Download Laboratory Exercise 10: Anatomy and Physiology of the Spinal Cord

Laboratory Exercise 10: Anatomy and Physiology of the Spinal Cord, Reflex Physiology
One of the fundamental problems in physiology is how to organize and unify the cells of the body into a
functional whole. Integration unifies the body cells. Two requirements of integration are:
1. Communication and
2. Control or modulation - regulating a function within specific limits.
The nervous system is specialized for communication using nerve impulses as its message code.
Communication makes possible control. Control permits integration of the body functions and plays a role in
homeostasis, i.e. the control allows for maintenance of a relatively constant internal environment.
A. Gross Examination of the Spinal Cord and Spinal Nerves
The spinal cord extends from the medulla oblongata of the brain to the 1st or 2nd lumbar vertebra (L1-L2). It is
protected and surrounded by bone and three membranes (meninges). External to the meninges there is
adipose tissue.
Spinal nerves enter and exit the spinal cord via intervertebral foramina.
Types of neurons structure and function – sensory, motor, interconnecting
There are 31 pairs of spinal nerves along the length of the vertebral column. Motor and sensory neurons are
contained in each spinal nerve. The peripheral ends of the spinal nerves are mixed, they contain dendrites of
the sensory neurons and axons of the motor neurons. After the spinal nerve divides into dorsal and ventral
roots there is a swelling on the dorsal surface of the spinal nerve, the dorsal root ganglion in which the cell
bodies of the sensory neurons are located. The dorsal (posterior) root carries axons of sensory (afferent)
neurons into the spinal cord and the ventral (anterior) root carry axons of motor (efferent) neurons out of
the spinal cord. The dendrites, cell bodies, beginning of the motor neuron’s axon are in the ventral horn of
the gray matter of the spinal cord.
C. Spinal Cord Reflexes
The basic structural and functional unit of the intact nervous system is the reflex. A reflex is a automatic,
predictable response to a stimulus. Reflex response is stereotypical, i.e. it produces same type of response
without conscious thought. Reflexes are homeostatic (protective) in nature. Most reflexes are at the
spinal cord level, but impulses can go to the cerebral cortex to inform you that the reflex occurred.
Components of the Reflex
1. Receptors (sense organs) - sensory neuron endings in the periphery of the body.
2. Dorsal root of Spinal Nerve - conducts sensory impulses to the spinal cord.
3. Integrating Center in spinal cord or brain contains interneurons between sensory and motor neurons.
4. Ventral root of Spinal Nerve - conducts motor impulses away from the spinal cord.
5. Effectors (muscle or gland) - produces movement or secretes.
Neurons communicate with each other at junctions, the synapses. The neuron on the receiving end at the
synapse can be excited or inhibited by the neuron delivering the impulse.
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Monosynaptic Ipsilateral (on same side) Reflex
The stretch reflex has one synapse between the sensory and motor neuron. The stretch reflex is initiated when
the stretch receptors or proprioceptors are stretched. The stretch receptors are stimulated on one side, cause
contraction of the muscle on that same side. This contraction is a protective response to overstretching of the
muscle. In the reflex, sensory impulses influence the motor neurons, causing contraction of some muscles and
inhibition of other muscles to produce a coordinated response. This is known as reciprocal innervation.
The patellar reflex (knee-jerk) and calcaneal reflex (Achilles reflex) are examples.
.Polysynaptic Ipsilateral Reflex
The withdrawal or flexor reflex has an interneuron between the sensory and motor neuron. Due to the
interneuron, it brings the stimulus to the level of consciousness. This reflex has at least two synapses. The
withdrawal reflex draws a body part away from a harmful stimulus to prevent damage to the body part.
Contralateral and Ipsilateral Reflex - Cross extensor reflex, muscle contractions occur on the opposite and the
same side of the body as the stimulus.
B., D. Histology and Functional Anatomy of the Spinal Cord
The spinal cord and brain make up the central nervous system (CNS). The CNS analyzes incoming impulses
from the peripheral nerves and integrates them with other neuronal activities to produce appropriate responses.
The spinal cord is subdivided into white and gray matter regions.
White Matter - consists of bundles of myelinated axons called tracts.
1. Ascending Tracts - Three neuron tracts, that convey sensory information from the spinal cord to the
brain. The sensory information has its origin in the sense organs.
2. Descending Tracts - Two neuron tracts that convey motor information from brain to the spinal cord. The
motor information ends at the effectors (muscles or glands).
An example of an ascending tract, is the Spinothalamic tract. This tract is located in the lateral and ventral
(anterior) surfaces of the spinal cord going to the thalamus of brain. The lateral part of the tract conducts
impulses to the opposite sides of the thalamus for pain, and temperature. The ventral part of the tract conducts
impulses to the opposite side of the thalamus for coarse touch, and pressure.
An example of a descending tract, is the Corticospinal tract. This tract is located in the lateral and ventral
surfaces of the spinal cord coming from the brain. The lateral part of the tract conducts motor impulses from one
side of the cerebral cortex to the opposite side of the ventral horn of the gray matter of the spinal cord and then
to the muscles on the opposite side of the body. The lateral tract crosses at the region of the medulla of the brain
known as the Pyramids, thus these tracts are also called Pyramidal tracts. This tract coordinates fine, precise and
discrete skeletal muscle movements.
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SPINAL NERVES
I. CERVICAL PLEXUS:
C1- C4
C2 - C3
C3 - C5
C1 - C5
Distribution:
Extrinsic laryngeal muscles
Skin of upper chest, shoulder, neck,
and ear
Phrenic nerve
Diaphragm
Sternocleidomastoid and Trapezius
II. BRACHIAL PLEXUS:
Distribution:
C5, C6
C5 - T1
Deltoid and Teres minor
Skin of shoulder
Extensor muscles on the arm and
forearm:
Triceps brachii
Extensor carpi u1naris
Extensor digitorum
Abductor pollicis
Skin over posterolateral arm
Flexor muscles on the arm:
Biceps brachii
Brachialis
Skin over lateral forearm
Flexor muscle on the forearm:
Palmaris longus
Pronator teres
Digital flexors:
Flexor digitorum superficialis
Flexor pollicis longus
Skin over anterolateral hand
Flexor muscle on the forearm:
Flexor carpi u1naris
Adductor pollicis
Skin over medial hand
Radial nerve
C5 - C7
Musculocutaneous nerve
C6 – T1
Median nerve
C8, T1
Ulnar nerve
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III. LUMBAR PLEXUS:
Distribution:
T12, L1
Abdominal muscle
External oblique
Skin over lower abdomen & buttocks
Skin over medial upper thigh
Skin over anteromedial thigh
Skin over anterior, lateral and
posterior thigh
Anterior muscles of thigh:
Sartorius
Rectus femoris
Vastus intermedius
Vastus lateralis
Vastus medialis
Adductors of thigh:
Adductor longus
Gracilis
Skin over medial leg
L1, L2
L2, L3
L2 - L4
Femoral nerve
L2 - L4
Obturator nerve
L2 – L4
Saphenous nerve
IV. SACRAL PLEXUS:
Distribution:
L2 – S2
L4 – S3
Abductor of thigh:
Tensor fasciae latae
Extensor of thigh:
Gluteus maximus
Leg flexor muscles:
Semimembranosus
Semitendinosus
Biceps femoris
Plantar flexors:
Gastronemius
Soleus
Flexor digitorum longus
Dorsiflexor:
Tibialis anterior
Extensor digitorum longus
External anal sphincter
Sciatic nerve
Tibial nerve
Peroneal nerve
S2 – S4
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