1. General organization of Nervous system

2. Organization of the Nervous System
Central Nervous System
(CNS)
Peripheral Nervous System
(PNS)
Brain
Spinal Cord
Cranial Nerves
(and their branches)
Spinal Nerves
(and their branches)
Somatic Nervous System (SNS)
Autonomic Nervous System (ANS)
Enteric Nervous System (ENS)
Sympathetic Division
Parasympathetic Division
Sensory: External stimuli
Somatic receptors (touch, pain)
Special senses (vision, hearing)
Sensory: Internal stimuli
(i.e. visceral organs)
Sensory: Internal stimuli
(i.e. GI tract)
Motor: Skeletal muscles
Motor: Visceral organs
Motor: GI tract

3. Cells of the Nervous System
Nervous tissue consists of 2 types of cells
Neurons:
Functional unit cells
“Unique functions” of the nervous system (e.g. sensation, information processing, control functions)
Communicate via electrical signals and chemical messengers
Neuroglia:
Support cells
Nourish and protect neurons
Preserve the physical and biochemical structure of nervous tissue

4. Neurons Dendrites Axon Cell body Axon terminals “INPUT” “OUTPUT”
Slender processes that branch out from the cell body
“OUTPUT”
Dendrites
Axon
Single long process extending out from the cell body
Cell body
Expanded portion that contains most of the typical cellular “bits”
Axon terminals
Transmit signals to a nerve/effector
“Typical neuron”

5. Ganglion (pl = ganglia)
Neuronal cell bodies and axons tend to group together throughout the nervous system
Cluster of Cell Bodies
Bundle of Axons
CNS
Nucleus (pl = nuclei)
Tract
PNS
Ganglion (pl = ganglia)
Nerve

6. Structural Classification of Neurons
Multipolar
Bipolar
Unipolar
Many dendrites and one axon
One dendrite and one axon
Dendrites and axon are fused
Most common
Special sense organs (eye, ear)
Sensory neurons of the PNS

8. Intricate web of dendrites Cell body shaped like a pyramid
Purkinje cells
Pyramidal cells
Intricate web of dendrites
Cell body shaped like a pyramid
Cerebellum
Cerebral cortex

9. Functional Classification of Neurons
Sensory (Afferent) Neurons:
Deliver signals from peripheral receptors to the CNS
Primarily unipolar neurons whose cell bodies are located in peripheral sensory ganglia
Motor (Efferent) Neurons:
Deliver signals from the CNS to effectors (e.g. muscles, glands) in the periphery
Primarily multipolar neurons whose cell bodies are located in spinal cord nuclei
Interneurons:
Transmit signals between sensory and motor neurons (i.e. distribute sensory information and coordinate motor activity)
Primarily multipolar neurons which are contained entirely within the CNS

10. Neuroglia Much more abundant than neurons
The CNS and PNS have different types of neuroglia
CNS:
Astrocytes
Oligodendrocytes
Ependymal cells
Microglia
PNS:
Satellite cells
Schwann cells

11. Neuroglia of the CNS Astrocytes Oligodendrocytes Ependymal cells
Guide neuron development
Create a supportive framework
Maintain the blood-brain barrier
Regulate the composition of the interstitial fluid
Oligodendrocytes
Produce the myelin sheaths of (some) CNS neurons
Ependymal cells
Line the canals and ventricles of the CNS
Produce cerebrospinal fluid (CSF) and assist in its circulation
Microglia
Act as macrophages (remove cellular debris and pathogens)

12. “Oligodendrocyte of the PNS”
Neuroglia of the PNS
Satellite cells
Create a supportive framework
Regulate the composition of the interstitial fluid
“Astrocyte of the PNS”
Schwann cells
Produce the myelin sheaths of (some) PNS neurons
“Oligodendrocyte of the PNS”

13. Myelin “White matter” “Grey matter”
Fatty substance that gets wrapped around the axons of some neurons
Functions:
Protects/insulates the axon
Increases the conduction speed of the axon
“White matter”
Region of myelinated axons
“Grey matter”
Region of unmyelinated axons and/or neuronal cell bodies

15. Electrical Signals in Neurons
Like most cells, neuronal cell membranes have a resting membrane potential (a difference in electrical charge between the inside/outside)
This is due to a difference in the number of positive and negative ions on either side of the cell membrane, with more negative ions being present within the cell (giving a negative value to the membrane potential)
Resting membrane potential
-70mV

16. Neurons and muscle cells are “electrically excitable”, meaning that their resting membrane potential changes in response to certain stimuli
In neurons, such changes take one of two forms:
GRADED POTENTIALS (Dendrites & cell bodies)
ACTION POTENTIALS (Axons)

17. Graded Potentials
Relatively small change in the resting potential that is localized to a small area of the cell membrane
Variable effect size
Can undergo summation
Diminish in intensity as they travel
Can be either excitatory or inhibitory

18. Action Potentials
Relatively large electrical impulse that is generated when the resting potential reaches a certain threshold (usually ~-55mv)
“All or none”
Travel along the length of the neuron at a constant intensity
Always excitatory

19. Signal Propagation Between Neurons
When an action potential travelling along an axon reaches a synaptic terminal, it is “transmitted” to an effector across a synapse
Synapses can be:
Electrical – Direct transmission via small connecting channels
Chemical – Indirect transmission via the release of neurotransmitters

21. Neurotransmitters have variable effects on an effector neuron
Excitatory – Produces a graded potential that brings the neuron’s cell membrane closer to threshold
Inhibitory – Produces a graded potential that brings the neuron’s cell membrane further from threshold
Neurons constantly receive input from
hundreds/thousands of other neurons
These inputs undergo summation,
and if the cell membrane reaches
threshold as a result, an action potential
will be generated in the neuron’s axon

22. External Anatomy of the Spinal Cord
In adults, the spinal cord extends from the medulla oblongata (the inferior portion of the brain stem) to the level of the L1-2 vertebrae
Therefore, the spinal cord is shorter than the vertebral column
Its caudal end tapers to a point called the conus medullaris
Extending inferiorly from this point is a strand of fibrous tissue (filum terminale) that anchors the spinal cord to the coccyx

23. Medulla oblongata
Spinal cord
Conus medullaris
L1-2
Filum terminale

24. The spinal cord has two regions of increased diameter:
The spinal cord is divided into 31 “segments”, each of which gives rise to paired spinal nerves:
8 pairs of cervical spinal nerves (C1-C8)
12 pairs of thoracic spinal nerves (T1-T12)
5 pairs of lumbar spinal nerves (L1-L5)
5 pairs of sacral spinal nerves (S1-S5)
1 pair of coccygeal spinal nerve (Co1)
The spinal cord has two regions of increased diameter:
Cervical enlargement (C3-T2 vertebrae) – Gives rise to the spinal nerves that innvervate the upper limbs
Lumbar enlargement (T9-T12 vertebrae) – Gives rise to the spinal nerves that innervate the lower limbs

25. Spinal nerves
Cervical enlargement
Lumbar enlargement
Spinal nerves

26. Internal Anatomy of the Spinal Cord
When looked at in cross-section, the spinal cord has a ring of white matter that surrounds a core of grey matter
There are two grooves that extend into the white matter and separate the spinal cord into left and right sides
In the centre of the grey matter is a central canal, which runs the length of the spinal cord and is filled with CSF
Posterior median sulcus
Central canal
White matter
Grey matter
Anterior median fissure

27. Regions of Grey Matter Posterior (dorsal) horn Lateral horn
Cell bodies of interneurons that relay sensory signals up the spinal cord
Posterior (dorsal) horn
Posterior
Lateral horn
Cell bodies of autonomic motor neurons
Only present in the T1-L2 and S2-S4 spinal cord segments.
Anterior
Anterior (ventral) horn
Cell bodies of somatic motor neurons

28. Regions of White Matter
Posterior columns
Lateral column
Lateral column
Anterior columns
Each of these columns contains ascending (sensory) tracts and/or descending (motor) tracts.

29. Spinal Nerves Posterior (dorsal) root Spinal nerve
Made up of axons of sensory neurons.
Cell bodies are within a posterior (dorsal) root ganglion.
Posterior (dorsal) root
Posterior (dorsal) root ganglion
Spinal nerve
Spinal nerves are “mixed”
(sensory and motor axons).
Anterior (ventral) root
Made up of axons of motor neurons.
Cell bodies are within the grey matter of the spinal cord.

30. Intervertebral foramen
The spinal nerves emerge from the vertebral column between adjacent vertebrae through the intervertebral foramina (IVF)
C1-C7 spinal nerves exit superior to their corresponding vertebrae
C8 spinal nerves exit between C7 & T1
Thoracic and lumbar spinal nerves exit inferior to their corresponding vertebrae
Intervertebral foramen

31. Since the spinal cord is shorter than the vertebral column, most spinal nerve roots angle inferiorly before joining within their respective IVFs
The nerve roots of the L2-Co1 spinal segments extend caudally from the conus medularis (along with the filum terminale) until they emerge from the vertebral canal at their respective levels
Since this collection of fibres resembles a horse’s tail, it is referred to as the cauda equina
Cauda equina