Transcript Central Nervous System (CNS)

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 Neuron
a)
b)
c)
d)
e)
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
Axon – leads away from cyton
Dendrite – brings stimuli into cyton
myelin sheath – lipoprotein (insulating)
Neuroglia – helper cells
1.
2.
•
Schwaan cells – ~1 mm long - make myelin in PNS
Oligodendrocytes – make myelin in CNS
Recall
–
–
grey matter = unmyelinated cells
white matter = myelinated cells
Parts of a Neuron
f)
g)
Nodes of Ranvier – speed up impulse
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)
Impulse
direction
Axon
myelin sheath
Impulse
direction
Synapse (gap)
Node of Ranvier
Schwann cell
(one internode)
Axon terminals
Dendrites
(new cell)
Types of Nerves
Sensory 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
Membrane potential (mV))
Figure 11.13: Propagation of an action potential (AP), p. 405.
Voltage
at 2 ms
+30
Voltage
at 0 ms
Voltage
at 4 ms
–70
(a) Time = 0 ms
Resting potential
Peak of action potential
Hyperpolarization
(b) Time = 2 ms
(c) Time = 4 ms
Saltatory Conduction
Protective Coverings
Axon
Blood vessels
Perineurium
Myelin sheath
Endoneurium
Perineurium
Epineurium
Fascicle
Fascicle
Blood
vessels
Nerve fibers
Endoneurium
page 498
(b)
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.
Neurotransmitter
Na+
Presynaptic
membrane
1
Mitochondria
Postsynaptic
membrane
Ion channel open
Synaptic vesicles
5
Axon terminal
Degraded
neurotransmitter
2
Synaptic
cleft
3
Dendrite
4
Neurotransmitter
molecules
Ion channel closed
Na+
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)
Subdural space
Pia
Arachnoid
Dura
Spinal meninges
Subarachnoid
space
Dorsal root
ganglion
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.
Lateral
ventricle
Lateral
ventricle
Third
ventricle
Cerebral
aqueduct
(Sylvius)
Anterior
horn
Third
ventricle
Interventricular
foramen
(Munroe)
Posterior
horn
Cerebral
aqueduct
Inferior
horn
Fourth
ventricle
Central
canal
Central
canal
(a) Anterior view
(b) Left lateral view
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.
Spinal cord (in cross section)
Stimulus
2 Sensory neuron
1 Receptor
4 Motor neuron
Skin
5 Effector
3 Integration center
Interneuron
The Knee Jerk
AKA The Patellar Reflex
1. 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.
Cervical
nerves
C1– C8
Thoracic
nerves
T1– T12
Lumbar
nerves
L1– L5
Sacral
nerves
S1– S5
Coccygeal
nerve
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.
Cervical plexus
Brachial plexus
Cervical
nerves
C1– C8
Cervical
enlargement
Intercostal
nerves
Thoracic
nerves
T1– T12
Lumbar
enlargement
Lumbar plexus
Sacral plexus
Cauda equina
Lumbar
nerves
L1– L5
Sacral
nerves
S1– S5
Coccygeal
nerve
C0
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