Diencephalon – The Hypothalamus Functions include the following Control of the ANS Control of emotional responses Regulation of body temperature Regulation of hunger and thirst sensations Control of behavior Regulation of sleep-wake cycles Control of the endocrine system Formation of memory The Diencephalon – The Epithalamus Forms part of the “roof” (top) of the third ventricle Consists of a tiny group of nuclei Includes the pineal gland (pineal body) Secretes the hormone melatonin Under influence of the hypothalamus Aids in control of circadian rhythm The Brain Stem Several general functions Produces automatic behaviors necessary for survival Passageway for all fiber tracts running between the cerebrum and spinal cord Heavily involved with the innervation of the face and head 25 10 of the 12 pairs of cranial nerves attach to it The Brain Stem Includes the midbrain, pons, and medulla oblongata The Brain Stem – The Midbrain The midbrain processes visual and auditory information and generates involuntary somatic motor responses Has reticular activating system - arousal of the whole brain Has nuclei for cranial nerves II and IV Has ascending and descending tracts Lies between the diencephalon and the pons Cerebral peduncles located on the ventral surface of the brain, contain pyramidal (corticospinal) tracts Superior cerebellar peduncles - connect midbrain to the cerebellum 26 27 The Brain Stem – The Midbrain Corpora quadrigemina (quad-ri-gemina) The largest nuclei Divided into the superior and inferior colliculi Superior colliculi – nuclei that act in visual reflexes Inferior colliculi – nuclei that act in reflexive response to sound 28 The Brain Stem – The Pons A “bridge” between the midbrain and medulla oblongata Pons contains the nuclei of cranial nerves V – Trigeminal nerve VI – Abducens nerve VII – Facial nerve Motor tracts coming from the cerebral cortex Pontine nuclei Connect portions of the cerebral cortex and cerebellum Send axons to cerebellum through the middle cerebellar peduncles 29 The Brain Stem – The Medulla Oblongata The core of the medulla contains Much of the reticular formation Nuclei then influence autonomic functions Cardiac center Vasomotor center The medullary respiratory center Centers for hiccupping, sneezing, swallowing, and coughing Functional Brain Systems: The Reticular Formation The Brain Stem – The Medulla Oblongata 30 Most caudal level of the brain stem Is continuous with the spinal cord Choroid plexus lies in the roof of the fourth ventricle Cranial nerves VIII–XII attach to the medulla External landmarks of medulla Pyramids of the medulla lie on its ventral surface Decussation of the pyramids - crossing over of motor tracts Inferior cerebellar peduncles - fiber tracts connecting medulla and cerebellum The Cerebellum Located dorsal to the pons and medulla Smooths and coordinates body movements Helps maintain equilibrium Consists of two cerebellar hemispheres Cortex – gray matter Arbor vitae - internal white matter Thick tracts connecting the cerebellum to the brain stem are superior, middle, inferior cerebellar peduncles Composed of Cortex – gray matter Arbor vitae - internal white matter Thick tracts connecting the cerebellum to the brain stem are Superior cerebellar peduncles Middle cerebellar peduncles Inferior cerebellar peduncles Fibers to and from the cerebellum are ipsilateral – run to and from the same side of the body Cerebellum receives information from the cerebral cortex On equilibrium On current movements of Limbs, neck, and trunk 31 Ventricles of the Brain Expansions of the brain’s central cavity Filled with cerebrospinal fluid (CSF) Lined with ependymal cells (glial cells) Continuous with each other Continuous with the central canal of the spinal cord Ventricles of the Brain Lateral ventricles – located in cerebral hemispheres Horseshoe-shaped from bending of the cerebral hemispheres Third ventricle – lies in diencephalon Connected with lateral ventricles by interventricular foramen Cerebral aqueduct – connects 3rd and 4th ventricles Fourth ventricle – lies in hindbrain Connects to the central canal of the spinal cord 32 Ventricles of the Brain Protection of the Brain The brain is protected from injury by The skull Meninges Cerebrospinal fluid Blood-brain barrier Cerebrospinal Fluid (CSF) Formed in choroid plexuses in the brain ventricles Choroid plexus is Located in all four ventricles Composed of ependymal cells and capillaries Arises from blood - 500 mL/day Cerebrospinal Fluid Fills the hollow cavities of the brain and spinal cord Functions: Provides a liquid cushion for the spinal cord and brain Nourishes brain and spinal cord Removes wastes Carries chemical signals between parts of the CNS 33 Cerebrospinal Fluid (CSF) Blood Brain Barrier Extensive impermeable capillaries & sinuses Perivascular feet of astrocytes cover and wrap around capillaries and promote tight junction formation Protects brain from hormones & circulating chemicals Prevents most blood-borne toxins from entering the brain Not an absolute barrier Nutrients such as oxygen pass through Allows alcohol, nicotine, and anesthetics through Many glucose transporters 34 Meninges Functions of meninges: Cover and protect the CNS Enclose and protect the vessels that supply the CNS Contain the cerebrospinal fluid between pia and arachnoid maters Meninges Dura Mater Strongest of the meninges Composed of two layers: periosteal layer & meningeal layer Arachnoid Mater Located beneath the dura mater Arachnoid villi - Project through the dura mater, allow CSF to pass into the dural blood sinuses Pia Mater Thin, delicate connective tissue, clings tightly to the surface of the brain Follows all convolutions of the cortex The Spinal Cord Functions of the spinal cord: Spinal nerves attach to it Provides two-way conduction pathway Major center for reflexes Location of the spinal cord: Runs through the vertebral canal Extends from the foramen magnum to the level of the vertebra L1 or L2 Cervical and lumbar enlargements - where nerves for upper and lower limbs arise Conus medullaris - the inferior end of the spinal cord Cauda equina - collection of spinal nerve roots Filum terminale - long filament of connective tissue, attaches to the coccyx inferiorly 35 Spinal Cord Segments Segments that indicate the region of the spinal cord from which spinal nerves emerge Designated by the spinal nerve that issues from it T1 is the region where the first thoracic nerve emerges The Spinal Cord Two deep grooves run the length of the cord Posterior median sulcus Anterior median fissure 36 White Matter of the Spinal Cord White columns: Dorsal (posterior) funiculus Ventral (anterior) funiculus Lateral funiculus Composed of myelinated axons Allow communication between spinal cord and brain White Matter of the Spinal Cord Fibers classified by type: Ascending fibers - afferent (sensory) Descending fibers – efferent (motor) Commissural fibers Major Fiber Tracts in White Matter of the Spinal Cord 37 Gray Matter of the Spinal Cord Shaped like the letter “H” Gray commissure – contains the central canal Dorsal horns consist of interneurons Ventral horns and lateral horns contain cell bodies of motor neurons Organization of the Gray Matter of the Spinal Cord Divided according to somatic and visceral regions SS – somatic sensory VS – visceral sensory VM – visceral motor SM – somatic motor Review of Protection of the Spinal Cord Protected by vertebrae, meninges, and CSF Meninges: Dura mater – a single layer surrounding spinal cord Arachnoid mater – lies deep to the dura mater Pia mater – innermost layer, delicate layer of connective tissue 38 39