Supporting Cells or Glial Cells of the Nervous System Types of Glial Cells 1. Radial Glial Cells: form tracks by which the neurons can migrate throughout the developing embryo 2.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 nonconductive In multiple sclerosis this myelin sheath is degrading and nerve impulse transmission is disrupted.
Figure 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.
How Does A Cell Maintain It’s Membrane Potential 1.There is a great tendency for K + to diffuse out of the cell. 2.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. 3.This buildup of negative charge creates a charge gradient for cations (potassium) to flow back in. 4.Eventually potassium would be entering the neuron at the same rate it is leaving. 5.Sodium is also moving across the membrane. There is a higher concentration outside the cell than inside so sodium moves into the cell. 6.This also would eventually end up at equilibrium such that the concentration gradient for sodium would also disappear.
7.To 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. 8.In this way, the concentration gradients are established for ions to flow. 9.Ions flow across this membrane during nerve impulse conduction. How Neurons Work
Figure 48.8 Graded potentials and the action potential in a neuron
Figure 48.9 The role of voltage-gated ion channels in the action potential (Layer 1)
Figure 48.9 The role of voltage-gated ion channels in the action potential (Layer 2)
Figure 48.9 The role of voltage-gated ion channels in the action potential (Layer 3)
Figure 48.9 The role of voltage-gated ion channels in the action potential (Layer 4)
Figure 48.9 The role of voltage-gated ion channels in the action potential (Layer 5)
Refractory Period Both gates, the activation and inactivation gates, are closed and so no stimulus can cause a depolarization and thus action potential. Strong vs. weak stimuli Strong 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.
Figure 48.10 Propagation of the action potential 1.Sodium ions enter the neuron’s axon. 2.The sodium ions then flow inside the axon and depolarize an adjacent region. 3.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. 4.This process occurs in unmyelinated axons. These can be found in humans, squids, lobsters. 5.Myelinated axons in many vertebrates, including humans, speed up conduction by saltatory conduction.
Neurotransmitters 1.Acetylcholine a)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. 2.Epinephrine and norepinephrine a)can be both excitatory or inhibitory 3.Dopamine a)lack of dopamine is implicated in Parkinson’s disease b)some hallucinogenic drugs act on dopamine receptors
4.Serotonin a)sleeeeeeep disorders as well as hallucinogenic effects 5.Gama aminobutyric acid or GABA 6.Endorphins
Figure 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 fluid White Matter: myelinated axons in the CNS Gray Matter: unmyelinated axons, dendrites and nuclei.
Figure 48.17 Functional hierarchy of the peripheral nervous system
Figure 48.18 The main roles of the parasympathetic and sympathetic nerves in regulating internal body functions
Figure 48.19 Embryonic development of the brain
The Hindbrain: Medulla and Pons and Cerebellum 1.Part of what is called the brainstem. 2.Develops from the embryonic hindbrain 3.Medulla: control centers for breathing, heart rate, digestion 4.Pons: breathing 5.Both the medulla and pons have nuclei or groups of nerve cells that send axons to other parts of the brain. a)Sensory info goes through medulla and pons on its way to cerebrum. b)Motor info goes through medulla and pons on its way to muscles so movement is coordinated. c)This is where axons from one side of brain cross such that the right side of brain controls left side of body.
6.Cerebellum a)coordinates movement with visual and motor senses b) may be involved in remembering motor responses c)received info about positions of joints and length of muscles during movement, and coordinates this with visual input. d)balance; hand-eye coordination
Midbrain 1.From the embryonic midbrain 2.Reticular Activating System is major component a)lots of nuclei b)regulates sleep, alertness c)receives input from all sorts of sensory neurons and filters it before sending it to cerebral cortex.
Forebrain: Cerebrum, Epithalamus, Thalamus and Hypothalamus 1.Epithalamus a)produces cerebrospinal fluid b)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 melatonin 2.Thalamus a) sensory input center and then sends signals on to the cerebrum b)motor relay center for signals coming from the cerebrum. 3.Hypothalamus
4.Cerebrum a)Right and left cerebral hemispheres b)connected by the corpus callosum (white myelinated fibers) c)outer area is gray matter
Figure 48.20x1 Cerebral cortex, gray and white matter
Figure 48.24 Structure and functional areas of the cerebrum
Figure 48.25 Primary motor and somatosensory areas of the human cerebral cortex
Figure 48.26 Mapping language areas of the cerebral cortex