BSCI201 FINAL

Organization of the Nervous System

Divided in the CNS and PNS

Central Nervous System

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

Peripheral Nervous System

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

Is the ANS (Autonomic Nervous System) part of the sensory division or the motor division?

Motor division

Why are neurons amitotic (a cell that doesn’t undergo mitosis and cannot reproduce cells)?

They contain the nucleus and all other cytoplasmic organelles, except centrioles, which are necessary for cell division.

Do primary brain tumors originate from neurons in the brain or the supporting cells (neuroglial cells)?

Supporting cells, due to neurons being unable to divide (amitotic)

Biosynthetic Center of a Neuron

Cell Body

Contains the nucleus and most organelles, including rough ER (Nissl bodies), which help make proteins and neurotransmitters that are sent to the axon terminals.

Receptive Center of a Neuron

Dendrites

Due to dendrites receiving and conveying electrical signals towards the cell body.

Conducting Region of a Neuron

Axon

Generates and transmits action potentials away from the cell body.

Secretory Region of a Neuron

Axon Terminals

Store and release neurotransmitters.

Nissl Body

Well-developed rough ER found in the cell body.

Ganglia

A clusters of neuron cell bodies located in the PNS (peripheral nervous system)

Nuclei

A cluster of neuron cell bodies found in the CNS (central nervous system)

Tract

A bundle of axons in the CNS

Nerve

A bundle of axons in the PNS

Neurilemma

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

Nodes of Ranvier

The spaces between adjacent myelin sheaths.

Structural organization of a tract or nerve

Each axon is wrapped in a delicate CT membrane called Endoneurium

Each bundle of endoneurium covered axons is called a fascicle

Each fascicle is covered by the coarse CT membrane called the Perineurium

A bundle of perineurium-covered fascicles form the nerve or a tract which is covered in a tough CT membrane called the Epineurium

Fascicle

A bundle of endoneurium covered axons in a tract or nerve

Astrocytes

Location: CNS, most abundant neuroglia

Function: Involved in forming the Blood-Brain barrier, regulates brain function

Microglia

Location: CNS

Function: Act as phagocytes to engulf/destroy pathogens and cell debris

Ependymal Cells

Location: Line the ventricles (fluid-filled cavities in the brain)

Function: These ciliated columnar cells create currents through the beatings of their cilia to circulate cerebrospinal fluid (CSF)

Oligodendrocytes

Location: CNS

Function: Their extensions myelinate axons of neurons in the CNS

Schwann Cells

Location: PNS

Function: Myelinate axons of neurons in the PNS

Satellite Cells

Location: Surround cell bodies of neurons in the PNS

Function: Control their chemical environment

What supporting cells are involved in the formation of the Blood-Brain-Barrier (BBB)?

Astrocytes

Blood-Brain-Barrier

A selective barrier that regulates the chemical environment of the brain, preventing harmful toxic substances in blood from crossing to the neurons in the brain

Which type of supporting cells in the PNS is analogous to the oligodendrocytes in the CNS?

Schwann Cells

Why can’t severed axons in the CNS regenerate?

• The microglia poorly clean up area of damage, so debridement is not complete

• No neurilemma to form a regeneration tube to guide growth of severed axon

• Presence of the growth-inhibiting proteins in the CNS inhibit regeneration of a severed axon

Why can severed axons in the PNS regenerate?

• Cells of the immune system clean up the damaged area (debridement) which sets the stage for regeneration

• The neurilemma of the Schwann cell forms a regeneration tube that guides regeneration of the severed axon

2 factors responsible for establishing the resting membrane potential

1. Differential ion permeability: K⁺ leaks out of the cell more easily than Na⁺ enters, making the inside of the cell negative.

2. The Na⁺/K⁺ pump, which uses ATP to move 3 Na⁺ out and 2 K⁺ into the cell, helping keep the inside negative.

3 Phases of an Action Potential

Depolarization

Repolarization

Hyperpolarization

Depolarization phase of a action potential is caused by…

The entry of sodium ions (Na+) into the axon.

Repolarization phase of an action potential is caused by…

1. Sodium channels closing (Na+ influx halts)

2. Potassium channels opening (K+ efflux begins)

Hyperpolarization phase of an action potential is caused by…

Excessive potassium (K+) efflux past the resting membrane potential.

How is the generation of an action potential affected in the presence of a NA+ channel blocker?

An action potential cannot occur because Na⁺ cannot enter the cell, so the membrane does not reach threshold and no nerve impulse is transmitted.

Absolute Refactory Period of an Action Potential

The period during the depolarization phase when no second action potential can be generated because voltage-gated Na⁺ channels are already open.

Relative Refractory Period of an Action Potential

The phase during repolarization when Na⁺ channels are closed and K⁺ channels are open, so only a very strong stimulus can trigger another action potential.

How do you tell the difference between a strong stimulus (such as an intense pain) and a weak stimulus (such as as mild pain) when both caused action potentials to be generated?

The stronger stimulus causes the impulse to be generated at a higher frequency than the weaker stimulus

3 Structural Classes of Neurons

Multipolar, Bipolar, Unipolar

Multipolar Neuron

Has at least 3 processes, many dendrites and one axon

Bipolar Neuron

Has 2 processes, one dendrite and one axon attached to the cell body

Unipolar Neuron

Has one process from the cell body, an axon. It. branches to connect to receptors and the spinal cord or brain.

Functional Classifications of a Neuron

Sensory/Afferent

Association/Interneurons

Motor/Efferent

Sensory/Afferent Neurons

Transmits impulses from sensory receptors towards the CNS

Association/Interneurons

Located in the CNS between the sensory and motor neurons

Makes up 99% of neurons in the body

Motor/Efferent Neurons

Transmits impulses away from the CNS to effector organs

Types of Nerve Fibers

Group A, B, and C

Based on diameter and degree of myelination

Most abundant structural class

Multipolar neurons

Most abundant functional class

Association/interneurons

Group A Fibers

Largest diameter and heavily myelinated

Transmit impulse at 150 m/s (335 mph)

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 mph)

Ex. Preganglionic autonomic fibers

Group C Fibers

Smallest diameters and unmyelinated

Transmit impulses at a rate of 1 m/s (2.2 mph)

Ex. Postganglionic autonomic fibers that innervate smooth muscle’s pain fibers

Nerve fiber with fasted conduction velocity

Group A, due to largest diameter and heavy myelination

Nerve fiber with slowest conduction velocity

Group C, due to small diameter and no myelination

Diameter of axon affect on rate of impulse transmission

Large axons transmit impulses at a faster rate than smaller axons due to the larger axon having a larger diameter and therefore having less resistance impulse transmission. The resistance in the smaller axons is higher, which prevents impulse transmission.

Degree of myelination affect on rate of impulse transmission

Myelinated axons use saltatory conduction, where action potentials are generated only at the nodes of Ranvier, whereas Unmyelinated axons use continuous conduction where action potentials developed stepwise across the entire axolemma

What happens to the conduction velocity when myelinated axons become demyelinated?

Conduction velocity decreases significantly or may stop because saltatory conduction is lost and the signal becomes slower continuous conduction with possible signal failure.

4 Structures Protecting the Brain

Cranium, Meninges, Blood-Brain-Barrier, Cerebrospinal fluid

3 Meninges

Dura Mater, Arachnoid Mater, Pia Mater

Dura Mater

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

Arachnoid Mater

Middle meninx separated from the underlying pia mater by the subarachnoid space. Has web-like extensions.

Pia Mater

Innermost meninx that clings to the surface of the brain

CSF Locations

Outside of the brain: Subarachnoid space

Inside the brain: Lateral ventricle, third ventricle, fourth ventricle, central canal

Location of Subarachnoid Space

Between arachnoid mater and pia mater

Location of 2 lateral ventricles

Cerebral hemisphere, connected together by the septum pellucidum and connected to the third ventricle by a channel called the interventricular foramen

Location of Interventricular Foramen

Between the 2 lateral ventricles and the third ventricle, connecting the 2.

Location of the third ventricle

Diencephalon

Location of cerebral aqueduct

Between third and fourth ventricles, connecting the 2

Location of fourth ventricle

Brain stem

4 Regions of the adult brain

Cerebrum, diencephalon, brainstem, cerebellum

Cerebrum

Superior region of the brain, accounting for 83% of the total brain mass

Gyrus

Elevated ridges

Sulcus

Shallow grooves

Fissures

Deep grooves

Gray Matter

Cell bodies and dendrites

White Matter

composed of tracts with myelinated axons that have a “whitish” appearance

Commissural Tracts

Connect corresponding areas in the 2 cerebral hemispheres

Projection Tracts

Connect the cerebrum to lower brain areas and the spinal cord

2 types:

1. Descending projection tracts: send info from the cerebral cortex

2. Ascending projection tracts: send sensory info to the cerebral cortex

Association Tract

Connect areas within the same cerebral hemisphere

What type of tract is the corpus callosum?

Commissural tract - connects the right and left cerebral hemisphere

What type of tract is the arcuate fasciculus?

Association tract - connects the Broca’s area and the Wernicke’s area both located in the same cerebral hemisphere.

What type of tract is the pyramidal tract?

Descending projection tract - sends info from the cerebral cortex

Longitudinal Fissure

A median fissure dividing the cerebrum into the right and left cerebral hemispheres.

Corpus Callosum

Holds the 2 cerebral hemispheres together medially

5 lobes in each cerebral hemisphere

Frontal, temporal, parietal, occipital, insula

Which lobe isn’t visible from the surface of the cerebral hemisphere?

Insula

Primary sensory cortex located in Insula

Primary gustatory cortex

Lateral Sulcus

Separates the temporal lobe from the parietal and frontal lobe

Parieto-occipital Sulcus

Separates the parietal lobe from the occipital lobe

Central Gyrus

Separates the frontal lobe from the parietal lobe

Precentral Gyrus

The gyrus in the frontal lobe in front of the central sulcus, containing the motor control area

Postcentral Gyrus

The gyrus in the parietal lobe immediately behind the central gyrus, containing the somatosensory area.

3 Functional Areas of the Cerebral Cortex

Motor areas, sensory areas, association areas

What functional areas are exclusively in the frontal lobe?

Motor areas

Motor Areas

Controls voluntary movements.

Consists of the primary motor cortex, premotor cortex, Broca’s area, frontal eye field. All located in the frontal lobes

Sensory areas

For the conscious awareness of sensation.

Consists of the Primary somatosensory cortex, Primary visual cortex, Primary auditory cortex, primary olfactory cortex, primary gustatory cortex

Association areas

Integrate and interpret sensory inputs from the sensory areas hence, each primary sensory area has its own associated area

Primary Motor Cortex

Located in the precentral gyrus of the frontal lobe, controls voluntary skeletal muscle movements.

Its pyramidal cells form the pyramidal tracts, which cross in the medulla (decussation of the pyramids), causing each cerebral hemisphere to control the opposite side of the body.