Neurons and the Nervous System

Two Regulatory Systems Nervous System Fast! Short duration effect Electric (ionic) signals …but also chemicals (neurotransmitters) Affects nearby cells (local) Endocrine System Slower to start Longer duration effect Chemical signals (hormones) Affects any cell (long distance)

Nervous and Endocrine are Related Neurosecretory Cells In brain, but secrete hormones Ex: epinephrine as hormone & neurotransmitter Each system affects outcome of other Ex: suckling…neurons…oxytocin…more milk Ex: chemoreceptors detect glucose in blood…pancreas secretes insulin/glucagon . Feedback Mechanisms Positive: suckling/oxytocin Negative: calcium levels/ PTH/calcitonin

Overview of the Vertebrate Nervous System Divisions of the Nervous System Central Nervous System (CNS) Brain Spinal cord video Peripheral Nervous System (PNS) Nerves

Overview of the Vertebrate Nervous System convey sensory stimulus to the brain “receive” convey motor stimulus to the muscles “act”

What Does What? 1. Fight or flight 2. Telling skeletal muscle what to do 3. Sending info to CNS 4. Telling glands what to do 5. Rest & digest 6. Telling smooth muscle what to do

PNS: Somatic and Autonomic Divisions

PNS: Parasympathetic & Sympathetic

Reflexes Simplest nerve circuit Adaptive value essential actions don’t need to think or make decisions about blinking balance pupil dilation startle Purpose: quickly respond to danger Quicker response time • Automatic: no integration with brain signal only goes to spinal cord does not reach brain before response occurs • Fewer neurons / synapses involved in reflex arc shorter distance for signal to travel • Involuntary no conscious control no processing by brain Response is innate (automatic response) rather than learned predetermined neuron pathway (hardwired)

The Reflex Arc Components Stimulus - signal detected by sensory neuron (receptor) Sensory neuron carries message to spinal cord (afferent path) by-passes the brain (shorter path, faster response) U-turn at the interneuron (relay neuron) Motor neuron sends back response (efferent path) Effector – completes response; muscle or gland video

Pain withdrawal reflex Reflex Arc Examples Pain withdrawal reflex Knee jerk reflex

The Human Brain Divisions Cerebrum – “thinking”; conscious actions; all the fancy stuff Cerebellum – coordinates voluntary muscle movements Brainstem – basic life functions (involuntary) Hypothalamus – homeostasis (hunger, thirst, etc)

Lateralization of Brain Function Left hemisphere language, math, logic operations, processing of serial sequences of information, visual & auditory details detailed activities required for motor control Right hemisphere pattern recognition, spatial relationships, non-verbal ideation, emotional processing, parallel processing of information

Nervous System Cells Neuron a nerve cell structure fits function many entry points for signal transmits signal in one direction neuron structure axon

Transmission of a Signal Transmission of signal is like dominoes: start the signal knock down line of dominoes by tipping 1st one (trigger the signal) propagate the signal do dominoes move down the line? no, just a wave through them! re-set the system before you can do it again, have to set up dominoes again (reset the axon) Neuron has similar system protein channels are set up once first one is opened, the rest open in succession all or nothing response “wave” action travels along neuron have to re-set channels so neuron can react again Bozeman video

How the Nerve Impulse Travels Membrane Potential the outside of the cell is more positive the difference in charge while it is not “firing” is called the “resting potential” resting potential video unstimulated neuron = resting potential of -70mV

Transmission of a Signal The Action Potential a momentary reversal in electrical potential across a plasma membrane (as of a neuron or muscle fiber) that occurs when a cell has been activated by a stimulus (a wave that moves down axon of nerve) nerve impulse

How the Nerve Impulse Travels Cells live in a sea of charged ions Anions (negative) more concentrated within the cell Cl-, charged amino acids (aa-) Cations (positive) more concentrated in the extracellular fluid Na+

How the Nerve Impulse Travels Propagation of the Action Potential Resting Potential Cells have voltage: voltage gated ion channels (opposite charges on opposite sides of cell membrane) Membrane is polarized negative inside; positive outside charge gradient present stored energy (like a battery) animation

Propagation of the Action Potential 2. Depolarization Stimulus: causes Na+ channels to open Na+ ions diffuse into cell Once enough Na+ moves in, membrane is “depolarized” charges reverse at that physical point on axon positive inside; negative outside

Propagation of the Action Potential Action potential: nerve impulse “wave” that travels down neuron brain  finger tips in milliseconds change in charge opens next Na+ gates down the line “voltage-gated” channels Na+ ions continue to diffuse into cell

Propagation of the Action Potential Charges re-set as action potential continues down neuron K+ channels open K+ channels open up more slowly than Na+ channels K+ ions diffuse out of cell charges reverse back at that physical point on axon negative inside; positive outside

Propagation of the Action Potential Combined waves travel down neuron wave of opening ion channels moves down neuron signal moves in one direction     flow of K+ out of cell stops activation of Na+ channels in wrong direction

Propagation of the Action Potential Action potential propagates Ion channels open & close in response to changes in charge across membrane

Propagation of the Action Potential Repolarization After firing a neuron has to re-set itself Na+ needs to move back out K+ needs to move back in both are moving against concentration gradients a pump is needed

How the Nerve Re-sets Itself Sodium Potassium Pump Active transport protein in membrane requires ATP 3 Na+ pumped out 2 K+ pumped in Re-sets charge across membrane Na-K pump maintains this difference

How the Nerve Re-sets Itself 4. Resting potential re-established: neuron is ready to fire again

Action Potential Graph 1. Resting potential 2. Stimulus reaches threshold potential 3. Depolarization Na+ channels open; K+ channels closed 4. Na+ channels close; K+ channels open 5. Re-polarization reset charge gradient 6. Undershoot K+ channels close slowly

Action Potential Graph

Saltatory Conduction Action potentials can only be generated at nodes of Ranvier axon coated with Schwann cells covered by myelin sheath (fatty covering) insulates axon found on most large sensory nerves Action potentials jump from node to node increasing conduction of impulse Multiple Sclerosis- immune system (T cells) attack myelin sheath- causes loss of signal 150 m/sec vs. 5 m/sec (330 mph vs. 11 mph) video

Propagation of Impulse to Other Nerves animation – chemical synapse Bozeman video Chemical Synapse: junction between neurons that allows transmission of signals between neurons Events at synapse: action potential depolarizes membrane - opens Ca++ channels neurotransmitter vesicles fuse with membrane - release neurotransmitter to synapse (diffusion) neurotransmitter binds with protein receptor ion-gated channels open neurotransmitter degraded or reabsorbed presynaptic neuron postsynaptic neuron CLEFT

Nerve Impulse in Next Neuron Each Neuron makes connections with many other neurons Post-synaptic neuron triggers nerve impulse in next nerve cell chemical signal opens ion-gated channels Na+ diffuses into cell K+ diffuses out of cell process starts over again

Neurotransmitters Chemical messengers more than 40 identified transmit signals across a chemical synapse, from one neuron to another "target" neuron Released from synaptic vesicles in synapses into the synaptic cleft received by receptors on the target cells.

Neurotransmitters Acetylcholine transmit signal to skeletal muscle Epinephrine (adrenaline) & Norepinephrine fight-or-flight response Dopamine widespread in brain affects sleep, mood, attention & learning lack of dopamine in brain associated with Parkinson’s disease excessive dopamine linked to schizophrenia Serotonin

Neurotransmitters Weak point of nervous system any substance that affects neurotransmitters or mimics them affects nerve function gases: nitrous oxide, carbon monoxide mood altering drugs depressants quaaludes, barbiturates stimulants amphetamines, caffeine, nicotine hallucinogenic drugs: LSD, peyote SSRIs: Prozac, Zoloft, Paxil neurotoxin poisons acetylcholinesterase: breaks down acetylcholine snake venom, sarin, insecticides

Neurological Disorder