BSCI201- Final Exam

CNS and PNS

What are the two division of the nervous system?

Central Nervous System (CNS)

Consists of the brain and the spinal cord; located in the dorsal body cavity surrounded by meninges.

Peripheral Nervous System (PNS)

Consists of the all neural structures outside the CNS including the cranial nerves, spinal nerves, ganglia and sensory receptors.

Nervous tissue, connective tissue, and blood vessels

What is the nervous system composed of?

Neurons and supporting cells

What two cells types is nervous tissue composed of?

Neurons

(Nerve cells) are conducting cells because they generate electrical signals.

Supporting cells

(Neuroglia) are non-conducting cells because they do not typically generate electrical signals.

Cell body, dendrites, and axon

What are the three regions of a neuron?

Cell body of a neuron

Also known as soma or perikaryon. Contains the nucleus and all other cytoplasmic organelles except centrioles hence, neurons are generally amitotic. Contains well-developed rough ER called Nissl Body or Chromatophilic substance; prominent nucleoli; they indicate a neuron is a of a secretory cell – neurotransmitter from the axon terminals. Neurotransmitters are synthesized in the cell body hence referred to as the “Biosynthetic region” a neuron.

The cell body

What is also know as the “biosynthetic region” of a neuron?

Dendrites

Tapering processes that act as the “receptive regions” of a neuron. Receive and convey electrical signals toward the cell body.

Dendrites

What is also known was the “receptive regions” of a neuron?

Axon

A single process extending from the cell body – each neuron can have only one; uniform diameter; unmyelinated or myelinated. Generates and transmits action potentials AWAY from the cell body hence, known as the “conducting region” of a neuron. Branches at the end into telodendria which end in bulbous ends called axon terminals (=synaptic knobs=boutons) – store and release neurotransmitter hence the terminals of these are referred to as the “secretory region” of a neuron.

Axon

What is also know was the “conducting region” of a neuron?

Axon terminals

What is also known as the “secretory region” of a neuron?

Axon terminals

Branches at the end into telodendria which end in bulbous ends that store and release neurotransmitters.

Structural and functional

What are the two types of classifications for neurons?

Multipolar, bipolar, and pseudounipolar

What are the three structural classifications of neurons?

Structural classification of neurons

Based on the number of processes extending from the cell body of the neuron.

Multipolar neuron

Has at least 3 processes – one axon and at least 2 dendrites; most abundant neuron in the human body.

Multipolar

What is the most abundant structural class of neuron in the human body?

Bipolar neuron

Has 2 processes – one axon and one dendrite.

Pseudounipolar neuron

Has one short process extending from the cell body that bifurcates into a central process and a peripheral process.

Functional classification of neurons

Based on the direction in which they transmit impulses relative to the central nervous system (CNS).

Sensory or afferent, association on interneurons, and motor or efferent

What are the three functional classes of neurons?

Sensory or afferent neuron

Transmits impulses from sensory receptors toward the CNS.

Association neurons or interneurons

Located in the CNS between the sensory neurons and the motor neurons. Most of the neurons (99%) in the body are these.

Association or interneurons

What is the most abundant functional class of neuron in the body?

Motor or efferent neuron

Transmits impulses away from the CNS to effector organs = glands, organs.

Nucleus

A cluster of neuron cell bodies in the CNS.

Ganglion

A cluster of neuron cell bodies in the PNS.

Tract

A bundle of axons in the CNS.

Nerve

A bundle of axons in the PNS.

Axolemma

The plasma membrane of an axon is called an…

Endoneurium

Each axon is wrapped in a delicate connective tissue membrane called…

Fascicle

A bundle of endoneurium-covered axons is called a…

Perineurium

Each fascicle is covered by the coarse connective tissue membrane called the…

Epineurium

A bundle of perineurium-covered fascicles form the nerve or a tract which is covered in a tough connective tissue membrane called the…

Neuroglia

Supporting cells also referred to as…

Astrocytes, microglia, ependymal cells, oligodendrocytes, schwann cells, and satellite cells

What are the six types of supporting cells?

Astrocytes, microglia, ependymal cells, and oligodendrocytes

What are the four supporting cells are located inside the CNS?

Schwann and satellite cells

What are the two supporting cells are located inside the PNS?

Astrocytes

What are the most abundant supporting cells?

Astrocytes

• Most abundant

• Numerous extensions that wrap around neurons

• Involved in forming the blood-brain barrier, a selective barrier that regulate the chemicals environment of the brain

• Regulate brain function

Microglia

Since the specific immune system does not have access to the CNS; these are phagocytes to engulf/destroy pathogens and cell debris.

Ependymal cells

Ciliated columnar cells that line the ventricles – cavities in the brain that contain cerebrospinal fluid (CSF). Currents created by beating of cilia to circulate the CSF.

Oligodendrocytes

Their extensions myelinate axons of neurons in the CNS.

Schwann cells

• = neurolemmocytes

• Myelinate axons of neurons in the PNS

Satellite cells

Surround cell bodies of neurons to control their chemical environment.

Myelination of axons in the PNS

The process of wrapping axons in a fatty, insulating sheath called myelin, produced by glial cells.

Axon

Each Schwann cell wraps around a segment of an _____.

Myelin sheath

Schwann cell squeezes around the segment of axon wrapping concentric rings of its plasma membrane called ______ ______ around the axon.

Neurilemma

The cytoplasm and the nucleus of the Schwann cell squeezed outside the myelin sheath.

Nodes of ranvier

The spaces between adjacent myelin sheaths.

Myelination of axons in the CNS

The process where oligodendrocytes create a fatty, insulating layer called myeline around neuronal axons, accelerating signal transmission and providing metabolic support.

Oligodendrocytes

The axons in the CNS are myelinated by extensions from the ___________ hence, neurilemma is absent.

Protection, electrical insulation, and saltatory conduction

What is the function of the myelin sheath?

Yes

Can severed axons in the PNS regenerate?

No

Can severed axons in the CNS regenerate?

When an axon is cut in the PNS, immune cells clean up the damaged area. Then, Schwann cells form a regeneration tube that helps guide the axon to grow back.

Why can severed axons in the PNS be regenerated?

In the CNS, cleanup of damage is incomplete because microglia don’t clear debris well. There’s no neurilemma to form a regeneration tube, and inhibitory proteins prevent the axon from growing back.

Why can’t severed axons in the CNS be regenerated?

The resting membrane potential

The stable, negative electrical charge difference (typically ~-70 mV- -90mV) across a neuron's membrane when not actively sending signals.

First is depolarization, second is repolarization, and third is hyperpolarization

What are the phases, in order, of generating an action potential?

Depolarization phase

1st Phase- entry of sodium ions (Na+) into axon referred to as sodium influx makes membrane potential less and less negative. When the Threshold Potential (-55mV) is reached, an action potential develops when the threshold potential is reached. AP is an all-or-none phenomenon Upshoot or spike due to an explosive entry of Sodium ions = a positive membrane potential is reached +30mV, the peak.

Repolarization phase

2nd Phase- 2 events occur at the peak: (i) sodium channels close (Na+ influx halts) (ii) potassium channels open (K+ efflux begins) and potassium ions (K+) rush out of the axon referred to as potassium efflux; this results in reversal of the membrane potential toward a negative membrane potential.

Hyperpolarization phase

3rd Phase- more K+ efflux occurs past the RMP driving the membrane potential below the RMP (RMP is restored by the Na+/K+ pump which pumps 3 Na+ out and 2 K+ in).

All-or-none phenomenon

An action potential will be generated if depolarization reaches a threshold potential.

Self propagating

Once generated by the axon, it is propagated down the axon to the axonal terminals; a propagated or transmitted action potential is called a nerve impulse.

Nerve impulse

A propagated or transmitted action potential.

Same

All action potentials traced have the same shape and the _____ amplitude (+30mV) irrespective of stimulus strength. Thus, the difference between a stronger stimulus that causes the generation of an action potential and a weaker stimulus that causes the generation of an action potential is that the stronger stimulus causes the impulse to be generated at a higher frequency than the weaker stimulus.

Absolute and relative refractory period

What are the two refractory periods occur during an action potential?

Absolute refractory period

Coincides with the depolarization phase of the action potential when sodium channels are opened and therefore another action potential cannot be generated because all the Na+ channels are already opened, and depolarization is already occurring.

Relative refractory period

Coincides with the repolarization phase of the action potential when the sodium channels are closed (potassium channels are open) thus, an exceptionally strong stimulus can cause sodium channels to open to allow for sodium ion influx leading to depolarization and the generation of another action potential.

Diameter of the axon and degree of myelination

What factors affect the rate of impulse transmission = conduction velocity?

Diameter of the axon

Larger axons transmit impulses at a faster rate than smaller axons because the larger axon have larger diameter and therefore presents with less resistance impulse transmission; the resistance in the smaller axons is higher which impedes impulse transmission.

Degree of myelination

Myelinated axons transmit impulses at a faster rate than unmyelinated axons. Myelinated axons use saltatory conduction where action potentials are generated only at the nodes of Ranvier hence, the impulse “jumps from node to node down the axon. Unmyelinated axons use continuous conduction where action potentials developed stepwise across the entire axolemma.

Saltatory conduction

Which type of conduction is used by myelinated axons?

Continuous conduction

Which type of conduction is used by unmyelinated axons

A, B, and C

What are the three types of nerve fibers?

Long

A nerve fiber is a _____ axon.

Group A fibers

Have the largest diameter and heavily myelinated: transmit impulse at the rate of 150 m/s ( 335 miles per hour). Ex. Motor neurons that innervate skeletal muscles.

Group B fibers

Intermediate diameter and lightly myelinated (with wider gaps of nodes of ranvier); transmit impulses at a rate of 15 m/s (33 miles per hour). Ex. Preganglionic autonomic fibers.

Group C fibers

Smallest diameters and unmyelinated; transmit impulses at a rate of 1 m/s (2.2 miles per hour). Ex. Postganglionic autonomic fibers that innervate smooth muscle’ pain fibers.

Group A nerve fibers

What are the fastest conducting nerve fibers in the human body?

Multiple sclerosis (MS)

An autoimmune disease that results in demyelination of axons in the CNS. As the disease progresses, impulse transmission slows down (fast saltatory conduction -slow continuous conduction- this interferes with communication/control between the brain and the rest of the body.

Brain and spinal cord

What structures make up the central nervous system (CNS)?

Cranium, meninges, cerebrospinal fluid, and the blood-brain barrier

What are the four protective structures of the brain?

Cranium

Bony helmet composed of the 8 cranial bones – frontal, parietal(paired), temporal(paired), occipital, sphenoid and the ethmoid bones.

Meninges

3 connective membranes surround the brain- dura matter, arachnoid matter, and pia matter.

Dura mater

Outermost meninx; double-layered – outer periosteal layer which lines the internal surface of the cranium and the inner meningeal layer separated from the underlying arachnoid mater by the sudural space.

Arachnoid mater

Middle meninx separated from the underlying pia mater by the subarachnoid space. Web-like extensions from the arachnoid mater to the subarachnoid space gives this meninx its name (= spider family).

Cerebrospinal fluid

What does the subarachnoid space contain?

Pia matter

Innermost meninx that clings to the surface of the brain. Forms two extensions -the denticulate ligaments and the filum terminale.

Dura matter → arachnoid matter → pia matter

What is the correct order of the meninges from outermost to innermost?

Cerebrospinal fluid (CSF)

Filtered from blood; located in the ventricles and also in the subarachnoid space hence, CSF is found inside and outside of the brain acting as a “liquid” cushion; provides buoyancy to the brain; provides nutrients; removes metabolic wastes.

Blood-Brain barrier

A selective chemical barrier that prevents harmful, toxic substances in blood from crossing to the neurons in the brain.

Ventricles

Cavities in the brain that contain cerebrospinal fluid.

Lateral ventricles

Each cerebral hemisphere contains one of these; the 2 of these are connected by a median membrane called the septum pellucidum; 2 of these are connected to the third ventricle below by a channel called the interventricular foramen.

Septum pellucidum

What structure separates the two lateral ventricles?

Interventricular foramen

What connects the lateral ventricles to the third ventricle?