The Nervous System Nervous System allows organisms to respond to external and internal stimuli - Consists of: –Brain and spinal cord – Central Nervous System –Peripheral Nerves – Peripheral Nervous System –Neurons – functional unit of the nervous system, specialized cells for transmitting electrical and chemical signals

Anatomy of a Nerve cell: 1. Cell body - contains nucleus, most of the cytoplasm and most of the organelles Dendrites and axon extend from the cell body 2.Dendrites - short and highly branched receive stimulus and send to cell body 3.Axon - conducts impulses away from the cell body to another neuron or to a muscle or gland microscopic in diameter but may extend a meter or more in length may divide forming branches – axon collaterals divides at the end to form terminal branches that end in synaptic terminals synaptic terminals release neurotransmitters (chemicals) that transmit impulse across the tiny gap between neurons – synapse

4.Myelin sheath – fatty mat’l surrounding the axons of neurons outside the CNS (sheath made of neuroglia in CNS) speed up transmission of impulse –composed of Schwann cells that form insulation –Nodes of Ranvier – gaps between Schwann cells (myelin sheath)

Nerve – consists of hundreds or thousands of axons wrapped together in connective tissue in the CNS, bundles of axons are called tracts or pathways instead of nerves Ganglia – outside the CNS, cells bodies are usually grouped together in masses called ganglia inside CNS, collection of cell bodies called nuclei

Types of Neurons 1.sensory (afferent) neurons – conduct impulses into CNS from the periphery (sensory impulses) Pick up stimulus from sensory receptors – mechanoreceptors, chemoreceptors, thermoreceptors, photoreceptors 2.Relay Neurons (interneurons) – afferent neurons usually transmit impulses to relay neurons –located within CNS –neurons that integrate input and output –integration involves sorting and interpreting incoming sensory information and determining the appropriate response –forms connecting lines b/w sensory and motor neurons 3.motor (efferent) neurons – transmit messages from CNS to effectors (musc. or gland) 4.sensory receptors, afferent and efferent neurons are part of the Peripheral Nervous System

Relay Neuron

How Neurons Work Membrane potential (resting potential) difference in electrical charge across the plasma membrane Resting Potential (not conducting an impulse)  more negatively charged inside the cell compared to the interstitial fluid outside  membrane of neuron is polarized due to unequal distribution of ions – as a result the cell can produce an action potential (impulse)  Electrochemical potential -70mV  slight excess of positive ions outside the membrane and slight excess of negative ions inside the membrane

 Na + concentration is 10x greater outside the cell and K + concentration 10x greater inside the cell  ion pumps, ion channels and gates cause a specific distribution of ions across the cell membrane  sodium-potassium pumps in the membrane pump Na + out and K + into cell – both are pumped against their concentration gradient (ATP) – for every 3 Na + pumped out, 2 K + are pumped in (more positive ions outside than in)  K + tends to leak out by diffusion through ion channels causing further negative charge inside as compared to outside of cell  ion channels that allow the passage of Na+ are closed at resting potential

Stimulation – all or none response (action potential) Threshold stimulus – minimum amount needed for depolarization to occur causes Na + ion channels to open allowing Na+ to rush into interior of cell (depolarization) disturbs adjacent areas – Na + channels open causing a depolarization wave – action potential polarity across membrane is momentarily reversed K + channels also open but more slowly allowing repolarization

Repolarization – after action potential passes, membrane begins to repolarize Na + channels close and membrane become impermeable to Na + open K + channels allow K + leak out of the neuron repolarizing the membrane impulse is actually a series of depolarization and repolarization waves sweeping down the axon (takes place in less than 1 millisecond)

resting conditions must be reestablished by sodium-potassium pumps Impulse conduction video

Impulse conduction impulse conduction is slower in unmyelinated axons – continuous conduction – entire axon must depolarize diameter of axon affects speed of transmission larger diameters transmit faster vertebrate neurons are myelinated – speeds up transmission depolarization occurs only at the nodes of Ranvier – action potential “jumps” from one node to the next – saltatory conduction transmission is much faster than in unmyelinated axons

Transmission across Synapses synapse – gap between axon of one neuron and dendrites of the next or between a neuron and an effector synapse between neuron and muscle cell is called a neuromuscular junction or motor end plate

neurotransmitters act as chemical messengers to conduct the signal across the synapse synaptic vesicles contain neurotransmitter When action potential reaches axon terminal, calcium ions begin to diffuse in – Ca + influx vesicle fuses to presynaptic membrane and dumps transmitter into synaptic cleft – diffuses across synapse neurotransmitter binds to highly specific receptors on the postsynaptic membrane (specific to the type of neurotransmitter) – binding begins depolarization and impulse continues enzymes in cleft decompose neurotransmitter to free up receptor sites for next impulse or the neurotransmitter is actively transported back into presynaptic vesicles (reuptake)

Neurotransmitters each have a different function: Excitatory – stimulate neurons –acetylcholine (stimulate muscle contraction) –norepinephrine, dopamine, and serotonin (affect mood) Inhibitory – stop depolarization –Gamma-aminobutyric Acid (GABA) – inhibits neurons in brain and spinal cord excitatory postsynaptic potential (EPSP) – if a neurotransmitter is excitatory, it results in an EPSP) –causes partial depolarization bringing neuron closer to firing –one EPSP is probably too weak to trigger an action potential – EPSPs can be added together (summation) – results in firing of neuron inhibitory postsynaptic potential (IPSP) – occur when neurotransmitter causes postsynaptic membrane to hyperpolarize – brings membrane potential farther away from threshold and a stronger stimulus would be necessary to fire it

Important neurotransmitters – “classical” neurotransmitters that have been recognized for many years: 1. Acetylcholine –secreted at neuromuscular junctions, by autonomic nervous system, and central nervous system –only neurotransmitter released at synapses between neurons and muscles (always excitatory) –may be excitatory or inhibitory at other synapses

2. Noradrenaline (also called Norepinephrine) –secreted by autonomic NS, and CNS –chemically very similar to the hormone adrenaline (also called epinephrine) –prepares body for stressful situations

3.Dopamine secreted by CNS thought to affect motor function may be involved in causing schizophrenia degeneration of neurons that produce dopamine in a specific brain region causes Parkinson’s disease –characterized by difficulty in initiating conscious movements, uncontrolled tremors, shuffling gait, and muscle weakness –without dopamine, impulses cannot be transmitted properly –the drug levodopa (L-dopa) can be used by unharmed neurons in the brain to synthesize dopamine - reduces symptoms

Classifying synapses in the peripheral nervous system: Cholinergic synapses use acetylcholine –most synapses in the parasympathetic NS are cholinergic –neuromuscular junctions (between neurons and muscle fibers) are cholinergic Adrenergic synapses use noradrenaline –most synapses of sympathetic NS are adrenergic Central NS uses much wider range of neurotransmitters

Central Nervous System Vertebrate Nervous System is divided into the Central Nervous System (brain and spinal cord) and the Peripheral Nervous System (sensory receptors and the nerves which act as communication lines) Parts of the body are linked to the brain by cranial nerves and to the spinal cord by spinal nerves Afferent neurons “inform” the CNS of changing conditions, efferent nerves transmit the “decisions” of the CNS