introduction to neurology. functional anatomy of the spinal cord

Introduction to
Neurology.
Functional Anatomy
of the Spinal Cord
O.L. Zharikova
Associate Professor
BSMU 2016
Plan
Structural and functional divisions of the
nervous system
Principals of the structural organization of
the central (CNS) and peripheral (PNS)
nervous systems
Development of the CNS
Functional anatomy of the spinal cord
Nervous System
is a complex of specialized structures of
nervous tissue
• The main controlling and communicating system of
the body
• Coordinates and direct activities of other systems and
adjusts them to changing conditions of environment
• Provides
◦ Sensory input - detects changes occurring inside
and outside the body (from receptors);
◦ Integration - conducts, processes, and interprets
the sensory inputs in the nervous centers;
◦ Motor output - response to stimuli by activating
effector organs.
Organization of the nervous system.
Topographic division:
CNS (central nervous system) contains the
majority of nerve cell bodies and synaptic
connections
◦ Brain
◦ Spinal cord
PNS (periferal nervous system):
◦ Pared nerves: 12 cranial & 31 spinal
◦ Their roots and ramifications within the body
Nerve fibres and endings (motor and receptors),
nerves, roots, branches, plexuses
◦ Ganglia
CNS and PNS Structures
Functions
CNS provides:
◦ Regulation and coordination of the activity of all
organs and systems
◦ Functional integrity (unity) of the body
◦ Higher mental activity - thinking, learning, memory,
intelligence, est.
PNS constitutes the link between the CNS
and structures in the periphery of the body:
◦ transmits sensory information and motor impulses
Functional division of the nervous
system
Autonomic (vegetative), ANS:
Somatic (animal), SNS
Provide conscious
awareness of the external
environment
Acts largely voluntary
Innervates skin, sensory
organs, striated/skeletal
muscles, (and muscles of
some organs: tongue,
pharynx and larynx)
sympathetic & parasympathetic
monitoring and
controlling conditions in
the internal environment,
i.e. homeostasis
Acts largely involuntary
Innervates internal organs,
glands, vessels, regulates
contraction of smooth and
cardiac muscles
Sensory and Motor Divisions of PNS
Both SNS and ANS have 2 functional divisions:
Sensory (afferent) - transmits impulses to CNS
◦ Somatic (SNS) – from receptors in skin (exteroceptors),
skeletal muscles and joints (proprioceptors)
◦ Visceral (ANS) – from receptors in internal organs and
vessels (interoceptors)
Motor (efferent) - transmits impulses from CNS to effectors
◦ Somatic – skeletal, striated muscles
◦ Visceral - smooth muscle, cardiac muscle, and glands
Principles of Organization of the
Nervous Tissue
Neurons are basic structural & functional units of NS:
Have ability to
◦ receive (excitability) and integrate incoming
information
◦ transmit (conductibility) information to other
neurons or effectors
Highly specialized cells - lose their ability to divide after
birth
Last a life time, but if destroyed cannot be replaced
Neuroglial cells (Glia) are essential for the normal
functioning of nerve cells
Neuroglial cells are more numerous than neurons ≈10 : 1
Neuron Structure
Dendrites collect and
conduct stimuli to the body
• multiple branching
processes increase surface
area for collecting stimuli
Body
Axon (1 or 2) transmits the
receptive zone,
receives information signal away from the cell body
• at the end has nerve or
generates the
electrical
axon terminals (terminal
impulse/signal
buttons)
biosynthetic center
•
•
•
•
Synapse: Connection Between
Neurons
Information is passed between
neurons at synapses usually
by chemical means:
1. Release of
neurotransmitters from
pre-synaptic terminals of
pre-synaptic neurons into
the synaptic cleft
2. Binding neurotransmitters
by the receptors of the
post-synaptic neuron
Types of Synapses
NeuronTypes:
Structural Classification
Neurons show more variation in shape than any other cells
in the body
Bipolar
Multipolar
Interneurons &
Special sensory
neurons
Motor neurons &
interneurons
(olfactory, gustatory,
optic, acoustic,
vestibular)
Pseudounipolar
Most sensory neurons
Neuron Types:
Functional Classification
1. Afferent/Sensory neurons transmit impulses from
receptors to the CNS (cell bodies in sensory ganglia in PNS)
◦ Somatic
◦ Visceral
2. Efferent neurons transmit impulses away from the
CNS to effectors
◦ Somatic efferent/motor neuron (cell bodies in motor
nuclei in CNS) to skeletal muscles
◦ Visceral efferent/autonomic to smooth muscle, cardiac
muscle, and glands (cell bodies in autonomic ganglia in PNS)
3. Interneurons: link the afferent and efferent neurons in
a chain, majority form sensory (or relay) nuclei in CNS (up to
99.8%)
Neuroglial cells (Glia)
Provide structural and metabolic support for neurons:
Nourishment
Protection
Separation
Secretion
Reparation
•
•
•
•
In the CNS:
Ependymal
Oligodendrytes =
Astrocytes
Microglial cells
In the PNS:
• Schwann cells
• Satellite cells
Glial Cells of CNS
Astrocytes (from Latin astra)
form an additional layer around
capillaries that creates selectively
permeable Blood-Brain Barrier
Ependymal cells line
a cavity filled with CSF,
cerebrospinal fluid,
and participate in its
production
Oligodendrocytes
form around axons
a myelin sheath
Glial cells of the CNS: Microglia
Small phagocytes that protect CNS from
bacteria and viruses.
constitute 20% of the total glial cell population
within the brain.
Glial cells of the PNS:
Schwann & Satellite cells
Schwann cells
form a myelin sheath
around axons
Nodes of Ranvier –
gaps between
myelinating cells allow
electrical impulses to jump
from one node to another
(fast conduction)
Satellite cells surround
neurons within ganglia
Myelinated Fibers
Myelin sheath consists of plasma membrane
(lipoprotein gives white color) wrapped multiple
times around the axon
◦ Insulates the axon to prevent interference from
nearby neurons
◦ Increases rate of conduction of an impulse
Myelinated axons/fibers:
◦ In CNS - white matter (all cells’ axons)
◦ In PNS - axons of sensory and motor neurons
Unmyelinated fibers in the PNS
Bundles of axons that are wrapped very thinly
Velocity of conducting signal is slower
Neurons parts &
Structural Components of NS
In the PNS
Neurons’ bodies form
ganglia:
In the CNS
• Neurons’ bodies form
gray matter
◦ Sensory ganglia of cranial
and spinal nerves
◦ Autonomic (efferent)
• Neurons’ processes/fibers
ganglia
• Neurons’ processes/fibers
are bundled together to
form: nerves, nerve fibers,
roots, branches, plexuses
form white matter, which
consists of their bundles
called fibers, fascicles,
commissures, lemnisci,
tracts…
Structure of the Nerve
Endoneurium wraps a single axon including the myelin sheath
Perineurium wraps a fascicle, a small bundle of nerve
fibers/axons. (peri = around)
Epineurium wraps a bunch of fascicles. (epi = upon/top)
CNS: Gray Matter
Gray matter is formed by cells bodies of neurons
together with unmyelinated processes and neuroglia:
◦ Clusters of nerve cell bodies form nuclei of the
spinal cord and brain
◦ Neurons organized by layers form cortex of the
cerebrum and cerebellum
The certain regions of the cortex and nuclei
serve as nervous centers - aggregations of neurons
with similar anatomical connections and functions:
Sensory – receive signals
Integrative – integrate them
Motor – send signals out
Reflex and Reflex Arc
In the NS signals from receptors to effectors
travel along the chains of neurons that include
nervous centers
This chain of neurons is called reflex arc
◦ morphological basis of a reflex
Reflex is a stereotyped pattern of response to a
sensory stimulus:
◦ Somatic
◦ Visceral
Mono- and polysynaptic reflex arcs
• In the simplest (stretch) reflex sensory neurons
directly synapse on motor neurons - monosynaptic
• Most reflexes involve an interneuron - polisynaptic
Development of the NS & Spinal Cord
1.
2.
3.
4 successive stages in the development of the spinal cord:
Neural plate: thickening of the midline ectoderm; ≈16th
day of embryonic development (stage of gastrulation)
Neural groove: neural plate deepens forming the neural
groove & folds on its sides; at their dorsal aspects the
neural crests form;18th day
Neural tube: neural folds and crests fuse by the 21st day
(end of 3rd week); neural tube separates from the surface
and neural crest starts to delaminate from the neural tube
Future
Spinal
22nd day
The whole neural tube is enclosed normally by
4 weeks and the cells begin a period of rapid division.
The front part will become the brain and the rest will
become the spinal cord.
Cell Differentiation in the Spinal Cord
• Starts from the caudal
portion of the neural tube
5 ½ weeks
• By week 6, there are 2
clusters of neuroblasts:
1. Dorsal alar plate →
interneurons; become
Mantle layer
posterior horn
Marginal layer
2. Ventral basal plate →
efferent (motor) neurons;
become anterior horn
Ependymal layer
• Neural crest cells →
sensory neurons; form
Other derivatives of the neural
dorsal root ganglia
crest: autonomic neurons and
adrenal medulla cells, Schwann
cells, melanocytes in the skin est.
Spinal Cord Growth
At 2nd-3rd
antenatal
month:
Vertebral
column=
Spinal cord
Spinal cord
ends at L3;
Length = 14 cm
6th
At
antenatal
month:
Spinal cord ends
at sacrum
Spinal cord
ends at L2;
Lenth=43-45cm
Development of the Brain
32 days
25 days
newborn
Congenital Malformations of the CNS
Many different types of brain
congenital defects
(anomalies) are caused by
improper closure of the
neural tube:
Anencephaly (“no brain")
when the topmost portion of
the tube fails to close
Encephalocele is a
protrusion of a part of the
brain through a defect in the
skull
Congenital Malformation of the
Spinal cord
Failure of closure of the neural tube or its close position to
the surface prevents full development of the vertebral arch →
spina bifida (cleft spine); most often at the L5 or S1
◦ low folic acid ingestion in 1st trimester
It may cause protrusion of the spinal cord (or cauda
equina) and its meninges
Functional Anatomy of the
Spinal Cord
Functions of the Spinal cord
Transmission of neural signals between the brain and
the rest of the body through the conduction tracts
◦ sends sensations to the brain from the body
◦ returns motor commands to skeletal muscles and internal
organs
Coordination of somatic reflexes - segmental
innervation of the body
◦ maintenance of posture and muscle tone, coordination of
movements, protection
Coordination of autonomic reflexes regulating:
◦ work of internal organs;
◦ functions of the bowel, bladder and sexual organs
(controlled by autonomic centers in conus medullaris)
◦ blood pressure and body temperature
Spinal reflexes
Are reflexes, involuntary stereotyped patterns of
response to sensory stimuli, involving the spinal
cord
Somatic reflexes provide muscular responses
1. Contribute to ordinary muscle activity (such as
the “knee” reflex)
2. Protective responses (such as withdrawing the
hand after touching a hot surface)
* Visceral (autonomic) reflexes will be discussed
in ANS
Spinal Somatic Reflex Arcs:
Monosynaptic reflexes
Tendon / stretch reflex
mediated by a 2–neuron
chain
Caused by percussion of the
tendon of the muscle
Extensor contraction is
important for muscle tone
and upright body posture Biceps C5-6
Each is served by specific Brachioradial (supinator jerk) C5-6
Triceps C6-7
spinal cord segments
Quadriceps (knee jerk) L3-4
Achills (ankle jerk) S1-2
Spinal Somatic Reflex Arcs:
Polisynaptic reflexes
Flexor reflex mediated by a 3–neuron chain
Noxious cutaneous stimulation of the limb causes
its flexion withdrawal (protection)
Requires coordinated action of several joints
and involves several spinal cord segments
Autonomic and Somatic Spinal
Reflex Arcs
Functional Zones/Nuclei of the
Gray Matter
somatic sensory (SS) - posterior horn
visceral sensory (VS) - posterior horn
visceral motor (VM) - lateral horn autonomic
(sympathetic) neurons (C8-L2)
somatic motor (SM) - anterior horn
• gray commissure — encloses central canal, connects
masses of gray matter
Functional division of White
Matter in the Spinal Cord
White matter is divided into 3 funiculi
• Fibers run in 3 directions: ascending, descending, transversely
(horizontally)
• Bundles of fibers form conducting tracts
Conduction tracts (pathways) are formed by nerve processes
(axons) with similar functions and connections
•
◦ Tract names usually reveal their origin and destination
Long: afferent & efferent
Short conducting tracts form the proper fibers
-
Posterior funiculus: ascending afferent sensory tracts
Anterior funiculus: primary descending efferent motor tracts
Lateral funiculus: both
Next to the gray matter: proper fibers – connect centers within
the spinal cord
White commissura: proper fibers and fibers of some long
conducting tracts
Somatotopic Organization of the
Spinal cord
Segments of the Spinal Cord
Segmental Innervation of the Skin
Dermatome Map of the Body
Dermatome - skin area
innervated by the sensory
fibers from a single spinal
segment/root/nerve
◦ bands around the trunk and
along the limbs.
Cervical segments/spinal
nerves innervate neck and arms
Thoracic - chest and body,
Lumbar – lower back & legs
Each dermatom is supplied
from 3 segments
Segmental Innervation of Muscles
A group of muscles primarily innervated by
the motor fibers of a single nerve
segment/mtor root is called a myotome.
•
•
In a spinal cord injury, complete or partial paralysis
occurs in the areas of the body that are controlled by
the nerves associated with the damaged
segment and those below the damaged segment
The higher the injury the more area of paralysis
Reticular Formation - Is one of the
integrative systems of the CNS
loosely organized network, which consists of clusters of neurons
bodies interspersed among small bundles of myelinated axons
extends throughout the brain stem from the spinal cord to
hypothalamus
Includes small and large nuclei (more than 1000)
Receives collaterals from afferent tracts; being activated activates
cortex of the brain (RAS) via thalamus or directly
needed for arousal from sleep and maintaining wakefulness
helps to maintain consciousness
filters out insignificant information
Has afferent and efferent connections with all parts of the brain and
the spinal cord: process incoming sensations and sends
outgoing motor commands;
◦ Conducts and process nonlocalized pain
◦ Activates or inhibits motor neurons of the spinal cord, regulates muscle
tone
◦ Initiate involuntary motor responses to stimuli
◦ Regulates visceral reflexes
Reticular Formation
Location & Connections
51
Meninges of the brain and spinal cord
The CNS is protected and isolated by multiple
structures:
Bones: vertebral canal and bony cranium
Meninges: Three connective tissue membranes
surround the CNS – dura mater, arachnoid mater & pia
mater:
◦ Provide protection, stability and shock absorption for the CNS
parts and their blood vessels
Dura mater forms partitions for parts of the brain and venous sinuses
within the skull
Denticulate ligaments of pia mater help to suspend the spinal cord
The pia mater contains blood vessels and capillaries that are responsible
for nourishing the brain and formation of blood-brain barrier
prevents entry of harmful materials from the bloodstream
◦ Participate in the formation and circulation of the cerebrospinal
fluid (CSF)
1552
Cerebrospinal fluid
CSF bathes the inside and outside of the
spinal cord and the brain and provides a
liquid support and cushion
Nourishes brain and spinal cord, removes
waste
Carries hormonal and chemical signals
between parts of the CNS
• A lumbar puncture (spinal tap) is performed to test
CSF (or reduce pressure) between spinous processes
of L3-L4 or L4-L5 vertebrae
•
A needle should be inserted into the subarachnoid space
• The epidural anesthesia is made into the dural sac to
reach the roots of spinal nerves