Introduction to the Nervous System

Basic Divisions of the Nervous System Figure 12.2

Central Nervous System (CNS): --brain and spinal cord Peripheral Nervous System (PNS):                    -- 31 pairs of spinal nerves                    -- 12 pairs of cranial nerves

THREE BASIC FUNCTIONS OF THE NERVOUS SYSTEM Sensory -  gathers info Integrative - information is brought together Motor - responds to signals, maintains homeostasis

Peripheral Nervous System SOMATIC NERVOUS SYSTEM - skeletal muscles, under voluntary control AUTONOMIC NERVOUS SYSTEM - smooth muscles, glands, involuntary

Parasympathetic System Brain & Spinal Cord Afferent System Efferent System Sensory Receptors Somatic System Autonomic System Stimulus Sympathetic System Parasympathetic System Smooth muscles, glands Skeletal muscles

Neurons

Dendrites – shorter, more numerous Dendrites – shorter, more numerous. Form the receptive surfaces of neurons Axon – single, long fibre which conducts impulses away from the cell body. End has many fine branches.

Neuroglial Cells  - support cells for the neurons 1. Microglial : scattered throughout, digest debris or bacteria 2. Oligodendrocytes: provide insulation around the axons in CNS 3. Astrocytes: connect blood vessels to neurons 4. Ependymal Cells: form a membrane that covers certain cavities in brain

5.  Schwann cells:  form the insulating sheath around the neurons within the PNS *Myelin Sheaths - necessary for insulation of neurons

Supporting Cells in the CNS Creates the myelin sheath that insulates axons

Supporting Cells in the CNS

Supporting Cells in the CNS

Supporting Cells in the CNS

Neuron Structure Myelin -insulation surrounding axons Nodes of Ranvier - gaps in the insulation

White vs Grey Matter Myelinated (white matter) – myelinated axons Unmyelinated (grey matter) - unmyelinated

Interesting Facts about the Neuron Longevity – can live and function for a lifetime Do not divide – fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception High metabolic rate – require abundant oxygen and glucose The nerve fibers of newborns are unmyelinated - this causes their responses to stimuli to be course and sometimes involve the whole body.  Try surprising a baby!

END OF INTRO NOTE!!!

Electrochemical Impulse - weak electric current. - a series of action potentials along a nerve fiber.

Resting Potential -inside of a neuron's membrane has a negative charge a Na+ / K+ pump uses energy from ATP to pump Na+ out of the cell and K+ into it the cell membrane is a bit leakier to K+ than it is to Na+, so K+ ions leak out of the cell the inside of the membrane builds up a net negative charge relative to the outside called membrane potential if electrodes placed across the cell membrane, the resting membrane potential is about -70mV - this difference in charge means the membrane is polarized http://bcs.whfreeman.com/thelifewire/content/chp44/4401s.swf

Sodium- Potassium Pump For every 3 Na+ out, 2 K+ come in Uses ATP (splits into ADP and inorganic phosphate) to alter pump shape

Action Potential 1. Neuron membrane maintains resting potential (-70mV) 2. Threshold stimulus is received. 3. Volate-gated Na+ channels open. 4. Na+ ions diffuse inward, depolarizing the membrane (+40mV) 5. Na+ channels close and voltage-gated K+ channels open K+ ions diffuse outward, repolarizing the membrane Membrane becomes hyperpolarized to – 90mV. K+ channels close. Na+/K+ pump restores resting potential During refractory period, membrane cannot undergo another action potential. Entire process continues along length of axon until it reaches the end.

Myelinated Neurons Action potentials only occur at nodes of Ranvier, which contain many voltaged-gated channels When Na+ ions enter cell, they move quickly through cytoplasm to next node. This reduces net charge, causing a threshold stimuli that elicits depolarization. Since depolarization just occurred at previous node, it is in refractory period and cannot depolarize thus preventing action potential from moving backwards. Action potentials jump from node to node = SALTATORY CONDUCTION Travels at a rate of 120m/s versus 0.5m/s in unmyelinated neurons.

The Synapse Synapse - junction between two communicating neurons   To complete the signal, a NEUROTRANSMITTER is released at the gap to signal the next neuron

Structure of a Synapses

Neurotransmitters Excitatory - allow positive ions to flow into postsynaptic membrane causing depolarization Inhibitory – allows potassium ions to flow out causing hyperpolarization

Types of Neurotransmitters Acetylcholine - stimulates muscle contraction Monoamines - Norepinephrine & Dopamine (sense of feeling good, low levels = depression) Serotonin (sleepiness) Endorphins (reduce pain, inhibit receptors)