Nervous tissue notes PDF

Preview

Summary

Lectures will provide an overview of functional anatomy at all levels, from the systems to the tissues. The goal is to provide a mechanistic understanding of the different structures in our body as a foundation for human-health oriented studies....

Description

Chapter 12 Nervous Tissue 12.1 Overview of the Nervous System • Central nervous system o Brain/SC o SC connected to brain through foramen magnum of the occipital bone o Emotions, memories, thoughts • Peripheral Nervous System o Ganglia – small masses of nervous tissue o Enteric plexus –extensive networks of neurons located in walls of organs of GI tract o Sensory receptors – structure of nervous system that monitors changes in the external or internal environment o 12 pairs of cranial nerves form the brain o 31 pairs of spinal nerves from spinal cord o SOMATIC NS (SNS)  Sensory neurons that convey info to cns from somatic receptos in head, body wall, limbs and from receptors for the senses of vision, hearing, taste, and smell  Motor neurons that conduct impulses form the cns to skeletal muscles only.  voluntary o AUTONOMIC NS (ANS)  Sensory neurons that convey info to the cns from autonomic sensory receptors located primarily in visceral organs like stomach and lungs  Motor neurons that conduct impulses from cns to smooth muscle, cardiac muscle, and glands  Involuntary  Sympathetic – flight or fight  Parasympathetic – rest and digest o ENTERIC NS (ENS)  Brain of the gut  Consists of over 100 mil neurons in enteric plexus that extend length of GI tract  Monitor chemical changes and stretching of GI tract • Functions of nervous system o Sensory function– detecting changes, internal and external o Integrative function – processing o sensory info – making decision on appropriate responses (integration) o Motor function – activation of effectors • ***FIG 12.1**** 12.2 Histology of Nervous tissue • ***neuroglia are smaller cells but they greatly outnumber neurons. They support, nourish, and protect neurons, and maintain /intersticial fluid that bathes them. Neuroglia continue to divide over a lifetime • Neurons o Possess electrical excitability – ability to respond to stim and convert to AP

•

Parts of neuron o Cell body (soma) – nucleus surrounded by cytoplasm that includes lysosomes, mitochondria, golgi complex o Nissl Bodies – cell bodies that contain free ribosomes and prominent clusters of rough ER o Cytoskeleton  Neurofibrils – bundles of intermediate filaments that provide the cell shape and suport  Microtubules – assist in moving materials b/w cell body and axon

Lipofuscin – pigment that occurs as clumps of yellowish brown granules in the cytoplasm • Product of neuronal lysosomes that accumulates as the neuron ages, but does not seem to harm the neuron o Nerve fiber – two kinds of processes ; multiple dendrites and a single axon o Axon – joins cell body at “AXON HILLOCK”  Initial segment – closest to axon hillock • Nerve impulses arise at the junction of axon hillock and initial segment called the “trigger zone”  Axoplasm – cytoplasm of an axon  Axolemma – plasma membrane of an axon  Axon collaterals – branches off the neuron  Axon terminals  Synaptic end bulbs – swellings at the synapse • Also varicosities  Slow axonal transport – 1-5mm/day cell bodyaxon  Fast axonal transport – 200-400mm/day used [proteins as motors to move materials along surfaces of microtubules • Both anterograde and retrograde  Structural diversity of neurons o Great diversity in size and shape Classification of neurons o Structural classifications  Multipolar – several dindrites and one axon  Bipolar – one main dendrite, and one axon (retna of eye, inner ear, olfactory area)  Unipolar – dendrites and axon fused together to form a continuous process from cell body • Most unipolar neurons function as sensory receptors  Purkinji cells  Pyramidal cells o Functional Classification  Sensory (afferent) – sensory receptors at their distal ends or are located just after sensory receptors that are separate cells • AP formed in axon and conveyed into the CNS through cranial and spinal nerves  Motor (efferent) – AP away from CNS to effectors in PNS through cranial and spinal nerves 

• •

 •

•

Interneurons – integrate and process incoming info and elicit correct motor response

Neuroglia o Smaller than neurons but 5-25 more numerous o Do not generate or propagate AP o Can multiply and divide in the mature NS o Brain tumours derived from glia, called gliomas, tend to be highly malignant and to grow rapidly. Neuroglia of CNS o Astrocytes  Most numerous of neuroglia  Protoplasmic – many short branching processes and found in grey matter  Fibrous astrocytes – long unbranched processes and are lovaed mainly in white matter  Functions • Contain microfilaments – enables them to support neurons • Processes of astrocytes wrapped around blood capillaries isolate neurons of the CNS from various harmful substances in blood by secreting chemicals that maintain the unique selective permeability

characteristics of the endothelial cells of the capillaries. In effect the endothelial cells create a blood brain barrier, which restricts the movement of substances between the blood and interstitial fluid of the CNS • In embryo, they secrete chemicals to regulate growth, migration, and interconnection among neurons of the brain • Maintain appropriate chemical environment for the generation of nerve impulses. (potassium ions- K), take up transmitter, and serve as a conduit for the passage of nutrient and other substances between blood capillaries and neurons • May play a role in learning and memory o Oligodendrocytes  Resemble astrocytes, but smaller and contain fewer processes  Responsible for forming and maintaining the myelin sheath around CNS axons o Microglia  Small cells with slender processes that give off numerous spine like projections  Function as phagocytes – remove cellular debris o Ependymal cells  Cuboidal to columnar cells arranged in a single later that possess microvilli and cilia  Line ventricles of the brain and central canal of spinal cord  Produce, monitor, and assist in the circulation of Cerebrospinal fluid  Form blood-cerebrospinal fluid barrier • Neuroglia of PNS o Schwann cells  Encircle PNS axons, form myelin sheath  Each shwann cell myelinates a SINGLE o o Satellite cells  Surround cell bodies of PNS ganglia  Regulate the exchanges of materials between neuronal cell bodies and interstitial fluid Myelination • Myelin sheath – speeds nerve impulses • Produced by schwan cells and oligodendrocytes • Neurolemma – final coating of schwann cells around a sheath o When axon is injured, the neurolemma aids regeneration by forming a regeneration tube that guides and stimulates regrowth of the axon • Nodes of Ranvier – gaps in myelin sheath o Appear at intervals of axon • Each Schwann cell wraps one axon segment between two nodes • Oligodendrocyte myelinates parts of several neurons o Neurolemma is not present because the oligodendrocyte cell body and nucleus do not envelop the axon o Display little growth after injury – in part due to the absence of neurolemma • Amount of myelination increase from birth to maturity. Collections of nervous tissue • Clusters of bodies, bundles of neurons Clusters of neuronal cell bodies • Termed ganglion, closely associated with cranial and spinal nerves Bundles of axons • Nerve – bundle of axons located in PNS • Cranial nerves connect the brain to periphery • Spinal nerves connect the spinal cord to periphery • TRAT – bundle of axons located in CNS – tracts interconnect neurons in the spinal cord and brain Gray and White Matter

• •

White matter – myelinated axons Gray matter – neuronal cell bodies

12.3 Electrical signals in neurons • Nerve impulse example o Touch – graded potential develops in a sensory receptor of the skin o Graded pot triggers the axon of sensory neuron to form a nerve AP, which travels along the axon into the CNS and ultimately causes the release of neurotransmitter at a synapse with an interneuron o Neurotransmitter stimulates the interneuron to form a graded potential in its dendrites and cell body o In response to graded potential, axon of interneuron forms a nerve AP. Results in neurotransmitter release at the next synapse with another interneuron o This occurs over and over as interneurons in higher parts of the brain are activated o Once interneurons in the CEREBRAL CORTEX are activates, perception occurs and you are able to feel the smooth surface of the pen • Suppose you want to use the pen o A stim un the brain causes graded potential to form in dendrites and cell body of upper MN. This graded potential subsequently causes a nerve AP to occur in the axon of the upper MN, followed by neurotransmitter release o Neurotransmitter generates a graded potential in a lower MN graded potential triggers the formation of a nerve action potential and then release of the neurotransmitter at neuromuscular junctions formed with skeletal muscle fibers that control movements of the fingers o The neurotransmitter stimulates muscle fibers that control finger movts to form muscle AP. The muscle Aps cause these muscle fibers to contract, which allows you to write with the pen • Production of GPs and APs depends on two basic features of the plasma membrane of excitable cells: the existence of resting membrane potential and the presence of specific types of ion channels. • Membrane potential – electrical potential difference across the membrane • The membranes of neurons contain many different kinds of ion channels that open/close in response to specific stimuli • Ion Channels o Move from higher concentration to lower concentration o Positively charged cations move towards a negatively charged area – vice versa o Open and close due to “gates” o 4 types  Leak channels • Randomly alter between open and close • More leakier K channels then Na  Ligand gated channels • Responds to binding of a ligand stimulus • Neurotransmitters, hormones, and particular ions • Located in the dendrites of some sensory neurons and in dendrites and cell bodies of interneurons and motor neurons  Mechanically gated channels • Responds to mechanical stim in the form of vibration, touch, pressure, or tissue stretching • Examples o Auditory receptors in ears, internal organ stretching, touch receptors in skin  Voltage gated channels • Respond to change in membrane potential • Resting membrane potential o Build-up of negative ions in the cytosol on inside of membrane, and positive ions in ecf along outside of membrane o Form of potential energy

Normally -70 mV Arises from  Unequal distribution of ions in ECF and cytosol  Inability of most anions to leave the cell  Electro genic nature of the Na-K ATPases • Help pump Na out and K in as fast as they leak the opposite way Graded potential o Small deviation from resting potential o Hyperpolarizing graded potential – inside more negative o Depolarizing graded potential – inside more positive o Mainly occur in the dendrites and cell body of a neuron  This is where ligand gated channels are] o Decremental conduction – potentials die out as they spread along the membrane o Can be come stronger via Summination Generation of Action potentials o Depolarizing phase  Stimulus causes depolarization to threshold  Voltage gated Na channels open (activation state) - both activation and inactivating gate are open and Na inflow begins  Positive feedback occurs, with more and more channels opening  ~20 000 Na flow across membrane o Repolarizing phase  Shortly after voltage gate Na channels open, the inactivation gates close  Voltage gated K channels open more slowly than Na, therefore they are fully open when Na channels close o After-hyperpolarizing phase  As the voltage gated K channels close, membrane potential is roughly -90mV o Refractory period  Absolute refractory period • Period of time after an action potential begins during which an excitable cell cannot generate another action potential in response to a normal threshold stimulus.  Relative refractory period • Second AP can be initiated, but only by a larger than normal stimulus • Na channels have returned to resting state Propagation of APs o AP regenerates over and over at adjacent regions of the membrane from trigger xone to axon terminal o One way direction  Continuous and Saltatory conduction o Continuous described above o Salutatory conduction – along myelinated axons  Leaps from Each node of Ranvier to another  Opening a smaller nuber of channels only at the nodes, rather than many channels in each adjacent segment of the membrane, represents a more energy efficient mode of conduction Factors that affect propagation speed o Myelination o Axon diameter  Larger d propagate Aps faster than smaller ones due to surface area o Temperature  Lower speeds when cooled o o

•

•

•

•

•

•

Classification of nerve fibers o A fibers – largest diameters, and myelinated  conduct impulses at speed of 12-130m/sec  Touch pressure, position of joints, thermal, pain o B fibers

•

•

 Myelinated – speed of 15m/sec  Somewhat longer absolute refractory period than A fibers  Sensory impulses from the viscera to the brain and SC o C fibers  Smallest in dimeter  Unmyelinated – speed of 0.5-2 m/sec  Longest refractory periods  Autonomic motor fibers that extend from autonomic ganglia to stimulate heart, smooth musce and glands Encoding of stimulus identity o Depends on the frequency of action potentials. o Number of sensory neurons activated (recruited) Comparison of electrical signals produced by excitable cells o TABLE 12.2

12.4 Signal Transmission at Synapses • Axodendric = axon to dendrite • Axosomatic = axon to cell body • Axoaxonic = axon to axon • Electrical synapses o Aps conduct between adjacent cells via Gap Junctions o Gap junctions contains a hundred or so tubular connexions “tunnels” which connect cytosols o Main advantages of electrical synapses  Faster communication than chemical synapses  Synchronization • Large number of APs can be produced in unison if they are connected by gap junctions • Chemical Synapses o Do not touch adjacent cells – chemical synapse o Presynaptic neurons release neurotransmitter that binds to receptors on postsynaptic neuron, producing a postsynaptic potential o Synaptic delay of about 0.5msec o 1) nerve impulse arrives ay synaptic end bulb o 2) voltage gated Ca channels open o 3) increase conc. Of Ca triggers exocytosis of synaptic vesicles o 4) neurotransmitters diffuse across synaptic cleft and bind to neurotransmitter receptors in the postsynaptic plasma membrane o 5) binding on ligand gated channels opens the cannels and allows ions to flow across membrane o 6) creates postsynaptic potential – depending on which ions the channels admit, the potential may be a depolarization (excitation) or hyperpolarization (inhibition) o 7) when a depolarizing postsynaptic potential reaches threshold, it triggers an action potential in the axon of the postsynaptic neuron • Excitatory and Inhibitory Postsynaptic Potentials o EPSP – excitatory postsynaptic potential o IPSP – inhibitory postsynaptic potential • Structure of neurotransmitter receptors o Ionotropic receptor  Contains a binding site and a channel (part of the same protein)  Type of a ligand gated channel o Metabotropic receptor  Contains neurotransmitter binding site but lacks an ion channel  Coupled to a separate ion channel by a type of membrane protein called a G protein • Removal of Neurotransmitter o Diffusion

Enzymatic degredation – ex. acetylcholinerase Uptake by cells – (reuptake) • and temporal summation Spatial – multiple stim Temporal – numerous stim from same nuron REMEMBER – it is the NET SUMMINATION OF IPSP AND EPSP that determine if the postsynaptic neuron will reach threshold o SEE TABLE 12.3 12.5 Neurotransmitters • Small molecule neurotransmitters o Acetylcholine  Released by many PNS neurons and some CNS neurons  Binds to ionotropic receptors to open cation channels – excitatory response  Binds to metatropic receptors coupled to G proteins that open K channels – inhibitory response o Amino acids  Glutmate and Aspartate – excitatory affects  GABA – inhibitory neurotransmitters • Anti anxiety drugs like Valium enhance GABA o Biogenic Amines  Most bind to metabotropic reveptors  catecholamine  Norepinephrine • Released from adrenal medulla  Dopamine • Regulate skeletal muscel tone • Addiction, emotional response  Serotonin • Sensory perception, temp reg, control of mood,, appetite, induction of sleep o ATP and other Purines  ATP, ADP, AMP o Nitric Oxide  Excitatory response  Enzyme NOS (nitric oxide synthase) catalyzes formation of NO  NO not synthesized in advance and packaged into synaptic vesicles. Rather, it is formed on demand and acts immediately  Action brief because NO is highly reactive free radical  Memory and learning o Carbon Monoxide  Formed as needed and diffuses out of cells that produce it into adjacent cells  Excitatory neurotransmitter produced by brain in response to some neuromuscular and neuroglandular functions  Diltion of BV, memory, olfaction (smell), vison, thermoreg, insulin release, anti inflammatory • Neuropeptides o Neurotransmitters consisting of 3-40 AA linked by peptide bonds o Bind to metabotropic receptors o Formed in neuron cell bond, packed into vesicles and transported to axon terminals o Enkephalins – 200x stronger than morphine o Opoid peptides – endorphins and dynophins  Though to be the bodies natural pain killer o Substance P – resleased by neurons that transmit pain related input from peripgeral pain receptors into the CNS  Also known to counter the effects of certain nerve damaging chemicals, prompting speculation that it might prove useful as a treatment for nerve degeneration o TABLE 12.4 o o Spatial o o o

12.6 Neural Circuits • Simple series circuit – presynaptic neuron stimulate a postsynaltic neuron o Divergence – single presynaptic neuron stimulating several post synaptic neurons at the same time • Diverging circuit – nerve impulse form a single presynaptic neuron causes the stimulation of increasing number of cells along the circuit o Ex. small amount of neurons in brain govern a particular body part causes stimulation of increasing number of cells in spinal cord • Converging circuit – postsynaptic neuron receives nerve uimpulses from several differen sources o Ex. single motor neuron that synapses with skeletal MF at NM junctions receives input from several pathways that originate from different brain regions • Reverberating Circuit – stimulation of the presynaptic cell causes the postsynaptic cell to transmit a series of nerve impulses – branches from later neurons synapse with earlier ones. This arrangement sends the impulse in a loop o Ex. coordinated muscle activities, waking up, short term memory • Parallel after discharge circuit o Single presynaptic cell stimulates group of neurons, each of which synapse with a common postsynaptic cell o Ex. mathematical calculations 12.7 Regeneration and Repair of nervous Tissue • Plasticity – capability to change based on experience o Sprouting of new dendrites, synthesis of new proteins, changes in synaptic contacts with other neurons • Neurogenesis in CNS o Epidermal Growth Factor (EGF)  Stimulated cells taken from the brains of adult mice to proliferate into both neurons and astrocytes o Nearly complete olack of neurogenesis results from  Inhibitory influences from neuroglia, particularly oligodendrocytes  Absense of growth-stimulating cues that were present during fetal development • Damage and repair in the PNS o Axon and dendrites that are associated with a neurolemma may undergo repair if the ell body is intact, fi the Schwann cells are functional, and if scar tissue formation does not occur too rapidy..  A person who injures axons of a nerve in upper limb has a good chance of regaining nerve function o Chromatolysis  24-48hrs after injury, Nissl bodies break up into fine granular masses  Following chromatolysis, signs of recovery become evident4  Shwann cells multiply  RNA generation increases o Wallerian degeneration  Degeneration of the distal portion of the axon and myelin sheath o Regeneration ube  Shwann cells multiply across injured area  Tube guides growth of a ner axon deom the proximal ...