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Summary BHCS1002 - Human Anatomy/CNS
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1121. Nervous system – general data. General organization of the nervous system.
I. Nerve cells
• Variety of nerve cells - the cell body is where the neurotransmitters are generated - transported to the terminal part of the
nerve with protein carriers
• Peripheral nerves include those that supply skin, muscles, and joints of the body wall and limbs, and those that supply
visceral structures, e.g., heart, lungs, stomach, etc.
• Each of these sets of peripheral nerves is intimately associated with the brain and spinal cord.
• Cell body is where neurotransmitters are generated and they are transported to terminal part of nerve with protein
carriers. Neurons consist of a central axon and myelin for insulation.
• Myelin - nerves of the CNS - formed of oligodendrocytes
• Schwann cells - nerves of the PNS
• All nerve fibers are myelinated except Group C [pain fibers]
• Sensory neurons:
• specialized to signal sensory stimuli
• connected to peripheral receptor - transmit to interneurons that lie within the spinal cord
• These signal to motor neurons - connect to muscle and leave via the ventral horn of the spinal cord
A. Neuroglia
• Function:
• Provide neurones structural support • Regulation of synapses
• Production of myelin • Create blood-brain barrier
• Remove debris –scavenger • Produce cytokines –defence
• Acts as buffer and maintains K+ ion • Guide the migration of neurons and direct
concentration outgrowth of axons during development
• Types
• I - IV found in CNS; V - VI: found in PNS
• Astrocytes
‣ most numerous ‣ egulate ion, nutrient, and dissolved gas
‣ Maintains blood-brain barrier concentrations
‣ Provides structural support R ‣ Absorb and recycle NT
• Microglia
‣ Specialised immune cells
‣ acts as macrophages in CNS {remove cell debris, wastes and pathogens}
• Oligodendrocytes
‣ Myelinate CNS axons and provides structural framework
• Ependymal cells
‣ Line ventricles (brain) and central canal (spinal cord)
‣ Assist in producing, circulating and monitoring CSF
• Satellite cells
‣ Surround neuron cell bodies in ganglia
‣ Regulate CO2, O2, nutrient and NT levels around neurons in ganglia
• Schwann cells
‣ Surround axons in PNS –responsible for myelination of peripheral axons
‣ Participate in repair process after injury
II. Saltatory conduction
• Stimulut is generated from the receptors [temp., pressure]
‣ cause sodium influx -> cause a local depolarization
‣ Once the membrane potential rises above -45 mV, the sodium voltage gated channels open -> sharp influx
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‣ Stopped when the membrane potential reaches +40 mV
‣ The signal is spread and jumps along the nerve.
‣ There is more sodium influx at the Nodes of Ranvier and the word saltatory relates to the ‘leaping’ of the sodium ions
along the myelinated segments of nerve cell - speeds up transmission.
III. Synapses
• Na ions arrive to the terminal part of the nerve -> depolarization -> triggers a calcium influx
‣ causes pre-stored vesicles containing acetylcholine (in somatic nerves, interneurons and motor end plates) or
noradrenaline (in postsynaptic sympathetic nerves) to fuse with the presynaptic membrane and be released into the
synaptic cleft.
• The neurotransmitter now binds with postsynaptic receptors, causing depolarization, and the signal is sent on.
IV. Neuromuscular junction
• juction between the motor neuron and skeletal muscle
• have the same basic structure as synapses but send signal to a motor end plate and muscle fibers
‣ via: T Tubules and specially adapted receptors
‣ The net result of which is calcium release from the sarcoplasmic reticulum to promote contraction
V. Central Nervous system
1. Brain
• Master organ of the CNS
• Icoordinates the functioning of the muscles and limbs, the hormones we release to adapt, grow and change with the
environment.
• CNS integrates information from PNS, information is processed to generate behavior which is communicated back to
PNS - Spinal cord connects directly with PNS
has several divisions, lobes:
•
A. Frontal lobe - inhibition of impulsive behaviors; contain the pre-central gyrus [primary motor cortes], the
Broca’s area [ form words and body language]
B. Temporal lobe - contain transverse temporal gyri [auditory information], left side [understand words
and comprehend information]
C. Partietal lobe - [visual interpretation, eye movement] contain post central gyrus [primary sensory strip;
Wernicke’s area
D. Occipital lobe - [primary visual cortex]
2. Brain stem
• attached to the inferior aspect of the brain
• consist of the Midbrain, Pons and Medulla oblongata
• lies within the cranial cavity, against the clivus on the inferior aspect - it continuous with the spinal cord
3. Cerebellum
• balance and coordination
• gives smoothness to the movements
• reprograms itself with a feed forward system according to the stimuli it faces
4. Spinal cord
• lies within the ventral canal
• lies deep to the three layers of the meninges - has 31 pairs of spinal nerves
• Nerves exit via the intervertebral foramina - merge to form plexi and go on to innervate different muscles
VI. Peripheral nervous system
• consist of peripheral nerves and ganglia associated with them
1. Spinal nerves
• 31 pairs of spinal nerves
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• consist of:
• an incoming sensory components - enters at the dorsal horn
• outgoing motor component - leaves via the ventral horn
• Both components are contained within the spinal nerve along with the autonomic signals
• After the spinal nerves leave the intervertebral foramen -> form anterior and posterior rami
• Anterior rami - supplies the limbs and trunks
• Posterior rami - back muscle
2. The cranial nerves
• originate from the brainstem and brain
1. Olfactory - sense of smell
2. Optic - sense of sight
3. Oculomotor - moves the eye, eyelid, constricts the pupil
4. Trochlear - moves the eye down and out, innervated superior oblique
5. Trigeminal - [V1 - ophthalmic, V2-maxillary, V3 Mandibular] sensation to face and innervated chewing
muscles
6. Abducens - moves the eye laterally, innervate the lateral rectus
7. Fascial - moves the face, sense of taste of anterior 2/3 of tongue,
8. Vestibulocochlear - hearing and balance
9. Glossopharyngeal - taste of posterior 1/3 tongue, sensation of pharynx
10. Vagus - parasympathetic to whole body down to splenic flexure, motor part of cough reflex
11. Accessory - innervated the sternocleidomastoid and trapezius muscle
12. Hypoglossal - innervates all tongue muscles, except palatoglossus
VII. Somatic nervous system
• Def.: relating to the body
• Function: nerves supply the arms, legs, neck muscle and trunk
‣ carries sensory and motor information
• Part of the PNS - cranial and spinal nerves contribute to the SNS
• The ventral rami of the spinal nerves (except T2-T12) coalesce and form plexi, resulting in final nerves that go to innervate
muscles and provide sensation
• Plexuses
1. Cervical plexus
2. Brachial plexus
3. Lumbar plexus
4. Scaral plexus
VIII. Autonomic nervous system
• composed of sympathetic and parasympathetic nervous system
• causes fight and fright reactions
• is an involuntary system that controls and regulates the functions primarily of visceral organs e.g. blood pressure,
respiration, sweating
1. Gray and white rami communicantes
• course of communication between the different NS
A. White rami communicantes [preganglionic sympathetic neurons]
‣ short myelinated sections of nerves - connect the spinal nerve to the sympathetic paravertebral ganglion
‣ Enter the sympathetic trunk - terminates -> pass downwards/upward
‣ synapse with the cell bodies of postganglionic sympathetic neurons - located in the sympathetic ganglia
‣ synapse with the grey rami communicant -> runs with the spinal nerve to the peripheral target
B. Grey rami communicantes
‣ white rami communicantes synapse with the grey rami communicant
‣ runs with the spinal nerve to the peripheral target
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2. Sympathetic nervous systme
• emotion - increases its effect while flight or fight response
• Effects:
‣ increased arterial pressure, ‣ increased metabolic rate,
‣ increased blood flow to active muscles- ‣ increased blood glucose concentration
vasodilation, ‣ increased mental activity and alertness.
• SNS operates continuously to regulate the functions of many organ systems such as heart, blood vessels, gastrointestinal
tract, bronchi, and sweat gland
• Preganglionic neurons arise from nuclei in the thoracic and lumbar spinal cord segments, specifically from T1–L3
• collection of sympatheitc nerve fibers - many coalesce around the major branch of abdominal aorta
• incl.: coeliac, superior mesenteric, inferior mesenteric plexus
‣ These plexi follow the course of the arteries and provide sympathetic innervation to the same areas
3. Parasympathetic nervous system
• rest and digest function - slows down the heart, promotes peristalsis, increase bowel contractions
• Maintains homeostasis and bodily functions such as digestion and blood flow
• CN 3, 7, 9, 10 provide parasympathetic innervations
i. Oculomotor - constrictor pupillae
ii. Fascial - lacrimal and salivary glands
iii. Glossopharyngeal - parotid salivary gland
iv. Vagus - parasympathetic innervation all the way down to the splenic flexure of the large bowel
IX. Clinical Notes:
1. Cranial nerve palsies
• The 12 cranial nerves all leave/enter the skull through various foramina.
• Narrowing of these foramina or any constriction along the nerves course results in nerve palsy.
• E.g.: Bell’s palsy affects the facial nerve:
• hemiplegia
• dry eyesan absent corneal reflex,
• overloud hearing
• affected taste in the anterior 2/3 of the tongue
2. Limb nerve lesions
• Result from fracture, constriction or overuse
• E.g.: carpal tunnel syndrome
• affects the median nerve, and occurs when the nerve is compressed within the tunnel
• due to enlargement of the flexor tendons within the tunnel or swelling due to oedema
• often occurs in pregnancy and acromegaly
3. Hirschsprung’s disease
• colonic atony secondary to a failure of the ganglion cells (described in the enteric nervous system section) to migrate into
the enteric nervous system
• results in a severely constipated and malnourished child, which is in desperate need of corrective surgery
4. Spina bifida
• Failure of normal development of the meninges and/or vertebral neural arch results in a defect usually in the lumbar spine,
where part of the spinal cord is covered only by meninges and therefore sits outside the body.
• Both environmental and genetic factors contribute to its cause.
• Folate supplements are now given to all pregnant mothers in early pregnancy for its prevention
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