MPP 3202 - Elements of Physiology Exam 2 (Mizzou)

Peripheral Nervous System

cranial & spinal nerves

Neurons

conduct impulses but generally cannot divide

Neuron Structure

- cell body: nucleus + organelles

- dendrites: receive impulses & conduct them to the cell body

- axons: action potentials away from cell body

Ganglia

clusters of cell bodies (of neurons) in PNS

Glial Cells

also called neuroglia

support the neurons - cannot conduct impulses but can divide

Axonal Transport

- fast: component moves vesicles (neurotransmitters)

- slow: moves microfilaments, microtubules, & proteins

Anterograde Transport

movement of particles outward from cell body of neuron

Retrograde Transport

movement of particles inward toward cell body of neuron

Sensory Neurons

conduct impulses from sensory receptors to the CNS

Motor Neurons

impulses from CNS to organs/muscles/glands

Somatic Motor Neurons

control reflexes & voluntary control of skeletal muscles

Autonomic Motor Neurons

involuntary control of smooth muscles, cardiac muscles, and glands

can by sympathetic or parasympathetic

Sympathetic Motor Neurons

autonomic

fight or flight

Parasympathetic Motor Neurons

autonomic

controls normal function - "rest & digest"

Nerves

bundles of axons outside the CNS

most composed of both sensory & motor neurons (mixed nerves)

cranial nerves have only sensory fibers

Tract

bundle of axons in CNS

Schwann Cells

*glial cells in PNS

form the myelin sheath

Satellite Cells

*glial cells in PNS

support cell bodies within ganglia

Oligodendrocytes

*glial cells in CNS

form myelin sheath

Microglia

*glial cells in CNS

migrate around CNS tissue & phagocytize foreign/degenerated materials

Astrocytes

*glial cells in CNS

regulate the external environment of neurons

Ependymal Cells

*glial cells in CNS

line ventricles & secrete cerebrospinal fluid

Blood-Brain Barrier

capillaries in brain don't have pores between adjacent cells - substances can only be moved by selective processes of diffusion through endothelial cells, active transport, or bulk transport

*movement is transcellular (within the cell)

Resting Membrane Potential for Nerves

-70 mV

established by large negative molecules inside, K+, Na+/K+ pumps, permeability of membrane to positive, inorganic ions

high concentration of K+ inside & Na+ outside

Depolarization

positive ions enter cells (usually Na+)

excitatory

Hyperpolarization

positive ions leave cells (K+) OR negative ions enter (Cl-)

inhibitory

K+ Leakage Channels

K+ leakage channels are not gated (always open)

Action Potential

At threshold (-55 mV), voltage gated Na+ channels open & Na goes IN

As cell depolarizes, more Na channels open (positive feedback) - causes overshoot of potential (+30 mV)

At +30 mV, Na+ channels close and K+ channels open (K+ goes out) - repolarization of potential occurs (negative feedback)

K+ Voltage Gated Channels

Closed at resting membrane potential

Open at +30 mV - K+ rushes out & cell repolarizes back toward equilibrium potential (refractory period)

Na+ Voltage Gated Channels

Closed at resting membrane potential

Open at -55 mV (threshold) - Na rushes in due to electrochemical gradient & potential climbs to +30 mV, when channels are deactivated

All-or-None

Once threshold is reached, action potential WILL happen

Size of stimulus doesn't affect size of action potential (ALWAYS reaches +30 mV) - may recruit more neurons

Size of stimulus doesn't affect action potential duration (may make them occur more frequently)

Refractory Period

Time when neuron can't be fired/excited again

Absolute Refractory Period

During the action potential - Na+ channels are inactive (not just closed)

Relative Refractory Period

When K+ channels are still open - only very strong stimuli can overcome this

Conduction of Nerve Impulses

Voltage Gated Na+ channels open in wave down axon

Unmyelinated Conduction

Potentials produced down entire length of axon at every patch of membrane

Slow conduction

Myelinated/Saltatory Conduction

Provides insulation

Nodes of Ranvier every 1-2 mm - Na+ concentrated at these nodes

Action potentials "leap" from node to node

Conduction Speed

Increased by diameter (reduced resistance) & myelination

Thin, Unmyelinated - 1 m/s

Thick, Myelinated - 100 m/s

Synapse

The functional connection between neuron & cell its signaling

In CNS, next cell is another neuron

In PNS, next cell is muscle/gland

Myoneural/Neuromuscular Junction

Synapse between neuron & muscle/gland cell

Electrical Synapses

In smooth & cardiac muscle, some brain

Joined by gap junctions

Stimulation results in phosphorylation or dephosphorylation of connexin proteins to open/close channels

Chemical Synapses

Involves the release of neurotransmitter from an axon terminal

Release of Neurotransmitter

When action potential reaches end of axon, voltage gated Ca+ channels open and this stimulates fusing of synaptic vesicles to plasma membrane

Action of Neurotransmitter

Neurotransmitter (aka ligand) diffuses readily across synapse & binds to a specific receptor protein.

This opens chemically-regulated (ligand-gated) ion channels.

Acetylcholine/ACh

Directly opens ion channels when it binds to receptor

Excitatory in some areas of CNS, autonomic motor neurons, & all somatic motor neurons.

Inhibitory in some autonomic motor neurons

Acetylcholinesterase/AChE

Enzyme that inactivated ACh activity shortly after it binds to the receptor.

Hydrolyzes ACh into acetate and choline which is taken back into presynaptic cell for reuse

Nicotinic ACh Receptors

Stimulated by nicotine

Found on motor end plate of skeletal muscle cells, autonomic ganglia

*Voluntary

Muscarinic ACh Receptors

Stimulated by muscarine (from poisonous mushrooms).

Found in CNS and plasma membrane of smooth & cardiac muscles & glands

Monoamines

Regulatory molecules derived from amino acids

Catecholamines - derived from tyrosine (dopamin, norepinephrine, epinephrine)

Serotonin is derived from L-Tryptophan

Histamine derived from histidine

Monoamine Oxidase/MAO

Quickly takes back monoamines into presynaptic cell (reuptake) and degrades them

Brain + Spinal Cord

Input from sensory neurons

Directs activity of motor neurons

Association Neurons

Integrate sensory info & helps direct the appropriate response to maintain homeostasis & respond to the environment.

Frontal Lobe/Precentral Gyrus

Located in the frontal lobe.

Responsible for motor control (upper motor neurons).

Hypothalamus & Pituitary Gland

Important for maintaining homeostasis & regulating autonomic systems

Centers for hunger/thirst, body temperature, sleep/wakefulness, arousal, emotions, controls endocrine system, hormone secretion from pituitary gland.

Posterior Pituitary

Extension of neural tissue

ADH and Oxytosin transported along hypothalamo-hypophyseal tract to the posterior pituitary where it is stored until needed.

Anterior Pituitary

Hypothalamus produces releasing/inhibiting hormones to the anterior pituitary via the circulation (portal vessels) to regulate the secretion of pituitary hormones.

Myencephalon/Medulla

All ascending and descending tracts between brain and spinal cord pass through here.

Contains nuclei for regulation of breathing and cardiovascular response (vital centers)

Vasomotor center control heartrate

Respiratory center words with pons to control breathing

Ascending Tracts

Conveys sensory info from skin/muscles/glands/joints/organ TO CNS

Cortiospinal/Pyramidial Descending Tract

Descend directly WITHOUT synaptic interruption between cerebral cortex and spinal cord.

Extrapyramidial Tracts

Originate in brain stem and controlled by motor circuits of corpus striatum, substantia niagra, and thalamus

Cranial Nerves

In PNS

*12 pairs

Arise directly from nuclei in brain

Most are mixed nerves with sensory & motor neurons

Nerves associated with vision, olfaction, & hearing have only sensory neurons (cell bodies in ganglia located near sensory organ).

Somatic Nervous System

Motor neurons have cell bodies in spinal cord & one neuron from cord to effectors (skeletal muscle)

Uses Acetylcholine always - excitatory

Autonomic Nervous System

Has 2 neurons in the PNS

One has cell bodies on brain/spinal cord & synapses in autonomic ganglion

Other has cell bodies in ganglion and synapses on effector.

Uses ACh and Norepinephrine (excitatory & inhibitory)

Autonomic Neurons

*In PNS

Involuntary

Cardiac & smooth muscle, blood vessels, & glands

Can stimulate or inhibit - depends on organ & receptors

Preganglionic Neurons

Originate in midbrain/hindbrain, thoractic, lumbar, and sacral spinal cord

Postganglionic Neurons

In head, neck, & abdomen as well as chains along either side of spinal cord

Sympathetic Nervous System

Part of ANS

Preganglionic neurons from thoracic and lumbar regions of spinal cord

Synapse in sympathetic ganglia & parallel to spinal cord (paravertebral ganglia) - in chains

Adrenal Glands

Secretes epinephrine and norepinephrine when stimulated as part of mass activation

Modified ganglion - innervated directly by preganglionic sympathetic neurons

Parasympathetic Nervous System

Part of ANS

Preganglionic neurons from brain or sacral region of spinal cord (up high or down low)

Synapse on terminal ganglia (near/in effector organ)

Short postganglionic neurons

Cranial Nerves in the PNS

Occulomotor (III) Nerve - innervate ciliary muscle of eye

Vagus (X) Nerve - control critical functions

Dual Innervation

Many functions controlled by both division of the ANS

In most cases, SNS and PNS act in opposing ways

Heart rate, digestion, pupil diameter

Functions of the SNS

Fight vs. Flight

Release epinephrine or norepinephrine

Increase heart rate

Increase blood glucose

Increase blood flow to skeletal muscles

Functions of PNS

Rest and Digest

Releases ACh

Decreases heart rate

Increases digestion

Cholinergic Synaptic Transmission

*Actylcholine

ALL preganglionic neurons

Most parasympathetic postganglionic neurons

Some sympathetic postganglionic neurons

Cholinergic Receptors

Nicotinic - autonomic ganglia, stimulated by ACh from PREganglionic neurons, ligand-gated receptors

Muscarinic - visceral organs, stimulated by POSTganglionic neurons, G proteins

Adrenergic Synaptic Transmission

*Norepinephrine

Most sympathetic postganglionic neurons

Adrenergic Receptors

Alpha 1 and Alpha 2 - norepinephrine

Beta 1 and Beta 2 - blood epinephrine

Varicosities

Swelling along postganglionic axons that release neurontransmitters along length of axon for refined/faster control

Sensory Receptors

Transduce (change) different forms of energy into nerve impulses

Afferent Pathways

Chemoreceptors

Functional receptor

Sense chemicals in environment or blood

Photoreceptors

Functional receptor

Sense light

Thermoreceptors

Functional receptor

Response to cold or heat

Mechanoreceptors

Functional receptor

Stimulated by mechanical deformation of receptor (pressure, touch, force)

Proprioceptors

Information receptor

Provide sense of body position and allows for fine muscle control.

In muscles, tendons, and joints

Cutaneous Receptors

Information receptor

Skin receptors

Respond to touch, pressure, heat, cold, pain

Special Senses

Information receptor

Vision, taste, smell, equilibrium

Exteroreceptor

Origin receptor

Respond to stimuli from outside the body (cutaneous and special senses)

Interoceptors

Origin receptor

Respond to internal stimuli (organs, blood pressure, pH, oxygen concentrations)

Phasic

Burst of activity that quickly adapts to stimulus (decreasing response)

Sensory adaptation

Tonic

High firing rate as long as stimulus is applied

No adaptation (ex. pain)

Gustation/Taste

*Chemoreceptors respond to chemicals in food/drink

Receptors (taste buds) have 50-100 specialized epithelial cells with long microvilli (increases surface area) that extend through pore

Microvilli have ligand-gated channels

Cells behave like neurons - depolarize and release neurotransmitters onto sensory neurons

Olfaction/Smell

*Chemoreceptors respond to chemical molecules in air

Olfactory receptors are bipolar neurons with ciliated dendrites in nasal cavity

Odorant molecule stimulates protein

Vestibular Apparatus

Provides a sense of equilibrium/balance

In Inner ear - otolith organs and semicircular canals

Otolith Organs

Utricle and saccule - In vestibular apparatus

Sense linear acceleration

Semicircular Canals

In vestibular apparatus

Sense rotational acceleration

Inner ear

Bony labyrinth surrounding membranous labyrinth

Between 2 labyrinths is perilymph

Within membranous labyrinth is *endolymph - unusually high K+ concentration

Sensory Hair Cells

*Mechanoreceptors

Modified epithelial cells with 20-50 stereocilia (cilia-like) and one kinocilium (true cilia)

1. Stereocilia bend toward kinocilium and K+ goes INTO cell (depolarization)

2. Cells release neurotransmitter

3. Neurotransmitter depolarizes sensory dendrites

Lens Accommodation

Ability of the lens to keep an object focused on the retina as distance changes

Close vision - lens is thick and round

Distant vision - lens is thin and flat

Rods and Cones

Outer segments full of flattened discs with photopigment molecules

Rods - black and white, low light vision (*rhodopsin)

Cones - color vision and acuity (*various photopsins)

Vision Mechanism

1. Na+ channels close (can't leak out)

2. Photoreceptors hyperpolarize

3. Inhibition of bipolar cells lifted

4. Bipolar cells activate ganglion cells

5. Action potential to brain