central nervous system (cns) bio 161 chapter 11 & 12 – part 1
TRANSCRIPT
Central Nervous System (CNS)
BIO 161 Chapter 11 & 12 – part 1
Review –Nervous Tissue
• Two types of tissues
– Neurons – excitable cells that transmit electrical signals
– Neuroglia - supporting cells – cells that surround and wrap neurons
Parts of a Neurona) Cyton (cell body)
– sometimes form ganglia – a group of cell bodies outside the CNS
– eg. sensory nerves have cell bodies located in the dorsal root ganglion
b) Axon – leads away from cytonc) Dendrite – brings stimuli into cytond) myelin sheath – lipoprotein (insulating)e) Neuroglia – helper cells
1. Schwaan cells – ~1 mm long - make myelin in PNS2. Oligodendrocytes – make myelin in CNS
• Recall– grey matter = unmyelinated cells– white matter = myelinated cells
Parts of a Neuronf) Nodes of Ranvier – speed up impulseg) Axon terminals – store neurotransmitter in
synaptic vessicles to release into synapse communicate impulse to next cell across the synapse.
h) Synapse – space between neurons– where neurotransmitter goes from one axon
to the next, or to a muscle, organ, or gland
Dendrites Cell body (Cyton)
Axon
Axon terminals
Node of Ranvier
Impulsedirection
Schwann cell(one internode)
myelin sheath Synapse (gap)
Dendrites (new cell)
Impulsedirection
Types of NervesSensory receptors are often unipolar• one short process from cell body, splits T-
like in two directions• distal process is the receptor end• proximal process goes into CNS• cell bodies usually in ganglion
Some sensory neurons are bipolar• cell body is central, one process from each
end of cell body• dendrite one way, axon the other• carry info from body to CNS – eg olfactory,
retina (RARE)
Motor neurons are often multipolar• multiple processes from cell body• carry info from CNS to body – eg Muscle
contraction (99% of all)
Resting Potential• Flux of Na+ and K+
is the action potential
• Normally, Na+ is pumped out of the cell, K+ is pumped in
• More Na+ out than K+ in inside is negative compared to outside
• the result -70 mV
..Nerve Conduction Theory
• The action potential– electrical event causes change in
ion gates– Na+ rushes in, making cell more
positive inside – disturbs adjacent area of cell to do
the same– disturbance moves along the axon
conduction
Figure 11.13: Propagation of an action potential (AP), p. 405.
–70
+30
(a) Time = 0 ms (b) Time = 2 ms (c) Time = 4 ms
Voltageat 2 ms
Voltageat 4 ms
Voltageat 0 ms
Resting potential
Peak of action potential
Hyperpolarization
Me
mb
ran
e p
ote
nti
al
(mV
))
Saltatory Conduction
Protective Coverings
(b)
Fascicle
Perineurium Blood vessels
Endoneurium Nerve fibers
Axon
Endoneurium
Perineurium
Epineurium
Myelin sheath
Bloodvessels
Fascicle
page 498
Synapse
• Connection between– nerve nerve (axon dendrite)– nerve muscle
• axon ends in terminal• space = “synapse”• chemicals cross synapse =
neurotransmitters– eg. acetylcholine (ACh),
norepinephrine (NE) aka adrenaline
Figure 11.18: Events at a chemical synapse in response to depolarization, p. 410.
Synaptic vesicles
Axon terminal
Synapticcleft
Presynaptic membrane
Postsynapticmembrane
Mitochondria
Ion channel closed
Ion channel open
Neurotransmitter
Degradedneurotransmitter
Na+
Na+
Action Potential
1
2
34
5
Neurotransmitter moleculesDendrite
Types of reflexes
• Monosynaptic– 2 neurons
•sensory neuron motor neuron•eg. knee jerk
• Polysynaptic– multiple neurons
•sensory neuron interneuron brain motor neuron
•eg. touching a hot stove (withdrawal)
Subdivisions of the Nervous System
• Central Nervous System - CNS– brain and spinal cord
• Peripheral Nervous System - PNS– everything outside the CNS– 12 pairs of cranial nerves– 31 pairs of spinal nerves– includes 2 divisions
• sensory – afferent (TO the CNS)• motor – efferent (AWAY FROM the CNS)
CNS- Spinal cord
– adult to ~L2– infant to ~L3 or L4
• conus medullaris– end of spinal cord
• filum terminale– end of pia
• cauda equina = “horse’s tail”– last spinal nerves
• 2 enlargements– cervical arms– lumbar legs
CNS - Brain
• 2% of body weight yet 20% of blood flow
• 3 main parts– forebrain
• cerebrum• diencephalon
– thalamus, hypothalamus, retina
– midbrain• midbrain
– hindbrain• pons• cerebellum• medulla oblongata
The Forebrain
• Cerebral cortex– 80%+ of brain mass– voluntary and conscious– memory, intelligence– 2 hemispheres
• left – usually dominant (90%)– problem solving, speech, logical
integration
• right– motor, intuition, creativity, emotion
The Forebrain• Sulcus
– valley
• Fissure– deep
valley
• Gyrus– bump
The Forebrain
• Cerebral lobes– Temporal
•hearing, memory of visual/auditory patterns,
– Occipital•vision
– Frontal•higher intelligence, motor action
– Parietal•Sensory – pain, temperature, touch,
pressure
The Forebrain
• Diencephalon• thalamus
– relays spinal info correct area of cortex
•hypothalamus– makes antidiuretic hormone– monitors hunger– controls temperature– monitors thirst– regulates sleep
•pituitary– anterior– posterior
•pineal gland
CNS
• Surrounded by – meninges - 3 layers
•dura mater– outermost – very tough
•arachnoid– diffuse fibres, spider web– space beneath is subarachnoid space – CSF
and blood vessels
•pia– very light film – “soft” – collagen, elastin,
reticulin– secretes CSF into subarachnoid space (rich
vascularization)
– what is meningitis?
Figure 12.31a: Anatomy of the spinal cord, p. 473.
Epidural space(contains fat)
PiaSpinal meningesArachnoid
DuraSubdural space
Subarachnoidspace
Dorsal rootganglion
CSF• derived from blood (choroid plexuses)
• ~150 ml in CNS, replaced ~ 3x/day 450-500 ml/day
• choroid plexuses in ventricles – capillary clusters
– functions• buoys brain (would crush under its own weight)• shock absorbance• nutritive
– locations• ventricles• subarachnoid space• central canal (in spine)
– returns to blood in subarachnoid space
Figure 12.5: Ventricles of the brain, p. 434.
(b)(a) Left lateral viewAnterior view
Lateralventricle
Posteriorhorn
Thirdventricle
Cerebralaqueduct
Anteriorhorn
Inter-ventricularforamen (Munroe)
Inferiorhorn
Centralcanal
Lateralventricle
Thirdventricle
Cerebralaqueduct(Sylvius)
Fourthventricle
Centralcanal
Lateralventricle
Posteriorhorn
Thirdventricle
Cerebralaqueduct
Anteriorhorn
Inter-ventricularforamen (Munroe)
Inferiorhorn
Lateralventricle
Thirdventricle
Cerebralaqueduct(Sylvius)
Fourthventricle
Peripheral Nervous System (PNS)
• Reflex arcs are commonly used– to test level of damage/disease in
spinal cord•eg. biceps reflex points to C5
Reflexes and Nerves p. 521
• Spinal reflexes– do NOT go to the brain – afferent goes to spine (“at”)– efferent comes from spine (“effect”)
• The spinal reflex is a 5 step process– stimulus– afferent signal to spine– integration at spine– efferent signal to effector– ‘effect’
Figure 13.14: The basic components of all human reflex arcs, p. 521.
Stimulus
Receptor
Skin
Sensory neuron
Spinal cord (in cross section)
Integration center
InterneuronMotor neuron
Effector
1
5
4
2 3
The Knee Jerk
AKA The Patellar Reflex1. tendon is struck
• rectus femoris is stretched• stretch receptor is stimulated
2. signal travels on afferent (sensory) neuron
• from receptor to spine• travels along fibres in femoral nerve
3. synapse with effector (motor) neuron
• in spine
The Knee Jerk
4. signal travels on efferent (motor) neuron
• from spine to receptor • travels along fibres in femoral
nerve
5. effect – rectus femoris contracts
• stops stretch that was initial stimulus
• Tests spine at level of L4-L6
Other spinal reflexes
• Biceps – C5, C6• Brachioradialis – C6• Triceps – C7• Knee jerk (patellar tendon) – L4
– L6• Ankle jerk (Achilles tendon) – S1
• Abdominal – light scratch in 4 quadrants – umbilicus should move
Other spinal reflexes
• Abdominal– light scratch in 4 quadrants – lateral to medial– umbilicus should move– Upper Quadrants – T7-T9– Lower Quadrants – T11, T12
• Plantar– toes should flex– Babinski sign – extension of toes
• Pupil response– shine light into eyes briefly– pupil should contract – CN III
(Occulomotor)
Spinal Nerves
• 31 pairs– supply body (not head)
– 8 cervical (C1-C8) – how come 8?
– 12 thoracic (T1-T12)
– 5 Lumbar (L1-L5)
– 5 Sacral (S1-S5)
– 1 Coccygeal (C0)
Figure 13.6: Distribution of spinal nerves, p. 509.
CervicalnervesC1– C8
ThoracicnervesT1– T12
LumbarnervesL1– L5
SacralnervesS1– S5
Coccygealnerve
C0
Spinal Nerve Anatomy
• Each spinal nerve is mixed fibres– at cord
• ventral root is motor fibres• dorsal root is sensory fibres
– come together to exit vertebral foramen as spinal nerve (mixed fibres)
– as it exits foramen, the nerve splits into– Ventral ramus - to limbs and ventrum– Dorsal ramus – to dorsum– NOTE: both rami are mixed fibres
Distribution of the Ventral Rami
• Ventral rami for plexuses (blending of fibres from several spinal nerves)– Note: ONLY IN VENTRAL RAMI
• Does not happen for Thoracic Spinal Nerves
• Fibres from Cervical, Lumbar, Sacral and Coccygeal mix and blend– fibres from several spinal nerves go to
one area– fibres from one spinal nerve arrive via
different paths
Figure 13.6: Distribution of spinal nerves, p. 509.
CervicalnervesC1– C8
ThoracicnervesT1– T12
LumbarnervesL1– L5
SacralnervesS1– S5
Coccygealnerve
C0
Cervical plexus
Intercostalnerves
Cervicalenlargement
Lumbarenlargement
Cauda equina
Brachial plexus
Lumbar plexus
Sacral plexus
Plexuses
• Cervical– C1-C4
• mostly to neck muscles• incl. phrenic nerve – diaphragm (mostly C4
fibres)
• Brachial– C5-T1
• to arm
• Lumbar– L1 – L4
• legs
• Sacral– L4-S4
• legs
Thoracic Rami
• Each Thoracic ramus, except T1, innervates muscles in a band at the same level of the spine– run between ribs T1-T11 are
intercostal nerves– only T12 is different (below the 12th
rib)•called subcostal nerve