Cell body Location in CNS-gray matter, in PNS-ganglia

1. Axon hillock- a cone shaped region where axon arises 2. Large nucleus with prominent nucleolus 3. Abundant RER and ribosomes – Nissl bodies - extend into dendrites but not into axon

hillock or axon 4. Numerous mitochondria 5. Large golgi apparatus 6. Cytoskeleton – neurofilament, microtubules to provide structural and intracellular support

Initial segment – initial unmyelinated part of axon between the axon hillock and beginning of myelin sheath, site which action potential is generated Collateral branches – arising at right angle from axon, end by dividing into small branches (terminal arbor=telodendria), at the end of terminal arbor is axon terminal button/bouton (button-like swelling)

Dendrites Arises from cell body and branches, unmyelinated Receive stimuli from other neurons or external environment (sensory receptor) and carry information (nerve impulses) to the cell body Axons Aka nerve fibre, a neuron has one axon, most are long and arise from cell body via axon hillock Myelinated or unmyelinated, transmit information away from cell body to another neuron or effector cell

Nervous tissues = neurons (soma/perikaryon) + microglia Neurons = Cell body (soma/perikaryon) + processes (dendrites and axons)

Synapse – sites where nerve impulses are transmitted from presynaptic cell (neuron) to a postsynaptic cell (another neuron/muscle cell/cell of gland) Classification Axodendritic (axon and dendrites) Axosomatic (axon and cell body) Axoaxonic (axon and axon) Components Presynaptic membrane – axon terminal/terminal button, contains synaptic vesicles contain neurotransmitters Synaptic cleft – narrow space between presynaptic membrane and postsynaptic membrane Postsynaptic membrane – part of plasma membrane of postsynaptic cell, contains receptors which neurotransmitters bind to

Classification of neurons

Functional classification

sensory neuron - receive stimuli from receptors and conduct nerve impulses to the CNS

motor neuron - originates from CNS and conduct nerve impulses to other neurons/effector cells

interneurons - located in CNS, form a communicative network between sensory and motor neurons

Morphological classification

Based on number of processes extending from cell body

multipolar neuron - most common, many processes (1 axon, >1 dendrites), motor, interneurons, pyramidal and purkinje cells

bipolar neuron - not common, 2 processes (1 axon, 1 dendrites), found in special sensory organs -retina of eyes and olfactory epithelium

pseudo-unipolar neuron - has only 1 process and divides into 2 processes (central and peripheral processes), sensory neurons which cell bodies located in sensory ganglia of cranial and spinal nerves (dorsal root ganglia)

Neuroglia

Central neuroglia

Astrocytes – largest neuroglial cell, stellate in shape with numerous long processes, to provide physical and nutritional support for neurons. Two types: protoplasmic astrocytes in gray matter and fibrous astrocytes in white matter. -some have foot processes called subpial feet which form glial limiting membrane (glial limitans) between the pia mater and the brain forming barrier surrounding CNS -some have foot processes called perivascular feet that cover capillaries and help to maintain blood-brain barrier, control the transfer of substances from blood to neurons Oligodendrocytes – to produce and maintain myelin in CNS, most numerous glial cells in white matter with few processes compared to astrocytes

Perivascular

feet

Microglia – smallest neuroglia with dark, elongated nuclei, short processes and phagocytic cells, part of mononuclear phagocytotic system, remove bacteria and injured cells Ependymal cells – cuboidal to columnar cells, 1 layer of cells lining ventricles of brain and central canal of spinal cord, apical surface has cilia (facilitate movement of CSF) and microvilli (absorbing CSF) - some in the ventricles of brain are modified together with the adjacent capillaries form the choroid plexus that secrete the CSF

Peripheral neuroglia

Schwann cells – produce myelin sheath in PNS, 1

forms myelin around 1 segment of 1 axon, thus 1

axon is covered by multiple Schwann cells

Satellite cells – small cuboidal cells, surrounds

cell bodies of neurons in ganglia in PNS, forms

layer around cell bodies

Axon

myelination

Myelination in PNS

-performed by Schwann cells

A Schwann cell envelops a segment of an axon, it wraps itself around the axon of >50 times result in enveloping the axon in many concentric layers of Schwann cell cytplasm and plasma membrane

As wrapping continues, cytoplasm between the concentric layers of plasma membrane is squeezed out into the cell body of Schwann cell (outermost parts), the overlapping concentric layers of plasma membrane forms the myelin sheath thus, myelin sheath actually composed of many concentric layers of plasma membrane of Schwann cell

small amounts of remnants of Schwann cell cytoplasm trapped within the layers of myelin -Schmidt Lanterman cleft

Myelination in CNS

-performed by oligodendrocytes

-oligodendrocytes have many cytoplasmic processes that extend towards each axons, each process envelops and wraps itself around a segment of an axon, thus 1 olidendrocyte can myelinate >1 axon

Clinical correlation – Demyelinating disease- myelin sheath is damaged in this disease, will slow down/block nerve conduction, symptoms= muscle paralysis, loss of cutaneous sensation (Guillain Barre Syndrome, multiple sclerosis) Unmyelinated axons – axons not wrapped with myelin sheath, several axons longitudinally invaginated by Schwann cell, unmyelinated axons are surrounded by only one layer of Schwann cell plasma membrane and cytoplasm, several unmyelinated axons may be enveloped by a single Schwann cell Node of Ranvier – segment of myelin formed is called intermodal segment of myelin, narrow gaps between them=nodes of Ranvier where axons is not covered by the myelin sheath and expose to extracellular space

Process by which an axon is wrapped

with a myelin sheath

Gray matter

- contains many cell bodies of neurons, dendrites of neurons, beginnings and

endings of axons of neurons (very little myelinated axons) and neuroglial cells

and its processes (gray due to many cell bodies of neurons)

CNS is made up of gray matter and white matter

Gray matter = most are cell bodies (neurons+neuroglial cells), less processes of dendrites and parts of axons and neurons, neuroglial cells processes White matter = majority of processes (mostly myelinated axons and neuroglial cell processes) with a few cell bodies of neuroglial cells

- Background between the cell

bodies is called neuropil which

consists of

Neuronal processes (axons

and dendrites

Neuroglial processes

White matter

Consists mostly axons myelinated and unmyelinated, mostly

myelinated

Neuroglial cells, less cells compared to gray matter, only has

neuroglial cells with small, round nuclei, cytoplasm is not evident

White colour due to lipid in myelin sheaths surround the axons

Does not contain the neuronal cell bodies

Gray matters Cerebral cortex – gray matter on the surface of cerebrum Basal ganglia – masses of gray matter situated deep in the cerebrum, surround by the white matter White matter – deep to the cerebral cortex, made up of axons and neuroglial cells, axons can be myelinated or unmyelinated, mostly are myelinated, three types of axons: commissural, association, projection fibres

5 types of neurons

Pyramidal cell

Fusiform cell

Stellate cell (granule cell)

Horizontal cell

Martinotti cell

2 categories

Principal neurons – their axons leave cerebral cortex, pyramidal and Fusiform cell

Interneurons – their axons do not leave the cerebral cortex, the rest of neurons

Pyramidal cell

Most abundant, main principal neuron in

cerebral cortex, has pyramid shape cell bodies

Axons leave the cerebral cortex and enter

white matter to form commissural,

association and projection fibres

Cells layers of cerebral cortex

-divided into 6 layers based on most cell type present, no clear cut borders between each

layers

Layer I: The Molecular Layer -most superficial layer -contains mostly dendrites and axons of neurons, not many cells, horizontal and neuroglial cells -large numbers of synapses between neurons occur here Layer II: The External Granular Layer -mostly stellate cells, pyramidal cells and interneurons made up of Martinotti cells and neuroglial cells -the axons of pyramidal cells extend to enter the white matter Layer III: The External Pyramidal Layer -mostly pyramidal cells, interneurons of stellate and Martinotti cells, neuroglial cells -the axons of pyramidal cells extend to enter the white matter Layer IV: The Internal Granular Layer -mostly stellate cells, pyramidal cells, other interneurons are Martinotti cells and neuroglial cells -main afferent (input) station -also called input layer -thickest in the primary sensory areas Layer V: The Internal Pyramidal Layer -mostly pyramidal cells, interneurons of stellate cells and Martinotti cells, neuroglial cells -most efferent (output) fibers from pyramidal cells, contribute to corticospinal and corticonuclaer tracts -also called output layer -thickest in the motor areas Layer VI: The Multiform Layer -mainly fusiform cells, pyramidal cells, interneurons of stellate and Martinotti cells and neuroglial cells -deepest layer -deep to it is the white matter, no clear cut demarcation between the layer VI and the white matter

Cerebellum

Gray matter – form

Cerebellar cortex – gray matter on the surface of cerebellum Deep cerebellar nuclei – masses of gray matter situated deep in the cerebellum and is surrounded by the white matter White matter – deep to the cerebral cortex and similar to cerebral white matter, contains densely packed axons (mostly myelinated) and neuroglial cells, do not contain the cell bodies

Neurons in cerebellar cortex – 5 types

Purkinje cell Stellate cell Basket cell Granule cell Golgi cell 2 categories Principle neurons – axons leave the cerebellar cortex, Purkinje cell Interneurons – axons do not leave the cortex, stellate, basket, granule and golgi cells

Division of cerebellar cortex – 3 layers

Molecular layer- outermost layer, contains mostly dendrites and axons with a few neurons (stellate cells

and basket cells)

The Purkinje cell layer – purkinje cells have very large cell bodies in flask shaped, arranged in single layer,

its axon passes through granular layer to enter white matter and synapse in deep cerebellar nuclei

The granular layer- deepest layer, consists of granule cells which are the most abundant and densely

packed and also Golgi cells

Gray matter – lies in the centre of spinal cord, H or butterfly in shape, volume is greatest in cervical and lumbosacral enlargement due to innervations of upper and lower limbs (sensory and motor innervations of limbs) - divided into 4 parts Anterior gray horn – contains large cell bodies of neurons, axons of motor neurons exit the spinal cord by passing through the white matter and enter the anterior root of spinal cord, motor neurons have large, spherical, pale staining nuclei with densely stained nucleolus, cytoplasm is dark staining due to Nissl bodies corresponds to RER Posterior gray horn – cell bodies of 2nd order sensory neurons of anterolateral system and interneurons Lateral gray horn – cell bodies of preganglionic sympathetic neurons, at thoracic, upper lumbar spinal cord segments (T1-L2) Intermediate zone – gray commissure and central canal which contains CSF, lined by Ependymal cells which are ciliated to facilitate the circulation of CSF White matter – located at periphery, contains bundle of axons, mostly myelinated and form ascending and descending motor tracts -amount largest in cervical spinal cord segment, smallest in sacral spinal cord segment, amount increases from sacral to cervical as sensory fibres are added to each level, decreases from cervical to sacral because motor fibres leave spinal cord