The Nervous System Chapter 8 – Overview and Neural Tissue

The Nervous system has three major functions: Sensory – monitors internal & external environment through presence of __________ Integration – interpretation of sensory information (information processing); complex (higher order) functions Motor – response to information processed through stimulation of ____________ _________________ _______________

General Organization of the nervous system Two Anatomical Divisions Central nervous system (CNS) __________ Peripheral nervous system (PNS) All neural tissue outside CNS – includes cranial nerves and spinal nerves __________ division (sensory input) __________ division (motor output) Somatic nervous system Autonomic nervous system

General Organization of the nervous system Brain & spinal cord

Histology of neural tissue Two types of neural cells in the nervous system: Neurons - For processing, transfer, and storage of information Neuroglia – For support, regulation & protection of neurons

Neuroglia (glial cells) CNS neuroglia: astrocytes oligodendrocytes microglia ependymal cells PNS neuroglia: Schwann cells (neurolemmocytes)

Astrocytes create supportive framework for neurons create “blood-brain barrier” monitor & regulate interstitial fluid surrounding neurons secrete chemicals for embryological neuron formation stimulate the formation of scar tissue secondary to CNS injury

Oligodendrocytes create myelin sheath around axons of neurons in the CNS. Myelinated axons transmit impulses faster than unmyelinated axons Microglia “brain macrophages” phagocytize cellular wastes & pathogens

Ependymal cells line ventricles of brain & central canal of spinal cord produce, monitor & help circulate __________

Schwann cells surround all axons of neurons in the PNS creating a neurilemma around them. Neurilemma allows for potential regeneration of damaged axons creates myelin sheath around most axons of PNS

Neuron structure

Most axons of the nervous system are surrounded by a myelin sheath (myelinated axons) The presence of myelin speeds up the transmission of action potentials along the axon Myelin will get laid down in segments (internodes) along the axon, leaving unmyelinated gaps known as “__________________” Regions of the nervous system containing groupings of myelinated axons make up the “_____________” “gray matter” is mainly comprised of groups of neuron cell bodies, dendrites & synapses (connections between neurons) of Ranvier

Anatomical organization of neurons Neurons of the nervous system tend to group together into organized bundles The axons of neurons are bundled together to form nerves in the PNS & tracts, columns & pathways in the CNS. The cell bodies of neurons are clustered together into ganglia in the PNS & nuclei/centers in the CNS.

Classification of neurons Structural classification based on number of processes coming off of the cell body: Multipolar neuron multiple dendrites & single axon most common type

Bipolar neuron two processes coming off cell body – one dendrite & one axon only found in eye, ear & nose Unipolar neuron single process coming off cell body, giving rise to dendrites (at one end) & axon (making up rest of process)

Classification of neurons Functional classification based on type of information & direction of information transmission: Sensory (afferent) neurons – transmit: ________________________________________ most sensory neurons are unipolar, a few are bipolar Motor (efferent) neurons – (muscles/glands/adipose tissue) in the periphery of the body all are multipolar Association (interneurons) – transmit information between neurons, located entirely within the CNS; analyze inputs, store information, coordinate outputs are the most common type of neuron (20 billion) are all multipolar

Reflex arc (p. 283-286) Reflex – a quick, unconscious, automatic response to a stimulus to protect or maintain homeostasis. e.g. stretch reflex, withdrawal reflex, pupillary light reflex, etc. Reflex arc – neural pathway involved in the production of a reflex. Structures include: receptor sensory neuron integrating center (brain or spinal cord; may or may not involve association neurons (interneurons)) motor neuron effector

Stretch reflex - simplest type of reflex - no association neuron involved Figure 8-29

Simplified Withdrawal reflex Figure 8-28

Neuron Function Neurons at rest have an unequal distribution of charged ions inside/outside the cell, which are kept separate by the plasma membrane more Na+ ions outside more K+ ions inside large negatively charged proteins & phosphate ions inside The sum of charges makes the outside of the membrane positive, & the inside of the membrane negative

Because of the difference of ionic charges inside/outside the cell, the membrane of the resting neuron is “polarized” The difference in charges creates a potential electrical current across the membrane known as the “membrane potential (transmembrane potential)”

At rest, the transmembrane potential can also be referred to as the “_______________________” (RMP) The RMP of a neuron = -70mV

For ions to cross a cell membrane, they must go through transmembrane channels “leakage channels” – open all the time, allow for diffusion “gated channels” – open & close under specific circumstances (e.g. voltage changes) When a stimulus is applied to a resting neuron, gated ion channels can open

If a stimulus opens gated K+ channels in a resting neuron, positive charges leave cell  membrane potential becomes more negative (-70mV  -90mV) This change from the cell’s resting membrane potential is known as hyperpolarization

When a stimulus causes Na+ gates open, Na+ diffuses into the cell This changes the electrical charge inside the cell membrane, bringing it away from its RMP of -70mV toward 0mV This change in membrane potential is known as depolarization

If a stimulus only affects a few Na+ gates at a specific site of the axon, the depolarization is small & localized only to that region of the cell. This is known as a graded potential But if the stimulus reaches a certain level (threshold level), voltage controlled Na+ gates will begin to open in sequence along the length of the axon. The depolarization will propagate (spread) along the entire surface of the cell membrane The propagated change in the membrane potential is known as an action potential (nerve impulse)

Action potentials Propagated change in transmembrane potential with two phases: Depolarization - movement of Na+ ions into the cell through voltage controlled Na+ gates; followed immediately by Repolarization - movement of K+ ions out of the cell through voltage controlled K+ gates Only nerve cells & muscle cells are excitable, i.e. can generate APs. Once an AP begins, it will propagate down the entire cell at a constant & maximum rate. This is known as the “__________” principle

Transmembrane potential (mV) Action Potential Conduction Activation of voltage- regulated sodium channels and rapid depolarization Depolarization to threshold Sodium ions Local current Potassium ions Inactivation of sodium channels and activation of voltage-regulated potassium channels +30 DEPOLARIZATION REPOLARIZATION 3 2 Transmembrane potential (mV) The return to normal permeability and resting state _ Threshold 60 _ 1 70 4 Resting potential REFRACTORY PERIOD 1 2 3 Time (msec) Figure 8-8 1 of 5 Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Transmembrane potential (mV) Depolarization to threshold Nerve cell at rest (RMP= -70mV) Stimulus applied to cell Na+ gates at axon hillock cause localized depolarization (graded potential) If stimulus is strong enough, flow of Na+ ions into cell reach threshold level triggering opening of voltage gated Na+ channels & formation of an action potential (nerve impulse) Sodium ions Local current +30 DEPOLARIZATION Transmembrane potential (mV) _ Threshold 60 _ 1 70 Resting potential 1 2 3 Time (msec) Figure 8-8 2 of 5 Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Transmembrane potential (mV) Activation of voltage- regulated sodium channels and rapid depolarization Depolarization to threshold Sodium ions Local current Potassium ions Once threshold is reached, Na+ will quickly diffuse into the cell causing a rapid depolarization of the membrane (- 70 mV  0 mV  +30 mV) this depolarization will spread to adjacent parts of the membrane, activating more voltage controlled Na+ gates in succession +30 DEPOLARIZATION 2 Transmembrane potential (mV) _ Threshold 60 _ 1 70 Resting potential 1 2 3 Time (msec) Figure 8-8 3 of 5 Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Transmembrane potential (mV) Activation of voltage- regulated sodium channels and rapid depolarization Depolarization to threshold Sodium ions Local current Potassium ions Inactivation of sodium channels and activation of voltage-regulated potassium channels +30 DEPOLARIZATION REPOLARIZATION 3 2 When the transmembrane potential reaches +30mV, Na+ gates will close & K+ gates will open Transmembrane potential (mV) _ Threshold 60 _ 1 70 Resting potential K+ will quickly exit cell resulting in repolarization of membrane & return to resting state Figure 8-8 4 of 5 1 2 3 Time (msec) Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Transmembrane potential (mV) Activation of voltage- regulated sodium channels and rapid depolarization Depolarization to threshold Sodium ions Local current Potassium ions Inactivation of sodium channels and activation of voltage-regulated potassium channels +30 DEPOLARIZATION REPOLARIZATION 3 2 Transmembrane potential (mV) The return to normal permeability and resting state _ Threshold 60 _ 1 70 4 Resting potential REFRACTORY PERIOD 1 2 3 Time (msec) Figure 8-8 5 of 5 Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Propagation of an Action Potential Continuous propagation (continuous conduction) Involves entire membrane surface Proceeds in series of small steps (slower) Occurs in unmyelinated axons (& in muscle cells) Figure 8-9(a)

Propagation of an Action Potential Saltatory propagation (saltatory conduction) Involves patches of membrane exposed at nodes of Ranvier Proceeds in series of large steps (faster) Occurs in myelinated axons Figure 8-9(b)

“Information” travels within the nervous system primarily in the form of propagated electrical signals known as action potentials. An action potential occurs due to a rapid change in membrane polarity (depolarization followed by repolarization) Depolarization is due to: ____________________; repolarization is due to: ________________________

Conduction across synapses In order for neural control to occur, “information” must not only be conducted along nerve cells (via action potentials), but must also be transferred from one nerve cell to another across a synapse Most synapses within the nervous system are chemical synapses, & involve the release of a neurotransmitter

The Structure of a Typical Synapse Figure 8-10

Events at a Typical Synapse An action potential arrives & depolarizes the synaptic knob (end bulb) Before repolarization can occur, _____ channels open & diffuses _______ end bulb Repolarization occurs An action potential arrives and depolarizes the synaptic knob PRESYNAPTIC NEURON Action potential Synaptic vesicles EXTRACELLULAR FLUID ER Synaptic knob AChE POSTSYNAPTIC NEURON CYTOSOL Figure 8-11 2 of 5 Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

An action potential arrives and depolarizes the synaptic knob Extracellular Ca2+ enters the synaptic cleft triggering the exocytosis of ACh PRESYNAPTIC NEURON Action potential Synaptic vesicles EXTRACELLULAR FLUID ACh ER Synaptic knob Synaptic cleft Ca2+ Ca2+ AChE Chemically regulated sodium channels POSTSYNAPTIC NEURON CYTOSOL Ca+2 causes the synaptic vessicles to fuse with the end bulb membrane causing ___________ of the neurotransmitter Figure 8-11 3 of 5 Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

An action potential arrives and depolarizes the synaptic knob Extracellular Ca2+ enters the synaptic cleft triggering the exocytosis of ACh PRESYNAPTIC NEURON Action potential Synaptic vesicles EXTRACELLULAR FLUID ACh ER Synaptic knob Synaptic cleft Ca2+ Ca2+ AChE Chemically regulated sodium channels POSTSYNAPTIC NEURON CYTOSOL The neurotransmitter diffuses across the synaptic cleft & binds to its receptors on the post synaptic membrane, causing an effect on the post synaptic cell ACh binds to receptors and depolarizes the postsynaptic membrane Initiation of action potential if threshold is reached Na2+ Receptor Na2+ Na2+ Na2+ Na2+ Figure 8-11 4 of 5

Excitatory (e.g. Ach, norepinephrine (NE)) The effect on the post synaptic neuron will depend on whether the neurotransmitter released is Excitatory (e.g. Ach, norepinephrine (NE)) Inhibitory (e.g. seratonin, dopamine, GABA) Excitatory neurotransmitters cause Na+ gates to open in the post synaptic membrane  ________________  impulse conduction Inhibitory neurotransmitters cause K+ or Cl- gates to open in the post synaptic cell  _______________  no impulse conduction

The effects of neurotransmitters on the post synaptic neurons are usually short lived because most neurotransmitters are rapidly removed from the synaptic cleft by enzymes or reuptake