Neurons

The Nervous System We learned in Bio 11 that animals have the ability to respond to their environment Animal response But how do they do that? We rely on specialized cells called neurons

Neuron 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 neurons Long 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 Ranvier Sheath are formed by Schwann cells

Types of Neurons Three classes: sensory, interneurons and motor Sensory These communicate info from environment (sensory input) Usually synapse (connect) with interneurons

Motor Neurons These convey impulses (motor output) to effector cells (those that respond) ◦ Basically, what the body is doing in response to the environment Interneurons Integrate sensory input with motor output Job is to connect sensory with motor neurons

CNS and PNS The central nervous system (CNS) consists of brain and spinal cord Peripheral 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 Arc AKA, the knee-jerk reflex Simplest neural circuits Sensory 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 Signals All cells have an electrical charge difference ◦ Typically, there is more K + and less Cl - inside the cell Creates 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

Movement Now a stimulus can be received The 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 potential K + channels open, floods out, restoring potential This event jumps from node to node Stimulated neuron