Nerve Fiber

Types of Nerve Fiber

Classification	Type	Subtype	Example	Relative Diameter	Myelination
Sensory & Motor	A	\(\alpha\)	\(\alpha\) motor neurons	Large	Heavily myelinated
		\(\beta\)	touch, pressure	Medium	Heavily myelinated
		\(\gamma\)	intrafusal fibers	Medium	Heavily myelinated
		\(\delta\)	touch, pressure, temperature, fast pain	Small	Heavily myelinated
	B			Small	Lightly myelinated
	C			Small	Unmyelinated
Sensory	I	a	Muscle spindle afferents	Large	Myelinated
		b	Golgi tendon organ afferents	Large	Myelinated
	II		secondary afferents of muscle spindles; touch, pressure	Medium	Myelinated
	III		touch, pressure, fast pain, temperature	Small	Myelinated
	IV		pain, temperature, olfactory	Small	Unmyelinated

Spinal Cord Nerve Fiber

Schematic of the histologic structure of a peripheral nerve. Adapted from (Schmitd et al., 2021)

	Diameter(mm)		Number of nerve fibers
Segment	Ventral root (Motor)	Dorsal root (Sensory)	Ventral root (Motor)	Dorsal root (Sensory)
C1	0.97±0.16	1.21±0.28	2751±639	6430±1606
C2	1.34±0.30	2.61±0.51	3116±724	11947±2977
C3	0.80±0.23	2.87±0.52	2460±471	21876±1916
C4	1.39±0.24	2.49±0.34	3833±408	10647±887
C5	2.50±0.55	3.43±0.77	7841±1020	23300±2856
C6	2.23±0.73	3.99±0.75	7048±1157	36353±7451
C7	2.22±0.50	4.61±0.87	8467±1019	39653±8458
C8	1.71±0.60	3.92±0.62	5883±1000	31156±8273
T1	1.03±0.23	2.18±0.31	5788±1186	26507±7617
T2	0.75±0.11	1.30±0.14	3576±398	10234±1728
T3	0.78±0.10	1.35±0.16	5499±1126	14888±2514
T4	0.77±0.17	1.13±0.13	5485±973	10849±1832
T5	0.64±0.08	1.21±0.30	5326±1314	8355±1390
T6	0.71±0.18	1.07±0.16	3666±1407	10015±1666
T7	0.78±0.15	1.25±0.27	4297±1130	9123±1178
T8	0.83±0.25	1.29±0.18	3643±1340	7619±903
T9	0.81±0.15	1.33±0.25	5209±704	8369±967
T10	0.72±0.08	1.27±0.15	5269±963	11329±2724
T11	0.69±0.08	1.26±0.16	4870±895	9713±1824
T12	0.76±0.14	1.45±0.19	6538±892	10420±802
L1	0.81±0.07	1.55±0.28	5384±833	16820±3456
L2	0.96±0.16	1.93±0.27	7374±720	18615±±3284
L3	1.19±0.07	2.24±0.30	9169±1160	26191±2772
L4	1.04±0.07	2.48±0.38	7878±1386	31175±2686
L5	1.37±0.16	2.66±0.40	8657±1396	34455±2740
S1	1.43±0.16	2.95±0.57	8253±1419	41543±3036
S2	0.93±0.11	2.02±0.53	4766±1035	18642±1716
S3	0.55±0.07	1.32±0.60	2233±299	11971±964
S4	0.34±0.03	0.52±0.17	1356±193	3402±304
S5	0.14±0.02	0.27±0.13	906±111	2206±197

*Table adapted from (Liu et al., 2015).

Dermatomes and Cutaneous Nerves (Wikipedia)

Optic Nerve Fibers

Image adapted from (Bose et al., 2005)

• Axon diameter: Mean=\(0.683 \mu m\), SD=\(0.369 \mu m\), SEM=\(0.026 \mu m\)​
• Fig A: Normal human left optic never fiber cross-section (about 1million retinal ganglion cell axons).
• Fig B: Transverse section.

Miscellaneous

Lower Bound of Axon Diameter

Axon diameters are pushed toward the channel-noise limit of \(0.1 \mu m\)​ (mature neurons) due to Spontaneous Action Potentials (SAPs) (Faisal et al., 2005).

Thickness of the Cell Membrane

\(7.5 - 10 nm\) (Elert, 2023).

Depolarisation and Hyperpolarization Voltage

Phase	Voltage	Duration
Depolarization	110mv	1ms
Repolarization	110mv	2~3ms
Hyperpolarization	5mv	1ms
Refractory Period	/	2~3ms

Firing Rate

From \(<1Hz\) to \(200Hz\) (minimum \(5ms/spike\)).

Axonal Conduction Velocity

From \(0.1m/s\) (unmyelinated) to \(200m/s\) (myelinated) (DeMaegd et al., 2017).

Endocytosis Size Limit

Max 500nm spheres in diameter - energy-dependent process (Rejman et al., 2004).

3nm Process

	Samsung	TSMC
Transistor type	MBCFET	FinFET
Transistor density (MTr/mm2)	150	197 (theoretical) 183 (A17 Pro)
SRAM bit-cell size (μm2)	Unknown	0.0199
Transistor gate pitch (nm)	40	45

Transistor density: \(197\times 10^8 Tr/mm^2 = 197 \times 10^8 \div 1000^2 = 19700 Tr/{\mu m}^2\)

Cross-section area of a matured neuron (\(d=0.1 \mu m = 100 nm)\): \(3.14 \times 0.05^2 = 0.00785 {\mu m}^2\)​

The number of transistors that can be fitted into the cross-section: 154 (upper bound)

Minimum transistor thickness: 0.34 nm (reference)

Size of multi-walled carbon nanotube: \(10-40nm, 5-20 \mu m\) (Abdallah et al., 2020).

Reference

1. Abdallah, B., Elhissi, A. M. A., Ahmed, W., & Najlah, M. (2020). Carbon nanotubes drug delivery system for cancer treatment. In Advances in Medical and Surgical Engineering (pp. 313–332). Academic Press. https://doi.org/10.1016/B978-0-12-819712-7.00016-4
2. Bose, S., Dhillon, N., Ross-Cisneros, F. N., & Carelli, V. (2005). Relative post-mortem sparing of afferent pupil fibers in a patient with 3460 Leber’s hereditary optic neuropathy. Graefe’s Archive for Clinical and Experimental Ophthalmology, 243, 1175–1179.
3. DeMaegd, M. L., Städele, C., & Stein, W. (2017). Axonal conduction velocity measurement. Bio-Protocol, 7(5), e2152–e2152.
4. Elert, G. (2023). The Physics Factbook. https://hypertextbook.com/facts/2001/JenniferShloming.shtml
5. Faisal, A. A., White, J. A., & Laughlin, S. B. (2005). Ion-channel noise places limits on the miniaturization of the brain’s wiring. Curr. Biol., 15(12), 1143–1149. https://doi.org/10.1016/j.cub.2005.05.056
6. Liu, Y. T., Zhou, X. J., Ma, J., Ge, Y. B., & Cao, X. (2015). The diameters and number of nerve fibers in spinal nerve roots. J. Spinal Cord Med., 38(4), 532. https://doi.org/10.1179/1079026814Z.000000000273
7. Rejman, J., Oberle, V., Zuhorn, I. S., & Hoekstra, D. (2004). Size-dependent internalization of particles via the pathways of clathrin- and caveolae-mediated endocytosis. Biochem. J., 377(Pt 1), 159. https://doi.org/10.1042/BJ20031253
8. Schmitd, L. B., Perez-Pacheco, C., & D’Silva, N. J. (2021). Nerve density in cancer: Less is better. FASEB BioAdvances, 3(10), 773.