Lecture #21Date ______ n Chapter 48 ~ Nervous System

Nervous systems n Effector cells~ muscle or gland cells n Nerves~ bundles of neurons wrapped in connective tissue n Central nervous system (CNS)~ brain and spinal cord n Peripheral nervous system (PNS)~ sensory and motor neurons

Structural Unit of Nervous System n Neuron~ structural and functional unit n Cell body~ nucelus and organelles n Dendrites~ impulses from tips to neuron n Axons~ impulses toward tips n Myelin sheath~ supporting, insulating layer n Schwann cells~PNS support cells n Synaptic terminals~ neurotransmitter releaser n Synapse~ neuron junction

Simple Nerve Circuit n Sensory neuron: convey information to spinal cord n Interneurons: information integration n Motor neurons: convey signals to effector cell (muscle or gland) n Reflex: simple response; sensory to motor neurons n Ganglion (ganglia): cluster of nerve cell bodies in the PNS n Supporting cells/glia: nonconductiong cell that provides support, insulation, and protection

Neural signaling, I n Membrane potential (voltage differences across the plasma membrane) n Intracellular/extracellular ionic concentration difference n K+ diffuses out (Na+ in); large anions cannot follow….selective permeability of the plasma membrane n Net negative charge of about -70mV

Neural signaling, II n Excitable cells~ cells that can change membrane potentials (neurons, muscle) n Resting potential~ the unexcited state of excitable cells n Gated ion channels (open/close response to stimuli): photoreceptors; vibrations in air (sound receptors); chemical (neurotransmitters) & voltage (membrane potential changes) n Graded Potentials (depend on strength of stimulus): n 1- Hyperpolarization (outflow of K+); increase in electrical gradient; cell becomes more negative n 2- Depolarization (inflow of Na+); reduction in electrical gradient; cell becomes less negative

The Resting Potential

Neural signaling, III n Threshold potential: if stimulus reaches a certain voltage (-50 to -55 mV)…. n The action potential is triggered…. n Voltage-gated ion channels (Na+; K+) n 1-Resting state both channels closed n 2-Threshold a stimulus opens some Na+ channels n 3-Depolarization action potential generated Na+ channels open; cell becomes positive (K+ channels closed) n 4-Repolarization Na+ channels close, K+ channels open; K+ leaves cell becomes negative n 5-Undershoot both gates close, but K+ channel is slow; resting state restored n Refractory period~ insensitive to depolarization due to closing of Na+ gates

The Action Potential

Neural signaling, IV n “Travel” of the action potential is self-propagating n Regeneration of “new” action potentials only after refractory period n Forward direction only n Action potential speed: n 1-Axon diameter (larger = faster; 100m/sec) n 2-Nodes of Ranvier (concentration of ion channels) ; saltatory conduction; 150m/sec

Synaptic communication n Presynaptic cell: transmitting cell n Postsynaptic cell: receiving cell n Synaptic cleft: separation gap n Synaptic vesicles: neurotransmitter releasers n Ca+ influx: caused by action potential; vesicles fuse with presynaptic membrane and release…. n Neurotransmitter

Signal transmission

Neurotransmitters n Acetylcholine (most common) skeletal muscle n Biogenic amines (derived from amino acids) norepinephrine dopamine serotonin n Amino acids n Neuropeptides (short chains of amino acids) endorphin

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

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