Nervous System Endocrine and nervous systems cooperate to maintain homeostasis

Overview Sensory input Integration Motor output

CNS Brain and spinal cord Integration of sensory input Association stimuli with appropriate motor output

PNS Network of nerves extending through out body Carry sensory input to CNS Carry motor output from CNS

Unit of Structure -- neuron Cell body Dendrite – signals to cell body Short, branched (surface area)

Axons signals away from cell body Long single process Schwann cells  myelin sheath Axon hillock – where impulses generated Telodendria tipped with synaptic terminals Release neurotransmitters

Types of neurons Sensory neurons Interneurons Motor neurons

Supporting Cells Reinforce, protect, insulate and assist neurons Do not conduct impulses Much greater in number than neurons

Glial cells – supporting cells of the CNS Astrocytes Encircle capillaries in the brain Contribute to blood-brain barrier Restricts passage of most substances into CNS Oligodentrocytes-forms myelin sheaths in CNS

Schwann Cells—supporting cells of PNS Grow around axon forming concentric layers Provides electrical insulation Membranes mostly lipids  poor conductors Increases speed of nerve impulse propagation MS-deterioration of myelin sheaths

Electrical Membrane Potential Membrane potential range: -50 to – 100mV Indicates the charge of cytoplasm side of membrane compared to extracellular fluid -70mV is a resting neuron

Membrane Potential results from: 1. Differences in the ionic composition of the intracellular and extracellular fluids Principal cation INSIDE cell is K+ Principal cation OUTSIDE cell is Na+ Principal anions inside cell: proteins, amino acids, sulfate, phosphate Principal anions outside cell: Cl-

2.Selective permeability of the plasma membrane Ions cannot readily diffuse through hydrophobic core of phospholipid bilayer Ion channels - carrier mediated transport Large anions inside cell cannot cross membrane

Ion Channel Allows specific ion to cross membrane Passive - open all the time Gated – require stimulu to change conformation to open Selective for specific ion – Na+, K+, Cl- Membrane permeability is a function of TYPE and NUMBER OF ION CHANNELS

Maintaining Membrane Potential K+ diffuses out of cell Down concentration gradient Membrane has high permeability to K+ Transfers positive charges to outside Cell’s interior becomes more negative K+ move into cell down electrical gradient Na+ move into cell down both gradients

What prevents weakening of electrical gradient? Sodium-potassium pump Uses ATP Pumps Na+ out of cell against both gradients Pumps K+ into cell restoring concentration gradient

Membrane potential changes All cells exhibit membrane potential Only neurons and muscles cells can change membrane potentials in response to stimuli These are excitable cells Resting potential – unexcited state, at rest

Gated ion channels change the membrane’s permeability Effect on the neuron depends on type of gated ion channel the stimulus opens Stimuli that open K+ channels Hyperpolarize the neuron K+ effluxes from the cell Increases the electrical gradient more negative inside cell

Stimuli that open sodium channels Depolarize the neuron Na+ influxes into the cell Reduce the electrical gradient Inside of cell becomes more positive

Graded potentials Voltage changes caused by stimulation Depends on strength of stimuls Threshold stimulus must be above this critical intensity to stimulate the axon If depolarization reaches threshold Then cell triggers an action potential

Action Potential Rapid change in membrane potential