5Supporting Cells or Glial Cells of the Nervous System Types of Glial Cells1. Radial Glial Cells: form tracks by which the neurons can migrate throughout the developing embryo2. Astrocytes: these are in the CNS and provide nutritional support for mature neurons; they also form tight junctions between capillaries in the brain forming a blood-brain barrier.3. Oligodendrocytes (in CNS) and Schwann cells (in PNS) form myelin sheaths around axons.Myelin is mostly lipid and is nonconductiveIn multiple sclerosis this myelin sheath is degrading and nerve impulse transmission is disrupted.
7Figure 48.6 Measuring membrane potentials So we say that the RESTING POTENTIAL of a neuron is -70 mV.It exists because of differences in the ion composition in the intracellular and extracellular environment.
8Figure 48.7 The basis of the membrane potential Cell Permeability to ions is dependent upon the number of channels available for a specific ion.-most cells, including neurons, have greater permeability to K+ than Na+.
9How Does A Cell Maintain It’s Membrane Potential There is a great tendency for K+ to diffuse out of the cell.As this occurs, there is a greater and greater negative charge within the cell due to the loss of K+ and the anions left inside.This buildup of negative charge creates a charge gradient for cations (potassium) to flow back in.Eventually potassium would be entering the neuron at the same rate it is leaving.Sodium is also moving across the membrane. There is a higher concentration outside the cell than inside so sodium moves into the cell.This also would eventually end up at equilibrium such that the concentration gradient for sodium would also disappear.
10To avoid reaching this equilibrium and then have no net tendency for either ion to flow in or out, there is the sodium potassium pump that pumps sodium out of the cell and potassium into the cell.In this way, the concentration gradients are established for ions to flow.Ions flow across this membrane during nerve impulse conduction.How Neurons Work
11Figure 48.8 Graded potentials and the action potential in a neuron
12Figure 48.9 The role of voltage-gated ion channels in the action potential (Layer 1)
13Figure 48.9 The role of voltage-gated ion channels in the action potential (Layer 2)
14Figure 48.9 The role of voltage-gated ion channels in the action potential (Layer 3)
15Figure 48.9 The role of voltage-gated ion channels in the action potential (Layer 4)
16Figure 48.9 The role of voltage-gated ion channels in the action potential (Layer 5)
17Refractory PeriodBoth gates, the activation and inactivation gates, are closed and so no stimulus can cause a depolarization and thus action potential.Strong vs. weak stimuliStrong stimuli set off action potentials with great frequency and the nerve depolarizes as fast as the refractory period will allow.Weak stimuli do not generate as frequent of action potentials.So it is the number of action potentials per second that indicate the intensity of the stimulus.
18Figure 48.10 Propagation of the action potential Sodium ions enter the neuron’s axon.The sodium ions then flow inside the axon and depolarize an adjacent region.The action potential cannot flow in the reverse direction because that area is repolarizing and cannot generate an action potential. Both activation and inactivation gates for sodium are closed.This process occurs in unmyelinated axons. These can be found in humans, squids, lobsters.Myelinated axons in many vertebrates, including humans, speed up conduction by saltatory conduction.
23NeurotransmittersAcetylcholinea) Receptor for this NT determines whether it is excitatory or inhibitory. Between a neuron and a muscle cell it is excitatory but the receptors on cardiac muscle cells generate an inhibitory responses to reduce strength of contraction and rate of heart beat.Epinephrine and norepinephrinea) can be both excitatory or inhibitoryDopaminea) lack of dopamine is implicated in Parkinson’s diseaseb) some hallucinogenic drugs act on dopamine receptors
24Serotonina) sleeeeeeep disorders as well as hallucinogenic effectsGama aminobutyric acid or GABAEndorphins
25Figure 48.15 Diversity in nervous systems Nerve Nets of Hydracontrol the movement of the body cavitySea Starnerve ring and radial nerve allows for movement of each radial arm.Planarianbeginning of cephalization where sensory neurons are localized to form a small brainnerve cords develop that help with directional movementthis is the beginning of a CNSMollusks and Insects:ventral nerve cords and ganglia or collections of nerve cell bodies outside the CNS from which nerves will radiate
26Figure 48.16 The nervous system of a vertebrate Brain and spinal cord have a fluid-filled space called the ventricles and central canal, respectively.These cavities are filled with cerebrospinal fluidWhite Matter: myelinated axons in the CNSGray Matter: unmyelinated axons, dendrites and nuclei.
27Figure 48.17 Functional hierarchy of the peripheral nervous system Sympathetic Division: heart rate speeds up, bronchi dilate, digestion slows, pupils dilate; liver converts glycogen to glucose,adrenaline is secreted.Parasympathetic Division: decrease in heart rate, digestion occurs, no adrenaline released, pupils constrict.
28Figure The main roles of the parasympathetic and sympathetic nerves in regulating internal body functions
30The Hindbrain: Medulla and Pons and Cerebellum Part of what is called the brainstem.Develops from the embryonic hindbrainMedulla: control centers for breathing, heart rate, digestionPons: breathingBoth the medulla and pons have nuclei or groups of nerve cells that send axons to other parts of the brain.Sensory info goes through medulla and pons on its way to cerebrum.Motor info goes through medulla and pons on its way to muscles so movement is coordinated.This is where axons from one side of brain cross such that the right side of brain controls left side of body.
32Cerebelluma) coordinates movement with visual and motor sensesb) may be involved in remembering motor responsesc) received info about positions of joints and length of muscles during movement, and coordinates this with visual input.d) balance; hand-eye coordination
33MidbrainFrom the embryonic midbrainReticular Activating System is major componenta) lots of nucleib) regulates sleep, alertnessc) receives input from all sorts of sensory neurons and filters it before sending it to cerebral cortex.
35Forebrain: Cerebrum, Epithalamus, Thalamus and Hypothalamus a) produces cerebrospinal fluidb) associated with it is the pineal gland which is involved in regulating bio. rhythms such as reproduction, biol. clocks. The main hormone produced by the pineal gland is melatoninThalamusa) sensory input center and then sends signals on to the cerebrumb) motor relay center for signals coming from the cerebrum.3. Hypothalamus
36Cerebruma) Right and left cerebral hemispheresb) connected by the corpus callosum (white myelinated fibers)c) outer area is gray matter
37Figure 48.20x1 Cerebral cortex, gray and white matter
38Figure 48.24 Structure and functional areas of the cerebrum
39Figure 48.25 Primary motor and somatosensory areas of the human cerebral cortex
40Figure 48.26 Mapping language areas of the cerebral cortex