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