THE NERVOUS SYSTEM

Homeostasis Strand

Introduction

• In general, a nervous system has three overlapping functions :

1. Sensory input – conduction of signals from sensory receptors to integration centres

2. Integration – information is interpreted and associated with proper responses

3. Motor output – conduction signals from the processing centre to effector cells

Organization of The Nervous System

http://www.youtube.com/watch?v=dOYOdJG0E0s

Structure of the Neuron

Nervous System Cells

- the CNS uses two types of cells : • glial - non-conducting cells - often surround neurons - functions :

– hold neurons in place – supply nutrients and oxygen to neurons – insulate one neuron from another – destroy / remove carcasses of dead neurons

• neurons

- functional units of nervous system

- three different kinds

1. sensory (afferent neurons)

- sense and relay info. from environment to CNS

- located in clusters called ganglia

- i.e. photoreceptors, chemoreceptors, baroreceptors

2. interneurons (association neurons)

- connect sensory neurons to motor neurons

- interpret info.

3. motor (efferent neurons)

- relay info. to effectors - muscles, organs,

glands

The simplest nerve pathway is called the reflex arc.

The Electrochemical Impulse

- around 1900 : nerve impulses are an electrochemical message created by the movement of ions through the nerve cell membrane

Inside A Nerve Cell - neurons have a large supply of both positive and negative ions inside

and outside the cell (rare in cells of other types) - the electrochemical gradient is caused by an unequal concentration

of positive ions across the membrane - NOTE : neg. ions play only a small role since they are mainly large and

cannot cross the membrane

- the electrochemical impulse takes place in four basic steps : 1. The Resting Potential - K+, concentrated inside the membrane, want to diffuse out - Na+, concentrated outside the membrane, want to diffuse in - diffusion is unequal - however, because the resting membrane is 50X more permeable to

K+, the membrane maintains an external (outside) pos. charge - the exterior of the membrane becomes pos. relative to the interior

(neg.) - this resting potential is about -70mV

2. Depolarization - a nerve impulse can be initiated in a neuron by mechanical, chemical,

thermal, or electrical stimulation - when excited, the nerve cell membrane becomes more permeable to

Na+ (than K+), their ion “gates” open, K+ gates shut, and Na+ rushes in by diffusion and charge attraction

- this change in potential is called the action potential and lasts about

3 ms - when an action potential occurs, the axon is said to be depolarized

(the interior is now pos. relative to the exterior) - the resting potential changes from -70mV to about +40mV

3. Repolarization (and Hyperpolarization) - K+ channels open in the membrane and K+ ions diffuse out

along a concentration gradient, starting repolarization - at the same time, sodium channels in the membrane close,

preventing any further influx of Na+ ions - the Na+ gates are slow to respond to the changes in voltage - in fact, so many K+ ions leave that the charge on the inside of

the membrane becomes more negative than it was originally (referred to as the “undershoot”, or hyperpolarization)

4. The Na+/K+ Pump - the Na+/K+ pump restores the resting polarized membrane

by transporting Na+ from the inside to the outside, and K+ from the outside in (3Na+ : 2K+)

- this is a form of active transport - adjoining areas of the nerve membrane become permeable

to Na+ and the action potential moves away from the site of origin

- the electrical disturbance causes Na+ channels to open in the

adjoining area of the nerve cell membrane and a wave of depolarization moves along the nerve membrane

http://www.youtube.com/watch?v=7EyhsOewnH4

Important Notes

• nerves conducting an impulse cannot be activated until the condition of the resting membrane is restored (the refractory period)

• different neurons have different threshold levels • smaller diameter axons result in faster impulse transmission • the response is all or none • greater stimulus intensity produces more frequent impulses • greater stimulus intensity produces impulses in more neurons

The Synapse

- small (~20 nm) spaces between neurons are called synapses - in axon endplates there are chemicals called neurotransmitters - when the action potential reaches a terminal, it depolarizes the

terminal membrane, opening calcium (Ca2+) channels in the membrane

- calcium diffuse into the terminal and cause neurotransmitter release - they are released when the nerve impulse reaches the end of the

axon and they diffuse across this space - these neurotransmitters, released from the presynaptic membrane,

depolarize the dendrites of the postsynaptic membrane

- the nerve impulse slows (slightly) at each synapse - excitatory neurotransmitters open Na+ channels on

dendrites, thereby increasing the membrane potential toward threshold

- inhibitory neurotransmitters open K+ channels on dendrites,

thereby decreasing the membrane potential away from threshold

- chemicals are released by the postsynaptic membrane to

destroy neurotransmitters after they have completed their function

- why is this important?

- examples of neurotransmitters : • acetylcholine

– inhibitory or excitatory, depending on receptor – excitatory on most muscle cells, inhibitory on cardiac muscle – low levels linked to Alzheimer’s disease

• serotonin – regulation of mood, appetite, sleep, memory, learning – mostly inhibitory effects – LSD binds to serotonin receptors and blocks inhibitory effect which

leads to hallucinations (trippy!) – low levels linked to depression

• dopamine – normally released when a need is filled, causing a feeling of pleasure

or satisfaction – low levels linked to Parkinson’s disease – schizophrenia possibly linked to high levels

• norepinephrine – can excite and inhibit – low levels linked to depression

http://www.youtube.com/watch?v=LT3VKAr4roo

The Autonomic Nervous System

• all autonomic nerves are motor nerves that regulate the organs of the body without conscious control – smooth and cardiac muscle, organs of gastrointestinal, cardiovascular, excretory, and endocrine systems

• convey signals that regulate the internal environment (as opposed to the somatic n.s. which carry signals in response to external stimuli)

• made up of two distinct, and often opposing, units :

– the sympathetic nervous system

• generally increase energy consumption and prepare an individual for action

– the parasympathetic nervous system

• enhance activities that gain and conserve energy

• ANS animation :

Paul Andersen, I challenge you!

You’re a dork. Copycat.