6.5 Neurons and synapses Essential idea: Neurons transmit the message, synapses modulate the message. Nature of science: Cooperation and collaboration between groups of scientists— biologists are contributing to research into memory and learning. (4.3)

Understandings: Neurons transmit electrical impulses. The myelination of nerve fibres allows for saltatory conduction. Neurons pump sodium and potassium ions across their membranes to generate a resting potential. An action potential consists of depolarization and repolarization of the neuron. Nerve impulses are action potentials propagated along the axons of neurons. Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential. Synapses are junctions between neurons and between neurons and receptor or effector cells. When presynaptic neurons are depolarized they release a neurotransmitter into the synapse. A nerve impulse is only initiated if the threshold potential is reached.

Applications and skills: Application: Secretion and reabsorption of acetylcholine by neurons at synapses. Application: Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors. Skill: Analysis of oscilloscope traces showing resting potentials and action potentials.

Nervous System The master controlling and communicating system of the body Functions Sensory input – monitoring stimuli Integration – interpretation of sensory input Motor output – response to stimuli

Organization of the Nervous System Central nervous system (CNS) Brain and spinal cord Integration and command center Peripheral nervous system (PNS) Paired spinal and cranial nerves Carries messages to and from the spinal cord and brain

Histology of Nerve Tissue The two principal cell types of the nervous system are: Neurons – excitable cells that transmit electrical signals Supporting cells – cells that surround and wrap neurons

Neurons (Nerve Cells) Structural units of the nervous system Composed of a body, axon, and dendrites Long-lived, amitotic, and have a high metabolic rate Their plasma membrane function in: Electrical signaling Cell-to-cell signaling during development

Cell body (soma) Contains the nucleus and a nucleolus Contains an axon hillock – cone-shaped area from which axons arise Figure 11.4b

Dendrites Short, tapering, and diffusely branched processes receptive regions of the neuron

Axons Slender processes of uniform diameter arising from the hillock Usually there is only one unbranched axon per neuron Axonal terminal – branched terminus of an axon

Neuron Classification Functional: Sensory (afferent) — transmit impulses toward the CNS Motor (efferent) — carry impulses away from the CNS Interneurons (relay neurons) — shuttle signals through CNS pathways

Neurophysiology Neurons are highly irritable Action potentials, or nerve impulses, are: Electrical impulses carried along the length of axons Always the same regardless of stimulus The underlying functional feature of the nervous system

Gated Channels When gated channels are open: Ions move quickly across the membrane Movement is along their electrochemical gradients An electrical current is created Voltage changes across the membrane

Electrochemical Gradient chemical gradient - when ions move from high concentration to low concentration electrical gradient - when ions move toward an area of opposite charge electrochemical gradient – the electrical and chemical gradients taken together

Resting Membrane Potential (Vr) potential difference (–70 mV) across the membrane of a resting neuron Differential permeability to Na+ and K+ sodium-potassium pump

Changes in Membrane Potential Changes are caused by three events Depolarization – the inside of the membrane becomes less negative Repolarization – the membrane returns to its resting membrane potential Hyperpolarization – the inside of the membrane becomes more negative than the resting potential

Action Potentials (APs) brief reversal of membrane potential only generated by muscle cells and neurons do not decrease in strength over distance

Resting Potential Na+ and K+ channels are closed Leakage accounts for small movements of Na+ and K+

Action Potential: Depolarization Phase Na+ gates are opened; K+ gates are closed positive sodium flows in, depolarization begins. Then the influx creates a self-propagating depolarization. Explosive positive feedback. Lasts 1 millisec.

Action Potential: Repolarization Phase Sodium channel close, K+ channel open K+ exits the cell and internal negativity of the resting neuron is restored Inactivation gates “swing shut” sodium influx stops. SLOW voltage gated potassium channels open and K+ flows OUT of cell. Repolarization “overshoots”

Action Potential: Hyperpolarization Potassium gates remain open, causing an excessive efflux of K+ This efflux causes hyperpolarization of the membrane (undershoot) The neuron is insensitive to stimulus and depolarization during this time THIS RESTORES POTENTIAL BUT NOT THE CHEMICAL GRADIENTS!!! Figure 11.12.4

Action Potential: Role of the Sodium-Potassium Pump Repolarization Restores the resting electrical conditions of the neuron Does not restore the resting ionic conditions Ionic redistribution back to resting conditions is restored by the sodium- potassium pump

Phases of the Action Potential 1 – resting state (-70 mV) 2 – depolarization phase (-70 +30 mV) 3 – repolarization phase (+30 -70 mV) 4 – hyperpolarization ( overshoots -70 mV) graph shows the action potential and the permiability of the plasma membrane to Na and K Figure 11.12

Propagation of an Action Potential (Time = 0ms) Na+ influx causes a patch of the axonal membrane to depolarize Positive ions in the axoplasm move toward the polarized (negative) portion of the membrane

Propagation of an Action Potential (Time = 2ms) Ions of the extracellular fluid move toward the area of greatest negative charge A current is created that depolarizes the adjacent membrane in a forward direction The impulse propagates away from its point of origin

Propagation of an Action Potential (Time = 4ms) The action potential moves away from the stimulus Where sodium gates are closing, potassium gates are open and create a current flow