NOTES: CH 48 Neurons, Synapses, and Signaling

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) 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 sodiumpotassium pump uses ATP to maintain the ionic gradients across the membrane (3 Na + out; 2 K + in)

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

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

**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

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 epinephrine norepinephrine dopamine serotonin endorphins Neuromuscular junction; can be inhibitory or excitatory 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 Decrease perception of pain by CNS; (heroin & morphine mimic endorphins)

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