Bio117 Chapter 8: Nervous System

Nervous Tissue

excitable cells that transmit electrical signals

Interneurons

act as integrating center

Central and Peripheral Nervous System act as?

homeostatic reflex arc

Peripheral Nervous System

sensory receptor (dendrites), axons (peripheral process) afferent neuron

efferent neuron, axon terminal, effectors (muscle, gland, or neuron)

Central Nervous System

Cell body, interneurons, axons (central process)

What occurs between nerve cells?

synapse (decisions being made)

Soma

body of nerve cell

Dendrites

Hair-like attachments at the ends of nerve cells

Axon

Long tube connecting nerve cell body to axon terminals (neurotransmitter (NT) vesicles

Axon Hillock/Initial Segment

Thickening segment connecting nerve cell to axon

Schann Cells

• cell that wraps around one part of the axon

• can use myelination

Oligodendrocyte

• cell that wraps around one or more parts of different axons (up to 40 axons)

• can use myelination

Myelin

• phospholipid/protein complex

• helps nerve signals travel faster

Why are neurons excitable?

they exist in a state of chronic, unstable eletrochemical disequilibrium

Resting Membrane Potential for Typical Nerve Cell

-70 mV

Na⁺ Extracellular:Intracellular Concentration (mmol/L)

150:15

Cl^- Extracellular:Intracellular (mmol/L)

100:5-10

K⁺ Extracellular:Intracellular (mmol/L)

5:150

Electrical:Chemical Gradient for Na⁺

influx:influx

Electrical:Chemical Gradient for Cl^-

efflux:influx

Electrical:Chemical Gradient for K⁺

influx:efflux

Theory on why interior cytoplasm is more negatively charged than extracellular fluid even with more Cl^- being outside the cell

Intracellular anionic proteins (maybe, just a theory)

Nernst Equation

E_ion = (61/z)log([ion]_out/[ion]_in)

E_ion = equilibrium potential

z = valence of ion (charge and magnitude)

E_Na⁺

+60 mV

E_K⁺

-90 mV

E_Cl^-

-70 mV

Action Potential

• rapid, temporary changes in electrical voltage across cell membrane

• resting membrane is polarized

Action Potential: Depolarization

• increase in membrane potential

• has to be less than zero still

Action Potential: Overshoot

• increase in membrane potential

• greater than zero

Action Potential: Repolarization

• decrease in membrane potential

• still above resting membrane potential (-70 mV)

Action Potential: Hyperpolarization

• decrease in membrane potential

• below resting membrane potential (-70 mV)

Highest and Lowest Cell Membrane Potential of Typical Nerve Cell

+30 mV to around -80 mV

Graded Potentials

• stepped

• initiate action potentials

• can be depolarization or hyperpolarization

• amplitude varies with size on initiating event

• can be summed

• has no threshold

• has no refractory period

• conducted decrementally, amplitude decreases with distance (fades)

• duration varies with initiating conditions

• initiated by environmental stimulus (receptor), by neurotransimitter (synapse), or spontaneously

• mechanism depends on ligand-gated channels or other chemical/physical changes

Action Potentials

• positive feedback

• Initiated by graded potentials

• can only be depolarization

• amplitude independent of size of initiating event (all or none)

• cannot be summed

• has a threshold (15 mV above the resting potential)

  • -55mV for resting membrane potential (-70 mV)

• has refractory period (to prevent summation)

• conducted without decrement (does not fade)

• duration is constant for a given cell type under constant conditions

• mechanism depends on voltage-gated channels

Environmental Stimuli Effect on Action Potentials

changes membrane potential, but action potential does not occur until graded potential reaches -55mV threshold

Na⁺ Channel Types

• Opened

• Closed

• Inactivated (opened but plugged by inactivation gate)

Positive Feedback of Na⁺ Channels

more Na⁺ channels open → more Na⁺ ions influx → more positive membrane potential → more Na⁺ channels open

When does refractory period occur?

• begins after peak of depolarization

• absolute refractory period ends after the lowest point of depolarization

  • Highest difference occurs here between current membrane potential and threshold - hard to reach threshold but not impossible

• relative refractory period ends after membrane potential returns to -70mV

When Na⁺ channel goes from inactive to closed, ___

action potential can occur again

Why does membrane go down after peak?

comes down because K⁺ is leaving the cell

Where does action potential start and end?

• starts when membrane potential reaches threshold of -55mV

• ends when membrane potential returns to stable -70mV (after hyperpolarization)

Graded potentials occur where?

somas and dendrites

Where does first action potential occur

initial segment of the axon

Flow of action potentials

one action potential can create signals for other action potentials as Na⁺ diffuses down the axon

action potentials cannot go backwards due to the refractory periods of the cells previously affected an action potential prior

Two things that hasten nerve impulses

1. More myelination

2. Longer axon diameter

Two Types of Synaptic Junctions

1. Electric synapses (gap junctions)

2. Chemical synapses (synaptic cleft)

• Could be Na⁺ channels

  • depolarizing GP

  • excitatory postsynaptic potential (EPSP)

• Could be K⁺ channels

  • hyperpolarizing GP

  • inhibitory postsynaptic potential (IPSP)

Events at Synapse

1. Axon potential reaches axon terminal

2. Voltage-gated Ca²⁺ channels opens

3. Ca²⁺ activates SNARE proteins

4. SNARE proteins lead chemical-filled vesicles into exocytosis

5. chemicals reach receptors on different neuron

Neurotransmitters/Cotransmitters

• chemicals secreted by one neuron’s axon into a synapse to stimulate another neuron on its dendrites

• cotransmitters are two or more neurotransmitters secreted by the same neuron at the same time with different effects

• induce EPSPs and IPSPs

• activate second messenger systems

EPSP

excitatory postsynaptic potential

IPSP

inhibitory postsynaptic potential

Neuromodulators

• secreted together with neurotransmitter

• alter signal of neurotransmitter

  • intensity or meaning

• needs neurotransmitter to have an effect

Acetylcholine (ACh)

• binds to cholinergic receptors

  • muscarinic and nicotinic cholinergic receptors

Biogenic Amines

• Epinephrine (Epi) / Norepinephrine (NE) use adrenergic receptors

  • alpha-adrenergic and beta-adrenergic receptors

Dopamine

uses dopaminergic receptors

Serotonin

uses serotonergic receptors

Histamine

uses histaminergic receptors

Melatonin

uses melatonergic receptors

Neuropeptides

• use peptidergic receptors

  • specific peptides use specific peptide receptors

    • oxytocin and endorphines

Amino Acids

• Glutamate uses glutamatergic receptors

  • major excitatory NT of mammalian CNS

• GABA uses GABAergic receptors

  • major inhibitory NT of the mammalian CNS

Other Neurochemicals

• ATP

• NO (nitric oxide)

• CO (carbon monoxide)

  • NO and CO are gases at body temperature

    • can go backwards to presynaptic neurons

Eicosanoids

• usually act as neuromodulators

  • Prostaglandins and leukotrienes

• Exception: Endocannabinouds

  • neurotransmitters that cross-react with THC (compound found in cannabis)

Factors Affecting Synaptic Strength

• Amount of neurotransmitter secreted

• Amount of Ca²⁺ influx into axon terminal

• Amount of neurotransmitter reaching receptros

• Rate of neurotransmitter loss, degradation, and recycling (reuptake)

  • NE: reuptake into presynaptic neuron for reuse

  • Acetylcholine: degraded by acetylcholinesterase to choline, then reuptake into presynaptic neuron to form new ACh

• Number of receptors on postsynaptic membrane

  • up-regulation or down-regulation

• Prior state of postsynaptic neuron (if it is refractory, neurotransmitter has no effect)

• Simultaneous effects of other presynaptic neurons

• Other neurotransmitters/cotransmitters

• Disease or drugs

Types of Neural Networks

• Convergent pathways (multiple inputs, one output)

• Divergent pathways (one input, multiple outputs)

• Parallel pathway

• Reverberating (feedback) pathway

  • reinforces output through positive feedback

• Serial pathway (input across multiple neurons in a line, one output)

Spinal Reflexes

• sensory neurons come in through dorsal side

• motor neurons exit through ventral side

How many neurons are needed for the simplest complete reflex arc?

2 neurons

Ex. Patellar Reflex

3-Cell Reflex Arc

• Ex. withdrawal reflex

  • one neuron excites muscles in bicep to contract

  • one neuron inhibits muscle in triceps causing them to relax

  • one neuron sends message to the brain telling it about the event

Brain Stem

• Reticular Formation

  • Sleep wake cycles

  • Arousal

  • Consciousness

• Midbrain

  • superior colliculi - visual

  • inferior colliculi - auditory

• Medulla oblongata

  • cardiovascular control center

  • respiratory control center (along with pons)

  • balance

  • other involuntary processes ( with part of autonomic n.s.)

    • swallowing, vomiting digestive secretion, and contraction

  • networking center for neural pathways to/from spine and cerebrum

    • control of movement and balance

Cerebellum

• helps maintain posture and balance

• some types of learning/memory

• receives voluntary motor commands and integrates this with sensory information to regulating skeletal muscle contraction and coordination of movements

Forebrain

• Cerebrum

• Corpus callosum

  • thick path of white matter axons connecting left and right hemispheres

• Diencephalon

• Cerebral cortex

  • left/right hemispheres

  • convoluted cerebral cortex of gray matter

  • conscious thought, complex motor movements, memory, reasoning

• (Subcortical) Basal nuclei

  • integrates motor and sensory signals for movement, part of limbic system

• Epithalamus/pineal gland

  • source of melatonin

• Thalamus

  • integrating/transfer pathway between spinal cord and cerebrum

• Hypothalamus

  • controls pituitary

  • houses many homeostatic set points

  • interacts with autonomic nervous system

Limbic System

• parts of cerebral cortex

• thalamus

• hypothalamus

• hippocampus

• amygdala

• olfactory pathway

Afferent division

• somatic sensory

  • skin, muscles

• visceral sensory

  • internal organs (heart, blood vessels, reproductive tract, and urinary tract)

• special sensory

  • eyes, ears, nose, mouth

Efferent division

• Somatic motor

  • skeletal muscles

• Autonomic motor

  • sympathetic

  • parasympathetic

  • enteric

Somatic Division

• consists of a single neuron between CNS and skeletal muscle cells

• innervates skeletal muscle

• can lead only to muscle excitation

• Voluntary

Autonomic Division

• Has two-neuron chain (connected by a synapse) between CNS and effector organ

• Innervates smooth and cardia muscle, glands, and GI neurons

• Can be either excitatory or inhibitory

• Involuntary

Parasympathetic Dvision

• Ganglion closer to effector cells

• ACh and then ACh

• Rest or Digest Response

  • reduces body’s state of alertness, helps body recover after emergencies, maintains homeostasis

Sympathetic Division

• Ganglion closer to first neuron/spine

• ACh and then NE/Epi

• Flight or Fight Response

  • Mobilizes energy and increases the body's level of alertness, interrupts body’s homeostatic mechanisms

    • alpha receptors to constrict

    • beta receptors to relax