Nerve- Overview Flashcards
Learning outcomes
Define and briefly describe the terms central nervous system (CNS) and peripheral nervous system (PNS); motor and sensory; ganglion; neuron, dendrite, axon Label the principal parts of the spinal cord and describe its relationship to its associated spinal (dorsal root) ganglia.
Describe the component parts of a peripheral nerve and its relationship to the spinal cord.
Describe the structural features of different types of neuron and where they occur.
List the types of supporting cells (neuroglia) in the nervous system and describe the relationship between neuronal processes/axons and the supportive cells involved in myelinated and unmyelinated nerves.
Describe the structural features of a synapse.
Describe other examples of neuronal endings, both sensory and motor e.g. Pacinian corpuscles, muscle spindles and motor end plates.
Give an account of the structure and roles of neuroglialcells including oligodendrocytes, astrocytes, ependymal and microglia.
Describe the structure of the meninges.
Principles of nerve tissue
Neurons •100 billion in the nervous system •Nerves undergo regeneration •Receive, process & integrate information •Conduct nerve impulses to target organs
Neuroglial cells
•1000 billion in the nervous system•They maintain homeostasis and provide support and protection for the neurons Schwann cells Oligodendrocytes Ependymal cells Microglia Satellite cells Astrocytes
The neuron
There are many kinds of neurons , but all consist of:
•a cell body/soma (perikaryon) containing the nucleus,
•a long cytoplasmic extension called the axon, specialised in generating or conducting nerve impulses to other cells that terminates in dilations known as end bulbs
•and one or more shorter processes called dendrites specialised in receiving stimuli from other cells –the ‘aerial’
Neurons use the common cell property of excitability to produce and move an action potential (nerve impulse) along the axon to excite another neuron or other effector cell. This nerve communication is transmitted to another neuron or effector cell via a synapse , where a neurotransmitter is released at the presynaptic membrane and binds receptors on the postsynaptic cell , initiating a new action potential there
What is the classification of nerves?
Structural
Anaxonic: where axon cannot be distinguished from dendrites.
Bipolar: axon and single dendrite on opposite ends of the soma.
Unipolar or pseudounipolar: dendrite and axon emerging from same process.
Multipolar: two or more dendrites, separate from the axon:
Functional
Sensory neurons
Motor neurons
Interneurons/association neurons
Neuron cell body
The cell body (soma) is the factory of the neuron. It produces all the proteins for the dendrites, axons and synaptic terminals
- Axon hillock =a specialized domain of the neuronal cell body from which the axon originates. A high amount of protein synthesis occurs in this region, as it contains a large number of Nissl granules *
- Neuropil = Mass of interwoven nerve endings & dendrites from other neurons
*Nissl bodies = Combination of free ribosomes and RER
Axon
Individual axons are microscopic in diameter -typically about one micrometre across -but may extend to 1 metre in length.
*Neurofilaments are a major component of the neuronal cytoskeleton, they provide structural support for the axon and regulate axon diameter
Axons are in effect the primary transmission lines of the nervous system, and as bundles they help make up nerves.
Axolemma- sheath surrounding axon
Neurotubules act like conveyor belts to transport numerous substances along an axon between the soma and the axon terminal.
CT of peripheral nerves
Individual axon = surrounded by endoneurium
Nerve fascicle (group of axons) = surrounded by perineurium Nerve (as seen in gross anatomical specimens) = surrounded by epineurium
Spinal cord
Consists of 31 segments, each of which give rise to a pair of spinal nerves
8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal
(only diff in vert. no to nerves is 7 cervical vertebrae- first comes off at spinal cord at c1)
The spinal begins at the base of the medulla oblongata.
It then travels inferiorly within the vertebral canal, surrounded by the spinal meninges containing cerebrospinal fluid.
At the L2 vertebral level the spinal cord tapers off, forming the conus medullaris.
The spinal nerves that arise from the end of the spinal cord are bundled together, forming a structure known as the cauda equina(horses tail).
Outer layer –white matter Inner layer –grey matter
What is a lumbar puncture?
A lumbar puncture is a commonly performed diagnostic test for; bacterial meningitis, viral encephalitis and idiopathic intracranial hypertension.
The procedure is typically performed between L3/L4 vertebrae (adults) and L4-L5 (children) due to a combination of ease and perceived safety.
In one study anaesthetists could only correctly identify the correct spinal level 29% of the time and were one space higher than anticipated more than 50% of the time.
The third National Audit Project performed by the Royal College of Anaesthetists showed that, pessimistically, there was permanent harm from spinal anaesthesia in 1 in 38,000 cases. This harm may include paralysis and death.
Nerves of the spinal cord
The spinal nerves are mixed nerves that originate from the spinal cord, forming the peripheral nervous system.
Each spinal nerve begins as an anterior/ventral (motor) and a posterior/dorsal (sensory) nerve root. These roots arise from the spinal cord, and unite at the intervertebral foramina, forming a single spinal nerve.
The spinal nerve then leaves the vertebral canal via the intervertebral foramina, and then divides into two:
Posterior rami –supplies nerve fibres to the synovial joints of the vertebral column, deep muscles of the back, and the overlying skin.
Anterior rami –supplies nerve fibres to much of the remaining area of the body, both motor and sensory.
Pointed horn is posterior horn, pointy things hurt, sense(sensory nerves)
Rounded horn is anterior horn (motor nerves)
What are the two horns of the grey matter?
Pointed horn is posterior horn, pointy things hurt, sense(sensory nerves)
Rounded horn is anterior horn (motor nerves)
Motor neurons are multipolar, cell body will be found in anterior horn, axon extends out
Sensory neurons are pseudounipolar- bodies are located in dorsal root ganglion, not posterior horn etc
Central canal, allows passage of CSF around spine
What are ganglia?
•Ganglia are typically ovoid structures containing neuronal cell bodies and their surrounding glial satellite cells supported by delicate connective tissue and surrounded by a denser capsule.
•Because they serve as relay stations to transmit nerve impulses, at least one nerve enters and another exits from each ganglion.
•The direction of the nerve impulse determines whether the ganglion will be a sensory or an autonomic ganglion.
Ganglia are a collection of nerve cell bodies
Glial cells of the PNS (spinal and cranial nerves)
Schwann cells (neurolemmocytes) enclose all axons in nerves of the PNS, producing myelin sheaths around large-diameter axons, whose impulse conductivity is augmented at the nodes of Ranvier between successive Schwann cells. (Diff. to oligodendrocyte)
Satellite cells are located within PNS ganglia, aggregated sensory or autonomic neuronal cell bodies, where they enclose each perikaryon and regulate its microenvironment.
Glial cells of the CNS ( brain/ spinal cord)
In contrast to a Schwann cell, an individual oligodendrocyte will form the myelin sheath for several axons insulating the axons and facilitating nerve impulses.
Astrocytes are the most numerous cell of the CNS
Two types: Fibrous (located in the white matter) and Protoplasmic (located in the grey matter)
Functions
•Blood brain barrier –end-feet around blood vessels are thought to be basis of control of the exchange of metabolites between blood and neurons
•Restraint of spread of electrical disturbance by absorption of K+ ions around synapses, unmyelinated axons and nodes of Ranvier
Ependymal cells are low columnar epithelial-like cells, lacking basement membranes, which line the cavities of the CNS i.e. the central canal of the spinal cord and the cerebral ventricles
Microglia differ from all other glial cells in originating fromblood monocytes, not from neural tissue precursors; Small elongated cells with irregular processes. When activated, they become the macrophage cells and antigen-presenting cells of the CNS mediating immune defence activity
Neural regeneration
Certain regions of the CNS, such as near the ependyma, retain rare neural stem and progenitor cells that allow some replacement of neurons throughout life.
The complexity of the neuronal and glial interconnections with the CNS make regeneration after major injury very difficult.
-The more simply organized peripheral nerves have better capacity for axonal regeneration, a process involving reactivation of the soma, Schwann cells, and macrophages.
