NOTES: CH 48 Neurons, Synapses, and Signaling

 A nervous system has three overlapping functions: 1) SENSORY INPUT: signals from sensory receptors to integration centers 2) INTEGRATION: information from sensory receptors is interpreted and associated with appropriate responses   3) MOTOR OUTPUT: conduction of signals from the integration center to effector cells (muscle cells or gland cells)

*CENTRAL NERVOUS SYSTEM (CNS)  integration center  brain and spinal cord

*PERIPHERAL NERVOUS SYSTEM (PNS) (ropelike bundles of neurons)  made up of nerves (ropelike bundles of neurons)  nerves communicate motor and sensory signals to and from CNS and rest of body

Two Main Classes of Cells: 1) NEURONS:  functional unit of the nervous system  transmits signals from one location to another  made up of: cell body, dendrites, axon  many axons are enclosed by an insulating layer called the MYELIN SHEATH  include: sensory neurons, interneurons, motor neurons

2) GLIAL CELLS (“GLIA”) - SUPPORTING CELLS  10 to 50 times more numerous than neurons  provide structure; protect, insulate, assist neurons  example: Schwann cells and oligodendrocytes form myelin sheaths in the PNS and CNS, respectively

MYELIN SHEATH:  produced by Schwann cells in the peripheral nervous system;  gaps between successive Schwann cells are called NODES OF RANVIER…. ***the #10 term!!! 

NODES OF RANVIER! ***word #10 on my list!!! 1) Okazaki fragments 2) plasmodesmata 3) ??????? 4) ??????? 5) ??????? 6) rubisco 7) oxaloacetate 8) islets of Langerhans 9) Batesian mimicry 10) nodes of Ranvier

2) GLIA (SUPPORTING CELLS)  example: astrocytes: responsible for blood-brain barrier

Astrocyte Nerve cells

ACTION POTENTIALS & NERVE IMPULSES all cells have an electrical charge difference across their plasma membranes; that is, they are POLARIZED.  this voltage is called the MEMBRANE POTENTIAL (usually –50 to –100 mV)   inside of cell is negative relative to outside   arises from differences in ionic concentrations inside and outside cell

**A- = group of anions including proteins, amino acids, sulfate, phosphate, etc.; large molecules that cannot cross the membrane and therefore provide a pool of neg. charge that remains in the cell

How is this potential maintained?  the sodium-potassium pump uses ATP to maintain the ionic gradients across the membrane (3 Na+ out; 2 K+ in)

of a nerve cell is approx. –70 mV  the “resting potential” of a nerve cell is approx. –70 mV    neurons have special ion channels (GATED ION CHANNELS) that allow the cell to change its membrane potential (a.k.a. “excitable” cells)

 when a stimulus reaches a neuron, it causes the opening of gated ion channels (e.g.: light  photoreceptors in the eye; sound waves/vibrations  hair cells in inner ear)

 HYPERPOLARIZATION: membrane potential becomes more negative (K+ channel opens; increased outflow of K+)    DEPOLARIZATION: membrane potential becomes less negative (Na+ channel opens; increased inflow of Na+) **If the level of depolarization reaches the THRESHOLD POTENTIAL, an ACTION POTENTIAL is triggered.

ACTION POTENTIALS (APs):  the nerve impulse    all-or-none event; magnitude is independent of the strength of the stimulus

5 Phases of an A.P.: 1) Resting state 2) Depolarizing phase 3) Rising phase of A.P. 4) Falling phase of AP (repolarizing phase) 5) Undershoot

State of Voltage-Gated Sodium (Na+) Channel Phase of A.P. State of Voltage-Gated Sodium (Na+) Channel State of Voltage-Gated Potassium (K+) channel Activation gate Inact. Gate Entire channel 1) Resting closed   open 2 & 3) Depolari-zation 4) Repolar-ization 5) Undershoot

**during the undershoot, both Na+ channel gates are closed; if a second depolarizing stimulus arrives during this time, the neuron will NOT respond (REFRACTORY PERIOD)    strong stimuli result in greater frequency of action potentials than weaker stimuli

How do action potentials “travel” along an axon?  the strong depolarization of one action potential assures that the neighboring region of the neuron will be depolarized above threshold, triggering a new action potential, and so on…

“Saltatory Conduction”

-motor neuron & muscle cell -neuron & gland cell  SYNAPSE: junction between a neuron and another cell; found between: -2 neurons -sensory receptor & sensory neuron -motor neuron & muscle cell -neuron & gland cell

Motor Neuron and Muscle Cell

 Presynaptic cell = transmitting cell  Postsynaptic cell = receiving cell

Electrical Synapses: allow action potentials to spread directly from pre- to postsynaptic cell *connected by gap junctions (intercellular channels that allow local ion currents) **Most synapses are… Chemical Synapses: cells are separated by a synaptic cleft, so cells are not electrically coupled; a series of events converts: elec. signal  chem.signal  elec.signal HOW???...

NEUROTRANSMITTERS: intercellular messengers; released into synaptic cleft when synaptic vesicles fuse with presynaptic membrane    specific receptors for neurotransmitters project from postsynaptic membrane; most receptors are coupled with ion channels  neurotransmitters are quickly broken down by enzymes so that the stimulus ends

 the electrical charge caused by the binding of neurotransmitter to the receptor can be:   EPSP (Excitatory Postsynaptic Potential): membrane potential is moved closer to threshold (depolarization) IPSP (Inhibitory Postsynaptic Potential): membrane potential is hyperpolarized (more negative)

 most single EPSPs are not strong enough to generate an action potential  when several EPSPs occur close together or simultaneously, they have an additive effect on the postsynaptic potential: SUMMATION -temporal vs. spatial

Examples of neurotransmitters:   **acetylcholine  Neuromuscular junction; can be inhibitory or excitatory epinephrine norepinephrine dopamine serotonin endorphins  Decrease perception of pain by CNS; (heroin & morphine mimic endorphins) epin. & norep. also function as hormones; “fight or flight response” dop. & ser. both affect sleep, mood, attention, learning; LSD & mescaline bind to ser. & dop. receptors

Neurotransmitters: Ach  ACETYLCHOLINE: triggers skeletal muscle fibers to contract…  so, how does a muscle contraction stop???

Neurotransmitters: Ach  a muscle contraction ceases when the acetylcholine in the synapse of the neuromuscular junction is broken down by the enzyme….. wait for it………………….

ACETYLCHOLINESTERASE!! It’s term #4!!!!!  ACETYLCHOLINESTERASE = the enzyme the breaks down the neurotransmitter acetylcholine.

ACETYLCHOLINESTERASE! ***word #4 on my list!!! 1) Okazaki fragments 2) plasmodesmata 3) ???????? 4) acetylcholinesterase 5) ???????? 6) rubisco 7) oxaloacetate 8) islets of Langerhans 9) Batesian mimicry 10) nodes of Ranvier