The document provides a comprehensive overview of the nervous system, detailing its structure, functions, and components, including the central and peripheral systems. It describes the different types of neurons, their classifications, cellular components, and functional aspects, such as neurotransmitters and synapses. Furthermore, it covers the brain's anatomical regions and roles, particularly highlighting the cerebellum and its subdivisions.

1. Nervous System
Dr. Rabia Inam Gandapore
Assistant Professor
Head of Department Anatomy
(Dentistry-BKCD)
B.D.S, M.Phil. Anatomy,
Dip.Implant, CHPE, CHR, Dip. Implant

2. Nervous System
 Coordinates voluntary and involuntary
actions of the body and transmits signals
between different parts of the body
 Together with endocrine system controls
and integrates activities of the body
 Nervous system allows us to perceive,
understand, and respond to our
environment

3. Components of Nervous System
 Divided into 2
1. Central nervous system
 Brain
 Spinal cord
2. Peripheral nervous system
 12 pairs of cranial nerves
 31 pairs of spinal nerves and their associated ganglia
Divided into 2 main components (functionally)
a. Somatic nervous system
 Controls voluntary activities
b. Autonomic nervous system
 Controls involuntary activities

4. Sensory
Motor
Cranial & Spinal Nerves
Visceral

6. Cellular Components of the Nervous
System
Comprised of two different types of cells:
1. Nerve cells (neurons): form functional
basis of nervous system, transmits signals
as electrical or chemical signals
2. Glial cells: provide functional and
structural support for neurons.
Example: Schwann cell: produces lipid
sheath of peripheral neurons

7. Central Nervous System (CNS)
 Composed of nerve cells (neurons) & processes (dendrites: Short & Axons: Long)
 Supported by specialized tissue called neuroglia (outnumber neurons by 10-50 times)
 Interior of the CNS is organized into:
 Gray matter (nerve cells, dendrites and synaptic contacts embedded in neuroglia)
 White matter (nerve fibers- axon embedded in neuroglia)
 Cell bodies of neurons are often grouped together in discrete areas termed as nuclei
 Embryologically: CNS is derived from the neural tube

8. Nerve cell/ Neuron
 Gray matter substantia grisea is
where the processing is done (cell
bodies, dendrites, axon terminals for
synapses) and the white matter
substantia alba is the channels of
communication (Nerve fibers-axons)

9. Neuronal Structure
 Different types, sizes of neurons found in nervous system
 Neurons are post-mitotic cells and, with few exceptions, are not replaced when lost
 All contain the same key structural components
1. Cell body
2. Dendrites,
3. Axon
4. Axon terminals

10. 1. Cell Body/ perikaryon/ soma
 Soma is central mass of cytoplasm, Holds the nucleus
 Site of protein synthesis, which occurs on small granules
of rough endoplasmic reticulum called nissl substance
 In the nervous system, many neuronal cell bodies can
group together to form a distinct structure.
a) CNS: this is known as a nucleus (clustered into nuclei,
columns or layers , dispersed within grey matter)
b) PNS: as a ganglion

12.  Plasma membrane of the soma is unmyelinated
 Cytoplasm is rich in rough and smooth endoplasmic reticulum and free
polyribosomes that congregate in large groups associated with the RER, many
mitochondria and moderate numbers of lysosomes.
 These aggregates of RNA-rich structures are visible by light microscopy as
basophilic Nissl (chromatin) bodies or granules
 More obvious in large, highly active cells such as spinal motor neurons
 The apparatus for protein synthesis (including RNA and ribosomes) occupies
the soma and dendrites but is usually absent from axons.

13. Contacts of Soma
 Contacted by inhibitory and excitatory axo-somatic
synapses
 Very occasionally, somasomatic and dendrosomatic
contacts may be made
 Non-synaptic surface is covered by astrocytic or
satellite, oligodendrocyte processes
 Various trans-membrane channel proteins and
enzymes are located at the surfaces of neurons, where
they are associated with ion transport

15. 2. Dendrites
 Dendrites are highly branched, usually short
processes that project from the soma/ body
 Dendritic trees may be expanding and contracting
as traffic of synaptic activity varies through afferent
axo-dendritic contacts.
 Extend outwards, receiving input from the
environment and from other neurons

16. 3. Axons
 A long, thin structure that conducts action potentials (the nerve impulse)
 Neurons have many dendrites, most cells only have one axon
 Axon is coated in myelin – a layer of insulating lipid. Myelin is formed by cells
wrapping themselves around the nerve axon
a) CNS: this is performed by oligodendrocyte cells
b) PNS: Schwann cells
 There are gaps ,nodes of Ranvier, between the myelin sheaths formed by different
cells. They allow for saltatory conduction of impulses

18.  Originates either from the soma or from the proximal segment of a dendrite, at a
specialized region called the axon hillock which is free from Nissl granules
 The axonal membrane (axolemma) is undercoated at hillock by a concentration of
cytoskeletal molecules i.e spectrin and actin fibrils
 Spectrin and actin fibrils helps in anchoring numerous voltage-sensitive channels to
the membrane
 Nodes of Ranvier are specialized constricted regions of myelin-free axolemma where
action potentials are generated and where an axon may branch

19. Axonal Ion Channels
 Density of sodium channels in the axolemma is highest at the nodes of Ranvier
and very low along internodal membranes.
 Sodium channels are spread more evenly within axolemma of unmyelinated axons.
 Fast potassium channels are also present in the paranodal regions of myelinated
axons.

21. Neuronal Skeleton
 A prominent feature of neuronal cytoplasm
 Gives shape, strength and rigidity to the dendrites and axons.
 Neurofilaments (the intermediate filaments of neurons) and
microtubules (motor protein call kinensin; transportation via
axon) are abundant
 Bundles of neurofilaments constitute neurofibrils
 Some axons are almost filled by neurofilaments.
 Dendrites usually have more microtubules than axons.

22. Kinesin protein and transport, enzyme,
neurotransmitter, membrane protein etc.
Dynein protein
transports dead
mitochondria, nerve
growth factors,etc.

23. 4. Axon Terminals
 Most distal part of the axon
 In here neuron sends chemical signals to other
cells – usually via neurotransmitter release ( hence
contain large number of mitochondria
 Reuptake of neurotransmitters
 The terminals of an axon are unmyelinated
Neuron

24. Pattern of axonal termination
 Vary considerably
 Single axon may synapse with one neuron
Example: climbing fibres ending on
cerebellar Purkinje neurons
 More often, synapses with many
Example: Cerebellar parallel fibres

25. How dendrites are different from axons
 Represent afferent neuron
 Receive both excitatory and
inhibitory axo-dendritic contacts
 May also make dendrodendritic and
dendrosomatic connections
 Synapses occur either on small
projections called dendritic spines or
on the smooth dendritic surface

26. Coverings of the nerves
 In PNS: axons of neurons are grouped together to form nerves
 The axons are enclosed by several connective tissue layers:
Endo-neurium: Surrounds the axon of an individual neuron.
Peri-neurium: Surrounds a fascicle, which is a collection of neurons.
Epi-neurium: Surrounds the entire nerve, which is formed by a collection of fasicles
 Contains numerous small blood vessels, which supply the nerve fibres
 Epineurium appears on the nerve as it exits the intervertebral foramen
 Created by the fusion of arachnoid and pia mater, which are layers of the meninges

28. Classification of Neurons (Structural)
1. Structural classification of neurons
 Unipolar: cell body is at one end of a single un-branched axon,& no dendrites.
Example: cochlear nucleus of brain
 Pseudounipolar: axon (not distinguished) with two branches (peripheral &
central process) from the cell body. Example: Sensory neurons, Dorsol root
ganglion, Cranial nerve 5 (trigeminal ganglion)
 Bipolar: have two processes arising from a central cell body – typically one
axon and one dendrite. Example: found in retina, olfactory nerves, inner ear.
 Multipolar: They have one axon and many dendrites, with a cell body displaced
to one side of the axon. Example: Motor Cortex neurons , cerebellum

29. Structural classification of neurons

30. 2. Functional Classification of Neurons
3 types:
1. Sensory (afferent): Towards CNS
Viscera to CNS: 1. General Visceral (lungs, heart, GIT), 2. General Somatic (skin, skeletal
muscle, joints), 3. Special Sensory ( vision & hearing), 4. Special Viscera (Smell, Taste)
2. Intermediate- Interneurons: Between sensory & motor (Motor Pathway: Motor cortex
down to neurons in spinal cord (lower motor neurons) and goes to skeletal muscles,
Sensory Pathway: Sensory receptors from skin or muscle to neuron (Nucleus gracilis) in
spinal cord and then to another neuron (Thalamus nuclei) and final to the cerebral cortex
(motor neuron))
3. Motor (efferent): Away from CNS to Effector Organ
1. General Visceral (smooth muscle in bronchi, cardiac muscle, gland- ANS), 2. General
Somatic (Skeletal Muscle). 3. Special Visceral (Muscles of Head & neck cranial nerves
5T,7F,9G,10V)

32. There are key structural differences between these three types:
1. Sensory nerves: small axons and pseudo-unipolar structure.
2. Motor nerves: larger axons and multipolar structure.
3. Intermediate neurons: central cell body and many dendrites.
 Sensory and motor nerves are located within the PNS, whereas
intermediate nerves are found in the CNS.

33. Neurotransmitters
Two Classes
A. Excitatory neurotransmitters
 Cause depolarization of post-synaptic membrane
 Promote action potentials
B. Inhibitory neurotransmitters
 Cause hyperpolarization of post-synaptic membranes
 Suppress actions potentials

34. The effect of a
neurotransmitter on a
postsynaptic membrane
depends on the
receptor, Not on the
neurotransmitter
 For example,
Acetylcholine (ACh)
: promotes action
potentials, But
inhibits cardiac
neuromuscluar
junctions

35. Neurotransmitters
 Asymmetric synapses (Excitatory) with relatively
small spherical vesicles are associated with
acetylcholine (ACh), glutamate, serotonin
(5hydroxytryptamine, or 5-HT) and some amines.
Those with dense-core vesicles include many
peptidergic synapses and other amines (e.g.,
noradrenaline, adrenaline, dopamine)
 Symmetric synapses ( Inhibitory) with flattened or
pleomorphic vesicles contain either gamma-
aminobutyric acid (GABA) or glycine

36. Neurotransmitters
50 neurotransmitters other than Ach, including: Biogenic amines, Amino
acids, Neuropeptides, Dissolved gases
Important Neurotransmitters
 Norepinephrine (NE): Released by adrenergic synapses, Excitatory
and depolarizing effect. Widely distributed in brain and portions of ANS
 Dopamine: A CNS neurotransmitter, May be excitatory or inhibitory
Involved in Parkinson's disease and cocaine use
 Serotonin: A CNS neurotransmitter Affects attention & emotional states

37. Difference between neurotransmitter and
neuromodulator
 Neuronal Responses to injuries Axons respond differently to injury,
depending on whether the damage occurs in the CNS or PNS.

38. Receptors for Neurotransmitters
 1. Cholinergic receptors for ACh : Named after drugs that bind to them and mimic
ACh effects. Two types of receptors bind ACh
1. Nicotinic: Found on Motor end plates of skeletal muscle cells, All ganglionic
neurons (sympathetic and parasympathetic), Hormone-producing cells of the adrenal
medulla, Effect of ACh at nicotinic receptors is always stimulatory
2. Muscarinic: Found on All effector cells stimulated by postganglionic cholinergic
fibers, The effect of ACh at muscarinic receptors can be either inhibitory or excitatory,
Depends on the receptor type of the target organ

40.  2. Adrenergic receptors for NE: Effects of Norepinephrine depend on which
subclass of receptor predominates on the target organ. Two types
1. Alpha (subtypes 1, 2)
2. Beta (subtypes 1, 2 , 3)

42. Classification of Glial cells
CNS Glia PNS Glia Basic Function
Astrocyte Satellite Cell Support
Oligodendrocyte Schwann Cell Insulation , Myelination
Microglia Immune surveillance & phagocytosis
Ependymal Cell Creating CSF

44. Brain
(ENCEPHALON)

45. Brain (Encephalon)
 Lies within the cranium
 Receives information & controls activities of
trunk and limbs via spinal cord
 Brain is divided into major regions based on:
 Ontogenetic growth (morphological)
 Phylogenetic principles (evolutionary)
 Principal division from spinal cord to higher up:
 Hindbrain or rhombencephalon
 Midbrain or mesencephalon
 Forebrain or prosencephalon

46. Brain (Encephalon)
1. Telencephalon: Cerebral Hemisphere, Basal ganglia,
Lateral Ventricles, Cerebral Cortex , White matter
2. Diencephalon: Epithalamus, Dorsal Thalamus,
Hypothalamus, Sub thalamus and Third Ventricle+
associated structures
3. Mesencephalon: Anterior : Cerebral Peduncle & Inter
peduncular fossa (acculomotor nerve) , Posterior :
Superior Colliculus (Visual) , Brachium SC, Inferior
Colliculus, Brachium IC, Trochlear nerve
4. Metencephalon: Pons & Cerebellum
5. Myelencephalon: Medulla Oblongata

47. 1. Hindbrain/ Rhombencephalon
Divided into:
 Myelencephalon: Medulla ablongata
 Metencephalon: Pons/ Cerebellum
• Medulla ablongata
• Pons
• midbrain
Brain stem

48.  Brain stem: lies over the basal portions
of the occipital and sphenoid bones
 Medulla ablongata: is the most caudal
part of the brain stem. Continuous with
the spinal cord below the level of foramen
magnum
 Pons: lies rostral to the medulla and is
distinguished by the mass of transverse
fibers which connect it to cerebellum

49. Cerebellum
 Cerebellum “little brain”
 Role in motor control, with cerebellar dysfunction often presenting with motor signs
 Active in coordination, precision, timing of movements, and in motor learning
Anatomical Location
 Located at the back of the brain, immediately inferior to the occipital and temporal
lobes, and within the posterior cranial fossa
 Separated from these lobes by the tentorium cerebelli, a tough layer of dura mater
 Lies at the same level of and posterior to the pons, from which it is separated by the
fourth ventricle

52. Anatomical Structure of Cerebellum
 Consists of two hemispheres which are connected by the vermis, a narrow midline
area
 Grey matter: located on the surface of the cerebellum. Tightly folded, forming the
cerebellar cortex.
 White matter: located underneath the cerebellar cortex. Embedded in the white
matter are the four cerebellar nuclei (Dentate, emboliform, globose, and fastigi
nuclei).
 3 subdivisions: Anatomical lobes, zones and functional divisions

54. 1. Lobes and fissures of Cerebellum
Anatomical Lobes
 3 anatomical lobes
 Anterior lobe,
 Posterior lobe
 Flocculonodular lobe
 Lobes are divided by two fissures
1. Primary fissure
2. Posterolateral fissure

55. 2. Zones of the Cerebellum
3 cerebellar zones
 Midline of cerebellum is vermis
 Either side of the vermis is the intermediate zone
 Lateral to the intermediate zone are the lateral hemispheres
There is no difference in gross structure between the lateral
hemispheres and intermediate zones

56. Vasculature of Cerebellum
Arterial Supply:
 Superior cerebellar artery (SCA) branch of basilar artery
 Anterior inferior cerebellar artery (AICA) branch of basilar artery
 Posterior inferior cerebellar artery (PICA) branch of the vertebral artery
Venous drainage: superior and inferior cerebellar veins, drain into the superior
petrosal, transverse and straight dural venous sinuses

58. 2. Midbrain/Mesencephalon
 Short segment of brainstem lying rostral to the pons
 Associated with vision, hearing, motor control, sleep awake, arousal & temperature
regulation
 Located below cerebral cortex and above hindbrain, placing it center of brain
 Comprises of:
Tectum
Cerebral aqueduct
Cerebral peduncles
Several nuclei and fasciculi
 Caudally it adjoins pons and cerebellum, rostrally it adjoins thalamus and
hypothalamus

61. 3. Forebrain-Prosencephalon
Subdivided into
Diencephalon
• Thalamus
• Hypothalamus
• Smaller epithalamus
• Subthalamus
Telencephalon
• 2 cerebral hemisphere or cerebrum

62. Forebrain Conti..
 Diencephalon: completely embedded in cerebrum & largely hidden from exterior
 Human cerebrum constitutes the major part of the brain
 Occupies the anterior cranial fossae and is directly related to the cranial vault
 Consists of 2 cerebral hemispheres
 Surface of each hemisphere is convoluted into a complex pattern of ridges called
gyri and furrows known as sulci
 Internally each hemisphere has an outer layer of grey matter- cerebral cortex
 Beneath which lies a thick mass of white matter

63. Sulci and Gyri
Sulci: (Furrows)
 Central sulcus: groove separating frontal and parietal lobes
 Lateral sulcus: groove separating frontal and parietal lobes from temporal lobe
 Lunate sulcus: groove located in the occipital cortex
Gyri: (Ridges)
 Precentral gyrus: ridge directly anterior to central sulcus, location of primary motor
cortex
 Postcentral gyrus: ridge directly posterior to central sulcus, location of primary
somatosensory cortex
 Superior temporal gyrus: ridge located inferior to lateral sulcus, responsible for the
reception and processing of sound

64. Sulci and Gyri
Lunate Sulcus

67. Cerebral Hemisphere
1. Frontal lobe: (premotor cortex, frontal eye field, motor speech area)
 Occupies area anterior to central sulcus and superior to lateral sulcus
2. Parietal lobe: (primary somesthetic area)
 Occupies area posterior to the central sulcus and superior to lateral
sulcus
3. Temporal lobe: (primary auditory area)
 Occupies the area inferior to the lateral sulcus
4. Occipital lobe: (primary visual area)
 Occupies the small area behind the parieto-occipital sulcus

69. Vasculature of the Cerebrum
Arterial Supply:
 Anterior Cerebral Arteries: branches of internal carotid arteries, supplying the
antero-medial aspect of the cerebrum
 Middle Cerebral Arteries: continuation of internal carotid arteries, supplying most of
lateral portions of cerebrum
 Posterior Cerebral Arteries: branches of vertebral arteries, supplying both medial
and lateral sides of cerebrum posteriorly
Venous drainage:
small cerebral veins , empty into dural venous sinuses – endothelial lined spaces
between outer and inner layers of dura mater

71.  Internal capsule is an important component of cerebral white matter
 Contains nerve fibers which pass to and from cerebral cortex and lower levels of
neuraxis
 Several large nuclei of grey matter called basal ganglia, are partly embedded in
the subcortical white matter
 Corpus callosum midline commissure that links the corresponding regions of the
two sides of the brain via fibers passing through it
 Basal Ganglia is group of subcortical nuclei responsible primarily for motor
control, motor learning, executive functions and behaviors, and emotions.

73. Basal Ganglia

76. Ventricular System
 Interconnected cavities in the brain
 choroid plexus are present, secrete cerebrospinal fluid which fills the ventricles
 Parts: Lateral ventricles ,3rd ventricle & 4th ventricle
Lateral ventricles: Large cavities located on each side of the cerebral
hemisphere. Communicates caudally with 3rd ventricle via a small aperture
3rd ventricle: Caudal end of lateral ventricles opens into a narrow slit-like midline
cavity called 3rd ventricle, Bounded laterally by diencephalon, Continuous caudally
with 4th ventricle with a narrow channel called cerebral aqueduct, which passes
through the midbrain

78. Ventricular System conti..
4th ventricle:
 Cavity of rhombencephalon expands to form 4th ventricle
 lies dorsal to pons and upper half of medulla
 Rostrally: connected to 3rd ventricle via cerebral aqueduct
 Caudally: it is continuous with a canal in the caudal medulla and through this
with the central canal of the spinal cord
 It is continuous with sub arachnoid space through 3 openings in its roof
(Foramina of Luschka and of Magendie)

79. Meninges of the Brain

80. Meninges of the Brain
 Membranous coverings of brain and spinal cord
 3 layers of meninges:
Dura, Arachnoid & Pia mater
Two major functions:
 Provide a supportive framework for the cerebral
and cranial vasculature
 Acting with cerebrospinal fluid to protect the
CNS from mechanical damage

81. Dura Mater
 Outermost layer of the meninges, lying directly underneath the bones of the skull
and vertebral column
 It is thick, tough and inextensible
 Within the cranial cavity, the dura contains two connective tissue sheets:
1. Endosteal layer: Lines the inner surface of the bones of the cranium.
2. Meningeal layer: Lines the endosteal layer inside the cranial cavity and the only
layer present in the vertebral column
 Between these two layers, the dural venous sinuses are located
 They are responsible for the venous vasculature of the cranium, draining into the
internal jugular veins

82.  In some areas within skull, meningeal layer of dura mater folds inwards as dural
reflections
 They partition the brain, and divide the cranial cavity into several compartments.
Example: tentorium cerebelli divides the cranial cavity into supratentorial and
infratentorial compartments.
 Dura mater vasculature: middle meningeal artery and vein
 Innervation: trigeminal nerve (V1, V2 and V3)

84. Arachnoid Mater
 Middle layer of the meninges, lying directly underneath the dura mater
 Consists of layers of connective tissue, is avascular, and does not receive any
innervation
 Underneath the arachnoid is a space known as the sub-arachoid space,
Contains cerebrospinal fluid, which acts to cushion the brain
 Small projections of arachoid mater into the dura (known as arachnoid
granulations) allow CSF to renter the circulation via the dural venous sinuses

86. Pia Mater
 Located underneath the sub-arachnoid space
 Thin & tightly adhered to surface of brain and spinal cord
 Only covering to follow contours of brain (the gyri and fissures)
 Highly vascularised, with blood vessels perforating through the membrane to
supply the underlying neural tissue

87. Spinal Cord

88. Spinal Cord
 Lies within the vertebral column, in the upper 2/3rd of the vertebral canal
 Extends from upper border of atlas vertebra to the junction between first and 2nd
lumbar vertebrae
 Continuous rostrally with medulla ablongata of the brain stem
 It narrows caudally to conus medullaris (L1, L2), whose apex is attached to
connective tissue filament filum terminale
 Cauda Equina (L2, Co1): Nerves go down as bunch /bundle (Horsetail)
 Cervical enlargement (C5-T1), more grey matter in ventral grey horn of spinal cord
(Supply Skeletal muscle of upper Limbs)
 Lumbar enlargement (L2-S3): Ventral grey matter-Skeletal muscles of lower limbs

90. Terminates at L1 or L2
C1
Spinal cervical nerves 8
(Above Vertebrae)

91.  Receives afferent inputs and controls the
functions of the trunk and limbs
 Afferent and efferent connections
between periphery and spinal cord travel
via segmentally arranged spinal nerves
 Consists of central core of grey matter
surrounded by white matter
 Grey matter has a characteristic “H” or
butterfly shape

92. White Matter
 White matter decrease C1-C01), Grey matter Increase (C1-Co1)
 White matter: Sensory (Ascending), Motor (Descending) tracts
 White matter more in cervical and less in coccygeal (sensory information from tail
bone) because the sensory information ascends up from co1 medially (midline
portion of dorsal column) up to the cerebral cortex , followed by sacral (genital
area), Lumber ,thoracic (trunk), and cervical (Neck,Limbs) more laterally.
 Same concept with the descending pathway(motor) and information exit out at ever
portion of spinal cord. The more information is received at the cervical region of the
spinal cord that’s why white matter is more in cervical region. Hence grey matter
(ventral grey horn) thickens at Co1.

94.  Grey matter has projections:
1. Dorsal horn: Neurons=Sensory Function
2. Ventral horn: Neurons= Motor activity
3. Intermediate horn: Small lateral horn is additionally present,marks location of cell
bodies of preganglionic sympathetic neurons
 Central canal: a vestigial component of ventricular system lies in center of spinal
grey matter and runs the length of the cord
 White matter: consists of ascending and descending tracts that link spinal cord
segments to one another and spinal cord to brain

96. Grey Matter (Rex Laminae and Nuclei)
Intermediate Zone

97. Spinal cord cross-section
 Axon appear white due to myelin sheath (White Matter), while dendrites are
unmyelinated and give grey color (Grey matter).
 Spinal cord cross-section:
1. Posterior median sulcus
2. Grey matter: Dendrites, Synapses
3. Posterior (dorsal) grey horn: Sensory Neurons
4. White matter: Axons
5. Anterior median fissure
6. Intermediate zone : Visceral motor function (smooth, cardiac, glands-Sympathetic
& Para-Sympathetic)

98. 7. Anterior (ventral) grey horn: Motor Neurons-somatic (skeletal muscle) motor neurons,
CNS= nuclei
8. Lateral grey horn: T1 to L2, Pre-ganglionic motor neurons of the sympathetic nervous
system, Comes out of intermediate zone only at T1 to L2
9. Dorsal (posterior) white column or funiculus: Ascending tracts- Somatic-Sensory
Information, Visceral sensation
10. Lateral white column or funiculus: Ascending (Sensory) & Descending (Motor) tracts
11. Ventral (anterior) white column or funiculus: Ascending (Sensory) & Descending
(Motor) tracts
12. Anterior white Commissure: crossing of fibers occurs, when fibers come in dorsal
horn and has to go to opposite side, it cross this structure
13. Grey Commissure: Connection 2 side (Dorsal, ventral and intermediate zone)
together in middle.

99.  Axon bundles in CNS is called tracts
 Cell bodies with Dendrites bundles in horn in the CNS is called nuclei
 Ganglia: group of cell bodies in PNS
 Spinal Nerve
Dorsal Root
Ventral Root
 Spinal Nerve splits and one go towards back is called dorsal rami (Supply: Bac;, neck,
vertebra , Anterio-lateral supply of limbs are called ventral rami (Supply: Anterior
trunk,Lateral trunk, Limbs)
 White and grey rami communicans and connected to a ganglia
Spinal Nerve

101.  Sensory Information from dorsal or ventral rami will move through the spinal nerve,
go in dorsal root and enter the dorsal root ganglia. It then enter the posterior grey
horn (sensory neurons) and inter-neuronal synapse occur. It activate somatic motor
neuron present in the ventral grey horn and enter ventral root and then enter spinal
nerve and go to dorsal rami (back) or go to ventral rami (trunk, Limbs)

104. Spinal Dorsal Horn
 Involved in the processing and transmission of sensory information to the brain
 The circuitry involved is complex and still poorly understood

105. Neuronal Components
 Primary afferents: Myelinated, Unmyelinated
 Projection neurons: Supraspinally projecting axons , Ascend mostly via
anterolateral tract
 Interneurons: Axons remain within spinal cord, Excitatory, Inhibitory
 Descending axons

106. Functions of the Spinal Dorsal Horn
Transmits/modifies inputs that it receives from primary afferents
•Local interneurons
•Descending axons
Provides outputs to
•Brain (projection neurons)
•Spinal circuits involved in Reflexes,
•Autonomic functions
Melzack & Wall (1965) Science 150 971-‐-979

107. Overview of Ascending Sensory
Pathways
 Responsible for the transmission and interpretation of sensory modalities
(special and general senses)
 Special senses:
 Olfaction
 Vision
 Taste
 Hearing Vestibular function
 General senses: Touch,Thermal sensation, Pressure, Proprioception,
Vibration, Pain

109. Ascending Pathway conti…
 Stimuli from the external and internal environments activate a diverse range of
receptors in the skin, viscera, muscles, joints and tendons
 Afferent impulses from the trunk and limbs are conveyed to the spinal cord in
spinal nerves while those from head are carried to brain in cranial nerves
 Ascending pathways in general consists of a sequence of 3 neurons that extend
from peripheral receptor to the contralateral cerebral cortex
 They are referred to as:
 Primary (first order) o Secondary (second order) o Tertiary (third order)

110. 1. Tract of Lissauer
 White matter tract
 Ascending & descending pathway
 Pain & temperature pathway
 Example; T1-T3 = Receptors picks pain & temperature sensation
and go through sensory neuron to posterior grey horn but NO, it
enter the tract instead on posterior grey horn and allows to ascend
1 or 2 spinal cord segments , it will not synapse in the posterior
grey horn at T3 instead synapse at T1 level. It will than crossover
and ascends up via the Spinothalamic tract.

111. Spinothalamic Tract
1st order neuron:
 Primary afferents carrying pain, temp & touch
 Peripherally located sensory endings and cells bodies that lie in the dorsal root
ganglia or sensory ganglia associated with cranial nerves
 Axons enter the CNS through spinal or trigeminal nerves
 Terminate by synapsing on cell bodies of ipsilateral 2nd order neurons
 Precise location of termination depends upon the modality they carry

112. 2nd order neuron:
 Cell bodies are located either in the dorsal horn or trigeminal sensory nucleus of
the brain stem
 Axons decussate and ascend to the ventral posterior nucleus of the contralateral
thalamus as spinothalamic or trigeminothalamic tract
 Axons synapse with the cell bodies of 3rd order neurons in thalamus

113. 3rd order neuron:
 Axons of third order neuron pass through
the internal capsule to reach cerebral
cortex
 They terminate in the post central gyrus
of the parietal lobe, also known as
primary somatosensory cortex

114. Primary afferents carrying propioception
and fine touch
 Fibers from trunk and limbs ascend ipsilaterally in the spinal cord as dorsal
column (fasciculus gracilis & fasciculus cuneatus)
 They end by synapsing 2nd order neurons in the dorsal column nuclei (nucleus
gracilis and cuneatus) of medulla
2nd order neuron:
 Axons of second order decussate in medulla and ascend as medial leminiscus to
VP nucleus of contralateral thalamus to synapse with cell bodies of 3rd order
neuron 3rd order neuron: Axons of these cells pass through internal capsule to
reach cerebral cortex and terminate in the somatosensory cortex

117. Overview of Descending Pathways
Corticospinal fibers/pyramidal tract:
 Originates from widespread regions of the cerebral cortex, including primary
motor cortex
 These fibers descend through out the length of the brain stem
 Majority cross to the contralateral side in the decussation of the pyramids in the
medulla
 The continue to descend as the lateral corticospinal tract of the spinal cord
 Terminates in association with interneurons and motor neurons of the spinal grey
matter
 Take part in control of the fine movements such as movement of hand

119. Fine touch,
Vibration,
proprioreception
Stimulate lower motor
neurons to allow voluntary
muscles of skeletal muscles
Stimulate lower motor
neurons to allow voluntary
muscles of Limb flexors
Proprioception (3D)
Proprioception to inferior olives to
cerebellum (Climbing fibers)
Pain & Temperature

120. Peripheral Nervous System

121. Peripheral Nervous System
 Consists of cranial and spinal nerves and their associated ganglia
 Bundles of axons (nerve fibers) supported by delicate areolar tissue
 Specifically composed of
 Efferent axons (fibers) of motor neurons situated inside CNS
 Cell bodies of sensory neurons grouped together as ganglia
 Afferent processes of these sensory neurons
 Ganglionic neurons of ANS
 Neuronal cells bodies in peripheral ganglia are derived embryologically from
cells that migrate from neural crest

122. Cranial Nerves

123. Cranial Nerves
 12 pairs of cranial nerves
 Leave the brain and pass through foramina in the skull
 All nerves are distributed in the head and neck except vagus which supplies the
structure of thorax and abdomen

127. Spinal Nerves

128. Spinal Nerves
 31 pairs of spinal nerves
 Leave the spinal cord and pass through intervertebral foramina in the vertebral
column
 Named according to the region of the vertebral column they are associated with
 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal

129. Structure of a Spinal Nerve
 Spinal nerve is connected to spinal cord by 2 roots
1. Anterior root
2. Posterior root
1. Anterior (ventral) root:
 Consists of bundles of nerve fibers carrying nerve impulses away from central
nervous system (also called efferent fibers)
 Those efferent fibers that innervate skeletal muscles are called motor fibers
 Their cells of origin lie in the anterior horn of the spinal cord

130. 2. Posterior (dorsal) root:
 Consists of bundles of nerve fibers carrying nerve impulses to CNS
 They are also called afferent fibers
 They carry information about the sensation of touch, pain, vibration and
temperature, hence also termed as sensory fibers

132. Structure of a Spinal Nerve
 Cells bodies are present in a swelling of the posterior root called posterior/dorsal root
ganglion
 At each intervertebral foramen, both anterior and posterior root unite to form a spinal
nerve
 Spinal nerve is a mixture of motor and sensory fibers
 It emerges from foramen and divides into
1. Large anterior ramus (supplies muscle and skin of anterolateral body wall, muscles
and skin of limbs)
2. Smaller posterior ramus (supplies muscle and skin of the back)
 Spinal nerves also give a small meningeal branch that supplies the vertebrae and the
coverings of the spinal cord (the meninges)

134. Nerve Plexuses
 Complicated arrangement of nerve fibers at the root of the limbs
 Formed by the anterior rami of spinal nerves
1. Upper Limb: Cervical and brachial plexuses
2. Lower Limb: Lumbar and sacral plexuses

136. Autonomic Nervous System

137. Autonomic Nervous System
 Concerned with innervation of involuntary structures such as heart, smooth muscles
and glands. Represents general visceral motor component
 Distributed through out central and peripheral nervous system
 Hypothalamus controls ANS and integrates activities of ANS and endocrine system
 Activity is regulated at
1.Higher centres of brain stem,
2.Cerebrum
3.Several nuclei of brain stem reticular formation,
4.Thalamus and hypothalamus,
5.Limbic lobe,
6.Prefrontal neocortex & ascending descending pathways that connect these regions

138. Divided into
1.Sympathetic nervous system
2.Parasympathetic nervous system
3.Enteric (neurons are intrinsic to the wall of GIT)
 ANS and Somatic differs in
1. Effectors
2. Efferent pathways (and their neurotransmitters)
3. Target organ responses to neurotransmitters

139. Efferent pathways (& neurotransmitters)
Somatic nervous system
A, thick, heavily myelinated somatic motor fiber makes up each pathway
from the CNS to the muscle
ANS pathway is a two-neuron chain
1. Preganglionic neuron (in CNS) has a thin, lightly myelinated
preganglionic axon
2. Ganglionic neuron in autonomic ganglion has an unmyelinated
postganglionic axon that extends to the effector organ

140. Neurotransmitter Effects
 Somatic nervous system: All Somatic motor neurons release
acetylcholine (ACh) & Effects are always stimulatory
 ANS:
Preganglionic fibers: release Ach
Postganglionic fibers: release norepinephrine or ACh at effectors.
Effect is either stimulatory or inhibitory, depending on type of
receptors
 Somatic Nervous System: Skeletal muscles
 ANS: cardiac muscles, smooth muscles and glands

142. Realistically both the systems are integrated and are capable of discrete activation
Autonomic neurotransmission:
 Preganglionic neurons of both systems and post ganglionic neurons of
parasympathetic system are cholinergic (release acetylcholine “Ach”-
Excitatory). Stimulates post ganglionic fibers
 Post ganglionic neurons of sympathetic system are noradrenergic
(norepinephrine-Inhibitory) – Principal co-transmitters: ATP, neuropeptide Y,
substance P . Stimulates the effector organs.

143. Central nervous system (CNS) Peripheral nervous system (PNS)
Motor (efferent) division
Sensory (afferent)
division
Somatic nervous
system
Autonomic nervous
system (ANS)
Sympathetic
division
Parasympathetic
division

145. Sympathetic Nervous System

146. Sympathetic Nervous System
General features:
 Prepares the body for emergency
 Accelerates heart rate
 Causes constriction of peripheral blood vessels
 Inhibits peristalsis of the intestinal tract
 Closes sphinchters

147. Sympathetic Nervous System Cont..
Anatomical features:
 Sympathetic trunks are 2 ganglionated nerve cords
 Extend from the cranial base to the coccyx
 Ganglia are joined to spinal nerves by short connecting nerves called grey and white
rami communicantes
 Preganglionic axons join the trunk through the white rami communicantes, while
postganglionic axons leave the trunk in the grey rami
 Cell bodies of preganglionic sympathetic neurons are located in the lateral horn of the
spinal grey matter
 Cell bodies of post ganglionic sympathetic neurons are located mostly in the ganglia of
the sympathetic trunk

156. Parasympathetic Nervous System

157. Parasympathetic Nervous System
General features:
 Aims at conserving and restoring body energy
 Slows the heart rate
 Increases intestinal peristalsis
 Opens sphinchters

158. Parasympathetic Nervous System
Anatomical features: (Cranio-sacral)
Preganglionic parasympathetic neuron cell bodies are located in the:
 cranial nerve nuclei (3,7,9,10) of the brain stem
 In the grey matter of the second to fourth sacral segments of the spinal cord
Post ganglionic parasympathetic neuron cell bodies are sited distant from the CNS
either in :
 Discrete ganglia/ terminal ganglia: located near the innervating structure
(target organ)
 Or Intramural ganglia: dispersed in the walls (tissues) of the viscera

161. Divisions Origin of
Fibers
Length of
Fibers
Location of
Ganglia
Type of Ganglia
Sympathetic Thoracolumbar
region of the
spinal cord
(T1-L2)-
Intermediate
lateral grey
Horn
Short
myelinated
preganglionic
(Ach) and long
unmyelinated
postganglionic
(NorEpi)
Close to spinal
cord
• Sympathetic Ganglia/
Paravertebral/ Lateral/
Chain
• Collaterals Ganglia/
prevertebral
• Specialized neurons in
anterior of suprarenal gland
Parasympathetic Brain (Cranial
nerve 3,7,9,
10) and sacral
spinal cord
(S2,S3,S4)
(craniosacral)
Long
myelinated
preganglionic
(ach) and short
unmyelinated
postganglionic
(ach)
In visceral
effector organs
• Within tissue of target
organ (intramural ganglion)
• Close to target organ
(Terminal Ganglion)

165. Enteric Nervous System

166. Enteric Nervous System
 Consists of ganglionic plexuses localized in the walls of gastrointestinal tract
 Contains reflex pathways through which contractions of the muscular coats of
alimentary tract and other GIT functions are controlled
 Capable of sustaining local reflex activity independent of CNS

168. Thank You
ANY QUESTIONS?