1. Chapter 48 – Nervous System

2. Types of Nervous Systems
Nerve net – branching network of nerves
Ganglia – segmentally arranged clusters of neurons
Cephalization – advancement of brain and nerve tissue towards the head; brain formation

3. Nervous system Organization
Nerves - bundles of neurons wrapped in connective tissue
Central nervous system (CNS) - brain and spinal cord
Peripheral nervous system (PNS) - sensory and motor neurons
Effector cells - muscle or gland cells

4. Structural Unit of Nervous System
Neuron - structural and functional unit
Cell body - nucleus and organelles
Dendrites - impulses from tips to neuron
Axons - impulses toward tips
Axon hillock – axon joins cell body, typically where signals are generated
Myelin sheath - supporting, insulating layer around axons in vertebrates
Synaptic terminals - neurotransmitter releaser
Synapse - neuron junction

5. Types of Vertebrate Neurons
Sensory neurons – receives signals from internal and external stimuli and transmits signal to CNS
Interneurons – in CNS, integrates sensory input and motor output
Motor neurons – transmits signals from brain/spinal cord to muslces/glands

6. Supporting Cells (Glia)
Astrocytes – in CNS, structural support for neurons, regulate concentrations of ions and neurotransmitters
Radial glia – in embryos, give rise to CNS
Oligodendrocytes – in CNS, form the myelin sheaths around axons of vertebrate neurons
Schwann cells – same as Oligodendrocytes, but in PNS

7. Simple Nerve Circuit – reflex arc
Sensory neuron: convey information to spinal cord
Interneurons: information integration
Motor neurons: convey signals to effector cell (muscle or gland)
Reflex: simple response; sensory to motor neurons
Ganglion (ganglia): cluster of nerve cell bodies in the PNS

8. Membrane Potential
Membrane potential – voltage differences across the plasma membrane, typically between -60 and -80 mV (millivolts)
Resting potential – the membrane potential of a neuron that is not transmitting
Intracellular/extracellular ionic concentration difference – K+ higher in cytosol; Na+ and Cl- higher in extracellular fluid

9. Action Potentials
Excitable cells - cells that can change membrane potentials (neurons, muscle)
Gated ion channels - open/close response to stimuli; 3 types: stretch-gated (open when membrane mechanically deformed), ligand-gated (chemical signals), and voltage-gated (membrane potential changes)
Graded Potentials (depend on strength of stimulus):
1- Hyperpolarization (outflow of K+); increase in electrical gradient; cell becomes more negative
2- Depolarization (inflow of Na+); reduction in electrical gradient; cell becomes less negative

11. Action Potentials (cont.)
Threshold potential: if stimulus reaches a certain voltage (-50 to -55 mV)….
The action potential is triggered….
Voltage-gated ion channels (Na+; K+)
1-Resting state - both channels closed
2-Depolarization - a stimulus opens some Na+ channels, influx of Na+ depolarizes the membrane, if reaches threshold will trigger an action potential
3-Rising Phase - Na+ channels open, K+ channels closed, Na+ influx making inside of membrane positive with respect to outside
4-Falling Phase - Na+ channels close, K+ channels open, K+ efflux making the inside negative with respect to the outside
5-Undershoot - both gates close, but K+ channel is slow; resting state restored
Refractory period - insensitive to depolarization due to closing of Na+ gates
animation

12. Action Potentials

13. Action Potentials (cont.)
Threshold potential: if stimulus reaches a certain voltage (-50 to -55 mV)….
The action potential is triggered….
Voltage-gated ion channels (Na+; K+)
1-Resting state - both channels closed
2-Depolarization - a stimulus opens some Na+ channels, influx of Na+ depolarizes the membrane, if reaches threshold will trigger an action potential
3-Rising Phase - Na+ channels open, K+ channels closed, Na+ influx making inside of membrane positive with respect to outside
4-Falling Phase - Na+ channels close, K+ channels open, K+ efflux making the inside negative with respect to the outside
5-Undershoot - both gates close, but K+ channel is slow; resting state restored
Refractory period - insensitive to depolarization due to closing of Na+ gates

14. Conduction of an Action Potential
Action potentials regenerate themselves along the axon for long distance signaling
Speed is determined by diameter of axon (larger = faster, 100m/s), insulation of axon with myelin (faster) called saltatory conduction – appears to “jump” between Nodes of Ranvier

15. Synaptic communication
Presynaptic cell: transmitting cell
Postsynaptic cell: receiving cell
Synaptic cleft: separation gap
Synaptic vesicles: neurotransmitter releasers
1-depolarize membrane
2-opens Ca+ channels, influx of Ca+
3-causes synaptic vesicles to fuse with presynaptic membrane
4-vesicles release neurotransmitter into synaptic cleft
5-binds to channels on postsynaptic membrane
6-neurotransmitter releases from receptors
Animation

16. Synapse

17. Neurotransmitters

18. Vertebrate CNS Derived from the dorsal embryonic nerve cord
In adult - central canal of spinal cord and the 4 ventricles of the brain
Cerebral spinal fluid – fill central canal and ventricles; supplies nutrients and hormones, removes waste, and acts as a cushion
White matter – myelinated axons
Grey matter – unmyelinated axons

19. Vertebrate PNS
Cranial nerves (brain origin) and Spinal nerves (spinal cord origin)
Somatic Nervous System: carries signals to and from skeletal muscles, external stimuli, voluntary
Autonomic Nervous System: regulate internal environment, smooth and cardiac muscles, generally involuntary
Sympathetic division – corresponds to arousal and energy “fight or flight”
Parasympathetic division – promotes calming and a return to self-maintenance functions “rest and digest”
Enteric division – parastalsis and pancreas, liver, gall bladder secretions

20. Autonomic nervous system

21. The Vertebrate Brain Forebrain Midbrain Hindbrain
cerebrum - memory, learning, emotion
cerebral cortex - sensory and motor nerve cell bodies
corpus callosum - connects left and right hemispheres
thalamus; hypothalamus
Midbrain
send sensory information to cerebrum
inferior (auditory) and superior (visual)
Hindbrain
cerebellum - coordination of movement
medulla oblongata/ pons - autonomic, homeostatic functions like breathing, heart and blood vessel activity, swallowing, digestion

22. Cerebral Cortex
Controls voluntary movements and cognitive functions