Chapter 12- Nervous Tissue*

Factors that Affect Propagation Speeds

Axon diameter Larger diameter propagate faster Amount of myelination Myelination increases propagation Temperature Increased temps increase propagation Action potentials in a neuron must travel from where they arise at the trigger zone of the axon to the axon terminals. Keeps its strength as it spreads along the membrane, this mode of conduction called propagation and it depends on positive feedback

Parts of a Neuron

Cell body (soma) Contains nucleus and other organelle Can receive input Dendrites Processes that receive input Contain receptor sites for binding chemical messages (short, tapering, highly branched) Axon Carries impulse away from cell body and toward another neuron, a muscle or a gland

Functions of Nervous System

General function of nervous system- responsible for our perceptions, behaviors and memories & initiates voluntary movements Sensory/ Afferent- Monitor changes through sensory receptors detect internal and external stimuli Incoming information Sensory info carried into brain and spinal cord through cranial and spinal nerves Motor/ Efferent- Once sensory info is integrated the nervous system may elicit an appropriate motor response by activating effectors (muscle & glands) through cranial & spinal nerves Respond to stimuli Outgoing information Integrative- Analyze incoming sensory information Store some aspects Make decisions regarding appropriate behaviors an activity known as integration

Graded Potentials-Summation

Graded potentials can be added together to become larger in amplitude

Signal Transmission at a Chemical Synapse

Picture Notes- Influx of Ca causes exocytosis of NeuroTransmitter

Important Neurotransmitters-Serotonin

Serotonin Thought to be involved with: Sensory perception Temp regulation Mood control Appetite Sleep Decreased amounts associated w/ depression

Neurogenesis in CNS

There is little or no repair due to: Inhibitory influences from neuroglia, particularly oligodendrocytes (CNS myelin from Oligo is inhibiting factor for regeneration of neurons) Absence of growth-stimulating cues that were present during fetal development Rapid formation of scar tissue (physical barrier to regeneration, cant carry impulses)

Action Potentials-Threshold

Action potentials can only occur if membrane potential reaches threshold 2 types of voltage gated channels open and then close during an action potential. Channels present mainly in axon plasma membrane and axon terminals. 1st channel the voltage gated Na+ channels allow Na+ to rush into the cell which causes depolarizing phase. Then voltage gated K+ channels open, allowing K+ to flow out, which produces the repolarizing phase Refractory period-after action potential begins cell cant regenerate another action potential in response to a normal threshold stimulus Absolute refractory period- even a very strong stimulus cannot initiate a 2nd action potential Relative refractory period- a 2nd action potential can be initiated by a larger than normal stimulus

Neuroglia of CNS

Astrocytes *most numerous kind Support neurons (contain microfilaments give them strength) Help to maintain proper chemical environment for neurons Serve as BRIDGE (ex: regulate concentration of K+, take up excess neurotransmitters, serve as conduit for passage of nutrients &other substances btw blood capillaries and neurons) Oligodendrocytes Processes surround CNS neurons responsible for forming and maintaining myelin sheath Microglia Remove cellular debris (ex: from tissue damage) *function as phagocytes Ependymal cells (cuboidal, single layer have microvilli/cilia) Produce and assist in circulation of Cerebral spinal fluid (in brain)

Multiple Sclerosis

Autoimmune disorder Immune system attacks myelin Affect women more than men Relapsing-remitting most common form

Synapses

Junction between neurons or between a neuron and an effector Electrical Synapse Gap junctions connect cells and allow transfer of information to synchronize activity of a group of cells (common in visceral smooth muscle, cardiac muscle and development of embryo) *faster than chemical (conducted directly through plasma membrane of adjacent neurons) Chemical Synapse One-way transfer of information from a presynaptic neuron (releases neurotransmitter that diffuses through the fluid and binds to receptors in plasma membrane of postsynaptic neuron) to a postsynaptic neuron.(plasma membranes of pre and post synaptic neurons in chem synapse do not touch, separated by synaptic cleft so must comm indirectly)

Ion Channels in Neurons-Leakage

Leakage channels are always open (contrary to what your book says) K+ channels are more numerous than Na+ channels Greater permeability to K+ than to Na+

Ion Channels In Neurons-Ligand-Gated

Ligand-gated channels respond to chemical stimuli (ligand binds to receptor) Open and close in response to the binding of a ligand (chemical) stimulus

Ion Channels in Neurons-Mechanically gated

Mechanically-gated channels respond to mechanical vibration or pressure stimuli Touch receptors work this way (such as sound waves, touch, pressure, or tissue stretching)

Resting Membrane Potential

Membrane of a non-conducting neuron is polarized Positively charged on outside Negative charged on inside Determined by: Unequal distribution of ions across plasma membrane and selective permeability of neuron's membrane to Na+ and K+ Most anions cannot leave the cell (cannot follow K+ out of cell bc they are attached to nondiffusable molecules such as ATP and large proteins) Na+/K+ pumps-pumps na out as fast as leaks in, expel 3 na for each 2 k imported

Neuroglia

Not electrically excitable Make up about half the volume of nervous system Can carry out mitosis 6 kinds total (4 in CNS, 2 in PNS) *Classified by size, cytoplasmic processes and intracellular organization

Types of Synapses Between Neurons

Presynaptic neuron-nerve cell that carries an impulse toward a synapse (sends a signal) Postsynaptic cell- cell that recieves a signal. It may be called a postsynaptic neuron that carries a nerve impulse away from the synapse or an effector cell that responds to the impulse at the synapse *potentials move in one direction Notes for picture Axodendritic-Axon to Dendrite Axosomatic-Axon to cell body

Summation responses

*the sum of all excitatory and inhibitory effects determines the effect on postsynaptic neuron which may respond in following ways: 1. EPSP If the total excitatory effects are greater than the total inhibitory effects but less than the threshold level of stimulation, the result is an EPSP that does not reach threshold. Following an EPSP, subsequent stimuli can more easily generate a nerve impulse through summation because the neuron is partially depolarized. 2. Nerve impulse(s). If the total excitatory effects are greater than the total inhibitory effects and threshold is reached, one or more nerve impulses (action potentials) will be triggered. (at trigger zone) Impulses continue to be generated as long as the EPSP is at or above the threshold level. 3. IPSP If the total inhibitory effects are greater than the excitatory effects, the membrane hyperpolarizes (IPSP). The result is inhibition of the postsynaptic neuron and an inability to generate a nerve impulse.

Continuous vs. Saltatory Conduction

Continuous conduction- step by step depolarazation and repolarization of ech adjacent segment on plasma membrane, occurs in unmyelinated axons and muscle fibers (slower) Saltatory conduction-occurs along myelinated axons bc of uneven distribution of voltage gated channels. Few voltage gated channels are present in regions where a myelin sheath covers the axolemma, by contrast at nodes of ranvier (where there is no myelin sheath) the axolemma has many voltage gated channels hence current carried by Na+ and K+ flows across the membrane mainly at nodes (leap from node to node) less ATP used (Faster)

Important Neurotransmitters-Dopamine

Dopamine (mostly in Brain) Used during emotional responses, addictive behavior and pleasure Regulates skeletal muscle tone and some movement Low dopamine associated w/ Parkinson's High dopamine associated w/ schizophrenia Dopamine and seratonion are considered Biogenic Amines where certain amino acids are modified and carboxyl group is removed

Neurons

Electrically excitable (can carry an impulse) Cellular structures Note direction of impulse

Electrical Signals in Neurons

Excitable cells communicate with each other via: Action potentials (AP) Graded potentials (GP) APs allow communication over short and long distances Action potentials can go a long way and maintain its strength only limited by amount of membrane GPs allow communication over short distances only Production of AP or GP depends upon existence of resting membrane potential and certain ion channels

Summation

If several presynaptic neurons release their neurotransmitter at about the same time, combined effect may generate a nerve impulse due to summation Summation may be spatial or temporal

Watch these animations

Introduction to structure and function of nervous system Membrane potentials Propagation of nerve impulses Events at the synapse

Myelination of Neurons

Myelin sheath is produced by: Schwann cells (PNS) Oligodendrocytes (CNS) Myelin sheath- multilayered lipid & Protein covering around some axons that electrically insulates the axon of a neuron and increases speed of nerve impulse conduction In PNS schwann cell wraps single axon spiraling many times (jelly roll) . Neurolemma only found around axons in PNS, aids regeneratation In CNS oligodendrocytes mylinate parts of several axons (no nerurolemma, fewer nodes of R & display little regrowth after injury) Nodes of Ranvier-Gaps in myelin sheath along intervals of axon

Nervous system-Definitions

Nerve- a bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels that lies outside the brain and spinal cord (located in PNS) Ganglia-small masses of nervous tissue, consisting primarily of neuron cell bodies, that are located outside of the brain and spinal cord (PNS) Tract-Bundle of axons located in CNS Nucleus-cluster of neuronal cell bodies located in PNS Sensory receptor- refers to a structure of the nervous system that monitors changes in the internal or external environment Enteric plexuses-extensive networks of neurons located in the walls or organs of the GI tract

Histology of the Nervous System: Neurons vs. Neuroglia

Nervous system is made up of 2 groups of cells Neurons: carry out transmission function (no mitosis) Neuroglial cells: assist neurons in their function (yes mitosis) Support, nourish and protect neurons Maintain extracellular environment (interstitial fluid that bathes them) Protection

Regeneration & Repair of Nervous Tissue

Neurons have a limited ability to regenerate themselves Regenerate: capability to replicate or repair Neurogenesis-the birth of new neurons from undifferentiated stem cells

Postsynaptic Potentials

Neurotransmitters can cause either an excitatory or inhibitory graded potential Excitatory postsynaptic potentials (EPSP) Depolarizing (more pos) excitatory bc it brings the membrane closer to threshold (ex: opening of na+ channels) Inhibitory postsynaptic potentials (IPSP) Hyperpolarizing (more neg) generation of action potential more difficult bc membrane potential becomes inside more neg and thus even further from threshold than in its resting state (Ex: opening K+ channels OR Cl- chlorine to make more neg) Postsynaptic neuron can receive many signals at once Can be a combination of EPSPs and IPSPs (summation) Greater summation of ESSPs the greater the chance threshold will be reached Postsynaptic potential-change in membrane voltage as ions flow through the opened channels *These are graded potentials

Removal of Neurotransmitter

Neurotransmitters do not remain for long in synaptic cleft Can be removed from cleft by: Diffusion from area (not shown in illustration) Enzymatic degradation (1) Reuptake into cells (2 & 3) reuptake by endocytosis

Parts of Neuron Contd

Nissl bodies-free ribosomes and clusters rough er in cell bodies, site of protein synthesis Cytoskeleton has neurofibrils (shape and support) & mictrotubules (assist in moving materials btw axon and cell body) AXON- long, thin, cylindrical joins cell body a cone shaped elevation called axon hillock. In most neurons impulses arise at the junction of axon hillock and the initial segment, and area called the trigger zone. No rough ER in axon, no protein synthesis. Cytoplasm of axon is axoplasm surrounded by plasma membrane called axolemma. End of axon=axon terminals

Graded Potentials contd

Occurs in response to opening of mechanically-gated or ligand-gated ion channel Vary in size depending on strength of stimulus, larger or smaller depending on how many ligand gated or mechanically gated channels are open (or closed) and how long each remains open Stimulus eventually dies out as the charges are lost across the membrane through leak channels

Peripheral nervous system

Peripheral nervous system- consists of all nervous tissue outside the CNS. Components of the PNS include nerves, ganglia, enteric plexuses and sensory receptors. PNS divided into- Somatic nervous system (SNS)-consists of sensory neurons that convey info to the CNS from somatic receptors in the head, body wall and limbs and from receptors for the special senses of vision, hearing, taste and smell AND motor neurons that conduct impulses from the CNS to the skeletal muscle only- VOLUNTARY Autonomic nervous system (ANS)- consists of sensory neurons that convery info to the CNS from the automatic sensory receptors located primarily in visceral organs such as the stomach and lungs AND motor neurons that conduct nerve impulses from the CNS to the smooth muscle, cardiac muscle, and glands- INVOLUNTARY Motor part of ANS has 2 divisions- Sympathetic- Support emergency actions "fight or flight" (ex: increase HR) Parasympathetic- Support "rest and digest" activities (ex: slow down HR)

Damage and Repair in PNS

Repair is possible if: Cell body is intact Schwann cells are functional (make myelin, can go through mitosis) Scar tissue formation does not occur too rapidly (repair can take place before scar tissue forms) Most nerves in the PNS consist of processes that are covered with a neurolemma. A person who injures axons of a nerve in an upper limb, for example, has a good chance of regaining nerve function. Steps involved in repair process are: Chromatolysis: Nissl bodies (rough ER) break up, myelin sheath deteriorates but neurolemma remains Wallerian degeneration: break down of distal portion of axon and myelin sheath Formation of a regeneration tube: via mitosis of Schwann cells Following chromatolysis, signs of recovery in the cell body become evident. Macrophages phagocytize the debris. Synthesis of RNA and protein accelerates, which favors rebuilding or regeneration of the axon. The Schwann cells on either side of the injured site multiply by mitosis, grow toward each other, and may form a regeneration tube across the injured area (Figure 12.29c). The tube guides growth of a new axon from the proximal area across the injured area into the distal area previously occupied by the original axon. However, new axons cannot grow if the gap at the site of injury is too large or if the gap becomes filled with collagen fibers. During the first few days following damage, buds of regenerating axons begin to invade the tube formed by the Schwann cells (Figure 12.29b). Axons from the proximal area grow at a rate of about 1.5 mm (0.06 in.) per day across the area of damage, find their way into the distal regeneration tubes, and grow toward the distally located receptors and effectors. Thus, some sensory and motor connections are reestablished and some functions restored. In time, the Schwann cells form a new myelin sheath.

Neuroglia of PNS

Schwann cells Entire cell wraps around portion of axon many times to create myelin sheath Outer portion called neurolemma Neurolemma can aid in axon regeneration by forming a regeneration tube that stimulates regrowth of axon Can enclose unmyelinated axons Satellite cells Surround cell body of neurons (of ganglia) (structural support) Regulate materials entering and exiting neuron cell body

Functional Classification of Neurons

Sensory/afferent neurons Incoming Sensory/afferent- once stiumulus activates forms an action potential in axon and is conveyed into CNS through cranial or spinal nerves. (most are unipolar in structure) Motor/efferent neurons Outgoing Motor/efferent- convery action potentials away from CNS to effectors (muscle and glands) in the periphery (PNS) through cranial or spinal nerves (most are multipolar) Inter/association neurons Integration/processing *classified according to the direction in which the nerve impulse (action potential) is conveyed

Action Potentials

Sequence of rapidly occurring events Decrease and eventually reverse membrane potential (depolarization) Eventually restore it to resting state (repolarization) *Depolarization- negative membrane potential becomes less negative, reaches zero, and then becomes positive

Graded Potentials

Small deviations in resting membrane potential that makes the membrane either more polarized (inside more neg) or less polarized (inside less neg) Hyperpolarized = more polar (inside more neg) Depolarized = less polar (inside less neg)

Temporal Summation

Stimuli occur at same location at different times * 1 stimulating

Spatial Summation

Stimuli that occur at different locations at the same time *multiple stimulation

Ion Channels in Neurons-Voltage-Gated

Voltage-gated channels respond to direct changes in membrane potential Muscles and neurons