The Nervous System Nervous Tissues and the Synapse Chapter 11

Nervous Tissues Two types of cells: Two types of cells: Neuroglia (glial cells) Neuroglia (glial cells) Support, insulate, help young neurons grow (page 390) Support, insulate, help young neurons grow (page 390) Ex: Schwann cells Ex: Schwann cells Neurons (nerve cells) Neurons (nerve cells) Conduct electrical impulses Conduct electrical impulses Do not divide (amitotic) Do not divide (amitotic) High metabolic rates High metabolic rates

Glial Cell Examples

Schwann Cell Formation

Myelin Sheath Nucleus neurilemma Node of Ranvier axon

Key Neuron Physiology Dendrites Dendrites receive incoming nerve signal (action potential) receive incoming nerve signal (action potential) Axon Axon transmits AP down length of neuron transmits AP down length of neuron Axon Terminals Axon Terminals pass AP to next neuron (via synapse) pass AP to next neuron (via synapse) Myelin Sheath Myelin Sheath insulates axon, increases speed of AP as it moves down axon insulates axon, increases speed of AP as it moves down axon Schwann cell Schwann cell wraps around axon, forms myelin (example of a glial cell) wraps around axon, forms myelin (example of a glial cell) Neurilemma Neurilemma plasma membrane of neuron plasma membrane of neuron Nodes of Ranvier Nodes of Ranvier gaps in myelin sheath, allows diffusion across neurilemma (ions*, wastes, nutrients) gaps in myelin sheath, allows diffusion across neurilemma (ions*, wastes, nutrients)

Structural Classification of Neurons Classified by # of processes extending from cell body (3 types) Classified by # of processes extending from cell body (3 types) 1. Multipolar 3+ processes 3+ processes Most common neuron type (99%) Most common neuron type (99%)

2. Bipolar 2 processes extend from cell body 2 processes extend from cell body Axon, dendrite Axon, dendrite More rare, found in CNS and some special sense organs More rare, found in CNS and some special sense organs

3. Unipolar Short extension of cell body split into “T” Short extension of cell body split into “T” Sensory neurons in PNS Sensory neurons in PNS

Functional Classification of Neurons Sensory (afferent) neurons Sensory (afferent) neurons Transmit impulses from receptors in body (PNS) to CNS Transmit impulses from receptors in body (PNS) to CNS (Usually) Unipolar (…sometimes bipolar…) (Usually) Unipolar (…sometimes bipolar…) Motor (efferent) neurons Motor (efferent) neurons Transmit impulses from CNS towards effector organs (muscles, glands) Transmit impulses from CNS towards effector organs (muscles, glands) Multipolar Multipolar Interneurons (aka association neurons) Interneurons (aka association neurons) Transmit signals through CNS Transmit signals through CNS Integration Integration Bipolar Bipolar

The Action Potential Nerve impulses travel as an electrical signal Nerve impulses travel as an electrical signal Dendrites -> cell body -> axon -> axon terminals Dendrites -> cell body -> axon -> axon terminals Due to a sudden change in voltage (amplitude) as ions travel in and out of the cell through protein channels Due to a sudden change in voltage (amplitude) as ions travel in and out of the cell through protein channels Ions create electrical charge! Ions create electrical charge!

The Synapse axon terminal of presynaptic neuron meets the dendrite of postsynaptic neuron.

At the synapse… Nerve impulses are passed from one neuron to the next Nerve impulses are passed from one neuron to the next May be: May be: Electrical synapses (less common) Electrical synapses (less common) Chemical synapses Chemical synapses

Electrical Synapse Use protein channels Use protein channels Ions flow directly from one neuron to the next Ions flow directly from one neuron to the next Very rapid signal transmission Very rapid signal transmission Sleep arousal, primitive emotions, some memory (all in brain) Sleep arousal, primitive emotions, some memory (all in brain) Early embryonic nervous tissue Early embryonic nervous tissue

Chemical Synapse Neurotransmitters (NTs) released from pre-synaptic axon terminal Neurotransmitters (NTs) released from pre-synaptic axon terminal NTs bind to receptors on post-synaptic neuron dendrites NTs bind to receptors on post-synaptic neuron dendrites Convert electrical signal --> chemical signal --> electrical signal Convert electrical signal --> chemical signal --> electrical signal Chemical Synapse Animation Chemical Synapse Animation

Neurotransmitters = Chemicals used to relay signals from axon terminal of pre-synaptic neuron to dendrites of the post-synaptic neuron (across the synaptic cleft / gap)

*In regards to this synapse PRE-synaptic neuron POST-synaptic neuron

At the chemical synapse… At rest = Neurotransmitters (NTs) are stored in synaptic vesicles until released in response to influx of Ca 2+ in axon terminal At rest = Neurotransmitters (NTs) are stored in synaptic vesicles until released in response to influx of Ca 2+ in axon terminal 1. Electrical nerve impulse (AP) opens voltage-gated calcium channels on pre- synaptic axon terminal neurilemma voltage-gated calcium channels voltage-gated calcium channels 2. Ca 2+ floods in to pre-syn. neuron 3. NTs released from vesicles, float across the synaptic cleft toward post-synaptic dendrite

Chemical Synapse cont… 4. NTs bind to receptors on post-syn. neuron Ion channels open (in post-syn. neurilemma) Ion channels open (in post-syn. neurilemma) 5. Ions flow through channels into post-synaptic neuron’s dendrite Membrane potential (voltage) changes, passing on electrical signal Membrane potential (voltage) changes, passing on electrical signal May be excitatory or inhibitory (based on NT released) May be excitatory or inhibitory (based on NT released)

Chemical Synapse Cont… Leftover NTs in synapse: Leftover NTs in synapse: *Taken back up by pre-synaptic neuron to be reused or… *Taken back up by pre-synaptic neuron to be reused or… Degraded by enzymes or… Degraded by enzymes or… Diffuse away Diffuse away Animation one more time Animation one more time

Just like with muscles… Summation! Summation - increase duration or intensity of nerve impulses Summation - increase duration or intensity of nerve impulses Temporal - post-synaptic neuron stimulated by 1 pre-synaptic neuron in rapid fire (similar to wave summation) Temporal - post-synaptic neuron stimulated by 1 pre-synaptic neuron in rapid fire (similar to wave summation) Spatial - post-synaptic neuron stimulated by many axon terminals at same time (similar to recruitment) Spatial - post-synaptic neuron stimulated by many axon terminals at same time (similar to recruitment)

Some Specific Neurotransmitters Excitatory: Glutamate Glutamate learning and memory learning and memoryInhibitory: GABA GABA main inhibitory neurotransmitter in brain main inhibitory neurotransmitter in brain effects exaggerated by alcohol effects exaggerated by alcohol Serotonin Serotonin “feel good” neurotransmitter “feel good” neurotransmitter involved in sleep, appetite, mood regulation involved in sleep, appetite, mood regulation Prozac blocks its uptake, keeps serotonin in synapse, relieves anxiety/depression Prozac blocks its uptake, keeps serotonin in synapse, relieves anxiety/depression

More Neurotransmitters Can be either excitatory or inhibitory: Dopamine Dopamine excitatory or inhibitory depending on receptor excitatory or inhibitory depending on receptor “feel good” neurotransmitter (reward pathway) “feel good” neurotransmitter (reward pathway) reuptake blocked by cocaine reuptake blocked by cocaine Plays role in motor control Plays role in motor control deficient in Parkinson’s patients deficient in Parkinson’s patients Acetylcholine Acetylcholine excitatory to skeletal muscles, may be either to visceral organs excitatory to skeletal muscles, may be either to visceral organs major neuromuscular junction neurotransmitter major neuromuscular junction neurotransmitter

Excitatory/Inhibitory Excitatory Neurotransmitters… Excitatory Neurotransmitters… increase activity of postsynaptic neuron increase activity of postsynaptic neuron increase amount of NT released increase amount of NT released Example: glutamate Example: glutamate Inhibitory Neurotransmitters Inhibitory Neurotransmitters decrease activity of postsynaptic neuron decrease activity of postsynaptic neuron decrease amount of NT released decrease amount of NT released Example: GABA Example: GABA