Lecture 6 - Fundamentals of the Nervous System and Nervous Tissue

What makes up the central nervous system?

brain and spinal cord

What makes up the peripheral nervous system?

cranial nerves, spinal nerves, ganglia

Somatic Sensory Fibers in Sensory (afferent) division of PNS

convey impulses from the skin, skeletal muscles, and joints TO CNS

Visceral Sensory Fibers in Sensory (afferent) division of PNS

convey impulses from visceral organs TO CNS

Motor (efferent) division of PNS

transmits impulses FROM CNS to effector organs (muscles/glands)

What division is the somatic and autonomic nervous systems part of?

Motor (efferent)

Somatic Nervous System

• voluntary

• somatic motor nerve fibers

• regulates skeletal muscle

Autonomic Nervous System

• involuntary

• visceral motor nerve fibers

• regulates smooth muscle, cardiac muscle, and glands

What division does sympathetic and parasympathetic nervous systems belong to?

Autonomic Nervous System

Which nervous system is fight or flight?

Sympathetic —> think S for stress

Which nervous system is rest and digest?

Parasympathetic —> think P for peaceful

Characteristics of neurons (nerve cells)

last a lifetime, amitotic, high metabolic rate

What is the function of dendrites?

• part of the neuron that receives signals (input region)

• conveys messages toward cell body as graded potentials (short distance signals)

• are short and branched

What is the function of cell bodies?

• synthesizes proteins, membranes, and chemicals

• rough ER

• has a nucleus and nucleolus

• plasma membrane receives info from other neurons

What are clusters of neuron cell bodies called in the CNS?

nuclei

What are clusters of neuron cell bodies called in the PNS?

ganglia

What is the function of axons?

• generates nerve impulses and transmits them to axon terminal (conducting region)

• can be short, very long, or even

• axon starts at a cone-shaped area called axon hillock

• endings are branched called axon terminals

What sends graded potentials?

dendrites —> runs down from dendrite TOWARDS the cell body then eventually to the axon

What sends action potentials?

axons —> runs down from the axon AWAY from the cell body all the way down to the axon terminal

What are bundles of neuron processes (axons) called in the CNS?

tracts

What are bundles of neuron processes (axons) called in the PNS?

nerves

What is the Myelin Sheath?

white fatty segments that individually wrap axons that protects and electrically insulates them to increase the speed of nerve impulse transmission

• good insulator bc less protein

Myelin sheath in the PNS

• formed by Schwann cells

  • 1 cell wraps 1 neuron = one bubble of the sheath

    • called myelinated fibers

  • spaces between cells are called myelin sheath gaps

Non-myelinated fibers in the PNS

one Schwann cell surrounds up to 15 thin fibers

Unmyelinated fibers in the CNS

thin fibers have no myelin surrounding them

Myelin Sheath in the CNS

• formed by oligodendrocytes

  • one cell can wrap up to 60 axons at once

  • spaces between cells called nodes of Ranvier

What is the function of axon terminals?

• secrete neurotransmitters that get released into extracellular space

  • these can excite or inhibit neurons it comes into contact with

• quickly decay if cut or damaged

• interact w many neurons at the same time to send signals

What is the resting membrane potential of a neuron?

• -70mV (inside is negative)

  • we have negatively charged proteins inside the cell that cannot pass through the plasma membrane to leave

• is a polarized membrane

• (the higher the voltage the greater the distance or difference in charges)

What cells can change their membrane potential?

muscles and nerves

What are leakage ion channels?

• NOT gated

• ions move DOWN their gradient

• passive process

What are gated ion channels?

• protein changes shape to open/close the channel

• passive process

Chemically gated ion channels

open in response to its appropriate neurotransmitter binding to its receptor

• passive

Voltage-gated ion channels

open in response to changes in the membrane potential

• passive

What is involved in maintaining resting membrane potential

• Higher concentration of sodium (Na⁺) outside the cell

• Higher concentration of potassium (K⁺) inside the cell

• we have more leakage ion channels for potassium so we are more permeable to potassium so potassium leaks out at a faster rate than sodium does

• overall: potassium (+) goes out and sodium (+) comes in

Sodium Potassium Pump

Moves ions AGAINST concentration gradients = active transport

Goal is to maintain the gradient so that ions can continue moving down their concentration gradients

• whatever sodium leaked in gets pumped out

• whatever potassium leaked out gets pumped back in

What is depolarization?

• membrane potential gets more positive than resting

• more likely to produce impulses

What is repolarization?

• membrane potential gets more negative than resting

  • has to be more negative than -70

• less likely to produce impulses

What is the plasma membrane of an axon?

Axolemma

Action Potentials

occurs at one part of an axon at a time

doesn’t lose strength as it travels down axon = propagation

1. Depolarization: opens Na+ voltage-gated ion channels and causes TONS of Na to rush in and causes a major depolarization (charge goes more positive)

2. Repolarization: closes Na⁺ channels and opens K⁺ channels

   1. K⁺ leaves going down its gradient

   2. Brings electrical condition back to normal but ions are in the wrong spot

   3. Hyperpolarization occurs where membrane potential is now slightly more negative than resting to ensure that the next gate will not cause the action potential to go backwards on the initial gate

3. Sodium Potassium Pump moves the ions to get back to reset ion position

Graded Potentials

short-lived changes in membrane potential that result in depolarization or hyperpolarization

• magnitude varies depending on stimulus strength

  • stronger stimulus goes further and bigger voltage change

• opens chemically gated ion channels

What is a synapse?

the point where two neurons meet

• axon terminal of one neuron forms a synapse with dendrites of next neuron

Presynaptic neuron

the neuron that is “before/ahead” the synapse point and sends information

Postsynaptic neuron

the neuron that is “after/behind” the synapse point and receives the information

What is the function of a chemical synapse?

• changes an electrical impulse into a chemical impulse across the synapse and then back to an electrical impulse

• releases chemical neurotransmitters

What is the structure of a chemical synapse?

Axon terminal of presynaptic neuron (has vesicles filled w neurotransmitters)

Fluid-filled gap called the synaptic cleft (the synapse)

Receptor region on dendrite or cell body of postsynaptic neuron’s membrane

What is the function of the synaptic cleft?

• prevents nerve impulses from directly passing from one neuron to the next

• ensures signals between neurons only travel in one direction

Process of Chemical Synapse

1. axon potential arrives at the axon terminal of presynaptic neuron

2. voltage-gated calcium channels open and Ca⁺ enters the axon terminal

3. the entry of Ca²⁺ causes synaptic vesicles (sacs filled w neurotransmitters) to fuse with the cell membrane (releases them via exocytosis = active!)

   1. the higher the frequency the more vesicles are exocytosed

4. neurotransmitter binds to the receptor protein on the postsynaptic neuron which opens chemically gated ion channels (protein changes shape)

5. graded potential triggered on postsynaptic neuron (excitatory or inhibitory)

6. neurotransmitter is terminated via one of three ways:

   1. re-uptake by presynaptic neuron

   2. degradation by enzymes

   3. diffusion where it is swept away from synaptic cleft

Why does the effect of a neurotransmitter need to be terminated quickly?

as long as the neurotransmitter is bound to a receptor the graded potentials will continue to occur

Acetylcholine Neurotransmitter

• neuromuscular junctions (ANS and CNS neurons use it)

• degraded by enzyme acetycholinesterase

What are the types of Biogenic Amines?

Catecholamines and Indolamines

What are the types of Catecholamines?

Norepinephrine and Epinephrine

• made from the amino acid tyrosine

• used by some ANS motor neurons (especially NE)

• used by the brain (CNS) for regulating emotional behaviors and biological clock

• imbalances associated w mental illness

What are the types of Indolamines?

• Serotonin: made from amino acid tryptophan

• Histamine: made from amino acid histidine

• used by the brain (CNS) for regulating emotional behaviors and biological clock

• imbalances associated w mental illness

What are the types of Peptides?

are derived from strings of amino acids

• Substance P

• Endorphins

• Gut-Brain Peptides

Substance P function

mediates pain signals (like capzasin)

Endorphins function

reduce pain perception

• “runner’s high”

• beta endorphin, dynorphin, enkephalins

Gut-brain peptides function

Somatostatin and cholecystokinin: regulate digestion

What are excitatory effects of neurotransmitters?

causing graded potentials in postsynaptic neuron

What are inhibitory effects of neurotransmitters?

causes hyperpolarization that inhibits either action or graded potentials