Nervous System Flashcards
(28 cards)
The nervous system has 2 major divisions
The nervous system has two major divisions, the central nervous system (CNS) and the peripheral nervous system (PNS).
What does the CNS consist of?
The CNS consists of the brain, spinal cord, optic nerve and retina, and contains the majority of neuronal cell bodies.
In the CNS, the cell bodies of neurones are often grouped together in discrete areas termed nuclei, or they may form more extensive layers or masses of cells; collectively they constitute the grey matter. Neuronal
dendrites and synaptic contacts are mostly confined to areas of grey matter and form part of its meshwork of neuronal and glial processes. In the spinal cord, cerebellum, cerebral cortices and some other areas, concentrations of tracts constitute the white matter, so called because the axons are often ensheathed
in lipid-rich sheaths of myelin, which is white when fresh
What does the PNS consist of?
The PNS includes all nervous tissue outside the CNS and consists of the cranial and spinal nerves, the peripheral autonomic nervous system (ANS) and the special senses (taste, olfaction, vision, hearing and balance). It is composed mainly of the axons of sensory and motor neurones that pass between the CNS and the body.
The PNS is composed of the efferent axons (fibres) of motor neurones situated inside the CNS, and the cell bodies of sensory neurones (grouped together as ganglia) and their afferent processes.
What does the ANS consist of?
The ANS is subdivided into sympathetic and parasympathetic components. It consists of neurones that innervate secretory glands and cardiac and smooth muscle, and is concerned primarily with control of the internal environment.
SYNAPSES
Transmission of impulses across specialized junctions (synapses) between two neurones is largely chemical and depends on the release of neurotransmitters from the presynaptic terminal. These neurotransmitters bind to cognate receptors in the postsynaptic neuronal membrane, resulting in a change of membrane conductance and leading to either a depolarization or a hyperpolarization.
Acetylcholine
Acetylcholine (ACh) is perhaps the most extensiely studied neurotransmitter of the classic type. Its precursor, choline, is synthesized in the neuronal soma and transported to the axon terminals, where it is
acetylated by the enzyme choline acetyl transferase (ChAT), and stored in clear spherical vesicles. ACh is synthesized by motor neurones and released at all their motor terminals on skeletal muscle. It is released by preganglionic fibres at synapses in parasympathetic and sympathetic ganglia, and many parasympathetic, and some sympathetic, ganglionic neurones are cholinergic. ACh is also associated with the degradatie extracellular enzyme AChE, which inactiates the transmitter by conerting it to choline. The effects of ACh on nicotinic receptors (i.e. those in which nicotine is an agonist) are rapid and excitatory. In the CNS, the nicotinic ACh
receptor mediates the effect of tobacco.
Noradrenaline and adrenaline
Noradrenaline is the chief transmitter present in sympathetic ganglionic neurones. The actions of noradrenaline depend on its site of action and vary with the type of postsynaptic receptor, e.g. it strongly inhibits neurones of
the submucosal plexus of the intestine and of the locus coeruleus is α2-adrenergic receptors, whereas it mediates depolarization, producing vasoconstriction, is β-receptors in ascular smooth muscle. Adrenaline is present in central and peripheral nerous pathways and occurs with noradrenaline in the suprarenal medulla. Both adrenaline and noradrenaline are found in dense-cored synaptic vesicles.
Nitric oxide
Nitric oxide (NO) is of considerable importance at autonomic and enteric synapses, where it mediates smooth muscle relaxation. NO is able to diffuse freely through cell membranes, and so is not under such tight quantal
control as vesicle-mediated neurotransmission.
Blood–brain barrier
Proteins circulating in the blood enter most tissues of the body except those of the brain, spinal cord and peripheral neres. This concept of a blood–brain or a blood–nere barrier applies to many substances –
some are actiely transported across the blood–brain barrier, others are actiely excluded. The blood–brain barrier is located at the capillary endothelium within the brain and is dependent on the presence of tight
junctions (occluding junctions, zonulae adherentes) between endothelial cells coupled with a relative lack of transcytotic vesicular transport. The tightness of the barrier is substantially supported by the close
apposition of astrocytes, which direct the formation of endothelial tight junctions, to blood capillaries.
There are certain areas of the adult brain where the endothelial cells are not linked by tight junctions, which means that a free exchange of molecules occurs between blood and adjacent brain. Most of these areas are situated close to the ventricles and are known as circumventricular organs; these areas make up
less than 1% of the total area of the brain. Elsewhere, unrestricted diffusion through the blood–brain barrier is only possible for substances that can cross biological membranes because of their lipophilic
character.
When does breakdown of the blood-brain barrier occur?
Breakdown of the blood–brain barrier occurs when the brain is damaged by ischaemia or infection, and is also associated with primary and metastatic cerebral tumours. Reduced blood flow to a region of the
brain alters the permeability and regulatory transport functions of the barrier locally; the increased stress on compromised endothelial cells results in leakage of fluid, ions, serum proteins and intracellular substances into the extracellular space of the brain.
How can integrity of the blood-brain barrier be evaluated?
The integrity of the barrier can be evaluated clinically using computed tomography and functional magnetic resonance imaging. Breakdown of the blood–brain barrier may be seen at postmortem in jaundiced patients who have had an infarction. Normally, the brain, spinal cord and peripheral nerves remain unstained by the bile post mortem, although the choroid plexus is often stained a deep yellow. However, areas of recent infarction (1–3
days) will also be stained by bile pigment because of the localized breakdown of the blood–brain barrier.
What does the choroid plexus produce?
The choroid plexus forms the CSF.
Age-related changes of the choroid plexus
Age-related changes occur in the choroid plexus, which can be detected by neuroimaging. Calcification of the choroid plexus can be detected by X-ray or CT scan. Visible calcification is usually restricted to the
glomus region of the choroid plexus, i.e. the vascular bulge in the choroid plexus as it curves to follow the anterior wall of the lateral ventricle into the temporal horn.
CLASSIFICATION OF PERIPHERAL NERVE FIBRES
Group A fibres are subdivided into α, β, γ and δ subgroups. Group Aα fibres are the largest and
conduct most rapidly, and C fibres are the smallest and slowest.
A fibres
The largest afferent axons (Aα fibres) innervate encapsulated cutaneous mechanoreceptors, Golgi tendon organs and muscle spindles, and some large alimentary enteroceptors. Aβ fibres form secondary endings on some muscle spindle (intrafusal) fibres and also innerate cutaneous and joint capsule mechanoreceptors. Aδ fibres innerate thermoreceptors, stretch-sensitive free endings, hair receptors and nociceptors, including those in dental pulp, skin and connectie tissue. Aγ fibres are exclusiely fusimotor to plate and trail endings on intrafusal muscle fibres.
B fibres
B fibres are myelinated autonomic preganglionic efferent fibres.
C-fibres
C fibres are unmyelinated and have thermoreceptive, nociceptive and interoceptive functions, including the perception of slow, burning pain and visceral pain.
Conduction velocity
The largest somatic efferent fibres (Aα) innervate extrafusal muscle fibres (at motor endplates) exclusively and conduct at a maximum of 120 m/s. Fibres to fast twitch muscles are larger than those to slow twitch muscle. Smaller (Aγ) fibres of gamma motor neurones, and autonomic preganglionic (B) and postganglionic (C) efferent fibres conduct, in order, progressively more slowly (40 m/s to less than 10 m/s).
SCHWANN CELLS
Schwann cells are the major glial type in the PNS. Schwann cells ensheathe peripheral axons, and myelinate those greater than 2 µm in diameter. In a mature peripheral nere, they are distributed along the axons in
longitudinal chains. Adult myelin-forming Schwann cells are characterized by the presence of seeral myelin proteins, some, but not all, of which are shared with oligodendrocytes and central myelin.
Schwann cells arise from Schwann cell precursors that, in turn, are generated from multipotent cells of the neural crest. Axon-associated signals appear to control the proliferation of developing Schwann cells and their precursors; the developmentally programmed death of those precursors in order to match numbers of axons and glia within each peripheral nere bundle; the production of basal laminae by Schwann cells. Few Schwann cells persist in chronically denerated nerves.
BLOOD SUPPLY OF PERIPHERAL NERVES
The blood vessels supplying a nerve, end in a capillary plexus that pierces the perineurium.
Blood–nerve barrier
Just as the neuropil within the CNS is protected by a blood–brain barrier, the endoneurial contents of peripheral nere fibres are protected by a blood–nere barrier and by the cells of the perineurium. The blood–nerve barrier operates at the level of the endoneurial capillary walls, where the endothelial cells are joined by tight junctions, and are non-fenestrated and surrounded by continuous basal laminae. The barrier is much less efficient in dorsal root ganglia and autonomic ganglia and in the distal parts of peripheral nerves.
FREE NERVE ENDINGS
Sensory endings that branch to form plexuses occur in many sites. They occur in all connective tissues, including those of the dermis, fasciae, capsules of organs, ligaments, tendons, adventitia of blood vessels, meninges, articular capsules, periosteum, perichondrium, Haversian systems in bone, parietal peritoneum, walls of viscera and the endomysium of all types of muscle. They also innervate the epithelium of the skin, cornea, buccal cavity, and the alimentary and respiratory tracts and their associated glands. Afferent fibres from free terminals may be
myelinated or unmyelinated but are always of small diameter and low conduction velocity. When afferent axons are myelinated, their terminal arborizations lack a myelin sheath.
Similar fibres in deeper tissues may also signal extreme conditions, which are experienced, as with all nociceptors, as ache or pain. Free endings in the cornea, dentine and periosteum may be exclusiely nociceptive.
Meissner’s corpuscles
Meissner’s corpuscles are found in the dermal papillae of all parts of the hand and foot, the anterior aspect of the forearm, the lips, palpebral conjunctia and mucous membrane of the apical part of the tongue.
They are most concentrated in thick hairless skin, especially of the finger pads.
