L2: NEUROBIOLOGY OF NEURON

NEURONS

• excitable cells for the reception of stimuli and the conduction of the nerve impulse.

• DOESN’T replicate or divide

(2) NERVE FIBERS:

• dendrites

• axons

DENDRITES

receiving information and conducting it toward cell body

AXONS

long tubular neurite, conduct impulses toward the cell body

TYPES OF NEURON: BASED ON NUMBER, LENGTH AND MODE OF BRANCHING TO NEURITES

• unipolar

• bipolar

• multipolar

TYPES OF NEURON: BASED ON SIZE OF NEURON

• golgi type 1

• golgi type 2

UNIPOLAR

• Single neurite that divides a short distance than the cell body into two branches.

• Examples found at posterior root ganglion

BIPOLAR

• have an elongated cell body, with a single neurite emerging from each end

• Example found at retinal bipolar cell and cells of sensory cochlear and vestibular ganglia

MULTIPOLAR

• Multiple neurites from cell body

• A long process: Axon

• Remainder: Dendrites

• Example found at the brain and spinal cord

GOLGI TYPE 1 NEURONS

• Axon: long (1m or more)

• Long fiber tracts of brain and spinal lcord and the peripheral nerve

GOLGI TYPE 2 NEURONS

• Axon: short that terminates about the cell body or absent

• Dendrites are star- shaped

NERVE CELL BODY

• Mass cytoplasm which is nucleus is embedded

• Small granular cells of cerebral cortex measure about 5 um in diameter

• Large anterior horn cells as 135 um in diameter

NUCLEUS

• stores the genes

• Centrally located within the cell body

• In mature neurons, the chromosomes no longer duplicate themselves and function only in gene expression but exist in uncoiled state.

NUCLEAR ENVELOPE

• is continuous with the cytoplasmic rough, or granular, endoplasmic reticulum (RER)

• Double layered and possesses fine Nuclear Pores

BARR BODY

In female, one of the two X chromosomes is compact

NUCLEAR PORES

• which materials can diffuse into and out of the nucleus

• Newly formed ribosomal subunits can be passed into the cytoplasm through these

CYTOPLASM

rich in rough (granular) and smooth (agranular) endoplasmic reticulum.

NISSL SUBSTANCE

• Has granules that are distributed throughout the cytoplasm except for those near the Axon: Axon Hillock

• Granules extend to proximal parts of dendrites

• Composed of RER stacked on top of the other.

• Basophilic

• responsible for synthesizing protein

• Chromatolysis - nissl substance disappeared

GOLGI COMPLEX

• it appears as clusters of flattened cisternae and small vesicles made up of smooth endoplasmic reticulum (SER)

• Protein from Nissl substance is transferred to the inside of the __ where the protein is temporarily stored and where glycoproteins can be formed.

• Active in lysosome production and cell membrane synthesis

MITOCHONDRIA

• Found scattered throughout the cell body, dendrites, and axon.

• Spherical or rod shaped

• Double membrane

• Inner membrane: thrown into cristae that project in the center

• for energy production

NEUROFIBRIL

• main component of the cytoskeleton

• neurofilaments are very stable and belong to the cytokeratin family

MICROFILAMENT

• concentrated at the periphery of the cytoplasm just beneath the plasma membrane

• role In the formation of new cell processes and the retraction of old ones

• also assist the microtubules in axon transport.

MICROTUBULES

• found interspersed among the neurofilaments

• Arranged in parallel

CELL TRANSPORT

• rapid transport

  • (100 to 400 mm/day)

• slow transport

  • (0.1 to 3.0 mm/day)

  • Slow axonal transport occurs only in the anterograde direction

ANTEROGADE MOVEMENT

• away from the cell movement

• kinesin-coated organelles

RETROGADE MOVEMENT

• toward the cell

• dynein-coated organelles

LYSOSOMES

• Acting as intracellular scavengers and contain hydrolytic enzymes

• has 3 forms:

  • primary - just been formed

  • secondary - contain partially digested material

  • residual bodies - enzymes are inactive,

CENTRIOLES

associated with the formation of the spindle during cell division and in the formation of microtubules

LIPOFUSCIN

• occurs as yellowish-brown granules within the cytoplasm

• harmless metabolic byproduct.

MELANIN GRANULES

Their presence may be related to the catecholamine- synthesizing ability of these neurons, whose neurotransmitter is dopamine

PLASMA MEMBRANE

• Continuous external boundary of the cell body

• site for the initiation and conduction of the nerve Impulse

• Has Inner and outer layer of very loosely arranged protein molecules

• Carbohydrate molecules are attached to the proteins or the lipids forming cell coat or glycocalyx

RESTING MEMBRANE POTENTIAL

• a nerve fiber is polarized so that the interior is negative to the exterior

• the potential difference across the plasma membrane (axolemma) is about -80 mV

ABSOLUTE REFRACTORY PERIOD

short time after the passage of a nerve impulse along a nerve fiber, while the axolemma is still depolarized, a second stimulus, however strong, is unable to excite the nerve.

RELATIVE REFRACTORY PERIOD

• period Is followed by a further short Interval during which the excitability of the nerve gradually returns to normal

SUMMATION

Multiple excitatory stimuli applied to a neuron's surface result

CONDUCTION VELOCITY

• is proportional to the cross-sectional area of the axon

• with the thicker fibers conducting more rapidly than those of smaller diameter

SODIUM AND POTASSIUM CHANNELS

• through which the sodium and potassium ions diffuse through the plasma membrane

• formed of the protein molecules that extend through the full thickness of the plasma membrane

• gating - twisting and distortion of the channel

DENDRITES

• diameter tapers as they extend from the cell body and they often branch profusely

• Dendritic spines-large numbers of small projections

• extensions of the cell body to increase the surface area

• conduct the nerve impulse toward the cell body

AXONS

• It arises from a small conical elevation on the cell body, devoid of Nissl granules, called the axon hillock

• proximal part of a dendrite

• tubular and is uniform in diameter

• Terminals- distal ends of the terminal branches of the axons often enlarged

VARICOSITIES

• swellings resembling a string of beads near their termination

• diameter varies:

  • larger - conduct impulses rapidly

  • smaller - conduct impulses very slowly

AXOLEMMA

plasma membrane of axon

AXOPLASM

cytoplasm of the axon

INITIAL SEGMENT

• axon is the first 50 to 100

  μ.m. after it leaves the axon hillock of the nerve cell body

• most excitable part of the axon

• site at which an action potential originates

(2) TYPES OF AXON TRANSPORT:

• anterograde transport

• retrogade transport

ANTEROGRADE TRANSPORT

• from cell body to axon terminals

• fast anterograde transport / slow anterograde transport

FAST ANTEROGRADE TRANSPORT

transport of proteins and transmitter substances (100 to 400 mm/day)

SLOW ANTEROGRADE TRANSPORT

transport of axoplasm and includes microfilaments and microtubules (0.1 to 3.0 mm/day

RETROGADE TRANSPORT

• from axon terminals to cell body

• explains how the cell bodies of nerve cells respond to changes in the distal end of the axons

SYNAPSE

• site where two neurons and functional interneuronal communication occurs

• takes place in one direction only

SYNAPTIC SPINES

• extensions of the surface of a neuron

• form receptive sites for synaptic contact with afferent boutons.

CHEMICAL SYNAPSE

• Presynaptic membrane

  - surface of the terminal axonal expansion

  - cytoplasm contains presynaptic vesicles, mitochondria, and lysosomes

• Postsynaptic membrane

  - surface of the neuron

  - cytoplasm contains parallel cisternae

• Synaptic cleft

  - separates the membranes

  - contains polysaccharides

presynaptic vesicles; mitochondria

__ and _ plays a key role in the release of neurotransmitter substances

NEUROMODULATORS

• These substances are capable of modulating and modifying the activity of the postsynaptic neuron

• principal neurotransmitters have a rapid, brief effect on the postsynaptic membrane

• do not have a direct effect on postsynaptic membrane

G PROTEIN

act through a second-messenger system, usually through a molecular transducer

ELECTRIC SYNAPSE

• gap junctions containing channels from the cytoplasm of the presynaptic neuron to that of the postsynaptic neuron

• rapid spread of activity from one neuron to another

• Bidirectional

(4) TYPES OF NEUROGLIA

• astrocytes

• oligodendrocytes

• microglia

• ependyma

ASTROCYTES

• type of neuroglia

• have small cell bodies with branching processes that extend in all directions

• Two types:

  • fibrous astrocytes

  • protoplasmic astrocytes

ASTROCYTE FUNCTIONS

1.Form a supporting framework for the nerve cells and nerve fibers

2. Serve as a scaffolding for the migration of immature neurons in the embryo

3. Serve as electrical insulators

4. Limit spread of neurotransmitter

5. Take up excess K+ ions

6. Store glycogen

7. Serve as phagocytes

8. Serve as a conduit for the passage of metabolites or raw materials from blood capillaries to the neurons

9. Secrete cytokines

10. Play a key role in BBB structure

OLIGODENDROCYTES

• type of neuroglia

• small cell bodies and a few delicate processes

• cytoplasm does not contain filaments

• found in rows along myelinated nerve fibers and surround nerve cell bodies

OLIGODENDROCYTE FUNCTIONS

• responsible for the formation of the myelin sheath of nerve fibers in the CNS

• They are thought to influence the biochemical environment of neurons

MICROGLIA

• type of neuroglia

• embryologically unrelated

  derived from macrophages outside the nervous system.

• Smallest

• found scattered throughout the CNS

MICROGLIA FUNCTION:

• appear to be inactive and are sometimes called resting microglial cells

• inflammatory disease of the CNS, they become the immune effector cells

• they proliferate and become antigen-presenting cells

• actively phagocytic

EPENDYMA

• type of neuroglia

• Line the cavities of the brain and the central canal of the spinal cord

• cuboidal or columnar in shape

• The cilia are often motile

• The bases lie on the internal glial limiting membrane

• Three Groups:

  • Ependymocytes

  • Tanycytes

  • Choroidal epithelial cells

EPENDYMOCYTES

line the ventricles of the brain and the central canal of the spinal cord and are in contact with the CSF

Functions:

• Assist in the circulation of the CSF within the cavities of the brain and the central canal of the spinal cord by the movements of the cilia

• Absorb CSF (microvilli on free surfaces)

TANYCYTES

• These cells have long basal processes that pass between the cells of the median eminence

• Line floor of third ventricle

  Functions:

  • Transport chemical substances from CSF to hypophyseal portal system

  • Control of the hormone production by the anterior lobe of the pituitary

CHOROIDAL EPITHELIAL CELLS

The sides and bases of these cells are thrown into folds and near their luminal surfaces

• cells are held together by tight junctions that encircle the cells

• Cover the surfaces of the choroid plexuses

Functions:

• The presence of tight junctions prevents the leakage of CSF into the underlying tissues.

• Production and secretion of CSF from the choroid plexuses

EXTRACELLULAR SPACE

A very narrow gap separates the neurons and the neuroglial cells

• CSF in the subarachnoid space externally

• CSF in the ventricles of the brain and the central canal of the spinal cord internally

• surrounds the blood capillaries in the brain and spinal cord Functions:

• provides a pathway for the exchange of ions and molecules between the blood and the neurons and glial cells