The Neuron and Action Potential http://www.blackwellpublishing.com/matthews/channel.html https://eee.uci.edu/08w/05717/week3.html

Basic structure of the neuron Dendrite – Recipient of incoming message Cell body (soma) – consists of nucleus and most organelles Axon – carries messages to the axon terminal primarily via the action potential Because axons are so long, transporting proteins down the axon has developed a special system Soma to terminal – anterograde transport – uses kinesin protein and microtubule to transport proteins Terminal to soma – retrograde transport – uses dynein protein and microtubules to transport proteins Myelin sheath – mostly lipid layer surrounding the axon, made from Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS) Node of Ranvier – regions of the axon where no myelin sheath found, high concentration of ion channels found here Axon terminal – action potential triggers the fusion of vesicles containing neurotransmitters to the cell membrane, causing release of neurotransmitters into the synapse Synapse – region between two neurons where communication occurs

Oligodendrocytes and Schwann cells Forms the myelin sheath Key differences Location Composition of myelin sheath Number of cells per axon Schwann cells wrap around one axon Oligodendrocytes wrap around many

The Neuron and Action Potential The action potential graph shown above is observed when you measure the electrical activity on the INSIDE of a neuron What is the resting potential voltage? When a neuron is resting, it is negatively charged.

What maintains the resting potential? Electrostatic pressures Charges like to remain neutral Diffusion Concentrations move from high to low in order to remain balanced Na+ Na+ Na+ Na+ -70mv concentration Sodium/Potassium pump helps maintain resting potential electrostatic - K+ Na+ Cl- Na+ + Cl- Na+ K+ K+

Na+/K+ pump 3 Na+ ions go out 2 K+ ions go in Requires ATP If 3 positive charges go out, and 2 come in, does the inside of the neuron become more negative or more positive?

The Neuron and Action Potential http://www.blackwellpublishing.com/matthews/channel.html https://eee.uci.edu/08w/05717/week3.html

The action potential – the stimulus dendrite axon -70mv myelin axon Na+ NT synapse

Muscle contraction Action potential Muscle cell nAChR Voltage gated calcium channel ACh Acetylcholine binds to nicotinic acetylcholine receptor (nAChR) Na+ comes through nAChR and causes an action potential (will discuss later) Voltage gated calcium channel opens and lets through calcium What does calcium do?

The Neuron and Action Potential If not enough positive charges reach the axon, then you get failed inititiations If enough positive charges reach the axon, this will cause voltage-gated Na+ channels to open and an action potential will begin. Once threshold occurs, the action potential will fire no matter what.

The action potential – threshold to peak dendrite axon -70mv myelin axon Na+ outside synapse inside Once the positive charges reach the axon, that’s when the action potential starts

The action potential – peak to hyperpolarization dendrite axon -70mv myelin axon outside synapse inside Na+ K+

The Neuron and Action Potential Resting potential = Na+/K+ pump Stimulus – neurotransmitter binds Threshold – Voltage gated sodium channels open Rising phase – Na+ flows in from voltage gated Na+ channels Peak – Na+ channels get blocked, K+ channels open Falling phase – K+ flows out Hyperpolarization – K+ channels close and neuron returns to resting potential

Release of neurotransmitter Action potential causes voltage-gated calcium channels to open Calcium comes into the neuron and causes neurotransmitter to be released from vesicles – starts the whole process over

Organization of the nervous system Nervous system divided into two types Central nervous system = brain and spinal cord Peripheral nervous system = sensory and motor neurons Motor neuron – this is what causes muscles to contract (we’ve seen this a couple times now) Sensory neuron – how we perceive the world around us (retina, nose, cochlea, etc.) Motor neurons can be divided into two types Somatic (voluntary) nervous system Autonomic (involuntary) nervous system Sympathetic – activated when an animal is stressed Parasympathetic – opposite of the sympathetic

Organization of the nervous system Cerebrum – where higher thinking takes place 4 lobes Frontal lobe – motor cortex Parietal lobe – Somatosensory (touch) cortex Occipital lobe – visual cortex Temporal lobe – auditory (hearing) cortex) Cerebellum – helps with coordinating movement Hippocampus – looks like a sea horse – involved in learning and memory Amygdala – what process emotions

Organization of the nervous system Hypothalamus – controls the release of hormones into the bloodstream Thalamus – process all the sensory information Brain stem – controls autonomic nervous system – heartbeat, respiration, etc. Brain stem

Reflex Muscle spindle – sensory receptor that detects stretch in muscle When a muscle in this type of stretch reflex is stretched, a sensory neuron fires That sensory neuron synapses with the motor neuron that goes back to the muscle that was originally stretched Sensory neuron stimulates motor neuron which stimulates the muscle to contract

Visual system – the eye Continuous with one another Cornea – focuses the light entering the eye Sclera – white part of the eye – protects the eye Iris – muscle that controls how much light can enter (eye color) Pupil – opening in the iris where light is let through Lens – focuses on near or distant objects Ciliary muscles – attached to the lens, they alter the shape of the lens so it can focus on near or distant objects Retina – interior lining of the back of the eye where sensory receptors are found (rods and cones – photoreceptors) Fovea – where most of our acute vision is regulated, only cones found here (part of the retina) Optic disk – where blood vessels and axons leave the eye, no receptors here so we have a blind spot Continuous with one another

Visual system Sensory receptors of the visual system (photoreceptors) Rods Detect only black and white (in other words, light or no light) help us see in the dark because they are more sensitive to light More prevalent in the peripheral retina Cones Detect color Daytime vision, higher acuity More prevalent in central retina

The retina Photoreceptor layer Bipolar cell layer Must be invisible ganglion cell layer light