A&P1 // Chapter 12: Nervous Tissue

Identify the presynaptic and postsynaptic cells at a synapse.

Anatomically, the presynaptic neuron is the neuron before the synapse, this neuron is delivering the "message" across the synapse to the postsynaptic neuron. The postsynaptic neuron is the "receiver" of the neurotransmitter "message".

List the structures that comprise a chemical synapse.

-presynaptic membrane -postsynaptic membrane -space between (synaptic cleft) CNS synapses: axodendritic, axosomatic, axoaxonic, dendrodendritic

Differentiate between a nerve and a CNS tract.

Nerves are found in the PNS and are nerve fibers held together by layers of connective tissue. Tracts are found in the CNS and are groups of nerve fibers without the coverings.

Define permeability

Property of the plasma membrane that determines what moves in/out of a cell.

Explain temporal and spatial summation of synaptic potentials.

temporal: same stimulus (single neuron) repeated close together in time spatial summation: different stimuli (mulitple neurons) overlap in time

Define threshold.

-55mV, must reach to get action potential

Restate the steps that lead from the action potential arriving in the synaptic terminal to the release of neurotransmitter from synaptic vesicles.

-A nerve signal (nerve impulse or AP) arrives at the synaptic knob -Voltage-gated Ca2+ channels in the synaptic knob open -Calcium ions enter the synaptic knob (Ca ions diffuse into the cell because their concentration in ECF is higher than the concentration in ICF) -Ca2+ triggers exocytosis of the NT ( Ach, Epi, substance P or other) from the synaptic vesicles into synaptic cleft -the neurotransmitter diffuses across the synaptic cleft -the neurotransmitter binds to ligand-gated channels on post synaptic cell -these channels open and allow ions like Na+ or Cl- to enter the post-synaptic cell -as ions enter the post-synaptic cell, they depolarize or hyperpolarize the postsynaptic cell (creating either IPSP or EPSP)

Describe the events of synaptic transmission in proper chronological order.

-Neurotransmitters synthesized and stored in a synaptic vesicle -Action potential propagates to the axon terminal -Voltage gated calcium channels open -Calcium entry triggers vesicle docking -Secretion of the neurotransmitter -Neurotransmitter diffuses across the synaptic cleft -Neurotransmitter binds to a receptor -Receptor changes permeability of the cell -Ions diffuse through the receptor -EPSP or IPSP occurs -Enzymatic degradation/reuptake

Describe saltatory conduction.

-occurs in myelinated axons -faster than continuous propagation and requires less energy -myelin prevents continuous propagation -local currents jump from node to node

Describe the major functions of the nervous system

3 major functions to maintain homeostasis: sensory input, integration and motor output

Compare and contrast chemical and electrical synapses.

A chemical synapse is a gap between two neurons where information passes chemically, in the form of neurotransmitter molecules. An electrical synapseis a gap which has channel proteins connecting the two neurons, so the electricalsignal can travel straight over the synapse.

Explain the physiological basis of the absolute and relative refractory periods.

Absolute refractory period happens during the the period when Na+ gates are open, but inactivated. As such, no action potential can be generated in that portion of the plasma membrane. Relative refractory period happens during the period when K+ gates are open and extra K leaving the cell hyper polarizes the plasma membrane. When the plasma membrane is hyper polarized, it is further away from the threshold and only a strong stimulus will excite the cell.

Describe the location and anatomy of the cell bodies of each type of neuron within the nervous system.

Anaxonic: more than two processes and may all be dendrites, located in the brain, functions poorly understood Bipolar: have two processes separated by the cell body, located in special sense organs, relay info about sight, smell or hearing from receptor cells to other neurons Unipolar: have a single elongated process, cell body located off to side, carry sensations from the tips of the toes to the spinal cord Multipolar: more than two processes, single axon and multiple dendrites, all motor neurons that control skeletal muscles are multipolar, carry motor commands from spinal cord to small muscles that move the toes

Discuss the role of positive feedback in generation of the action potential.

At a specific voltage, these gates open. Voltage regulated gates open and allow Na+ ions to rush in. If the threshold is reached, rapid depolarization occurs and action potential is reached. More gates open and more Na+ ions rush in. This is a positive feedback loop--as more sodium ions flood in, the neuron becomes more positive.

Describe the nervous system as a control system, identifying nervous system elements that are sensory receptors, the afferent pathway, control centers, the efferent pathway, and effector organs.

CNS (brain & spinal cord) -> information processing PNS (nervous tissue outside CNS & ENS) -> sensory info within AFFERENT DIVISION = afferent fibers extend from sensory receptors to CNS, motor commands within EFFERENT DIVISION = efferent fibers carry instructions from CNS to peripheral effectors (organs like skeletal muscles, glands, adipose tissue)

State which parts of each type of neuron receive information, which parts integrate information, and which parts conduct the output signal of the neuron.

Dendrite: neuron receives info from another neuron primarily at dendritic spines axon: propagates electrical impulse/action potentials telodendria: communicates with other neurons at synaptic/axon terminals

Define excitatory postsynaptic potential (EPSP) and inhibitory postsynaptic potential (IPSP) and interpret graphs showing the voltage vs. time relationship of an EPSP and an IPSP.

EPSP: Excitatory Post Synaptic Potential -more likely to produce action potential -depolarization -common NT: glutamate, Ach -open Na+/K+ channels or closed K+ channels IPSP: Inhibitory Post Synaptic Potential -less likely to produce an action potential -hyperpolarization, membrane stabilization -common NT: GABA -Open K+ or Cl- channels make it more negative

Compare and contrast synaptic potentials with action potentials.

Graded potentials (local potentials) occur in the receptive (dendrites & cell bodies) due to opening of chemically gated channels that allow small ion amounts to cross the membrane and the altered charge may result in a change in polarization. Degree of change depends on how many molecules make it through and decreases intensity over distance-usually only lasting a short time. Action potentials (AP) are the result of voltage gated channels opening (not chemically gated ones like GP's) and require a threshold value to be reached. Voltage below this is not sufficient to create an AP but once the threshold value is reached a temporary reversal of polarity across the plasma membrane occurs. AP's are self propagated or transmitted and maintain intensity along the synaptic knob because of the successive opening of other voltage gated channels. AP's obey the "all or none" law but not all APs have the same intensity under the same conditions.

Explain how movement of sodium ions alone, or movement of both sodium and potassium ions, across the postsynaptic cell membrane can excite a neuron.

SODIUM: -plays the main role in excitatory responses in cells -as it flows INTO the cell (INFLUX) it makes the inside of the plasma membrane less negative= depolarization= excitation, which is the first wave of the AP -if Na cannot enter the cell, no depolarization is possible POTASSIUM: -repolarization is caused by K ions EXCITING the cell (EFLUX) and taking positive charge with them -without repolarization, there cannot be another depolarization and AP

Explain how axon diameter and myelination affect conduction velocity.

The larger the axon diameter the greater the conduction velocity. larger diameter = lower resistance and faster speed

Explain how passive ion channels cause development of the resting membrane potential in neurons.

The leak channels allow Na+ and K+ to move across the cell membrane down their gradients. With the combined ion pumping and leakage of ions, the cell can maintain a stable resting membrane potential.

Discuss the consequence of a neuron having an absolute refractory period.

This is the time during which another stimulus given to the neuron (no matter how strong) will not lead to a second action potential. Thus, because Na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials. Theabsolute refractory period takes about 1-2 ms

Explain how ion channels affect neuron selective permeability.

When ion channels are open, permeability of plasma membrane is increasing. When ion channels are closed, solutes will not be able to pass through.

define electrochemical gradient

a gradient of electrochemical potential, usually for an ion that can move across a membrane gradient consisting of both chemical and electrical

Define absolute and relative refractory periods.

absolute: all voltage gated Na+ channels are already open or inactivated, membrane cannot respond further. relative: begins when Na+ channels regain resting condition, continues until membrane potential stabilizes, only a strong stimulus can initiate another action potential

Differentiate between passive and active ion channels.

active: open and close in response to stimuli, at resting membrane potential most are closed, three types - chemically gated, voltage gated, mechanically gated passive: leak channel, always open, permeability varies, ex: K+

Define the term nerve

bundles with associated blood vessels and connective tissues

Describe how local circuit currents cause impulse conduction in an unmyelinated axon.

circuit currents are slower in an unmyelinated axon

Discuss the relationship between a neurotransmitter and its receptor.

each neurotransmitter matches to a receptor like a lock and key

List four types of CNS glial cells, their functions and explain how the anatomy of each CNS glial cell supports its function.

ependymal - line ventricles (brain) and central canal (spinal cord), assist in producing, circulating and monitoring cerebrospinal fluid [simple cuboidal epithelial cells, cilia to help circulation] astrocytes - maintain blood brain barrier, provide structural support, regulate ion, nutrient and dissolved gas concentrations, absorb and recycle neurotransmitters, form scar tissue after injury [largest and most numerous, star shaped, packed with microfilaments and connected via gap junctions] microglial cells - remove cell debris, wastes and pathogens by phagocytosis [type of phagocyte oligodendrocytes - myelinated CNS axons, provide structural framework [cells w/sheet like processes that wrap around axons]

Explain how movement of potassium or chloride ions across the postsynaptic cell membrane can inhibit a neuron.

ligand-gated chloride channels allow Cl- to ENTER the cell bringing in negative charge= hyperpolarization= inhibition

Describe the mechanism by which neurotransmitters may have indirect (metabotropic) effects on postsynaptic cells.

metabotropic receptor: neutrotransmitter binds to receptor creating slow change leading to intracellular cascades and second messengers open ion channels

Explain how the receptors for neurotransmitters are related to chemically-gated (AKA ligand-gated) ion channels.

receptors are ligand gated channels. These channels will only open when the NT binds to the receptor each receptor is specifically built to only bind to a specific neurotransmitter.

List two types of PNS glial cells, their functions and anatomy

satellite cells - surround neuron cells bodies in ganglia; regulate oxygen, carbon dioxide, nutrient and neurotransmitter levels around neurons in ganglia schwann cells - surround all axons in PNS; responsible for myelination of peripheral axons; participate in repair process after injury

Identify each type of neuron and function.

sensory neurons: form afferent division of PNS, cell bodies groups in sensory ganglia, delivers info from sensory receptors to spinal cord or brain, two types: somatic monitor outside world and visceral monitor internal conditions motor neurons: form efferent division of PNS, two types: somatic in SNS and visceral in ANS, carry instructions from the CNS to peripheral factors interneurons: located between sensory and motor neurons within the brain and spinal cord, distributes sensory info and coordinates motor activity, involved in memory, planning, learning

Differentiate between the somatic and autonomic divisions of the nervous system.

somatic nervous system: somatic motor neurons are part of the peripheral nervous system associated with voluntary control of body movements through skeletal muscles. autonomic nervous system: visceral motor neurons are part of ANS and innervate all other peripheral effects including smooth and cardiac muscle, glands and adipose tissue. divided into two parts: parasympathetic and sympathetic.

Contrast the relative concentrations of sodium, potassium and chloride ions inside and outside of a cell.

the sodium concentration is higher outside the cell than inside the cell and the potassium concentration is higher inside the cell than outside the cell

Describe the voltage-gated ion channels that are essential for development of the action potential.

two types: • voltage gated Na channels-open first and cause depolarization (Na rushes in and brings positive charge) • voltage gated K channels are slower and cause the repolarization of the cell (K moves out of the cell and takes away positive charge), as well as the hyperpolarization.

Differentiate between voltage-gated and chemically-gated (AKA ligand-gated) ion channels.

voltage gated: open and close in response to a voltage change in membrane (ex: K+, calcium, Na+) chemically/ligand gated: open or close in response to stimulus, chemically gated and need binding to open (ex: Ach, Na+/K+)