NERVOUS SYSTEM CH 48

NERVOUS SYSTEM Central Nervous system –  Brain & spinal cord Peripheral nervous system- nerves that communicate motor & sensory signals through the body

NEURONS Sensory neuron – input from external stimuli Interneuron – integration: analyze & interprets input Motor neuron– signal sent to muscle or gland cells

Nucleus Dendrites Stimulus Axon hillock Cell body Presynaptic cell Signal direction Axon Synapse Neurotransmitter Synaptic terminals Postsynaptic cell Synaptic terminals Parts of a neuron

Axon Myelin sheath Schwann cell Nodes of Ranvier Node of Ranvier Layers of myelin Axon Schwann cell Nucleus of Schwann cell

NERVE SIGNALS Membrane potential - the electrical charge difference across a membrane Due to different concentrations of ions in & out of cell Resting potential – the membrane potential of an unstimulated neuron About -70 mV (more negative inside)

Key Na  KK Sodium- potassium pump Potassium channel Sodium channel OUTSIDE OF CELL

MAINTAINING RESTING POTENTIAL To keep sodium & potassium in the right gradients, the sodium- potassium pump uses ATP to maintain gradients The sodium-potassium pump pumps 2K+ in and 3Na+ out each time.

TYPES OF ION CHANNELS: Ungated ion channels – always open Gated ion channels – open or close in response to stimuli Ligand gated ion channels (chemically gated)–in response to binding of chemical messenger (i.e. neurotransmitter) Voltage gated ion channels – in response to change in membrane potential Stretch gated ion channels – in response to mechanical deformation of plasma membrane

HYPERPOLARIZATION When gated K + channels open, K + diffuses out, making the inside of the cell more negative Stimulus Threshold Resting potential Hyperpolarizations  50 0  50 Membrane potential (mV)

DEPOLARIZATION Opening other types of ion channels triggers a depolarization, a reduction in the magnitude of the membrane potential For example, depolarization occurs if gated Na + channels open and Na + diffuses into the cell Stimulus Threshold Resting potential Depolarizations  50 0  50  Membrane potential (mV)

ACTION POTENTIALS Signals conducted by axons, transmitted over long distances Occur as the result of gated ion channels that open or close in response to stimuli - “All or nothing”

ACTION POTENTIAL Steps: 1) resting state 2) threshold 3) depolarization phase 4) repolarization phase 5) undershoot Threshold Resting potential  50 0  50  Membrane potential (mV) 6 Action potential

OUTSIDE OF CELL INSIDE OF CELL Inactivation loop Sodium channel Potassium channel Action potential Threshold Resting potential Time Membrane potential (mV)  50  100  50 0 Na  KK Key Resting state Undershoot Depolarization Rising phase of the action potential Falling phase of the action potential

ACTION POTENTIAL hill.com/sites/ /student_view0/ch apter14/animation__the_nerve_impulse.htm l

HOW DO ACTION POTENTIALS “TRAVEL” ALONG A NEURON? Where action potential is generated (usually axon hillock), the electrical current depolarizes the neighboring region of membrane Action potentials travel in one direction – towards synaptic terminals

KK KK KK Na  Action potential Axon Plasma membrane Cytosol Action potential 213

Why doesn’t it travel backwards? The refractory period is due to inactivated Na+ channels, so the the depolarization can only occur in the forward direction.

SPEED OF ACTION POTENTIALS Speed is proportional to diameter of axon, the larger the diameter, the faster the speed Several cm/sec – thin axons 100 m/sec in giant axons of invertebrates such as squid and lobsters

Ganglia Brain Arm Nerve Eye Mantle Nerves with giant axons

SPEEDING UP ACTION POTENTIAL IN VERTEBRATES Myelination (insulating layers of membranes) around axon Myelin is deposited by Schwann cells or oligodendrocytes. Cell body Schwann cell Depolarized region (node of Ranvier) Axon

Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na + channels are found Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction

SYNAPSES Neurons communicate with other cells at synapses Electrical synapse- Direct communication from pre to post synaptic cell Gap junctions connect cells and ion currents flow between cells

CHEMICAL SYNAPSE Much more common in vertebrates & most invertebrates 1) Action potential reaches synaptic terminal 2) This depolarization causes Ca+ to rush into neuron through voltage gated calcium channels

3) Synaptic vesicles fuse with presynaptic membrane and release neurotransmitters. 4) Neurotransmitter diffuses across synaptic cleft and binds to ligand gated ion channels in second neuron. 5) Ligand gated ion channels open, generating a post-synaptic potential 6) Neurotransmitter is removed quickly – by enzymes or by surrounding cells uptake

Presynaptic cell Postsynaptic cell Axon Presynaptic membrane Synaptic vesicle containing neurotransmitter Postsynaptic membrane Synaptic cleft Voltage-gated Ca 2  channel Ligand-gated ion channels Ca 2  Na  KK

EXCITATORY SYNAPSES Some synapses are excitatory – they increase the likelihood that the axon of the postsynaptic neuron will generate an action potential Opens channel for both Na+ & K+ - allows Na+ to enter & K+ to leave cell, so this depolarizes the membrane EPSP – excitatory postsynaptic potential

INHIBITORY SYNAPSES Some synapses are inhibitory – they make it more difficult for the postsynaptic neuron to generate an action potential Opens channel that is permeable for only K+ or Cl-, so this hyperpolarizes the membrane IPSP – inhibitory postsynaptic potential

SUMMATION OF POSTSYNAPTIC RESPONSES A single EPSP is usually not enough to produce an action potential Summation = the additive effect of postsynaptic potentials The axon hillock is the neuron’s integrating center Temporal summation Spatial summation

NEUROTRANSMITTERS Many different types – 5 main groups: Acetylcholine biogenic amines amino acids Neuropeptides gases One neurotransmitter can have more than a dozen different receptors

ACETYLCHOLINE - One of the most common neurotransmitters in vertebrates and invertebrates - Can be inhibitory or excitatory - Released at neuromuscular junctions, activates muscles - inhibits cardiac muscle contraction -also involved in memory formation, and learning

BIOGENIC AMINES Biogenic amines are derived from amino acids They include Norepinephrine – excitatory neurotransmitter in the autonomic nervous system Dopamine – rewards increase dopamine levels Serotonin - helps regulate mood, sleep, appetite, learning and memory They are active in the CNS and PNS

ENDORPHINS - Decrease our perception of pain - Inhibitory neurotransmitters - produced during times of physical or emotional stress – i.e. childbirth, exercise Opiates (i.e. morphine & heroin) bind to the same receptors as endorphins and can be used as painkillers

VERTEBRATE BRAIN SPECIALIZATION Cerebrum – 2 hemispheres, higher brain functions such as thought & action

Brain Hemispheres

VERTEBRATE BRAIN SPECIALIZATION Cerebellum – helps coordinate movement, posture, balance

VERTEBRATE BRAIN SPECIALIZATION Brainstem – controls homeostatic functions such as breathing rate, heart rate, blood pressure. Conducts sensory & motor signals between spinal cord & higher brain centers

Allan Jones: A map of the brain f_the_brain.html The mysterious workings of the adolescent brain re_the_mysterious_workings_of_the_adolesce nt_brain.html