·         The spinal cord begins at the level of the foramen magnum and extends to L2 vertebral segment

·         Each spinal nerve leaves the spinal cord at the intervertebral foramen

o C1-C7 exit the spinal cord through the intervertebral foramen ABOVE the corresponding vertebrae

o All other spinal nerves leave through the intervertebral foramen BELOW the corresponding vertebrae

** What this means is that the spinal cord level below C7 DIFFERS from the vertebral level. Therefore, when there is injury to the spinal column (such as in a fall or auto accident) it can impact the spinal cord structure but the clinical outcome will be that of a lower segment of the cord versus the vertebral level.

Remember the organization of the spinal cord has a typical pattern which includes a center that has a butterfly or H-shaped pattern which is darker and is surrounded by a lighter area. The butterfly or H consists of gray matter. This gray matter is organized into discrete layers or columns. The lighter areas are made up of white matter: the tracts that ascend and descend.

• reflexes are integrated
• sensory info comes in the SC posteriorly in DR and synapses in gray matter to elicit segmental reflexes as well as head up to let brain know what is going on

·         Alpha Motor Neurons found in the ventral (anterior) horn

·         Gamma Motor Neurons found in the ventral horn

·         Interneurons found in all areas of the gray matter

o Renshaw Cells a type of interneuron found in the ventral horn (inhibitory- help regulate and smooth movement).

the ventral horn has a part of the homunculus where the motor neurons exit that innervate the trunk = medially

extremities = distally

Motor neurons to the extensor mms = caudally

flexors mms = rostrally in the horn

• muscle > muscle fiber > myofibril > myofilaments > actin(thin) and myosin(thick)

In a myofibril you can see a pattern of stripes, which are due to the actin and myosin.

Explain

Each stripe constitutes a sarcomere. There are dark and light bands in the stripe. The dark bands are called "A" bands and the light bands are "I" bands. The "I" band is bisected by a dark line called the "Z" line.

The actin and myosin are responsible for muscle contraction by binding (cross bridges) and moving over one another. The actin is pulled towards the center of the sarcomere sliding over the myosin during a muscle shortening contraction.

The synapse between a muscle and a nerve is called the neuromuscular junction or motor end plate. Neurotransmitter is used to transmit the signal across the junction (acetylcholine). The Ach travels across the junction and acts at specific acetylcholine receptor sites to depolarize the muscle membrane. The signal is propagated along the muscle fiber along the transverse tubules (T-tubules). The sarcoplasmic reticulum is responsible for regulating the calcium in the muscle cells. The release of calcium triggers the formation of actin-myosin bonds for contraction.

As the actin and myosin overlap tension develops. Maximum tension is developed at that point where the myofilament overlap has the maximal cross bridge formation. This is the optimal length. As you have less cross bridges there is less tension able to be developed in the muscle.

Muscle Spindle:

Intrafusal muscle fibers:

·         Nuclear bag fibers

o Dynamic

o Static

·         Nuclear chain fibers

The spindle is innervated by the gamma afferent fibers:

·         Group 1A -innervates the sensory portions of the bag and chain

·         Group II-innervates the static bag fibers and all the chain fibers

The nuclear bag and chain are in series with the intrafusal muscle fibers

Efferent innervation:

·         The extrafusal muscle fibers are innervated by alpha motor neurons

·         The intrafusal muscle fibers are innervated by gamma motor neurons (also called fusiform neurons: innervate the distal contractile part of the fiber)

The spindle lies in parallel with the extrafusal muscle fiber.

intrafusal muscle

As the spindle is put on tension it is depolarized and initiates an action potential in the group Ia and group II axons.

31%

so the CNS is monitoring the muscle spindle.

·         When a muscle is stretched the spindle is excited and a reflex contraction occurs to the muscle fibers that surround the spindle. Let's look at the circuitry of this reflex (as in a knee jerk reflex).

·         Muscle is put on stretch (you tapped the tendon)

·         The spindle is excited with the signal carried by a type Ia and group II sensory fibers (afferents) to the dorsal root of the spinal cord

·         The axon of the Ia goes to the ventral horn and synapses on an alpha motor neuron (monosynaptic pathway). The type II afferents make polysynaptic connections. (A rapid stretch will activate the Ia pathway more than a slow stretch)

·         From the alpha motor neuron the signal is carried back to the same muscle and facilitates it (resists the stretch-autogenic facilitation) the knee extends-"kicks"

·         The axon coming into the spinal cord also branches and synapses on an interneuron

·         The interneuron then synapses with an alpha motor neuron that then goes to the antagonist muscle and inhibits it (reciprocal inhibition). With the knee jerk it is the hamstrings that are inhibited so they don't interfere with the reflex.

·         The axon coming into the spinal cord also sends a branch up to the cerebellum and reticular formation (let's the brain know what is going on)

·         Higher centers influence the system by synapsing on alpha motor neuron and then down a gamma efferent to contract the muscle spindle. (This keeps the spindle shortened with the muscle so it can continue to monitor length). This is known as Coactivation. The alpha and gamma motor neurons are stimulated together resulting in contraction of both the extrafusal and intrafusal muscle fibers:

o Keeps the length of the spindle receptor constant so it doesn't oppose the muscle contraction

o Provides a dampening mechanism

1. CNS via presynaptic inhibition
2. inhibitory synapse of the IA preventing reciprocal inhibition
3. controlling the Y motor neuron affecting muscle spindle sensitivity

this loss can lead to abnormal reflex activity like spasticity

Flexor and Crossed Extension Reflexes:

What and How?

What = are protective reflexes that are a result of a pain stimulus

How = free nerve endings from foot carried by afferent nerves to SC > synapses on interneuron which fasciliates alpha motor neurons > for several segments so you can withdraw your foot from the sharp stimulus

ALSO > triggers the crossed extension reflex by > in SC polysynaptic connections through commissural spinal interneurons to the opposite side to fascilitate extensor mms for standing balance.

Reflexes of Posture and Locomotion:

continued

Positive Supportive Reaction = pressure on bottom of foot causes leg ext but if SC is transected the reflex is intensified and any pressure on foot can cause the leg to ext in such as way as to support weight bearing

Rhythmic Stepping Mvmt = triggers from sensory system from bottom of foot and position sensors can activate CPG producing rhythmic stepping

• found near m attachment and in series with extrafusal m fibers
• encaspulated sensory receptors IB axons
• TENSION monitors
• when m excessively contracts it comes to the rescue - does the reverse of the stretch reflex
• called a reciprocal reflex - faciliates the antagonist
• unloads the agonist (inhibits)
• synapse on interneuron - lamina 5 - 7

GTO:

clinical implications

• similar to skeletal -contains actin and myosin and ligands for chemical transmission
• contraction time is LONGER
• organized in sheets
• controlled by NT, hormones and stretch
• resting membrane potential = -60mV
• sympathetic and parasympathetic innervation
• uses ACh and Norepinephrine as NT

0 No response
1+ Present, but depressed, low normal
2+ average, Normal
3+ Increased, brisker than average; possible but  not necessarily abnormal
4+ Very brisk, hyperactive, with clonus; abnormal

resistance to passive stretch

It is due to mechanical (elasticity) and activation factors (number and rate of motor discharges). The number and rate of motor discharges is the critical variable. The number and rate of motor unit discharges depend on the stimulation of LMN's by muscle spindles and other receptors in the skin, tendons, joints, and bone and the stimulation of LMN's by pyramidal and extrapyramidal pathways. The muscle tone depends on the activity of motor units as determined by the algebraic sum of diverse excitatory and inhibitory impulses from the peripheral nervous system and the central nervous system on the LMN. The end result can be hypotonia or hypertonia.

Hypotonia

Hypertonia

Clonus

• tone is decreased below normal levels and stretch reflex is dampened
• increase in tone above resting levels
• cyclical, spasm alteration of contraction and relaxing when spastic m is put on sustained stretch. Often seen in PF and can be counted like a musical beat

Spasticity

Rigidity

• form of hypertonicity and is VELOCITY dependent - you feel a spastic catch and release like a ratchety mms feel
• form of hypertonicity that is NOT velocity dependent - commonly seen in Parkinson's

1. vascular tone
2. sweating
3. GI reflexes
4. bowel
5. bladder reflexes

Example = bladder control - holds about 300-500ml of urine

Recap: S2-4 is the parasympathetic component; T9-L2 is the sympathetic component.