Part 1 of 2 Nervous communication Coordination Chapter 15 Part 1 of 2 Nervous communication

Nervous communication In animals there are two types of information transfer Electrical impulses – nerves Chemical messengers – hormones Communication systems within animals Receptors (input) receive information from within and outside the organism Effectors (output) include muscles and glands

Receptors/effectors

Mammalian nervous system Central nervous system (CNS) – brain & Spinal cord Peripheral nervous system (PNS) – cranial & spinal nerves

Neurons Nerves = neurons 3 types of neurons Sensory neurons – impulse from receptor to CNS Intermediate neurons – impulse from sensory neurons to motor neurons (a.k.a. relay or connector neurons) Motor neurons – impulse from CNS to effector

Types of neurons

Motor neurons Body of motor neuron lies within the spinal cord or brain Dendrites – often short cytoplasmic extensions with many branches, impulse receiving end of neuron Axons – long extensions carrying the output impulse. Mitochondria are found along the axon with many mitochondria at the axonal terminal with vesicles containing neurotransmitter chemicals.

Sensory neuron Same basic structure as motor neuron Body near source of stimuli or ganglion (cluster of neurons) Long axon

Relay neurons Relay neurons or intermediate neurons are only found in the CNS

Pyramidal neurons Pyramidal cells are motor and relay neurons Pyramidal neurons are the primary excitation units of the mammalian prefrontal cortex. Pyramidal neurons are also one of two cell types where the characteristic sign, Negri bodies, are found in post-mortem rabies infection. Pyramidal neurons in the prefrontal cortex are implicated in cognitive ability.

Myelin (a.k.a. myelin sheath) Myelin is the term used to describe Schwann cells wrapped around the axon of neurons 1/3 of motor and sensory neurons are myelinated This wrapping is referred to as myelin sheath Myelin sheath is made of lipid and protein Myelin sheath affects the speed of impulse conduction

Myelin (a.k.a. myelin sheath) cont. The small uncovered areas between Schwann cells are called nodes of Ranvier Nodes of Ranvier are 2-3 micrometers long They occur every 1-3 millimeters

Myelin Sheath cont.

Oligodendrocytes Equivalent to the function performed by Schwann cells in the peripheral nervous system. Oligodendrocytes do this by creating the myelin sheath, which is 80% lipid and 20% protein. A single oligodendrocyte can extend its processes to 50 axons Schwann cells, on the other hand, can wrap around only one axon.

Oligodendrocyte

Nerve fiber

Reflex arc A reflex arc is the pathway by which an involuntary response to danger signals occurs (reflex action) Touching hot object Sight of an object flying towards you

Transmission of nerve impulse Fast traveling electrical impulses along the surface membrane of neuron Very brief changes in the distribution of electrical charge across the membrane (action potential) Rapid movement of sodium and potassium ions into and out of axon

Resting potential The inside of axons are always slightly negative compared to the outside (potential difference) 60-70 mV lower inside (-60 to -70 mV) Produced and maintained by sodium-potassium pumps in membrane 3 Na+ out for every 2 K+ in A form of active transport (ATP is hydrolysed)

Resting potential cont.

Resting potential cont. The axons of squid and earthworms are wide enough to insert tiny electrode into cytoplasm to measure the change in electrical charge https://youtu.be/CXCGqwdtJ78

Resting potential Membrane protein channels for K+ and Na+ remain open all the time More for K+ than for Na+ Membrane relatively impermeable to Na+ Steep concentration gradient + negatively charged inside make for an electrochemical gradient

Electrochemical gradient

Action potential In addition to the Na+/K+ pumps that are always open there are channels that allow the reverse movement of ions. These are called voltage-gated channels Voltage-gated channels remain closed until the action potential threshold is reached. -70 mV to +30 mV back to -70 mV in 3 milliseconds (ms)

Action potential (positive feedback) Action potentials in mammals are only generated if the stimulus causes a potential difference to reach about 55 mV – the threshold potential At 55 mV channels open and the inside of the axon reaches +30 mV compared to outside axon This is termed depolarization

Repolarization of axon After 1 millisecond, Na+ voltage gated channels close Simultaneously, K+ channels open returning the potential difference to – 70 mV As the action potential moves down the axon towards the axonal terminal, there is a refractory period when the axon is unresponsive to another stimuli

Consequences of refractory period Action potentials don’t merge into one another There is a minimum time between action potentials The length of the refractory period determines the maximum frequency impulses can be transmitted

Action potential

How action potentials carry information Action potentials do not change in size Nor they change in size due to intensity of stimulus no matter how long the axon is Speed of action potential is always the same The difference from strong or weak stimulus is the frequency – or how many neurons are carrying the signal Action potentials are an all-or-none response (all or none law) https://youtu.be/-h_kWFM2faQ https://youtu.be/liPWjnpEl_M

Speed of conduction Unmyelinated neurons – 0.5 meters per second (m s-1) Myelinated neurons – 100 m s-1 Depolarization can only occur at nodes of Ranvier where all the channel proteins and pump protein are concentrated Action potentials jump from node to node (saltatory conduction)

Speed of conduction cont. Diameter affects speed of transmission Thick axons faster than thin axons

Starting an action potential Receptor cells respond to a stimulus (light, touch, sound, temperature, chemicals) receptor potential Receptor cells turn stimulus into electrical impulse If receptor potential is large enough, it will open calcium ion (Ca2+) channels Ca2+ enter cytoplasm and cause exocytosis of vesicles containing neurotransmitters Neurotransmitters stimulate action potential in sensory neuron

synapses There is a 20 nm gap called the synaptic cleft separating neurons Action potential starts at the pre-synaptic neuron Action potential causes the release of neurotransmitters into the synaptic cleft Neurotransmitters bind to receptor proteins on the post synaptic neuron If the threshold is reached, then it will initiate depolarization of the subsequent neuron

neurotransmitters More than 40 neurotransmitters are known Noradrenaline and acetylcholine (ACh) are found throughout the nervous system Dopamine, glutamic acid and gamma-aminobutyric acid (GABA) only occur in the brain

Cholinergic synapses Cholinergic synapses use acetylcholine (ACh) Action potential arrives at axonal terminal Voltage-gated channels open for calcium ions (Ca2+) This causes the vesicles containing Ach to fuse with membrane and through exocytosis releases ACh which crosses the synaptic cleft ACh binds with receptors on in on the post-synaptic neuron causing Na+ channels to open This depolarizes membrane causing an action potential https://youtu.be/mItV4rC57kM

Synapse cont. ACh is recycled by acetylcholinesterase into acetate and choline Choline is taken back into the presynaptic neuron and combine with coenzyme A to make ACh again This all happens in 5-10 ms Synapses are a one-way street