Bio Ch 8

Synaptic transmission

The continuation of a signal at the end of a neuron by release of neurotransmitters into the synaptic cleft to cause hyperpolarization or depolarization of the next neuron

Soma

Cell body of the neuron

Axon

Carries the action potential from the cell body to the synaptic terminal

Bipolar Neuron

has one dendrite

multipolar neuron

has many dendrites

dendrites

receive chemical signals from the presynaptic neuron

synaptic knobs

end of axons that form connections with target cells via a synaptic cleft

synaptic cleft

space between synaptic knobs and dendrites that chemical messengers travel across

Nerve

large bundle of many different axons from many different neurons

Kinesin

protein that drives movement of vesicles and organelles along microtubules in the anterograde direction (soma to terminal knobs) down the axon

Resting membrane potential

-70 mV in most neurons

Two membrane proteins that maintain resting potential

1. Na⁺/K⁺ ATPase

2. Potassium leak channels

Na⁺/K⁺ ATPase

pumps 3 Na⁺ out and 2 K⁺ in with every cycle using 1 ATP — primary active transport

Potassium Leak Channels

Allow the potassium pumped in by the Na⁺/K⁺ ATPase to flow back out of the cell

Gradient of common ions in a cell

Lots more Na⁺ outside

Lots more K⁺ inside

Lots more Ca²⁺ outside

Lots more Cl^- outside

Polarization

The state of a neuron being negative on the inside and positive on the outside

Depolarization

Departure from the resting membrane potential to a more positive potential

Repolarization

Returning to resting potential after a period of depolarization

Threshold potential

The membrane potential a neuron must depolarize to before opening voltage gated sodium channels and depolarizing completely generating an action potential — usually around -50 mV

Reached by opening of ligand gated Na⁺ channels by binding of neurotransmitters in the axon hillock

Maximum Depolarization Membrane Potential

Usually around +35 mV

Voltage gated Na⁺ channels inactivate here

Voltage gated K⁺ channels open here allowing for repolarization

Hyperpolarization

When voltage gated K⁺ channels open at the peak membrane potential around +35 mV, they stay open for a while depolarizing the cell below resting potential to about -90 mV

After this overshoot of the resting potential, the K⁺ leak channels and Na⁺/K⁺ ATPase restore the resting -70 mV

Schwann Cells

Make myelin membranes that coat stretches of the axon of peripheral nervous system neurons — called oligodendrocytes in the central nervous system

Myelin

Several layers of specialized membrane that wrap around an axon in spaced lengths not allowing any ions to enter the axon portions they cover — myelination increases conduction velocity

Nodes of Ranvier

gaps in the myelin sheath that have high concentrations of ion channels — APs jump from node to node accounting for the faster conduction of myelinated neurons

Saltatory conduction

The rapid jumping of action potentials from node to node down a myelinated axon

Glial Cells

Specialized non-neuronal cells that provide structural and metabolic support to neurons such as Schwann Cells

Types of Glial Cells

Cell Type	Location	Primary Functions
Schwann Cells	PNS	Form myelin
Oligodendrocytes	CNS	Form myelin
Astrocytes	CNS	Guide neuronal development, Regulate neurotransmitter levels
Microglia	CNS	Fight CNS infections Remove dead cells and debris
Ependymal Cells	CNS	Produce and circulate cerebrospinal fluid (CSF)

Equilibrium Potential

Membrane potential at which an ion gradient (and thus driving force) no longer exists. Since the ion gradient is different for each ion, each ion has its own equilibrium potential

Na⁺ = +50 mV

K⁺ = -90 mV

Nernst Equation

See image

Absolute Refractory Period

Time during which a neuron will not fire another AP regardless of stimulus strength or depolarization magnitude

Happens when voltage-gated sodium channels are in the inactivated state — they cannot open again until they reach the close state

Relative Refractory Period

Time during which a neuron can be induced to fire an action potential but the depolarization required is greater than normal because the neuron is still hyperpolarized after coming down from the last potential and has not yet returned to resting potential

Synapse

Space between an axon terminus and the next neuron’s dendrites, soma, or axon

Can also be the space between the axon terminus and a target organ

Two main types of synapses

1. Electrical Synapses

2. Chemical Synapses

Electrical Synapse

Where the cytoplasm of two cells is joined by a gap junction allowing an action potential to seamlessly flow from one cell to the other

• Common in smooth and cardiac muscle

• Not common in the nervous system

Chemical Synapse

Where an electrical signal (action potential) is converted into a chemical signal (neurotransmitter) which can cross the synapse

Signal transmission steps across a chemical synapse

1. Dendrites bind neurotransmitter and open ligand gated channels

2. Ligand gated Sodium channels on axon hillock are bound causing depolarization to threshold

3. Voltage gated sodium channels open and generate action potential

4. Action potential propagates by continually opening voltage gated sodium channels as it travels down the axon

5. Voltage gated potassium channels open at peak depolarization creating a repolarizing wave following the wave of depolarization

6. Once depolarization reaches the synaptic terminal, it opens voltage gated Calcium channels

7. Calcium influx causes release of neurotransmitter into synapse

8. Repeat step 1

9. Neurotransmitter is reabsorbed through transporters (reuptake)

Neuromuscular Junction

Synapse between neurons and skeletal muscle where acetylcholine is released

Acetylcholinesterase (AChE)

Degrades acetylcholine in the synaptic cleft

Excitatory Neurotransmitter/Receptor

Depolarizes postsynaptic neuron

• The neurotransmitter itself doesn’t differentiate between excitatory and inhibitory effects, the receptor it binds to determines which happens

• Some neurotransmitters can bind to some excitatory receptors and some inhibitory receptors

Inhibitory Neurotransmitter/Receptor

Hyperpolarizes postsynaptic neuron

• The neurotransmitter itself doesn’t differentiate between excitatory and inhibitory effects, the receptor it binds to determines which happens

• Some neurotransmitters can bind to some excitatory receptors and some inhibitory receptors

“All-or-nothing” nature of Action Potentials

when fired, action potentials always have the same speed and magnitude of depolarization for a given neuron

Spatial Summation

Every synapse has several presynaptic neurons converging on it

The postsynaptic neuron’s ability to reach threshold potential and fire is dependent upon the summation of effects of all of the presynaptic neurons at any given moment

Some presynaptic neurons may fire inhibitory potentials at the same time others fire excitatory potentials

Excitatory Postsynaptic Potentials (EPSPs)

cause postsynaptic depolarization

Inhibitory Postsynaptic Potentials (IPSPs)

cause postsynaptic hyperpolarization

Temporal Summation

One presynaptic neuron fires a ton of excitatory or inhibitory potentials that all pile up at once creating enough of an effect to single-handedly excite or inhibit the postsynaptic neuron

PNS

Peripheral Nervous system

• Carries out sensory and motor functions

CNS

Central Nervous System

• Carries out integrative functions, taking sensory signals from the PNS and converting them into motor signals to be sent by the PNS

Effectors (2 types)

The tissues/organs acted upon by motor neurons

1. Muscles

2. Glands

Efferent

Out of CNS, motor

Afferent

Into CNS, sensory

Reflex

Direct motor response to sensory input without conscious thought

• Usually doesn’t involve the brain and only involves the spinal cord

• Usually monosynaptic with the synapse in the spinal cord

Disynaptic Reflex

A reflex where the sensory neuron stimulates two other neurons

• Seen in reciprocal inhibition where the sensory neuron stimulates a motor neuron to cause contraction and an inhibitory interneuron to cause relaxation

Reciprocal Inhibition

Sensory neurons allowing for a movement by stimulating a motor neuron to contract one muscle and an interneuron to relax another

• Seen with leg extension where quadriceps contract and hamstring relaxes

Organization of the Nervous System

See Image

• Sympathetic effects often occur as a result of epinephrine release from the adrenal medulla

The enteric system also exists :)

Effects of Autonomic Nervous System on various organs

See image

Tract / Column

White matter (axons) in the CNS

Nucleus

Gray matter deep in the brain

Cerebral Cortex

Gray matter on the surface of the brain

Horn

Gray matter in the spinal cord

Ganglion

gray matter in the PNS

Cerebrospinal Fluid (CSF)

Clear liquid that the CNS (brain and spinal cord) floats in for shock absorption and nutrient / waste exchange

Reticular Activating System (RAS)

responsible for arousal and wakefulness

Mostly contained within the midbrain

Hypothalamus

• Controls emotions and autonomic functions

• Links the nervous and endocrine systems through its control of the pituitary

Basal Nuclei / Basal Ganglia

Involved in procedural learning and habit forming

Works with the cerebellum to coordinate movement initiated by the frontal lobe

Sensory Homunculus

Depicts which part of the parietal lobe interprets sensation of each part of the body

pairs of cranial nerves

12

pairs of spinal nerves

31

vagus nerve

Parasympathetic cranial nerve

• Lowers heart rate

• increases GI activity

Somatic Motor Neuron Facts

What do they innervate?

What neurotransmitter do they all use?

Where are their cell bodies located

They innervate skeletal muscle using acetylcholine

Their cell bodies are all in either the brain stem or the ventral root of the spinal cord

ventral vs dorsal roots of the spinal cord

Dorsal = sensory = info into CNS

Ventral = motor = info out of CNS

Somatic Sensory Neuron Facts

Where does the dendrite end?

Where is the cell body?

Where is the first synapse

The dendrite extends from the sensory receptor at some place on the body, to the dorsal root ganglion where the sensory neuron cell body is

The sensory neuron soma is still outside of the meninges, the axon extends into the meninges

The first synapse is either within the spinal cord or within the brain

Meninges

Protective sheath around brain and spinal cord

Autonomic nervous system neurotransmitters

All preganglionic autonomic neurons use acetylcholine

All parasympathetic postganglionic neurons use acetylcholine

Most sympathetic postganglionic neurons use norepinephrine

Preganglionic and Postganglionic neuron locations in the efferent autonomic system

• Sympathetic starts in thoracic or lumbar vertebrae — thoracolumbar system

• Parasympathetic starts in cervical or sacral vertebrae — craniosacral

• Sympathetic has a very short preganglionic axon that ends just outside the spinal cord and the postganglionic axon travels most the way to the effector (target organ or tissue)

• Parasympathetic has a very long preganglionic axon that travels most the way to the effector with a short postganglionic axon that travels a small distance to innervate the effector

Short Reflex

When an autonomic afferent neuron synapses with an autonomic efferent neuron in the PNS to effect a non-conscious, quicker response than traveling all the way to the CNS

Inner portion of the adrenal gland

adrenal medulla

outer portion of the adrenal gland

adrenal cortex

Main hormones secreted by the adrenal cortex

Cortisol — a glucocorticoid

Aldosterone — a mineralocorticoid

Sex hormones

Hormone secreted by the adrenal medulla

Epinephrine — acts on sympathetic postganglionic neurons to enhance their activity resulting in increased heart rate and the fight-or-flight response

Exteroceptors

Sensory receptors that detect stimuli from the outside world

Interoceptors

Sensory receptors that respond to internal stimuli (within the body)

Mechanoreceptors

Respond to mechanical disturbances

Pacinian Corpuscles

Onion-shaped pressure sensors located deep in the skin

When the onion-like layers of the corpuscular membranes are compressed by pressure on the skin, a graded depolarization results in a nearby neuron that could start an action potential

Auditory Hair Cell

Found in the cochlea, detects vibrations generated by sound waves

Vestibular Hair Cell

Located within semicircular canals of the inner ear

Detect acceleration and position relative to gravity

Intestinal stretch receptor

Type of mechanoreceptor on the intestinal wall that feels the stretch when the intestine is full and stops the signal of hunger and induces satiety

Chemoreceptors

Respond to chemicals

Olfactory receptors

Type of chemoreceptor

Detect chemicals in air to allow us to smell things

Gustatory Receptors

Type of chemoreceptor — taste buds

Bind to molecules in food that cause us to taste

Aortic and Carotid Chemoreceptors

Chemoreceptors in the aortic and carotid arteries detect

• pH

• pCO₂

• pO₂

Nociceptors

Pain receptors — may be somatic or autonomic

• Consist of a free nerve ending that detects chemical signs of tissue damage

• Give unclear sensation of dull, aching pain

• Can create the illusion of pain on the skin when crossed with other somatic afferent neurons — referred pain

Referred Pain

Pain that is felt in one part of the body despite the tissue damage occurring somewhere else.

If the injury occurs at point A but the pain is felt at point B:

The nociceptor at point A crosses with a sensory neuron that originates at point B, thus causing the body to think that the pain signal from the nociceptor at point A came from the origin point of the somatic sensory afferent at point B

Thermoreceptors (3 categories)

Stimulated by changes in temperature — can be both autonomic and somatic

1. cold-sensitive

2. warm-sensitive

3. thermal nociceptors — detect painfully hot stimuli

Electromagnetic Receptors

Respond to electromagnetic waves — only consist of rods and cones of eye in humans

Four Properties of a Stimulus communicated to the CNS

1. Modality — type of stimulus — based on which receptor is firing (labeled line)

2. Location — origin of stimulus — based on which receptive field the sensory neuron comes from

3. Intensity — strength of stimulus — encoded by the frequency of action potentials

4. Duration — not always communicated — can be communicated by tonic response receptors that fire APs for the duration of the stimulus

Stimulus Adaptation

Decrease in AP frequency when stimulus intensity remains constant — allows us to get used to consistent stimuli

Does not apply to nociceptors (pain), they never adapt

Proprioceptors

Provide an “awareness of self”

Form the kinesthetic sense

Muscle Spindle

A type of mechanoreceptor and proprioceptor

• Detects muscle stretch

Golgi Tendon Organs

Proprioceptors that monitor tension in the muscles to make sure it doesn’t get too great and tear the muscle

Joint Capsule Receptors

Detect pressure, tension, and movement in joints