Topic 19 -Nervous System Histology and protective features Flashcards
2 divisions of nervous system
Central Nervous system
Peripheral Nervous System
Central Nervous System
- command centre
- brain + spinal cord
- processes + integrates information
Peripheral Nervous System
- consists of:
a) cranial nerves to/ from brain
b) spinal nerves to/ from spinal cord
2 divisions of peripheral nervous system
a) sensory/afferent division
- has sensory receptors that detect stimuli (changes in the internal or external environments
- nerves convey impulses toward CNS
b) motor/efferent division
- nerves convey impulses away from CNS
- innervates (supplies nerves to) effectors = muscles + glands (endocrine or exocrine)
relationship between CNS and PNS
Receptors detect stimuli -> PNS afferent (sensory) neurons -> CNS integrates -> PNS efferent (motor) neurons -> effector muscle or gland
Nervous system organization
stimulus -> sensory (PNS) -> CNS -> motor (PNS) -> somatic (skeletal muscle) OR autonomic (smooth and cardiac muscle, glands) -> sympathetic OR parasympathetic
Cell types of nervous system
Neurons
Neuroglia
Neurons
- conduct impulses
- make up CNS + PNS
- mostly amitotic (irreplaceable)
exceptions = taste, olfaction, memory
Neuron structure
a) cell body
- typical organelles
- RER called Nissl bodies
- groups/clusters in CNS = nuclei (gray matter)
- groups/clusters in PNS = ganglia
b) processes from cell body:
i) dendrites
- receive incoming messages + relay to cell body
ii) axon
- carries impulses away from cell body - axon hillock - where axon meets cell body
- axon terminal → typically branched with synaptic end bulbs (enlarged tips)
- may be:
① myelinated – wrapped in many layers of cell membrane from Schwann cells (PNS) or oligodendrocytes (CNS) - electrical insulation - gaps in myelin sheath are called nodes of Ranvier - myelinated axon bundles in: - CNS = tracts (white matter) - PNS = nerves
② unmyelinated - no myelin
Neuroglia (glial cells)
types
- support neurons = can undergo mitosis (prone to cancer – brain tumour)
CNS neuroglia
PNS neuroglia
CNS neuroglia
i) oligodendrocytes - produce myelin around axon
ii) microglia - protective ⇒ become phagocytic if detect infected, dead or damaged neurons (because immune cells can’t enter CNS)
iii) astrocytes
- surround blood capillaries to form part of blood brain barrier (BBB)
- help control capillary permeability
iv) ependymal cells - neural epithelium - line brain ventricles + central canal of spinal cord
- secrete cerebrospinal fluid (CSF) + circulate it (cilia)
PNS neuroglia
i) Schwann cells - form myelin around axons in PNS
ii) Satellite cells - surround neuron cells bodies in ganglia
- protection and support
Neuron types based on structural/anatomical
- based on number of processes coming off cell body
a) unipolar - 1 process that divides into two: central + peripheral
- peripheral end has dendrites = sensory receptors (pain, touch etc)
- remainder is axon
- always sensory
b) bipolar - 2 processes: 1 axon, 1 process with dendrites
- sensory ⇒ retina, nose (olfaction
c) multipolar - 3 or more processes: 1 axon, many dendrites
- all interneurons + motor neurons
Functional types of neurons
- based on direction of impulse conduction
a) Sensory/Afferent neurons - mostly unipolar
- from sensory receptors to CNS
b) Interneurons - within CNS (between sensory + motor)
- 99% of neurons (mostly multipolar)
c) Motor/Efferent neurons - CNS to effectors (all multipolar)
Neuronal junctions
1) Neuronal junction:
- neuron to neuron
- can be chemical (use neurotransmitters) or electrical (ions)
2) Neuromuscular junction:
- motor neuron to skeletal muscle cell
3) Neuroglandular junction:
- motor neuron to smooth or cardiac muscle, or a gland
Chemical Neuronal Synapses:
1) Presynaptic Neuron
- neuron bringing impulse
- has axon terminal with synaptic end bulbs
- inside end bulb = synaptic vesicles containing neurotransmitter (nt)
- presynaptic membrane (cell membrane facing space)
2) Synaptic Cleft
- space between neurons
3) Postsynaptic Neuron
- receives the signal
- has postsynaptic membrane = cell membrane of dendrites or cell body with receptors for nt
Central Nervous system Protective features
Bone meninges Cerebrospinal fluid Blood-brain barrier Cerebral arterial circle
bone
skull and vertebral column
Meninges
- CT around brain + spinal cord
- layers:
dura mater
arachnoid matter
Pia mater
Meninges: dura mater
- brain = 2 fused layers
- separated in some places to form spaces = venous (dural)
sinuses - contain blood - spinal cord - 1 layer
- deep to dura mater = subdural space
- filled with ISF
- superficial to dura mater = epidural space (spinal cord only)
- filled with fat, blood vessels, CT etc
Meninges: arachnoid mater
- avascular
- subarachnoid space
- contains cerebrospinal fluid (CSF)
- web-like strands of CT secure it to pia mater (below)
- has arachnoid granulations (villi) (brain only)
- project into dural sinuses
- CSF enters granulations (villi) to return to blood
Meninges: pia mater (inner)
- on surface of CNS (brain and spinal cord)
- vascular
meningitis
inflammation of meninges
Cerebrospinal fluid
Where is it
Formed by?
Produced by?
- surrounds brain + spinal cord
- in brain ventricles + central canal of spinal cord
- cushions CNS – brain buoyant
- formed from blood plasma (similar composition)
- produced by choroid plexuses (blood capillaries) found in each ventricle
ventricles of brain
- spaces inside brain (filled with CSF)
- 2 lateral ventricles (1st and 2nd) – in cerebrum
- 3rd ventricle - in diencephalon
- 4th ventricle - surrounded by pons, medulla oblongata, cerebellum
Central canal
space in spinal cord`
Circulation of CSF
Choroid Plexuses -> Lateral ventricles 1st and 2nd OR 3rd ventricle OR 4th ventricle
lateral ventricles 1st and 2nd -> interventricular foramina -> 3rd ventricle -> cerebral aqueduct -> 4th ventricle -> median (1) and lateral (2) aperatures -> subarachnoid space (of brain and spinal cord) -> arachnoid villi -> dural sinus (venous blood) -> internal jugular veins -> heart -> arterial blood -> choroid plexus
Blood brain barrier
Where is it not in brain?
- 2 cell types:
a) endothelial cells (of capillaries) with tight junctions
b) astrocytes - foot processes wrap around endothelial cells - selectively permeable
- allows: glucose, fat soluble material to cross - not toxins, antibiotics etc
- in most parts of brain except:
- posterior pituitary, hypothalamus, vomiting centre
Cerebral arterial circle
- cerebral arteries form circle at base of forebrain
- circles pituitary gland and optic chiasma (where optic nerves cross)
- unites the 2 major blood supplies to the brain (anterior and posterior)
- provides an alternate route for blood if vessels blocked
Blood flow to brain posteriorly means in circle
vertebral arteries -> basilar arteries -> posterior cerebral arteries
Blood flow to brain anteriorly means in circle
internal carotid arteries -> middle cerebral arteries OR posterior communicating artery OR anterior cerebral arteries -> anterior communicating arteries
Blood return from brain
Cerebral arteries -> capillaries -> cerebral veins -> dural sinuses -> superior vena cava
