physio chapter 07

What are the two major divisions of the nervous system?

Central nervous system CNS: brain and spinal cord peripheral nervous system PNS: cranial and spinal nerves

What are the two tissue cell types that compose nervous tissue?

Neurons (conduct impulses) and glial cells (neuroglia) that support neurons and can divide.

What is the structural and functional unit of the nervous system?

Neuron.

What are glial cells and what are their general characteristics?

Non-conducting supporting cells that can divide

ex. Schwann cells and Satellite cells in the PNS, and oligodendrocytes, microglia, astrocytes, and ependymal cells in the CNS.

Define the Central Nervous System (CNS) and Peripheral Nervous System (PNS).

CNS = brain and spinal cord

PNS = nerves, ganglia, and nerve plexuses outside the CNS.

What is an association neuron (interneuron)?

A neuron located within the CNS that integrates functions.

What is a sensory neuron (afferent neuron)?

Transmits impulses from a sensory receptor into the CNS.

What is a motor neuron (efferent neuron)?

Transmits impulses from the CNS to an effector organ (muscle or gland).

What is a nerve?

A cablelike bundle of axons located outside the CNS; usually mixed with sensory and motor fibers.

What is a tract?

A bundle of axons located inside the CNS.

Which cells form the myelin sheath in the PNS and CNS?

Schwann cells form myelin in the PNS

oligodendrocytes form myelin in the CNS.

What are nodes of Ranvier?

Gaps between myelin sheath segments where ion channels are concentrated.

What is saltatory conduction?

Propagation of action potentials by jumping from node to node, increasing conduction speed.

Describe axonal transport and its two directions.

Anterograde: cell body → axon terminal (carried by kinesin).

Retrograde: axon terminal → cell body (carried by dynein)

What is the axon hillock?

Initial segment of the axon where action potentials are generated.

What are the functional classifications of neurons?

Sensory (afferent), motor (efferent), and association/interneurons.

What are the two divisions of motor neurons?

Somatic motor neurons (skeletal muscle) and autonomic motor neurons (involuntary targets like smooth/cardiac muscle and glands).

What are the structural classifications of neurons?

Pseudounipolar, bipolar, and multipolar neurons.

What is the difference between a nerve and a tract?

Nerve = bundle of axons outside the CNS

tract = bundle of axons inside the CNS.

Name the four CNS glial cell types and a key function for each.

Oligodendrocytes (form CNS myelin)

Microglia (phagocytose)

Astrocytes (regulate extracellular environment, BBB)

Ependymal cells (line ventricles and produce CSF)

What is the function of Schwann cells?

Produce myelin around PNS axons and form the neurilemma; assist in regeneration.

What are satellite cells?

Support cells in PNS ganglia.

What is the difference between CNS myelin and gray matter?

Myelin in CNS is formed by oligodendrocytes

white matter = myelinated axons

gray matter = cell bodies and dendrites without myelin

What is neurilemma (Sheath of Schwann)?

The outer layer of Schwann cells surrounding a PNS axon, aiding regeneration.

Compare regeneration in the PNS and CNS.

PNS regenerates via Schwann cell growth tubes

CNS has limited regeneration due to oligodendrocyte death, inhibitory myelin, and astroglial scars.

What are neurotrophins and some examples?

Growth factors for neurons

ex. nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), glial derived neurotrophic factor (GDNF), neurotrophin-3, and neurotrophin-4/5.

List key functions of astrocytes.

Take up K+

remove excess neurotransmitters

supply glucose via end-feet; store glycogen

support synapse formation and neurogenesis

form the blood-brain barrier

release gliotransmitters.

What is the blood-brain barrier (BBB) and its significance?

Tight junctions between brain capillary endothelial cells selective diffusion with astrocyte influence

restricts drug entry, complicating chemotherapy.

What is resting membrane potential (RMP) in neurons?

Approximately -70 mV.

What factors establish resting membrane potential?

Negative intracellular molecules, Na+/K+ pumps, and selective membrane permeability.

Define depolarization and hyperpolarization.

Depolarization: membrane potential becomes more positive; Hyperpolarization: becomes more negative.

What is the threshold potential?

Around -55 mV; the membrane depolarizes enough to open voltage-gated Na+ channels.

What happens at the peak of an action potential (+30 mV)?

Voltage-gated Na+ channels inactivate and voltage-gated K+ channels open, causing repolarization.

What is the All-or-None Law?

Once threshold is reached, an action potential occurs with the same amplitude; stimulus strength does not affect amplitude.

How is stimulus intensity coded in neurons?

By firing frequency (frequency modulation) and by recruitment of more neurons.

What are absolute and relative refractory periods?

Absolute: Na+ channels are inactivated and cannot fire; Relative: K+ channels remain open and require a strong stimulus.

What factors increase conduction velocity of nerve impulses?

Increased axon diameter and myelination (saltatory conduction).

Contrast conduction in unmyelinated versus myelinated axons.

Unmyelinated: slower, impulses travel along entire membrane

myelinated: faster via saltatory conduction at nodes of Ranvier.

What are electrical synapses and gap junctions?

Electrical synapses use gap junctions for direct ionic coupling

faster and bidirectional

found in some brain regions, smooth/cardiac muscle

What are chemical synapses?

Most synapses; neurotransmitter release into a synaptic cleft

presynaptic and postsynaptic cells are joined by CAMs.

What triggers neurotransmitter release at chemical synapses?

An action potential opens voltage-gated Ca2+ channels

Ca2+ triggers vesicle fusion and exocytosis.

What is the role of SNARE proteins and synaptotagmin in neurotransmitter release?

SNAREs mediate docking/fusion of vesicles

synaptotagmin is a Ca2+ sensor that triggers fusion.

What are ligand-gated and G-protein-coupled (GPCR) receptors?

Ligand-gated receptors are ion channels opened directly by neurotransmitter binding

GPCRs activate second messenger pathways via G-proteins.

What is the nicotinic ACh receptor?

A ligand-gated ion channel at the NMJ with two ACh binding sites; opens Na+/K+ channel, causing depolarization (EPSP).

What is the muscarinic ACh receptor?

A G-protein-coupled receptor found in CNS and on smooth/cardiac muscles; modulates ion channels via second messengers.

What enzyme inactivates acetylcholine in the synaptic cleft?

Acetylcholinesterase (AChE).

What is an end plate potential?

Depolarization at the motor end plate of a muscle fiber due to ACh at the NMJ, leading to an action potential and muscle contraction.

Name some agonists and antagonists of acetylcholine receptors.

Agonists: nicotine (nicotinic), muscarine (muscarinic). Antagonists: curare (nicotinic), atropine (muscarinic).

What is a presynaptic inhibition?

A neuron inhibits the release of excitatory neurotransmitter from another neuron’s axon, often via inactivation of Ca2+ channels; involves endogenous opioids in pain modulation.

What is the difference between EPSP and IPSP?

EPSP: excitatory depolarization (Na+/Ca2+ entry) toward threshold; IPSP: inhibitory hyperpolarization (K+ efflux or Cl- influx) away from threshold.

What is synaptic plasticity?

Changes in synaptic strength: long-term potentiation (LTP) strengthens synapses; long-term depression (LTD) weakens them, often involving Ca2+ signaling and AMPA receptor trafficking.

What role do NMDA and AMPA receptors play in memory?

Both contribute to synaptic plasticity and memory formation; their trafficking and signaling are key to LTP. AMPA receptor insertion strengthens synapses.

What are endocannabinoids and their general function?

Retrograde neurotransmitters produced in postsynaptic neurons; inhibit neurotransmitter release from presynaptic neurons; modulate learning, memory, and appetite.

Describe nitric oxide (NO) and carbon monoxide (CO) as neurotransmitters.

Gaseous transmitters; NO diffuses to target cells and activates cGMP signaling; CO also stimulates cGMP; NO is involved in vasodilation and neural signaling; CO has roles in olfactory and cerebellar function.

What are ATP and adenosine as neurotransmitters?

Purines released as co-transmitters; act on purinergic receptors (P1 for ATP, P2 for adenosine); involved in blood vessel regulation, pain, and gut motility.

What is the general role of monoamines as neurotransmitters?

Regulatory neurotransmitters (dopamine, norepinephrine, epinephrine, serotonin, histamine) that act via second messenger systems rather than direct ion channels.

How are monoamines inactivated after release?

Reuptake into the presynaptic neuron and degradation by monoamine oxidase (MAO).

What is the role of dopamine in the brain?

Nigrostriatal system: motor control; Mesolimbic system: reward and addiction; imbalances associated with Parkinson’s disease and schizophrenia.

What is the role of norepinephrine in the nervous system?

Used in both CNS and PNS; implicated in arousal and sympathetic responses; influences alertness and attention; affected by stimulants like amphetamines.

What is glutamate and why is it important?

Major excitatory neurotransmitter in the brain; many synapses; NMDA/AMPA/kainate receptors; involved in memory and energy-demanding synaptic activity; astrocytes support uptake and blood flow.

What are glycine and GABA?

Glycine: inhibitory in the spinal cord via Cl- channels; GABA: main inhibitory neurotransmitter in the brain via Cl- channels; involved in motor control and preventing overexcitation.

What is the role of neuromodulators and neuropeptides?

Polypeptides that modulate classical neurotransmission; examples include CCK, Substance P, neuropeptide Y; can be co-released with classical transmitters and influence synaptic strength.

What is endogenous opioid signaling?

Endogenous opioid peptides (enkephalin, beta-endorphin, dynorphin) bind opioid receptors to modulate pain and reward pathways.

What is endocannabinoid signaling in the context of LTD and memory?

Endocannabinoids can mediate forms of LTD by suppressing neurotransmitter release; influence learning, memory, and appetite.

What effect does nitric oxide have on erection and which drug influences it?

Nitric oxide (NO) causes erection by relaxing penile blood vessels, increasing blood flow. Drugs like sildenafil (Viagra) enhance this effect by preventing the breakdown of cGMP, which NO produces.