Neural Tissue. Chapter 12 Part B

Transcription

1 Neural Tissue Chapter 12 Part B

2 CNS Tumors - Neurons stop dividing at age 4 but glial cells retain the capacity to divide. - Primary CNS tumors in adults- division of abnormal neuroglia rather than from the division of abnormal neurons. - Primary CNS tumors involving abnormal neurons occur in young children. - Secondary tumors arise from metastasis (spread of cancer cells that originate in other parts of the body) Injury Repair & Regeneration of Nervous Tissue If nervous tissue is damaged regeneration depends on the extent of injury and its location. Most injuries in nervous tissue are permanent. In CNS: - very limited axon regeneration occurs after injury. - astrocytes form scar tissue that prevents axon growth across damaged area. - astrocytes release chemicals that blocks regrowth of axons. - slower cellular debris clearance impede axonal regrowth.

3 Injury Repair & Regeneration of Nervous Tissue In PNS: If the cell body of the neuron is damaged no regeneration. If an axon is damaged Wallerian degeneration process is triggered. The axon past the injury breaks down macrophages clean up Schwann cells multiply to form a pseudo-tunnel axon grows through the tunnel full recovery including synapse.

4 Myelination (Review) saltare -means to jump

5 Neurophysiology Resting Neuron Resting neuron: refers to a neuron that is not yet stimulated. There is unequal distribution of ions outside (extracellular fluid) and inside (cytoplasm/intracellular fluid) of a neuron: Outside: greater concentration of Na +, Cl - Inside: greater concentration of K +, negative ions such as Cl -, PO 4, proteins -, etc.

6 Neurophysiology Resting Neuron Na + -K + pump Resting neuron: Outside: positive Inside: negative Membrane is called polarized. + outside -inside The difference in charges across the membrane of this resting neuron resting membrane potential (RMP).-70mV.indicating negative inside. How is RMP maintained (Why is the inside of a cell more negative than outside)? A. Many more K + leak channels than Na + leak channels K+ ions exit the cell more rapidly than Na+ ions entering the cell. B. Negatively charged larger proteins - and other negative ions cannot easily diffuse out. C. Na + - K + pumps use ATP to pump (3) Na + out and (2) K + in against their conc. gradient.

7 Neurophysiology Stimulated Neuron Threshold stimulus Action potentials occur only when the membrane is stimulated (depolarized) enough so that sodium (Na+) channels open completely. The minimum stimulus needed to achieve an action potential is called the threshold stimulus, which is around -55 mv (When the depolarization reaches about -55 mv, a neuron will fire an action potential). All-or-None Law Action potentials occur maximally or not at all. In other words, there's no such thing as a partial or weak action potential. Either the threshold potential is reached and an action potential occurs, or it isn't reached and no action potential occurs.

8 Action potential (AP) Neurophysiology Stimulated Neuron An action potential is a very rapid change in membrane potential that occurs when a nerve cell membrane is stimulated (stimulus e.g:-chemical signals/neurotransmitters). First step in the generation of an AP is the opening of voltage gated Na+ channels, at one site, usually the initial segment of axon. Movement of Na+ ions into the axon depolarizes adjacent sites, triggering the opening of additional voltage-gated Na+ channels. Result is a chain reaction that spreads across the length of the axon, ultimately reaching axon terminals.

9 Generation of an action potential Voltage-gated Na+ channel Voltage-gated K+ channel

10 Generation of an action potential depolarization repolarization Voltage-gated Na+ channel Voltage-gated K+ channel An impulse (also called an action potential) is a wave of depolarization followed by a wave of repolarization.

11 Types of Nerve Fibers Nerve fibers can be classified into type A, B, and C Depending on the diameter of the fiber, myelination and speed of conduction. 1. Type A fibers- Fastest conduction-largest myelinated axons, diameter- 4 to 20 microns fibers carry action potential at a speed of 120 meters/second, e.g. motor neurons that control skeletal muscles. 2. Type B fibers- Smaller myelinated axons, diameter-2 to 4 microns fibers carry action potential at a speed of 18 meters/second. 3. Type C fibers- slowest conduction- unmyelinated axons, diameter-less than 2 microns fibers carry action potential at a speed of 1 meter/second. Type B and C fibers carry instruction to smooth muscle, gland and peripheral effectors. Why not have all neurons myelinated, with large diameter axons and conduct fast?? 1. Our body does not respond to everything very fast! When you touch something hot you need to have fast conduction and fast action! If it gets hot outside and body temperature starts to go up you don t need to have fastest conduction to start sweating and cooling! 2. If all the neurons of the body were myelinated that will have tremendous affect on the size of the brain, spinal cord and the entire body!

12 How Do Neurons Pass Impulses To The Next Cells? Axon terminals Direction of action potential Presynaptic cell Postsynaptic cell When a neuron sends an impulse the wave of depolarization reaches the axon terminals now the impulse has to jump to the next cell.could be another neuron or an effector (a muscle fiber or a gland). Synapse: the point of contact between the two cells. Pre-synaptic cell: the neuron that brings the impulse. Post-synaptic cell: the cell to which the impulse is transferred.which could be a neuron, muscle fiber or a gland.

13 Synapse/Junctions A Neuron Neuron B Neuron Muscle fibers C Neuron Gland A. If a neuron connects with another neuron interneural synapse/junction. B. f a neuron connects with a muscle fiber neuromuscular synapse/junction. C. If a neuron connects with a gland neuroglandular synapse/junction. Notice: Presynaptic cell is always a neuron. Postsynaptic cell could be a neuron, muscle fiber or a gland.

14 Synapse/Junctions - Types How do synapse transfer impulses (wave of depolarization) from the presynaptic cell to the postsynaptic cell?? There are two types of junctions/synapse: 1. Electrical synapse 2. Chemical synapse

15 Synapse/Junctions Electrical Synapse Pre-synaptic cell Plasma membranes Post-synaptic cell Electrical synapse: Not common.found in eye, cardiac muscle tissue and certain areas of brain. Membranes of presynaptic cell and postsynaptic cell are physically connected by gap junctions.structures that also allow ionic flow through the gap. Wave of depolarization travels along the presynaptic cell membrane continues on to the membrane of the postsynaptic cell.continuous and faster transfer! Allows 2-way conduction in CNS.

16 Synapse/Junctions Chemical Synapse Synaptic vesicles Ca 2+ Ca 2+ Ca 2+ Chemical synapse: Most common in the body. Membranes of presynaptic cell and postsynaptic cell are not physically connected.space between synaptic bulb and membrane of the postsynaptic cell synaptic cleft has high concentration of Ca 2+. Mechanism is more complex.many more steps therefore, slower mechanism. Special chemicals (neurotransmitters) connect presynaptic cell with postsynaptic cell..chemical synapse! Presynaptic cell stores neurotransmitters inside small synaptic vesicles.

17 Synapse/Junctions Chemical Synapse Impulse/action potential/wave of depolarization travels along the membrane of the presynaptic neuron impulse reaches the synaptic bulb near the cleft Ca 2+ gates open Ca 2+ in the cleft diffuses into the synaptic bulb of presynaptic neuron Ca 2+ triggers the movement of the synaptic vesicles towards the cleft synaptic vesicles fuse with the plasma membrane of the synaptic bulb neurotransmitters are released into the cleft neurotransmitters travel across the cleft and bind to neurotransmitter receptors present on the membrane of the postsynaptic cell that triggers ionic diffusion and generation of a wave of depolarization/impulse on the postsynaptic cell! Presynaptic neuron Ca 2+ Ca 2+ Ca 2+ Ca 2+ Ca 2+ Postsynaptic neuron Since neurotransmitter release and binding to its receptor is one-way..chemical synapse works only one-way- from presynaptic membrane to the post-synaptic membrane! Presynaptic neuron Postsynaptic neuron

18 Synapse/Junctions Chemical Synapse If the neurotransmitter remains in the cleft postsynaptic cell will remain stimulated indefinitely! It is important to remove it after the transfer is done successfully. How is neurotransmitter removed from the cleft?? 1. Some of it diffuses out of the cleft region taken up by glial cells and destroyed. 2. Some of it is reabsorbed by the presynaptic cell and recycled to make more neurotransmitter. 3. Some of it is destroyed by enzymes present in the cleft. Example: Common neurotransmitter is acetylcholine and an enzyme acetylcholinesterase is present in the cleft for breakdown of acetylcholine.

19 Neurotransmitters Neurotransmitters are chemicals released by the presynaptic cell to stimulate impulse generation on the post synaptic cell. Presynaptic cells have synaptic vesicles containing neurotransmitters. Postsynaptic cells have neurotransmitter receptors to receive the neurotransmitters. Two major categories of neurotransmitters present in the synapses in the brain, spinal cord, neuromuscular and neuroglandular junctions: 1. Excitatory neurotransmitters: cause excitation of the postsynaptic cell membrane wave of depolarization/impulse/action potential is generated on the postsynaptic cell. 2. Inhibitory neurotransmitters: cause hyperpolarization of the postsynaptic cell inhibition of impulse generation on the postsynaptic cell. Some neurotransmitters are excitatory, some are inhibitory. Some neurotransmitters are excitatory in one location and inhibitory at another location (e.g. acetylcholine is excitatory in CNS and neuromuscular junction in the skeletal muscle, but acetyl choline is inhibitory in the neuromuscular junction in the heart-slows down heart rate & decreases blood pressure). The effect of a neurotransmitter on the postsynaptic membrane depends on the properties of the receptor that it binds to and not on the nature of the neurotransmitter. Some synapses may have 2-3 types of neurotransmitters.

20 Neurotransmitters 1) Acetylcholine (ACh): mostly excitatory sometimes inhibitory. Common in brain, spinal cord, and neuromuscular junctions.cholinergic synapses. Decreased level in Alzheimer patients (Ach is important for memory processing and learning). 2) Amino acid neurotransmitters: can be excitatory (glutamate, aspartate) or inhibitory (glycine, GABA). Found in brain and spinal cord. 3) Amines: neurotransmitters derived from amino acids.usually from tyrosine. a) Epinephrine/norepinephrine b) Serotonin- decreased level-depression. c) Dopamine- inhibitory (prevents overstimulation of neurons that controls skeletal muscle tone) or excitatory neurotransmitter. Decreased levels in Parkinson s disease involuntary tremors.treated by giving L-dopa to inhibit muscle contractions. 4) Neuropeptides: short proteins.3-40 amino acids. Enkephalins and endorphins.inhibitory.natural painkillers. 5) Gases: nitric oxide (NO) generated by axon terminals that innervate smooth muscles in the wall of blood vessels causes vasodilation.