Notes – Chapter 44
Nervous System
Nervous systems – 3 functions – sensory, integration, motor
-Sensory – PNS (peripheral nervous system)
-Input from five senses
-Integration
-How sensations are interpreted and responses determined
-Takes place in CNS (brain and spinal cord)
-Motor - PNS
-Actual movement in response
-In effectors – muscles or glands
Two kinds of cells
-Neurons
-Cell body, dendrites (from tips to cell body), and axons (cell body to tips)
-Dendrites branching – increases surface area of sensitivity
-Axon – usually one long one – sciatic nerve – one axon going from spinal cord to foot or lower leg
-Enclosed by Schwann cells – collectively called the myelin sheath
-Endings also branched – ends called synaptic terminals (where chemicals called neurotransmitters are released to be communicated to other effector cells – juncture called synapse
-Sensory neurons
-From sensory receptors
-Most synapse with “interneurons” in CNS which connect with other neurons
- Motor neurons
-from CNS neurons to effector cells
-Supporting cells
-Glial or “glue” cells
-Astrocytes – around capillaries in the brain
-Astrocytoma – cancer – John Travolta “Phenomenon” and Jim Moran’s daughter, I think
-Contribute to “blood-brain barrier” – why chemo doesn’t work on brain cancer
-Oligodendrocytes - Schwann cells, actually
-Form insulating barriers around axons
-Myelin sheath increases speed of neural impulses
-Multiple sclerosis – loss of coordination
Electrical Impulse conduction
-voltage measurement – membrane potential (mp)
-inside cell negative
-mp arises from differences in ions
-inside cell, K ions (cation) and proteins, sulfates, phosphates (anions)
-outside, mostly Na (cation) and Cl (anion)
-ions cannot directly diffuse, must be transported
-K readily moves out of cell, leaves a big, net negative charge inside
-A few Na’s move into the cell, but active transport moves them back out
-Some cells can change their mp’s – called excitable cells – have “gated ion channels”
-At rest, the mp is called the resting potential
-So, here come stimulus to sensory neuron from environment
-Gated ion channels then allow K to move out, makes electrical gradient between inside and outside greater – called hyperpolarization
-If Na channel is opened, lessens electrical gradient – depolarization
-So…..if depolarization occurs, then produces a threshold – then an action potential (which is the nerve impulse) is the response
-All – or – none
-(BIG QUESTION _ WHAT IS A HYPERPOLARIZING STIMULUS AND WHAT IS A DEPOLARIZING STIMULUS? Obviously, anything that produces a nerve impulse is a depolarizing stimulus but what does a hyperpolarizing stimulus do?
-Now…there’s repolarizing – a return of mp to the resting state
-But the K channels are slow so the cell can “overshoot” letting K out of cells – this is hyperpolarization – action potential/nerve impulse cannot happen so this is called “refractory period”
So – if nerve impulses are all or none, how is a strong stimulus distinguished from a weak one?
-if strong, neuron will fire rapidly, repeatedly, as soon as refractory period lets it, so it’s number of action potentials per second that codes for strength of stimulus
Action Impulse Transmission Speed
-function of diameter of axon – the bigger, the faster
-squids and octopuses have giant axons – have you seen it? Pull it out when cleaning squid - bigger axons present in neural pathways that require great speed of response – tail flip in lobsters, make a quick getaway
-vertebrate – myelin sheath – nodes of Ranvier where gated channels are – nerve impulses can actually leap from node to node – faster – called saltatory conduction
Synapses – electrical and chemical
-electrical – pre to post – cells connected by gap junctions – allows ion currents to flow between cells – giant axons of crustaceans – I think, also in the heart muscle, to communicate the contraction
-chemical – most are these kind
-synaptic cleft is junction
-action potential translates into release of chemicals, across cleft, translated back into an electrical signal, calcium plays a big part here
-vesicles at tips contain neurotransmitters, release by exocytosis, diffuses
-postsynaptic cells have special receptors, different for each neurotransmitter
-can deliver message only in one direction
-wow, still it’s awesome that with all the firing, or not, of nerve impulses, the system can keep straight all the fine responses
Different messages - Summation
-can be excitatory or inhibitory
-if inhibitory, then hyperpolarization takes place, actually results in fast uptake of chemicals across synapse
-summation is a collection of firings
-temporal – firings come fast, even before resting potential is established
-spatial – several neurons stimulate one postsynaptic cell at once
Neurotransmitters and Receptors
-acetycholine – most common, sometimes inhibits, sometimes excites
-see Table 44.2
-some amines – catecholamines – epinephrine and norepinephrine, dopamine, and that all important serotonin
-dopa and sero affect sleep, mood, attention, and learning
-Parkinson’s – low dopa Schizophrenia – too much dopa
-LSD and mexcaline – hallucinatory – bind to both
-GABA – most abundant
-Endorphins – analgesics, opium, morphine
-Actually naturally produce in times of high stress – childbirth, running
-Decrease urine, respiration, enhance euphoria
Gaseous Messengers
-NO released in sexual arousal, produces erection
-NO released, relaxes muscles – nitroglycerin
-Are synthesized on demand
Circuitry
-neurons arranged in circuits
-convergent – many different neurons come together – vision, touch, smell – to id an object
-divergent – take infor to several different parts of the brain
-reverberating – memory storage – round and round
-Cluster of cell bodies in PNS – called ganglia (in brain, nuclei) allow system to coordinate without involving the whole system
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