NeuronsNeurons

The Nervous SystemThe Nervous SystemWe learned in Bio 11 that

animals have the ability to respond to their environment

Animal responseBut how do they do that?We rely on specialized cells

called neurons

Neuron StructureNeuron Structure

There are many different neurons, but there are also many similarities

Largest feature is the cell body◦Contains nucleus and other organelles◦Site of most metabolic activity

Branching outwards are dendrites◦Conveys signals from environment, or

other neuronsLong extension from body is the

axon◦Carries signal away from body◦Ends in branching axon terminal

The myelin sheath covers (insulates) the axon in discrete sections◦The exposed sections in between are

called Nodes of RanvierSheath are formed by Schwann

cells

Types of NeuronsTypes of NeuronsThree classes: sensory,

interneurons and motor

SensoryThese communicate info from

environment (sensory input)Usually synapse (connect) with

interneurons

Motor NeuronsThese convey impulses (motor

output) to effector cells (those that respond)◦Basically, what the body is doing in

response to the environment

InterneuronsIntegrate sensory input with

motor outputJob is to connect sensory with

motor neurons

CNS and PNSCNS and PNSThe central nervous system (CNS)

consists of brain and spinal cordPeripheral nervous system (PNS)

consists of sensory and motor neurons

PNS conveys messages about outside, CNS decides what to do, tells PNS to respond

However, CNS is not always involved

Reflex ArcReflex ArcAKA, the knee-jerk reflexSimplest neural circuitsSensory neurons directly

attached to motor neurons◦No processing required

Signals transmits to spine, directly turned back to leg for response

Interneurons are still used, keeps hamstring from contracting and resisting the quads

Transmission of SignalsTransmission of SignalsAll cells have an electrical charge

difference◦Typically, there is more K+ and less

Cl- inside the cellCreates a membrane potential of

around -50mV to -100mV

Recall ions cannot pass through a membrane

So, there are transport proteins and ion channels◦Noted that there are more K+ than Na+

Both concentration and electrical gradients are important

K+ wants out because of concentration, wants to stay in because of electrical

This competition alone would lead to a potential of around -85mV

However, some Na+ can leak in, raising it to around -70mV◦This is the resting potential

This is all kept in check by the sodium-potassium pump

Ion channels

MovementMovementNow a stimulus can be receivedThe response causes opens Na+

gates, making the inside more positive

The membrane must reach around -50mV◦This is called the threshold

If the stimulus is large enough (enough Na+ gates are opened), a cascade of events will happen

Basically, Na+ channels open first, floods in

Membrane potential reverses◦This is what we call an action

potentialK+ channels open, floods out,

restoring potentialThis event jumps from node to

nodeStimulated neuron