Neurons and Neurological Cells: The Cells of the Nervous System

1. Neurons and Neurological Cells: The Cells of the Nervous System • The nervous system • Integrates and coordinates many of the body’s activities • The nervous system is divided into • The central nervous system (CNS) • The brain and spinal cord • The peripheral nervous system (PNS) • All of the nervous tissue in the body besides the brain and spinal cord

2. Cells of the Nervous System • The nervous system is composed of two types of specialized cells • Neurons • Excitable cells that generate and transmit messages • Neuroglialcells (also called glial cells) • More numerous and provide structural support, growth factors, and insulating sheaths around the nerves

3. Cells of the Nervous System • Sensoryneurons • Carry information toward the CNS from sensory receptors • Motorneurons • Carry information away from the CNS to an effector • Interneurons • Located between sensory and motor neurons within the CNS, where they integrate and interpret sensory signals

4. Neurons Have Dendrites, a Cell Body, and an Axon • The shape of a typical neuron is specialized for communicating with other cells. It has: • Many short, branching projections called dendrites (one or more) • A single long extension of the neuron called an axon (one) • A cell body (one) which contains the nucleus and other organelles, and functions to maintain the neuron

5. Neurons Have Dendrites, a Cell Body, and an Axon • Dendrites • Carry information toward the cell body of a neuron • A single long axon • Carries information away from the cell body

6. Dendrites, Cell Body, and Axon • Nerves • Consist of parallel axons, dendrites, or both from many neurons • Covered with tough connective tissue • Classified as sensory, motor or mixed (sensory and motor together) depending on the type of neurons they contain

7. Dendrites, Cell Body, and Axon • Most axons not found in the CNS, and some of those within, are electrically insulated by a myelin sheath which increases the rate of conduction of a nerve impulse

8. Dendrites, Cell Body, and Axon • In the PNS, Schwann cells form the myelin sheath, insulating it and allowing messages to travel faster as they jump from one node of Ranvier to the next in a type of transmission called saltatory conduction

9. Dendrites, Cell Body, and Axon • The myelin sheath also facilitates nerve repair outside the CNS • When an axon in the PNS is cut, the Schwann cells take part in regeneration as they wrap around an axon

10. Dendrites, Cell Body, and Axon • Multiple sclerosis (MS) • Results from the destruction of the myelin sheath that surrounds axons found in the CNS • The resulting scars (scleroses) interfere with the transmission of nerve impulses • Can result in paralysis and loss of sensation, including loss of vision

11. The Nerve Impulse Is an Electrochemical Signal • A nerve impulse, or action potential, is a bioelectrical signal involving sodium ions (Na+) and potassium ions (K+) that cross the cell membrane through ion channels • Each ion channel is designed to allow only certain ions to pass through

12. The Nerve Impulse • Ions are also transported across the membrane actively by the sodium-potassium pump (3Na+ out, 2K+ in) • When a neuron is not conducting a nerve impulse, it is in a resting state • There is a slight difference in charge across the membrane which is called the resting potential (mainly due to action of sodium-potassium pump)

13. The Nerve Impulse • When the neuron is stimulated, there is a sudden reversal of charge across the membrane because the sodium gates open and sodium ions enter the cell • The minimum charge that causes the sodium gates to open is called the threshold

14. The Nerve Impulse • Next, the potassium gates open and potassium ions rush out of the cell, causing the cell to return to the original state, or repolarize • Lastly, resting potential must be restored • Note the charge across the membrane actually dips below the resting potential as the potassium gates are slow to close (hyperpolarization)

15. The Nerve Impulse • An action potential is the sudden reversal of the charge across the membrane followed immediately by its restoration • These changes occur in a wave along the axon • For a very brief period following an action potential, the nerve cannot be stimulated again • This is called the refractory period

16. Synaptic Transmission Is Communication between Neurons • Communication between neurons is by neurotransmitters, which are chemicals that cross the gap between two neurons • Synapse • The junction between a neuron and another cell • Between two neurons • Presynaptic neuron sends a message to the postsynaptic neuron

17. Communication between Neurons • A signal travels down the axon • Calcium ions cause the membrane of the synaptic vesicles to fuse with the plasma membrane, and to release the neurotransmitter substances into the synaptic cleft

18. Communication between Neurons • Neurotransmitters diffuse across the synaptic cleft to the other side, where they bind with specific receptors on the postsynaptic cell, which causes ion channels to open • E.g., if another neuron is to be stimulated sodium channels will open and this will cause the axon membrane to depolarize

19. Communication between Neurons • If neurotransmission occurs at an excitatory synapse, and enough receptor sites bind with neurotransmitter substances to cause depolarization to the threshold value, an action potential is generated in the postsynaptic cell

20. Communication between Neurons • However, in an inhibitory synapse, the postsynaptic cell becomes more negatively charged, and there is no action potential generated

21. Summation • A neuron may have as many as 10,000 synapses with other neurons at the same time • Some have excitatory effects and some inhibitory effects • Summation is the combined effects of excitatory and inhibitory effects at any given moment to determine whether an action potential is generated

22. Communication between Neurons • After the neurotransmitter crosses the membrane, it is quickly broken down or pumped back into the synaptic knob of the presynaptic axon • The enzyme acetylcholinesterase removes acetylcholine from synapses

23. Communication between Neurons • Acetylcholine • Acts in both the PNS and the CNS as a neurotransmitter • Causes voluntary muscles to contract • Myasthenia gravis is an autoimmune disease that attacks the acetylcholine receptors, resulting in little muscle strength

24. Communication between Neurons • The neurotransmitters dopamine, norepinephrine, and serotonin affect our emotional state • Alzheimer’s disease, depression, and Parkinson’s disease are caused by deficiencies of these chemicals in the brain