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Mind, Brain & Behavior. Monday January 27, 2003. Connections Among Neurons. The growing tip of an axon is called a growth cone . Lamellipodia – flaps at the edge of the growth cone. Fold in to become the terminal synapse at destination.
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Mind, Brain & Behavior Monday January 27, 2003
Connections Among Neurons • The growing tip of an axon is called a growth cone. • Lamellipodia – flaps at the edge of the growth cone. • Fold in to become the terminal synapse at destination. • Filopodia – spikes take hold of the extracellular material and pull the cone forward.
Pathway Formation • Axons stick together due to fasciculation – expression of cell adhesion molecules (CAM). • Chemical markers in the axon and the targets guide axon growth. • Diffusable molecules called netrins also attract axons. • Absence of laminin at target may retard further growth.
Synapse Formation • Proteins are secreted by both the growth cone and the target membrane in a layer – basal lamina. • Interaction between these proteins results in receptor formation. • Agrin reception attracts ACh receptors. • Ca2 enters the growth cone and triggers neurotransmitter release.
Naturally Occurring Cell Die Off • Cells compete to innervate targets. Those not used die off. • Cell survival depends on activation at the target. • Neurotrophins travel back from target tissue to neuron cell body promoting survival. • Nerve growth factor (NGF) • Brain-derived neurotrophic factor (BDNF).
Activity-Dependent Rearrangement • At first cells are in no particular order and send axons everywhere. • Neural activity causes rearrangement of cells and synapses. • Hebb synapses – synapses that are active at the same time as the target is active are strengthened. • Things that fire together, wire together.
Plasticity • Critical periods are periods of plasticity. • Plasticity ends when axon growth ends. • Plasticity ends when synaptic transmission matures. • Plasticity diminishes when cortical activation is constrained. • Reduction of ACh or NE (norepinephrine)
Aging and the Brain • To study normal aging of the brain, researchers must control for health conditions. • Abnormal aging is affected by: • Dementia – usually caused by artherosclerosis (hardening of arteries) • Alzheimer’s disease
Mental Changes in Old Age • Cognitive processes slow down • Neuronal speed of transmission may be affected by loss of myelin • NMDA receptors decrease by 30% (important to learning & memory) • Variability is greater at 60 than at other times of life. • Loss of functioning is relative to someone’s original level of functioning.
Longitudinal Studies • Scores on IQ tests show little decline until age 70. • Declines in motor movements are not dramatic or disabling. • Remaining intellectually active protects against some cognitive decline. • Elderly professors do better than same-age controls, even on memory tasks.
Sensory Loss • Age-related changes in hearing and vision can affect performance. • Decline in sensory accuity affects: • Amount of information received • Rate at which information can be processed
Behavioral Consequences • Most elderly compensate for the gradual changes during aging so that no performance difference occurs. • Other ways can be found to do most tasks. • Elderly may continuously increase in “wisdom,” social and emotional skills, experience-based understanding.
Ion Channels Chapter 7
Ion Channels • Found in all cells throughout the body. • Open and close in response to signals. • Selectively permeable to specific ions • High rate of flow (conductance) • Resting channels – usually open • Gated channels – open and close • Refractory period – temporarily cannot be opened
Control of Gating • Binding of neurotransmitters, hormones, or second messengers from within the cell. • Phosphorylation – energy comes from a phosphate that binds with the channel. • Dephosphorylation – removal of the phosphate. • Voltage-gated – responds to a change in the membrane potential. • Stretch or pressure gated – mechanical forces.
Effects of Drugs • Exogenous ligands – drugs that come from outside the body. • Endogenous ligands – naturally occurring • Agonist – binds with and opens a channel. • Endogenous or exogenous (e.g., drug) • Antagonist – binds with and closes a channel. • Reversible (curare) or irreversible (snake venom)
Importance of Calcium • Voltage-gated calcium (CA2) channels permit CA to enter the cell. • As CA2 rises, it binds with the neuron, preventing additional calcium from entering. • Increased calcium concentrations can cause dephosphorylation or permanent inactivation of a channel. • Calcium signals neurotransmitter release.
