Download Spinal Cord

Spinal Cord
Lec:2
Assis.Professor Dr. Farah Nabil
Abbas
MBChB, MSc, PhD
Spinal Cord
• Within bony vertebral column.
• Cylindrical of soft tissue (little finger).
• Cross-section  central butterfly (gray matter):
i.
ii.
iii.
iv.
v.
Interneuron
Cell bodies
Dendrites of efferent neurons
Entering fibers of afferent neurons
Glial cells.
Spinal Cord
• Gray matter (Lacks myelin).
• White matter
• Myelinated axons of interneuron “fiber tracts or
pathways”.
1) Descending: Relay information from brain to
spinal cord.
2) Ascending: Transmit information to brain.
3) Others: Transmit information between different
levels of brain and spinal cord.
Spinal Cord
• Afferent fibers enter spinal cord via dorsal root.
• Cell bodies of the afferent neurons “dorsal root ganglia.
• Efferent neurons leaves spinal cord via ventral roots.
• Dorsal and ventral roots from same level combine 
spinal nerve, one on each side of spinal cord.
Motor Functions of the Spinal Cord
(Cord Reflexes)
Reflex
• An involuntary motor response to an adequate sensory stimulus that
occurs subconsciously, without the mediation of the cerebral cortex
• i.e., withdrawal reflex.
Characters of Reflex Action
1. Important role in protection of the body.
2. Responsible for maintenance of:
a. muscle tone.
b. body posture.
3. Center can be anywhere except cerebral
cortex.
4. Center can be in spinal cord or in brain stem.
Classification of the Reflexes
1. Anatomical – classified
involved into:• Segmental reflex
according to the spinal cord segment
• reflex arc involves one side of spinal segment.
• Receptors organ, afferent neuron, efferent neuron and effector organ are
located on same side of spinal segment, i.e., stretch reflex.
Classification of the Reflexes
• Intersegmental reflex
• Reflex arc involves both sides of spinal segment
• Efferent neuron and effector organ are on opposite sides of receptor organ,
i.e., crossed extensor reflex.
• Suprasegmental
• Segments above the level of reflex arc are involved, i.e., postural reflex.
Classification of the Reflexes
2. Physiological:
• Classified into
• flexor/ withdrawal reflex
• extensor / stretch reflex.
3. Clinical Reflexes: classified according to whether they were present
at birth or developed later into:
• Unconditioned & Conditioned reflexes
Classification of the Reflexes
A.
Conditioned reflexes:
• Not present at birth but acquired later on by training
• i.e., cycling and swimming.
B.
Unconditioned reflexes:
• Inborn - present since birth
• i.e., suckling reflex.
• They are of three types:
Unconditioned Reflexes
A. Superficial reflexes
• Elicited by stimulation of receptor organs that are present
superficially in the skin or mucous membrane
i.
Conjunctival reflex
ii. Abdominal reflex
iii. Planter reflex
Unconditioned Reflexes
B. Deep reflexes
• Elicited by tapping the tendon of a slightly stretched muscle
i.
Knee jerk
ii. Ankle jerk.
Unconditioned Reflexes
C. Visceral reflexes
• Concerned with the reflex activity of internal organs.
• Part of the reflex is made by autonomic NS.
i.
ii.
iii.
Cardiovascular reflex
Gastrointestinal reflex
Micturition reflex.
Reflex Arc
• Basic unit of integrated neural activity.
• Made up of sense organ, an afferent neuron, one or more synapses, and an
efferent neuron.
• Monosynaptic “single synapse” – simple arc
• Polysynaptic “multiple interneuron".
• Number of synapses in the arcs - 2 to 1000s
Organization of Spinal Cord for Motor
Functions
• Each spinal segment - several million neurons in the gray matter.
•Gray matter - integrative area of cord
reflexes.
• Neurons (2 types)
1. Anterior motor neurons
2. Internurons
Anterior Motor Neurons
• Anterior horn - Several 1000s neurons / each segment .
• 50-100% larger than most of others.
• Give rise to nerve fibers  leaves by way of ventral roots and
innervates skeletal muscle fibers.
• 2 types
Anterior Motor Neurons
• Alpha Motor Neurons
• Transmit impulse through Aα fibers (14μm)  innervate
large skeletal muscle fibers.
• Stimulation  excites 3-100s skeletal muscle fibers called
the "motor units".
• Gamma Motor Neurons
• Transmit impulse through Aδ fibers (5μm)  innervate
intrafusal fibers - part of the muscle spindle  help to
control basic muscle tone.
Interneuron
• All areas of gray matter.
• 30 times > anterior motor neurons.
• Small and highly excitable.
• Interconnections with each other
• Directly innervate anterior motor neurons.