nerve tissue

 CNS: consisting of the brain and the spinal cord;  PNS: consisting of nerves,ganglia(collection of nerve cell bodies) and specialized nerve endings. nervous system

Composition of nerve tissue nerve tissue neurons (nerve cells) supporting cells In the CNS: neuroglia cell (glia cell) In the PNS: Schwann cell satellite cell

Function of nerve tissue  Neuron: to receive stimuli and conduct electrical impulses.  Supporting cell: physical support (protection) for neuronal processes; electrical insulation for nerve cell body and processes; metabolic exchange pathways between vascular system and neuron.

neuron Neuron is the functional unit of the nerve system.It can receive stimuli and conduct electrical impulses. Structure of neuron Neuron is made up of cell body(soma or perikaryon), dendrite and axon. 1. Soma ： where ribosomes, mitichondria, and other subcelluar parts do the metabolic work of cells Cell membrane: There are some ion-channels and receptors in it. Cytoplasm: Nissl bodies, neurofibrils, ribosomes, mitochondria, etc. Nucleus: central,large and round, lightly stained, with an obvious nucleolus.

neuron

Nissl body in the soma ： Basophilic granules. Each Nissl body is made up of a stack of RER and functions in producing proteins.

2. Dendrite ： numerous tapering branches form the dendritic trees,which increase the receptor surface area of a neuron.There are Nissl bodies in the dendrites. Function: receiving stimuli and conducting them to the cell body.

neuron

3. Axon: thin and long, similar in diameter. few branches. without Nissl bodies. Function as transmitting stimuli to other neurons or effector cells. Hillock: continuous with the axon,the area of cell body where is free of large cytoplasmic organells: RER and so on. Axonal transport system ： A mode of intercellular communication between cell body and the terminal of axon. ① Anterograde transport ： from cell body to the terminal of axon （ materials synthesized in the soma) ② Retrograde transport ： (metabolic products and etc.)

neural structure

Axon Hillock

Axonal transport system

The classification of neurons 1.Based on the number of its processes ① multipolar neuron ： one axon and two or more dendrites ② bipolar neurons ： one axon and one dendrite. (limited to the retina and etc.) ③ unipolar (pseudo-unipolar) neuron ： one process, which divides close to the cell body into two long processes.

three types of neurons

2.Based on the function of the neuron ① sensory neuron ： convey impulses from receptors (unipolar) to CNS afferent neuron. ② motor neuron ： convey impulses from the CNS or (multipolar) ganglia to effector cells and control the activity of muscle and the secretion of the glands efferent neuron. ③ interneuron : form a communicating and integrating (multipolar) network between sensory and motor neurons intercalated neuron.

synapse synapse is some special cell junction between neurons or between neuron and its effector cell (muscle fiber or glandular cell). synapse electrical synapse chemical synapse gap junction: two-direction transmission independent of neurotransmitters consists of presynaptic element, synaptic cleft and postsynaptic element; one-direction transmission mediated by neurotransmitters;

chemical synapse 1.presynaptic element: bulb-like enlargements of the axon terminals which is called terminal button or synaptic bag. presynaptic membrane ： presynaptic cell membrane with voltage sensitive calcium channels. synaptic vesicles ： membrane bound vesicles which contain neurotransmitters. 2.synaptic cleft ： the space between the presynaptic and postsynaptic membranes. 3.postsynaptic element ： postsynaptic cell membrane with chemical sensitive ion channels and receptors of the neurotransmitters.

synaptic bag

synapse

the transmission of the nerve impulse Nerve impulse arrives at presynaptic cell membrane influx of the calcium ions through the presynaptic membrane Synaptic vesicles pour the neurotransmitters into the synaptic cleft Neurotransmitters reach and bind onto corresponding receptors The postsynaptic membrane becomes depolarized.

synapse

several morphological kinds of synapses  axodendritic: occurring between axons and dendrites;  axosomatic: occurring between axon and soma;  axoaxonic: occurring between axon and axon;  dendrodentritic: occurring between dendrites and dendrites.

kinds of synapses

supporting cells In the PNS: Schwann cell satellite cell In the CNS: neuroglia cell: astrocyte (glia cell) oligodendrocyte microglia ependymal cell nonconductive cells. function: insulate the neurons from each other, protect the neurons

supporting cells in the PNS 1.Schwann cell 2.satellite cell

1 Schwann cell and myelin sheath profile: flat sac in shape nucleus: flattened,stains darkly. function: surrounding the axon and constituting the neurilemma and myelin sheath. myelin sheath-forming cell in the PNS.

formation of myelin sheath myelin sheath: composed of multiple layers of Schwann cell membrane wrapped concentrically around the axon.  Each Schwann cell initially surrounds some part of an axon;  It wraps a short segment of axon,forming multiple Schwann cell layers,and cytoplasm is squeezed out from between the membrane of the concentric layers;  The inner leaflets of the plasma membrane then fuse.

formation of myelinated axon

myelinated axon

myelin sheath

myelinated axon （ SEM ）

longitudinal and cross sections of myelin sheath

myelinated axon: surrounded by myelin sheath unmyelinated axon: without myelin sheath in the PNS

( 1) myelinated axon in the PNS and its myelin sheath The myelin sheath is segmented because it is formed by numerous Schwann cells arrayed sequentially along the axon. These "breaks" in myelination allow for saltatory conduction. neurilemma( the sheath of Schwann): It is part of Schwann cell which is external to, contiguous with,the myelin sheath. myelin sheath: composed of multiple layers of Schwann cell membrane wrapped concentrically around the axon. Ranvier node: The junction where two adjacent Scheann cells meet is devoid of myelin.This site is called Ranvier node. internode(internodal segment): the myelin between two sequential Ranvier nodes.It is composed of part of axon and its myelin sheath which is formed by one Schwann cell.

(2) Unmyelinated axon in the PNS The Schwann cells are elongated,and axons fit into the grooves in the surface of the cell. A single axon or a group of axons may be enclosed in the surface of one Schwann cell.

2.satellite cell a layer of cuboidal or low columnar cells that surround the neuronal cell body of the ganglia. Function: providing electrical insulation and physical protection of the neuronal body.

satellite cell

supporting cells in the CNS 1. neuron 2. oligodendrocyte 3. blood vessel 4. Axon 5. astrocyte 6. ependymal cell 7. microglia

1.astrocyte(fibrous and protoplasmic) profile:largest, star-shaped glial cells of the CNS that have numerous processes. Many of these processes extend to blood vessels. The expanded endings of the astrocyte processes are known as end-feet. blood-brain-barrier : It is formed by tight junctions between endothelial cells, the end-feet function to induce and maintain the blood-brain barrier. supporting cells in the CNS

end feet of astrocytes and BBB BBB: to restrict the material permeation between blood and brain.

astrocyte

（ 1 ） fibrous astrocyte ： more common in white matter. Fewer processes which are relative straight.

（ 2 ） protoplasmic astrocyte ： more prevalent in gray matter They have numerous,short,branching processes.

2. oligodendrocyte Profile: smaller than astrocyte,processes are shorter and less than those of the astrocyte. Nucleus: smaller and stained darkly. Function:involved in the formation of the myelin sheath in the CNS.

oligodendrocyte and the myelin sheath in the CNS

(1) myelinated axon in the CNS 2. oligodendrocyte 4. Axon

(1)myelinated axon in the CNS  In the CNS,myelin sheath is formed by oligodendrocytes.  each process of the oligodendrocytes can surround some part of one axon:Processes of one neuron can surround the axons of several neurons.

(2)The unmyelinated axon in the CNS The axon is thinner. Usually there is no glia surrounding it.

3. microglia profile: the smallest glia cell.round or thin and long in shape. with fewer and shorter processes. nucleus: triangle in shape or looks like a pole. function: similar to that of macrophage.

microglia

4. ependymal cell ： column-shaped cells that line the ventricles of the brain and central canal of the spinal cord.it is an important part of the barrier between the brain and cerebrospinal fluid.

supporting cells in the CNS

myelinated axon

axons in the PNS

nerve fiber and nerve nerve fiber: It is composed of the axon with or without the myelin sheath. Based on the presence of myelin sheath, nerve fibers are divided into two categories: myelinated nerve fiber and unmyelinated nerve fiber.

nerve nerve: a bundle of nerve fibers held together by the connective tissue(C.T.). epineurium ： CT surrounding a nerve. perineurium ： CT surrounding each nerve fiber fascicle. endoneurium ： CT surrounding each individual nerve fiber.

peripheral nerve epineurium endoneurium perineurium

5. nerve ending sensory nerve ending the terminal part of the peripheral arborization of the sensory neuron.It can form kinds of sensory receptors in the tissue or organ. 1.free nerve ending 2.tactile corpuscle 3.lamellar corpuscle 4.muscle spindle

1. Free nerve ending terminal branches of the myelinated sensory nerve fiber. It is devoid of myelin sheath. Distribution:between the cells of epidermis,corneal epithelium or in the C.T. of dermis and hair follicles,etc. Function: receptor of pain.

2.tactile corpuscle(Meissner’s corpuscle) ellipse, free nerve endings with a capsule of C.T. Distribution: lie in the dermal papillae. Function: serving as touch receptors.

3. lamellar corpuscle (Pacinian corpuscle) Distribution: mainly lying in deeper dermis and hypodermis,internal organs,etc. Function: deep pressure receptor for mechanic and vibratory pressure.

Lamellar corpuscle

4. muscle spindle Distribution: lies in the skeletal muscle. Function: serve as a specialized stretch receptor within the skeletal muscle

motor nerve ending motor nerve ending : terminal branches of motor neuron distributing in muscle tissue and glands. one neuron can control many muscle fibers, while one muscle fiber is controlled by only one neuron. motor unit ： a neuron along with its specific muscle fibers that it innervates. motor end plate: the junction between motor nerve ending and skeletal muscle fibers.

motor end plate

Key point 1. The composition of nerve tissue:neuron and supporting cells() 2. basic structure of the neuron(soma,dendrite and axon) and the Nissl body 3. Classification of the neuron 4. Structure of the Chemical synapse 5. Myelin sheath and myelin sheath-forming cells in the PNS and CNS;Myelinated axon and unmyelinated axon 6. Nerve;the composition of sensory nerve endings and their function;motor unit;motor end plate.