Histology of the Nerve

The lowest part of the brainstem (medulla oblongata) is continuous with the spinal cord.

The nervous tissue consists of 2 main types of cells: 1. Neurons (nerve cells). 2. Neuroglial cells: non-neuronal, non-excitable cells form

the main component of the nervous tissue.

Spinal nerves leave the vertebral column through intervertebral foramina. (single = foramen)

Each pair of spinal nerves is coming out of a segment of the spinal cord. (Spinal segments are also 31 in no.)

Rootlets (axons) form ventral root (motor) + dorsal root (sensory) unit to form the trunk of a spinal nerve which gives ventral ramus + dorsal ramus.

Nervous System

Peripheral Central

Spinal N. (31 pairs) Cranial N. (12 pairs) Brain Spinal Cord

Cerebrum

Cerebellum

Brainstem

Midbrain

Pons

Medulla Oblongata

8 Cervical

12 Thoracic

5 Lumber

5 Sacral

1 Coccygeal

Effector organs of motor neurons: skeletal muscles.

Effector organs of sensory neurons: skin, muscles, joints…

Effector organs of sympathetic neurons: glands, smooth muscles and cardiac muscles.

Between CNS and PNS we have an independent portion of the CNS: it's the autonomic nervous system (sympathetic + parasympathetic).

Any neuron has many dendrites and a single axon.

The axon will leave the gray matter (and be a part of the spinal nerve) to the effector organ.

The gray matter is formed of: • 2 ventral horns. • 2 lateral horns. • Gray commissure which has a central canal.

In addition we have 2 lateral horns (intermidiolateral horns) only present in the 12 thoracic and upper 2 lumber segments.

Neurons in the ventral horn are mostly motor with large number of neuroglial cells.

Large motor neurons are called ( α ) and small ones are called ( γ ).

Sometimes we call the motor neurons (α + γ ): lower motor neurons (while the upper motor neurons are in the brain) or the final common path.

Axons of α will supply the main bulk of skeletal muscles. (Main bulk = extrafusal muscle fibers)

Intrafusal muscle fibers (muscle spindle) are specialized skeletal muscle fibers act as receptors (stimulus = stretch) and they are supplied by γ axons.

The structure of the spinal segment

If we hit the patellar tendon using a medical hammer, it will stretch the fibers of quadriceps muscle the intrafusal muscle fibers will respond to this stretch.

Types of neurons in the dorsal horn: sensory and very large number of interneurons.

The interneurons (يطة ا الوس connect sensory neurons (الخاليwith motor.

There must be coordination between sensory and motor neurons it’s called integration.

Lateral horn has the mother cells of the sympathetic (preganglionic cells).

The white matter is surrounding the gray matter and formed of axons (nerve fibers) myelinated and unmyelinated.

Group of axons Bundle.

Tract or Pathway: a bundle of axons of similar function.

A sensory pathway carries the impulses to the brain.

A motor pathway carries the impulses or A.P from the brain to the effector organ.

Whenever a muscle is stretched it will contract partially (Stretch Reflex).

The partial contraction of a muscle is called Muscle Tone.

REMEMBER THAT:

The shoulder joint is freely mobile so it’s easily dislocated but it’s stabilized by rotator cuff muscles (Supraspinatus, Infraspinatus, Teres Minor and Subscapularis) these muscles have their tendons very close to the capsule of the shoulder joint.

Forms

Muscle Tone

How do they stabilize the shoulder joint

We should keep the head of humerus close to the glenoid fossa of scapula by muscle tone (approximate the insertion to the origin by partial contraction) whenever a muscle contracts it will shorten; here we use the Stretch Reflex...

The distance between the origin and the insertion of these muscles is lesser than the length of the muscle so it's always stretched partially contract shorten and approximate the insertion to the origin.

Every spinal nerve has 2 roots (ventral + dorsal).

Each root is formed of large number of axons (nerve fibers).

The cell body (the soma) of the motor fiber is in the ventral horn –within the CNS– but the cell body of the sensory fiber is within a ganglion –outside the CNS– connected to the dorsal root and called dorsal root ganglion or spinal ganglion.

The Ganglion is a group of neurons outside the CNS.

Each sensory neuron in the ganglion has a single process divided into central part and peripheral part:

The central part enters the gray matter to end at an interneuron or directly at a motor neuron.

The peripheral part will go to the receptors.

The trunk of the spinal nerve is found in the intervertebral foramen and divides into anterior (ventral) primary ramus + posterior (dorsal) primary ramus.

Essential difference between the ventral and the dorsal rami that only the ventral rami form plexuses.

Every spinal nerve contains sympathetic, sensory and motor fibers.

Spinal nerve

IN THE THORACIC AND UPPER 2 LUMBER SEGMENTS:

The sympathetic neuron in the lateral horn gives preganglionic nerve fiber which will enter a sympathetic ganglion where it makes a synapse and leaves as postganglionic nerve fiber.

The sympathetic ganglion may be inside the sympathetic chain (trunk) or in the abdomen.

The preganglionic fiber has a myelin sheath while the postganglionic doesn't.

The preganglionic fiber is called white ramus communicans while the postganglionic fiber is called gray ramus communicans.

The difference between a sensory ganglion and a sympathetic ganglion is that there is no synapse in the sensory but there is a synapse in the sympathetic…

So if an A.P is formed in the preganglionic sympathetic fiber it will move through the postganglionic fiber after passing through it's soma (cell body) in the synapse but if an A.P is formed in a sensory fiber it will move from the peripheral part to the central part without going through the cell body which job is to provide the nutrition for the cell.

How does the spinal nerve acquire the sympathetic fibers

The preganglionic neuron in the lateral horn gives pre-ganglionic fiber which runs along the ventral root to the ganglion where it synapses and comes out of it as post-ganglionic.

Although the spinal nerves go to skeletal muscles, we need sympathetic fibers to regulate the function of sweat glands and control the smooth muscles of the blood vessels.

The excitable part of a neuron is the part which can create A.P due to the presence of voltage gated ion channels.

The molecular base of A.P is the behavior of Na+, K+, Cl- voltage gated channels.

The excitable parts of the motor neurons: axon and it's terminals.

We can't develop A.P in the soma or the dendrites because there is no voltage gated ion channels.

The excitable parts of the sensory neuron: the central part and the peripheral part except its peripheral processes.

At the dendrites and the cell body a sub-threshold potential is formed (local or graded potential). Sometimes it's called the electrotonic current.

In CNS we have receptive, integrative and motor functions.

Interneurons are the most abundant neurons.

The neuroglial cells affect the metabolism of the neurons.

The nucleus is pale basophilic (poorly stained) and has a well-developed nucleolus.

Neurons are protein-producing cells where the protein is a neurotransmitter.

A.P starts at the initial segment BECAUSE: 1. It has the lowest threshold. 2. It has voltage gated ion channels.

The pre-synaptic vesicles are only present in the axon terminals and contain neurotransmitter…

So the transmission of A.P is only in one way from the axon terminals of the pre-synaptic neuron to the dendrites of the post-synaptic neuron.

Microtubules are used to transport substances from the soma to the axon terminals (such as neurotransmitters).

Mitochondria are abundant in the terminals because they provide energy for releasing neurotransmitters.

The structure of the neuron