Nervous system. 3 Basic functions of the nervous system 1-Sensory

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1 Nervous system 3 Basic functions of the nervous system 1-Sensory Detects stimuli, external and internal Converts stimuli into nerve impulses Sends them to Central Nervous System (CNS) 2-Integration Combines signals within CNS Generates a response Based quality and quantity of signals. 3-Motor sends signals out of CNS to peripheral effectors (muscles and glands).

2 I. Central Nervous System (CNS) Two major parts Brain Within cranial cavity; three parts i) Cerebral hemispheres ii) Cerebellum iii) Brain Stem Spinal Cord Within the vertebral cavity. I. Peripheral Nervous System (PNS) 2) Afferent towards afferent sensory neurons 3) Efferent away efferent motor neurons Sensory information; towards CNS Neurons that carry sensory information Motor information; away from CNS Control muscles and glands.

3 efferent (con t) ii) Somatic nervous system Skeletal voluntary Somatic = body Neurons and nerves; control skeletal muscles Movement; maintain posture Conscious. efferent (con t) ii) Autonomic nervous system Smooth muscles Cardiac muscle Glands (endo/exocrine) Involuntary ANS; automatic. Signals to. digestive tract, cardiovascular, urinary, etc. Regulates heart function Regulates release of hormones, sweat, sebaceous Non-conscious.

4 2 subdivisions of the ANS b) Sympathetic c) Parasympathetic Antagonistic functions; both affect the same structures. See ANS later Histology Neuroglial cells Support cells of CNS and PNS. ~ 1 trillion. Maintain a viable environment for neurons Astrocytes a) Maintain ionic environment b) Mechanical support of neurons. c) Nutrient delivery to neurons d) Form scar tissue Oligodendrocytes Form myelin sheath around neuron axons

5 9 Histology Neuroglial cells Microglia CNS immune system. Ependymocytes Phagocytize dead cells, debris and pathogens gliomas Line the ventricles (fluid spaces) of the brain and cord. Circulate, produce and regulate cerebrospinal fluid (CSF) Cancer of neuroglial cells; neuroglial cells can replicate; neurons mostly can t. Example: anaplastic astrocytoma, glioblastoma multiforme

6 11 Neurons Cell body, soma, perikaryon Nissl bodies Neurofibrils Dendrites Protein synthesis. Contains nucleus, mitochondria Neurotransmitter synthesis occurs here. Also contains rer with ribosomes called Protein filaments and microtubules in the soma. Cells extensions; carry nerve impulses towards the soma. Multiple dendrites

7 Neurons Axon Structure Extension that carries nerve impulses away from the soma. Only one axon. Axon hillock Axoplasm Axolemma Bulge on the soma ; origin of the axon. Axon cytoplasm. Axon cell membrane. Neurons Axon collaterals Axon terminals (telodendria) Axon branches; usually connect to different cells. All collaterals carry the same signal. Dendrites do not. Multiple points of contact with the target cell. All telodendria carry the same signal. Telodendria

8 Neurons Synaptic end bulbs Synaptic vesicles Expanded end of a telodendrion; part of the synaptic junction (AKA, synapse) Within the end bulb; contain neurotransmitter (NT). Nerve impulse stimulates NT from the end bulb.

9 17 Axoplasmic flow Axons can be long. Need to deliver materials to the axon terminal Two mechanisms Delivery via diffusion; slow axoplasm movement. Uni-directional; towards end bulb. Slow; 1-2 mm/day. Axonal transport Requires neurotubules (microtubules) and kinesins (molecular motors). NT vesicles attached to kinesins; transported along the neurotubules. Recycled material is transported towards the soma. Bi-directional Fast; mm/day

10 19 Nerve fiber = axon + sheath. NERVE; many nerve fibers together covered by CT Myelin sheath Insulating cover Made of and by Schwann cells, AKA neurolemmocytes. Glial cells of PNS. Cells wrap around the axon many times; layers of PM. PM mostly lipid + transport or channel proteins Sheath is a good electrical insulator.

11 Myelin breaks down or not formed properly Multiple sclerosis nerve fibers myelin breaks down. Usually in the PNS. Speed of nerve impulse declines; signals become garbled. Possible paralysis and loss of sensory function. Nodes of Ranvier (neurofibril nodes) Gaps in the myelin. Actually speed up nerve transmission. See later Neurolemmocytes/ Schwann cells In PNS Neurolemma Regeneration Part of peripheral nerves Neurolemmocyte PM covers axons. No exposed axons. Even in the nodes. Schwann cells rebuild the sheath; allow axon regeneration. Peripheral nerves can regenerate; assuming no excess scarring.

12 Oligodendrocytes Form myelin in the CNS. No regeneration These cells are not very effective at repairing nerve damage. Mostly, CNS damage appears to be permanent. Generation of an Action potential (nerve impulse) At the resting state 1) The different concentrations of Na+ and K+ across the plasma membrane are maintained by the Na+/K+ ATPase with the expenditure of energy in the form of ATP. Membrane potential difference (PD)=-70mv, inside negative. The membrane is polarized.

13 At the resting state 2) Activation gate (called this because when the gate is stimulated it activates or opens the channel) on the voltage-sensitive Na + channel is closed. 3) The inactivation gate is open at present. 4) No Na + flow in or out of the cell. 5) The voltage-sensitive K + channel is slightly open, so there is a small K + leak out of the cell. 1) A stimulus opens the activation gate of the voltage-sensitive Na + channel. 2) The channel is now open and Na + flows inward, down its [Na + ] and electrical gradient. 3) As more Na + enters the cell, the inside of the cell becomes positive. The PD passes 0 mv, then approaches +30 mv. The membrane becomes depolarized.

14 1) The stimulus also causes the inactivation gate (called that because it inactivates the channel when it is stimulated) to close. This happens slowly, after the channel has been open for several microseconds. 2) The flow of Na + into the cell stops. 3) The membrane PD does not go beyond +30 mv. 1) The voltage-gated K + channels opens slowly, at the same time that the inactivation gate of the Na + channel closes. 2) K + flows out of the cell, down the [K + ] and electrical gradient: Remember- the inside of the cell in now more positive than the outside.

15 3) K + movements carries positive charges out of the cell and the inside becomes more negative. The cell becomes repolarized. 4) When PD= -70 mv the gates return to their resting state. 5) The Na + -K + ATPase returns the concentrations of the Na + and K + ions to the resting state levels. 30

16 Refractory period Absolute Def: time after an initial stimulus a second stimulus cannot generate another AP. During the time of Na + flow into the cell. Na+ channels are already open; you can t open them any further Relative During the time of K + flow out of the cell. Na + channels are closed; K + channels are open. Strong stimulus can force the Na + channels open early. Starts a new AP. Nerve AP vs. Muscle AP nerve muscle 32

17 Saltatory conduction Nodes of Ranvier Increase speed of conduction APs that jumps from node to node, down the axon Space between nodes is covered by myelin cannot transport ions. Ion transport only occurs at the nodes AP jumps node to node So the speed of the AP increases. Decrease in energy use Only the nodes depolarize and repolarize, reduces the number of transported ions requires less ATP

18 Nerve fiber types Fiber types A B C Size Myelination 4-20µm 2-4µm <2µm Yes Yes No Speed 140m/sec 18m/sec 1m/sec Example Some sensory and skeletal Temp. pain, smooth muscles Temp. pain, smooth muscles Neuroeffector junction Between a neuron and an effector (neuron, muscle, etc.) Neuromuscular junction (motor endplate) Between a neuron and muscle cell (the effector) Neuroglandular junction Between a neuron and gland (endocrine gland, sweat, sebaceous)

19 Synapse Junction between neurons AKA, neuro-neuronal junction. Synaptic end bulb of one neuron and dendrite, soma or axon of the next. Structure End bulb is separated from the surface of the next cell by the Synaptic cleft or gap; signal is transmitted across the cleft via neurotransmitters. Synaptic end bulbs Can form synapses at several spots on the surface of next cell Synapse on the dendrite. Axodendritic Axosomatic Axoaxonic On the soma. On the axon/axon hillock.

20 One-way impulse conduction Pre-synaptic cell: cell before the synapse Post-synaptic cell: cell after synapse Synaptic vesicles found only in the synaptic end bulbs of the pre-synaptic cell Neurotransmitter AP is converted to the NT chemical signal in pre-synaptic end bulb, Signals travel from pre-to post-synaptic cell only. Post-synaptic cell receives a pulse of NT; can be either stimulated or inhibited. Excitatory transmissions cause post-synaptic cell to become stimulated. Excitatory transmitter-receptor interaction AKA, ETRI An interaction where post-synaptic cell PD moves closer to depolarization; becomes more positive.

21 Two mechanisms can cause an ETRI 1- Receptors open Na+ channels Receptors in the synapse respond by the opening Na+ channels; which allows Na+ flow in; PD moves towards positive. 2- Adenylate cyclase camp-kinases More complex mechanism is activated; see next frame.

22 Caffine Drug is a Phosphodiesterase inhibitor Phosphodiesterase is inhibited; can t break down camp. camp remains high. Whatever processes the camp regulates stay active. Excitatory post-synaptic potential, EPSP EPSP; any post-synaptic potential where the PD moves towards positive. Increased entry Na+ or decreased loss of K+. Facilitation Post-synaptic PD moves towards positive; has not depolarized. Easier for a second EPSP to depolarize neuron. Summation Spatial Several EPSPs arrive at the SAME TIME from different synapses Temporal Several EPSPs arrive at slightly DIFFERENT times from the same synapse.

23 Inhibitory transmissions Inhibitory transmitter-receptor interaction or ITRI NT may inhibit the post-synaptic cell. Inhibitory post-synaptic potential IPSP PD of the post-synaptic cell becomes more negative; more difficult for the cell to depolarize. May require many more EPSPs for cell depolarization. Integrator post-synaptic neuron Cell which sums the EPSPs and the IPSPs. If EPSPs > IPSPs; integrator neuron fires. If EPSPs < IPSPs; neuron is inhibited. If EPSPs = IPSPs; neuron does not fire. Neurotransmitters > 50 different NTs identified. Acetlycholine (ACh) Peripheral mostly motor endplate and other synapses Peripheral NS Acetlycholine esterase Breaks Ach into Acetate and choline Choline is transported into the synaptic end bulb; recycled into ACh. Acetate diffuses out of the gap and eventually into the blood. More on NTs later.

24 Neural networks Collections of neurons. Information processing system. 100,000s of such interconnected networks. Simple Diverging Series of neurons linked together. One neuron connects to several others; in turn connect to more; signal diverges; single stimulus activates multiple neurons. Converging One neuron receives signals from many; signal converges; many summed stimuli causes a single response. Reverberating One axon sends an axon collateral to a pervious neuron; network is re-stimulated without a new signal; echo circuit.

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