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Chapter 10 The Nervous System Introduction • Types of neural tissue: • 1. Neurons – • 2. Neuroglia – support tissue with a variety of functions • Functions of nervous system: • 1. Sensory – • 2. • 3. Motor – use motor neurons to help the body react to stimuli Integrative – integrate signals from sensory & motor neurons to produce thought, memory, etc. Divisions of the Nervous System • Central Nervous System (CNS) – consists of the brain & spinal cord • Peripheral Nervous System (PNS) – Structure of a Neuron • Dendrites – pick up impulses • Cell body – • Axon – sends impulses • Schwann cells – • Myelin – lipid covering formed by Schwann cells; speeds rate of impulse • Axon terminals – Structure of A Neuron • Axon hillock – slight elevation where axon originates • Node of Ranvier – Structure of A Neuron • Neurofibrils – • Nissl bodies – consist of rough ER Neurilemmal sheath – formed by the cytoplasm & nucleus of the Schwann cell that remain on the outside Direction of Impulse • Impulse always travels from dendrites, through cell body, & down axon • Axon synapses w/next neuron or an effector (muscle or gland) Structural Classification of Neurons • Bipolar – • Unipolar – has 1 process from c.b. that divides into 2; in PNS • Multipolar – have many processes from c.b; in CNS Functional Classification of Neurons • Sensory (afferent) – unipolar & carry impulses from body parts to brain or s.c. • Interneurons (association neurons) – • Motor (efferent) – multipolar & carry impulses from brain or s.c. to muscle or gland Types of Neuroglia • Support tissue w/a variety of functions: 1. Astrocytes –star-shaped; found b/t neurons & b.v.; support, transport & communication b/t nerves & b.v. Types of Neuroglia 2.Microglia – Types of Neuroglia 3. Oligodendrocytes – Types of Neuroglia 4. Ependyma – columnar & cuboidal shaped cells; form inner lining of brain & s.c.; provide a layer for diffusion to occur Types of Neuroglia Cell suicide • Microglia can destroy cells that are old &/or damaged • A – healthy neuron • B – neuron being destroyed & DNA breaking apart • C – microglia removing debris Resting Potential • A resting neuron is one not sending an impulse & is in resting potential • The cell membrane of this neuron is polarized b/c of an un= distribution of ions on either side • Outside the neuron – • Inside the neuron – Resting Potential • K+ leak out of K+ channels at a slow rate leaving behind negatively charged proteins • • The voltage meter (next pg.) shows a charge of -70 mv & refers to the charge of a neuron in resting potential Resting Potential Movement of Ions • Ions follow the laws of diffusion (movement from high to low concentrations) when moving thru membranes • Ions enter & leave the membrane thru channels or gates that are specific for that ion Resting Potential The charge outside the cell is positive b/c: 1. 2. the movement of K+ ions to the outside Action Potential • An abrupt change in the electrical potential across the cell membrane that occurs after a stimulus (a.k.a. nerve impulse): 1. Resting neuron stimulated (remember – a resting neuron is polarized) 2. 3. Na+ channels close & K+ channels open; K+ move out & charge reverts back to negative (-70mv); cell is repolarized Resting Potential → Action Potential A)Resting potential (polarized) B)Action potential A.P. in the 1st region stimulates adjacent region (depolarized) C) Graphing Action Potential After repolarization a brief period of delay occurs when Na+ gates cannot temporarily open; called refractory period Graphing Action Potential Hyperpolarization when the cell becomes more negative than -70mv; depends on which ions are allowed to enter the cell, + or – ions (i.e. Cl- ions) Threshold – Impulse Conduction • Saltatory conduction – impulse jumps from 1 node of Ranvier to another; why? • Myelin covering – • channels - are located at nodes of Ranvier for ions to diffuse in & out • Myelinated axons (white matter) - The Synapse • Junction b/t 2 neurons • Presynaptic neuron – occurs before the syapse • Postsynaptic neuron – • Synaptic knob – enlargement of axon terminal • Synaptic vesicles – • Synaptic cleft – space b/t neurons Actual Synapse Events at the Synapse • Action potential travels down presynaptic neuron & arrives at synapse • • This causes vesicles to release ntm • Ntm causes A.P. to enter postsynaptic neuron • The Synapse Types of Neurotransmitters • The nervous system produces approx. 30 different types of ntm • Some open ion channels, others close them • Monoamines: Neuropeptides: - epinephrine - endorphins - enkephalins - dopamine Acetylcholine (ACh) Effects of Ntms • Epinephrine & norepinephrine – hormones when released in blood, but ntm in the n.s.; stimulate autonomic n.s.; incr. HR, resp. rate, etc.; “fight-or-flight” response • Dopamine – • Serotonin – inhibitory; insufficient levels associated with insomnia • Endorphins & enkephalins – generally inhibitory & influence mood; released under stress to reduce pain (blocks substance P) • Substance P – • ACh – stimulates muscles to contract Synaptic Potentials • Ion channels that respond to ntm are called chemically gated channels (as opposed to those that are voltage-gated & are involved in sending A.P.) • Changes in chem. gated channels create local changes called synaptic potentials (a small, temporary change in the potential charge of a neuron) • They allow one neuron to influence another The Synapse Synaptic Potentials • 2 types: 1. Excitatory postsynaptic potential (EPSP) – A true A.P. won’t occur, but will be more likely to occur if the neuron receives more subthreshold stimuli Synaptic Potentials 2. Inhibitory postsynaptic potential (IPSP) occurs when the neuron is hyperpolarized (or becomes more negative than -70mv). An A.P. will be less likely to occur. – Effects of Ntm on Synaptic Potentials • If a ntm opens Na+ channels & Na+ diffuse in, the membrane is depolarized (EPSP) • • A neuron can receive EPSP’s & IPSP’s simultaneously; the neuron responds to the algebraic sum of the + and - charges Synaptic Potential vs. Action Potential • 2 differences: 1. P.S.P. are graded (depends on amt. of ntm) & their effect adds up (called summation) whereas A.P. are all-or-none 2. P.S.P. decr. in intensity w/incr. distance from synapse • Facilitation – Convergence vs. Divergence • Convergence – impulses from 2 or more fibers converge on a single neuron (summation will occur) • Divergence – Convergence vs. Divergence Importance of Ions • Ca+ are needed for the release of ntm • • Insufficient Ca+ levels result in channels remaining open & impulses repeatedly transmitted; results in tetany • May occur in pregnancy (as fetus uses maternal Ca+), when diet lacks Ca+ or Vit D during dehydration Importance of Ions • An incr. in extracellular K+ causes neuron to be less negative; threshold is reached sooner & neurons are very excitable; may result in convulsions • Resting Potential Action Potential Saltatory Conduction EPSP IPSP Convergence vs. Divergence