Nerve Fibre Structure: Axon, Myelin Sheath &
Neurilemma
● Nerve fibre = centrally located axon + surrounding supporting cell (Schwann cell)
● Axon:
○ Major extension of the nerve cell’s membrane and axoplasm
○ From Greek axis (“axle/axis”)
● Neuron = nerve cell (Latin tendon = stretched out)
● Schwann cell:
○ Forms neurilemma (Greek: nerve + husk)
● Myelin sheath:
○ Fatty layer from Schwann cell
○ Internal to neurilemma > Outermost nucleated layer of schwaan cells
○ Increases conduction speed by saltatory conduction (impulses jump between
nodes of Ranvier)
● Conduction velocity:
○ Proportional to fibre diameter
○ Large myelinated fibres (e.g., motor to skeletal muscle) = fastest
○ Small non-myelinated fibres (e.g., visceral pain fibres) = slowest but
space-saving
Connective Tissue Layers of a Nerve: Endoneurium,
Perineurium, Epineurium
entire nerve
● Endoneurium:
○ Surrounds individual nerve fibres Bundle of nerves
○ Greek: “within nerve”
fascicle
● Perineurium:
○ Surrounds bundles (fascicles) of fibres
○ Greek: “around nerve”
● Epineurium:
○ Outermost sheath, surrounds entire nerve Fats
·
Individual
○ Greek: “upon nerve”
nerve
● Composed mostly of collagen fibres
● Fat may be present between epineurium and perineurium
● Provides protection at compression/stretch points (e.g., median nerve in carpal tunnel)
,Synapses
● Synapse = junction between axon of one neuron and another cell (Greek: touch)
● A neuron has only one axon, but receives many synapses
● Synapses occur:
○ On cell body
○ On dendrites (Greek: trees)
● Synaptic cleft:
○ Gap between pre- and post-synaptic membranes
○ Neurotransmitter released from presynaptic terminal crosses cleft and affects
excitability of post-synaptic membrane
inhibitory or excitatory
Sensory vs Motor Fibres > no dendrites
> Receptor
● Sensory fibres (afferent): carry impulses to CNS
heive
● Motor fibres (efferent): carry impulses from CNS
- in CNS
beripheral
● Classification: < near CNS
○ Most peripheral nerves are mixed
○ Sensory neuron: Congsingle axon from cell bod o
■ Has no dendrites ora
Merve
■ Has a short single axon with:
■ Proximal extension → synapses in CNS D
■ Distal extension → travels in peripheral nerve
■ Cell body in sensory ganglion, near CNS
○ Motor neuron:
■ Has single long axon from its cell body
■ Axon travels in peripheral nerve
■ Cell body & dendrites are in CNS
Receptors
● Receptor = origin of a sensory nerve fibre
○ Located at distal axon terminal
● Types:
○ Exteroceptors:
■ In skin and sense organs
■ Include:
■ Mechanoreceptors (touch, pressure)
■ Thermoreceptors (hot, cold)
■ Nociceptors (superficial pain)
○ Proprioceptors:
■ In muscles, joints, bones
■ Include:
, ■ Mechanoreceptors (stretch, joint position, vibration)
■ Nociceptors (deep pain)
○ Interoceptors:
■ In internal organs
■ Include:
■ Baroreceptors (BP)
■ Chemoreceptors (O2 tension)
■ Nociceptors (visceral pain)
Effector & Neuro-effector Junction
● Effector = target at motor axon termination
○ Can be muscle or gland > movement
or hor mone release
● Not continuous with neuron
● Neuro-effector junction:
○ Has a gap (junctional cleft)
○ Region exposed by local loss of myelin, neurilemma, endoneurium
● Neuromuscular junction (skeletal muscle):
○ Electrical impulse causes neurotransmitter release
○ Transmitter diffuses → excites motor end plate
Somatic & Visceral Fibre Types
● 4 major functional fibre groups:
○ Somatic afferent to CNS
○ Visceral afferent
○ Somatic efferent from CNS
○ Visceral efferent
● Somatic afferents:
○ Superficial (skin): touch, temp, sharp pain
○ Deep (muscles, joints): proprioception, vibration, dull pain
● Visceral afferents:
○ Non-myelinated, slow
○ Sense visceral stretch, colicky pain
● Somatic efferents:
○ To skeletal muscle
○ Large, fast
● Visceral efferents:
○ To smooth/cardiac muscle, glands
○ Small, slow
, Sympathetics & Parasympathetics
● May have:
○ Complementary actions (e.g. erection vs ejaculation)
○ Opposite actions (e.g. HR, pupil size)
○ Independent actions (some organs get only sympathetic input)
● Sympathetic targets: = fight or flight s more blood +O 2 to working muscles to minimize excess energy loss
○ Sphincters of viscera
○ Blood vessels (vasoconstriction)
○ Dilator pupillae (eye)
○ Ventricles & atria (↑ stroke volume, HR)
○ Suprarenal medulla → adrenaline release
○ Ejaculation pathway
● Parasympathetic targets: = calm
State where body functions
occur
○ Smooth muscle walls of viscera
○ Exocrine glands
○ Sphincter pupillae (eye)
○ Atria only (↓ basal HR)
● Both systems are not activated simultaneously
Thoracolumbar Outflow in Alarm
> fight or flight
● Sympathetic fibres emerge from T1–L2 spinal cord segments
● Travel via sympathetic trunk (skull base to coccyx)
● Activated in alarm/fight-flight responses
○ Diverts blood to skeletal muscle
○ Dilates pupils
○ Facilitates ventilation (bronchioles)
○ Stimulates GI/urinary sphincters
○ Triggers adrenaline release (suprarenal medulla)
Nerve Fibres & Reflex Arcs
● Nerve fibre abilities:
○ Excitability: change in polarity
○ Conductibility: wave propagation
● Nerve = fibres (ectoderm) + connective tissue (mesoderm)
● CNS: brain + spinal cord
● PNS: peripheral nerves + ganglia (from cranial/spinal nerves)
Neurilemma
● Nerve fibre = centrally located axon + surrounding supporting cell (Schwann cell)
● Axon:
○ Major extension of the nerve cell’s membrane and axoplasm
○ From Greek axis (“axle/axis”)
● Neuron = nerve cell (Latin tendon = stretched out)
● Schwann cell:
○ Forms neurilemma (Greek: nerve + husk)
● Myelin sheath:
○ Fatty layer from Schwann cell
○ Internal to neurilemma > Outermost nucleated layer of schwaan cells
○ Increases conduction speed by saltatory conduction (impulses jump between
nodes of Ranvier)
● Conduction velocity:
○ Proportional to fibre diameter
○ Large myelinated fibres (e.g., motor to skeletal muscle) = fastest
○ Small non-myelinated fibres (e.g., visceral pain fibres) = slowest but
space-saving
Connective Tissue Layers of a Nerve: Endoneurium,
Perineurium, Epineurium
entire nerve
● Endoneurium:
○ Surrounds individual nerve fibres Bundle of nerves
○ Greek: “within nerve”
fascicle
● Perineurium:
○ Surrounds bundles (fascicles) of fibres
○ Greek: “around nerve”
● Epineurium:
○ Outermost sheath, surrounds entire nerve Fats
·
Individual
○ Greek: “upon nerve”
nerve
● Composed mostly of collagen fibres
● Fat may be present between epineurium and perineurium
● Provides protection at compression/stretch points (e.g., median nerve in carpal tunnel)
,Synapses
● Synapse = junction between axon of one neuron and another cell (Greek: touch)
● A neuron has only one axon, but receives many synapses
● Synapses occur:
○ On cell body
○ On dendrites (Greek: trees)
● Synaptic cleft:
○ Gap between pre- and post-synaptic membranes
○ Neurotransmitter released from presynaptic terminal crosses cleft and affects
excitability of post-synaptic membrane
inhibitory or excitatory
Sensory vs Motor Fibres > no dendrites
> Receptor
● Sensory fibres (afferent): carry impulses to CNS
heive
● Motor fibres (efferent): carry impulses from CNS
- in CNS
beripheral
● Classification: < near CNS
○ Most peripheral nerves are mixed
○ Sensory neuron: Congsingle axon from cell bod o
■ Has no dendrites ora
Merve
■ Has a short single axon with:
■ Proximal extension → synapses in CNS D
■ Distal extension → travels in peripheral nerve
■ Cell body in sensory ganglion, near CNS
○ Motor neuron:
■ Has single long axon from its cell body
■ Axon travels in peripheral nerve
■ Cell body & dendrites are in CNS
Receptors
● Receptor = origin of a sensory nerve fibre
○ Located at distal axon terminal
● Types:
○ Exteroceptors:
■ In skin and sense organs
■ Include:
■ Mechanoreceptors (touch, pressure)
■ Thermoreceptors (hot, cold)
■ Nociceptors (superficial pain)
○ Proprioceptors:
■ In muscles, joints, bones
■ Include:
, ■ Mechanoreceptors (stretch, joint position, vibration)
■ Nociceptors (deep pain)
○ Interoceptors:
■ In internal organs
■ Include:
■ Baroreceptors (BP)
■ Chemoreceptors (O2 tension)
■ Nociceptors (visceral pain)
Effector & Neuro-effector Junction
● Effector = target at motor axon termination
○ Can be muscle or gland > movement
or hor mone release
● Not continuous with neuron
● Neuro-effector junction:
○ Has a gap (junctional cleft)
○ Region exposed by local loss of myelin, neurilemma, endoneurium
● Neuromuscular junction (skeletal muscle):
○ Electrical impulse causes neurotransmitter release
○ Transmitter diffuses → excites motor end plate
Somatic & Visceral Fibre Types
● 4 major functional fibre groups:
○ Somatic afferent to CNS
○ Visceral afferent
○ Somatic efferent from CNS
○ Visceral efferent
● Somatic afferents:
○ Superficial (skin): touch, temp, sharp pain
○ Deep (muscles, joints): proprioception, vibration, dull pain
● Visceral afferents:
○ Non-myelinated, slow
○ Sense visceral stretch, colicky pain
● Somatic efferents:
○ To skeletal muscle
○ Large, fast
● Visceral efferents:
○ To smooth/cardiac muscle, glands
○ Small, slow
, Sympathetics & Parasympathetics
● May have:
○ Complementary actions (e.g. erection vs ejaculation)
○ Opposite actions (e.g. HR, pupil size)
○ Independent actions (some organs get only sympathetic input)
● Sympathetic targets: = fight or flight s more blood +O 2 to working muscles to minimize excess energy loss
○ Sphincters of viscera
○ Blood vessels (vasoconstriction)
○ Dilator pupillae (eye)
○ Ventricles & atria (↑ stroke volume, HR)
○ Suprarenal medulla → adrenaline release
○ Ejaculation pathway
● Parasympathetic targets: = calm
State where body functions
occur
○ Smooth muscle walls of viscera
○ Exocrine glands
○ Sphincter pupillae (eye)
○ Atria only (↓ basal HR)
● Both systems are not activated simultaneously
Thoracolumbar Outflow in Alarm
> fight or flight
● Sympathetic fibres emerge from T1–L2 spinal cord segments
● Travel via sympathetic trunk (skull base to coccyx)
● Activated in alarm/fight-flight responses
○ Diverts blood to skeletal muscle
○ Dilates pupils
○ Facilitates ventilation (bronchioles)
○ Stimulates GI/urinary sphincters
○ Triggers adrenaline release (suprarenal medulla)
Nerve Fibres & Reflex Arcs
● Nerve fibre abilities:
○ Excitability: change in polarity
○ Conductibility: wave propagation
● Nerve = fibres (ectoderm) + connective tissue (mesoderm)
● CNS: brain + spinal cord
● PNS: peripheral nerves + ganglia (from cranial/spinal nerves)
