THE NERVOUS SYSTEMmrvolkmann.weebly.com/uploads/8/8/9/7/8897773/the...Nervous System Cells - the CNS...
Transcript of THE NERVOUS SYSTEMmrvolkmann.weebly.com/uploads/8/8/9/7/8897773/the...Nervous System Cells - the CNS...
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
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
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
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.