Chapter 48 ~ Nervous System

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Nervous System Overview Sensory Input Integration Motor Output-signal conducted from processing center to effector cells Signals Conducted by Nerves-extensions of nerve cells Nervous System Composition: Neurons and Glia (supporting cells) Neurons communicate information via electrical and chemical signals

Both Divisions of the Nervous System Involved 1. Central nervous system (CNS)~ brain and spinal cord; Integration 2. Peripheral nervous system (PNS)~ sensory (input) and motor neurons (output) Effector cells~ muscle or gland cells Nerves~ bundles of neurons wrapped in connective tissue

Neuron structure Neuron- structural and functional unit Cell body- nucelus and organelles Dendrites- signals to cell body. Short, numerous Axons- away from cell body. Long, Myelin sheath- supporting, insulating layer produced by Schwann Cells Schwann cells-PNS support cells; surround axons Axon hillock-Hillock-axon extends from here Synaptic terminals~ neurotransmitter releaser Synapse- gap / neuron junction

3 Classes of neurons 1. Sensory neuron: receive & convey from sensory environment information to spinal cord 2.Interneurons: information integration; located in CNS. Synapse only with other neurons. 3. Motor neurons: convey impulses from CNS to effector cell. (muscle or gland)

Neurons Grouped into Nerve Circuit The Reflex Arc Simplest : Knee-Jerk Reflex (Patellar Reflex) Stretch receptor simple response; sensory to spinal cord to motor neurons—knee contracts

Neural Signaling Signal transduction depends on voltages across neuron plasma membranes. Membrane Potential: voltage differences across the plasma membrane). Net negative charge of about -70mV Ions Intracellular ( -) ; K+ principal cation Large organic ions- anions Extracellular (less negative) Na+- principal cation Cl- main anion. Ion channels- ungated, gated; all selective K+ diffuses out (Na+ in); large anions cannot follow….selective permeability of the plasma membrane

Creating & Maintaing the Membrane Potential Na + - K + Pumps --pump against their conc. gradients ATP K+ pumped back in Na+ pumped back out

Changes in membrane potential key to neural transmission Only neurons and muscle cells can change their membrane potentials in response to stimuli Excitable Cells Sensory neurons-environmental stimuli Interneurons stimuli transmitted via other neurons Resting Potential: M.P. of excitable cell at rest. Change due to flow of ions as gated ion channels open. stimuli cause ion channels to open Stimuli that open K+ channels HYPERPOLARIZE the neuron Stimuli that open NA+ channels DEPOLARIZE the neuron

Graded Potentials –these voltage changes 1- Hyperpolarization (outflow of K+); increase in electrical gradient; cell becomes more negative 2- Depolarization (inflow of Na+); reduction in electrical gradient; cell becomes less negative

Clusters allow coordination of activities by all parts of Nervous System Nucleus=cluster of nerve cell bodies in the brain Ganglion (ganglia)= cluster of nerve cell bodies in the PNS Supporting cells/glia: nonconductiong cell that provides support, insulation, and protection

The Types of Glia Functions: Reinforce, protect 1. Astrocytes-encircle capillaries of brain—blood-brain barrier 2. Oligodendrocytes—form mylen sheaths—insulate CNS 3. Schwann Cells—form the myelin sheaths around axons in PNS

Mylenation Electrical insulation—lipid is poor conductor Increasing speed of nerve impulse propagation Multiple Sclerosis: myelin sheaths deteriorated-los of coordination

Neural signaling Gated ion channels (open/close response to stimuli): photoreceptors; vibrations in air (sound receptors); selective 1. chemically-gated (neurotransmitters) 2. voltage-gated (membrane potential changes)

Normal Membrane Potential Resting Potential: Resting Neuron -70 mV Cytoplasm is negatively charged relative to cell interior Resting potential~ the membrane potential of the unexcited nerve. A change in voltage MAY result in an electrical impulse.

The rapid change in membrane potential in an excitable cell When the Threshold potential is reached, usually sl. More positive (-50 to -55 mV)…. The action potential is triggered…. The rapid change in membrane potential in an excitable cell b/c stimulus triggered the selective opening and closing of voltage-gated ion channels

Action Potential- All Or None change in the Membrane Potential Phases 1. Resting stage •both channels closed 2-Depolarization: •a stimulus opens some Na+ channel gates Na+ influx reverses membrane polarity. Threshold reached. (cell interior sl. positive) Action potential generated . 3-Repolarization •Na+ channels close. K+ channels open; K+ leaves cell returns to resting potential—then .. 4-Undershoot •K+ channels still open-temporarily HYPERPOLAR. --------------------------------------------

The Action Potential Followed by a Refractory period~ insensitive to stimulus. Amplitude not affected by stimuli Intensity

Action Potentials are self-propagating Action Potential regenerated along axon membrane begins at Axon Hillock “Travel” of the action potential is self-propagating One direction only. Nodes of Ranvier-action potential jumps from one node to the next Gaps, ion sensitive channels concentrated here, extracellular fluid contact here Forward direction only

Action potential speed: 1) Axon diameter (larger = faster; 100m/sec) 2) Saltatory Conduction: Mylenation Nodes of Ranvier (concentration of ion channels in gaps of the myelin). A.P. “jumps” from node to node. 120m/sec

Chemical or Electrical Communication between cells occurs at synapses Synapse-tiny gap Presynaptic cell: transmitting cell Postsynaptic cell: receiving cell 1) Electrical Synapses-via gap junctions; no delay or less in signal strength; less common; fish tail-swim away quickly from predator 2) Chemical Synapses: synaptic cleft separates pre and post-synaptic cells. Not electrically coupled

Synaptic communication Synaptic cleft: separation gap Synaptic vesicles: neurotransmitter releasers When an Action Potential arrives at synaptic terminal of presynaptic cell Causes Ca++ influx; Synaptic vesicles fuse with presynaptic membrane and release…. Neurotransmitter Neurotransmitters quickly degraded

Neurotransmitter may do one of the following 1. Excite the membrane by depolarization Or 2. Inhibit the postsynaptic cells by hyperpolarization

Types of Neurotransmitters Acetylcholine (most common) may be excitatory or inhibitatory skeletal muscle Biogenic amines (derived from amino acids) •norepinephrine , epinephrine •dopamine •serotonin (from tryptophan) Amino acids GABA—most abundant inhibitory transmitter in brain Neuropeptides (short chains of amino acids) •endorphin-natural analgesics for the brain

Gaseous Signals of the Nervous System NO (nitric oxide)—blood vessel dilation. Acetylcholine stimulates blood vessel walls to release NO; neighboring smooth muscles relax & dilate heart’s blood vessels. Nitroglycerine is converted to NO—similar response

Nervous system organization tends to corrolate with body symmetry

Vertebrate PNS Cranial nerves (brain origin) Spinal nerves (spine origin) Sensory division Motor division •somatic system voluntary, conscious control •autonomic system √parasympathetic conservation of energy √sympathetic increase energy consumption

The Vertebrate Brain Forebrain •cerebrum~memory, learning, emotion •cerebral cortex~sensory and motor nerve cell bodies •corpus callosum~connects left and right hemispheres •thalamus; hypothalamus Midbrain •inferior (auditory) and superior (visual) colliculi Hindbrain •cerebellum~coordination of movement •medulla oblongata/ pons~autonomic, homeostatic functions