Chapter 13

The Spinal Cord and Spinal Nerves

Functions of the nervous system

• Sensory (input):– Light– Sound – Touch– Temperature– Taste– Smell– Internal Chemical– Pressure– Stretch

Functions of the nervous system (cont’d)

• Integration:– Integration means making sense of

sensory input. Analyzing stimuli based on experience, learning, emotion & instinct and reacting in a useful way (you hope).

• Motor (output):– The response to the sensory input

and subsequent integration. Sending signals to the muscles and other organs of the body instructing them how to respond to the stimuli.

Nervous System Organization

Overview of Ch 13 & Ch 14

The Spinal Cord & Nerves

• The spinal cord is part of the Central Nervous System.

• The spinal nerves are part of the Peripheral Nervous System.

• The lowest level of integration occurs in the spinal cord and peripheral ganglia.

Spinal cord: gross

anatomy

Spinal cord anatomy:

the meninges

Spinal cord and

associated structures

Functional arrangement of the spinal cord tissues

Cross section of the spinal cord

White Matter in the Spinal Cord

• Fibers run in three directions – ascending, descending, and transversely

• Divided into three funiculi (columns) – posterior, lateral, and anterior

• Each funiculus contains several fiber tracks– Fiber tract names reveal their origin

and destination– Fiber tracts are composed of axons with

similar functions

Spinal nerves• Thirty-one pairs of mixed nerves

arise from the spinal cord and supply all parts of the body except the head

• They are named according to their point of issue– 8 cervical (C1-C8)

– 12 thoracic (T1-T12)

– 5 Lumbar (L1-L5)

– 5 Sacral (S1-S5)

– 1 Coccygeal (C0)

Spinal nerve

structure

Spinal Nerve structure• Each axon is also called a “nerve

fiber”– These are covered by an endoneurium– The endoneurium is made of areolar c.t.

• Fibers are bundled into “fascicles”– These are covered by perineurium– This is an extension of the outer layer of

collagen fibers

• Nerves are bundles of fascicles– Each nerve is covered by dense

irregular tissue of collagen fibers

Components of the Peripheral Nervous

System• Motor pathways

– Leave the spinal cord via the ventral nerve roots.

– Have multipolar cell bodies found in the gray matter.

– Carry motor impulses to skeletal muscle (somatic pathways) and to glands, smooth muscle, the heart, organs, etc (autonomic pathways).

– Are also called efferent pathways.

Motor pathways

Components of the PNS • Sensory Pathways

– Enter the cord via the dorsal roots.– Have unipolar cell bodies found in the

dorsal root ganglia.– Carry sensory inputs into the CNS via the

central processes of their axons. They begin at the general sensory receptors of the skin (somatic sensory) and internal organs (visceral sensory).

– Are also known as afferent pathways.– Special sense will be covered in Chapter

17

Sensory pathways

What’s a damn

dermatome?

Nerve plexuses

Nerve plexuses

• Fibers travel to the periphery via several different routes

• Each muscle receives a nerve supply from more than one spinal nerve

• Damage to one spinal segment cannot completely paralyze a muscle

Cervical plexus

Table 13-1

Summary: Cervical Plexus

Brachial plexus

Fig. 13.13a

The brachial plexus

Some common injuries to the brachial plexus

Study question: Which nerves are affected here?

Table 13–2 (1 of 2)

Summary: Brachial Plexus

Table 13–2 (2 of 2)

Summary: Brachial Plexus

Lumbar and sacral

plexuses

Table 13-3 (1 of 2)

Summary: Lumbar and Sacral Plexuses

Table 13-3 (2 of 2)

Summary: Lumbar and Sacral Plexuses

Lumbar & sacral plexus

nerves

Neural circuits

Neural Circuits• Divergent – spread information from

one source to several destinations.– Examples: visual input being processed

at a conscious level (the horizon is tilting) and a subconscious level (I adjust my body so that I don’t fall over).

• Convergent – multiple sources of input into one neuron. – Examples: conscious – I contract my

rectus femoris to step over a pile of dog poop. Unconscious – my rectus femoris automatically contracts as the bus moves

Neural Circuits• Serial – a series of neurons in a

sequence.– Example: Pain pathways

• Parallel – Divergence followed by serial.– Example – reflexes that result in a complex

series of responses simultaneously

• Reverberation – positive feedback loops– Examples – many of the complex processes

of the brain.

Reflexes

• Rapid, automatic responses to stimuli.

• Can be visceral (e.g. swallowing) or somatic (“knee-jerk”).

• Have little variability

Components of a reflex arc

1. Stimulus activates a receptor.2. Impulse travels along a sensory

pathway.3. Integration occurs in an

integration center (most often in the CNS)

4. Impulse then travels by a motor pathway.

5. An effector responds.

4 Classifications of Reflexes

1. By early development2. By type of motor response3. By complexity of neural circuit4. By site of information

processing

Development• How reflex was developed:

– innate reflexes:•basic neural reflexes •formed before birth

– acquired reflexes:•rapid, automatic•learned motor patterns

Response• Nature of resulting motor response:

– somatic reflexes:• involuntary control of nervous system

– superficial reflexes of skin, mucous membranes

– stretch reflexes (deep tendon reflexes) e.g., patellar reflex

– visceral reflexes (autonomic reflexes):• control systems other than muscular system

e.g., glands smooth muscle and cardiac muscle

Complexity

• Complexity of neural circuit:– monosynaptic reflex:

•sensory neuron synapses directly onto motor neuron

– polysynaptic reflex:•at least 1 interneuron between

sensory neuron and motor neuron

Processing

• Site of information processing:– spinal reflexes:

•occurs in spinal cord– cranial reflexes:

•occurs in brain

Monosynaptic: Stretch reflex

Monosynaptic Reflexes

• Have least delay between sensory input and motor output:– e.g., stretch reflex (such as patellar

reflex)

• Completed in 20–40 msec

Muscle Spindles

• The receptors in stretch reflexes• Bundles of small, specialized intrafusal

muscle fibers:– innervated by sensory and motor neurons

• Surrounded by extrafusal muscle fibers: – which maintain tone and contract muscle

Muscle spindles

Postural Reflexes

• Postural reflexes:– stretch reflexes– maintain normal upright posture

• Stretched muscle responds by contracting:– automatically maintain balance

Polysynaptic Reflexes

• More complicated than monosynaptic reflexes

• Interneurons control more than 1 muscle group

• Produce either EPSPs or IPSPs

Polysynaptic: Flexor

The Tendon Reflex

• Prevents skeletal muscles from:– developing too much tension– tearing or breaking tendons

• Sensory receptors unlike muscle spindles or proprioceptors

Withdrawal Reflexes• Move body part away from

stimulus (pain or pressure):– e.g., flexor reflex:

•pulls hand away from hot stove

• Strength and extent of response:– depends on intensity and

location of stimulus

Reflex Arcs

• Ipsilateral reflex arcs:– occur on same side of body as

stimulus– stretch, tendon, and withdrawal

reflexes

• Crossed extensor reflexes:– involves a contralateral reflex

arc– occurs on side opposite stimulus

Figure 13–18

The Crossed Extensor Reflex

5 General Characteristics of Polysynaptic Reflexes

1. Involve pools of neurons2. Are intersegmental in distribution3. Involve reciprocal inhibition4. Have reverberating circuits:

– which prolong reflexive motor response

5. Several reflexes cooperate:– to produce coordinated, controlled

response

Figure 13–19

The Babinski Reflexes

• Normal in infants• May indicate CNS damage in adults

Spinal Cord Trauma: Paralysis

• Paralysis – loss of motor function• Flaccid paralysis – severe damage

to the ventral root or anterior horn cells– Lower motor neurons are damaged

and impulses do not reach muscles– There is no voluntary or involuntary

control of muscles

Spinal Cord Trauma: Paralysis

• Spastic paralysis – only upper motor neurons of the primary motor cortex are damaged– Spinal neurons remain intact and

muscles are stimulated irregularly– There is no voluntary control of

muscles

Spinal Cord Trauma: Transection

• Cross sectioning of the spinal cord at any level results in total motor and sensory loss in regions inferior to the cut

• Paraplegia – transection between T1 and L1

• Quadriplegia – transection in the cervical region

Spinal cord transection

Poliomyelitis

• Destruction of the anterior horn motor neurons by the poliovirus

• Early symptoms – fever, headache, muscle pain and weakness, and loss of somatic reflexes

• Vaccines are available and can prevent infection

Some effects of Polio

Amyotrophic Lateral Sclerosis (ALS)

• Lou Gehrig’s disease – neuromuscular condition involving destruction of anterior horn motor neurons and fibers of the pyramidal tract

• Symptoms – loss of the ability to speak, swallow, and breathe

• Death often occurs within five years• Linked to malfunctioning genes for

glutamate transporter and/or superoxide dismutase

Some Famous Victims of ALSLou Gehrig

Steven Hawking,renowned physicist

Axonal degeneration of motor neurons evident in lateral corticospinal (pyramidal) pathways, especially

in the loss of

myelinated fibers of the corticospinal tracts

SUMMARY (1 of 7)

• General organization of nervous system: – CNS, PNS

• Gross anatomy of spinal cord:– enlargements, dorsal and ventral

roots, filum terminale, conus medullaris

SUMMARY (2 of 7)

• Afferent (sensory) and efferent (motor) fibers

• Structures and functions of spinal meninges

• Gray matter and horns of spinal cord

SUMMARY (3 of 7)

• White matter and columns (tracts) of spinal cord

• 3 layers in spinal nerves• Distribution (rami) of spinal nerves:

– white, gray, dorsal, ventral

SUMMARY (4 of 7)

• 4 major nerve plexuses:– cervical, brachial, lumbar, sacral

• Neuronal pools and neural circuit patterns:– divergence, convergence, serial,

parallel, reverberation

SUMMARY (5 of 7)

• Reflexes and reflex arcs• Classifications of reflexes:

– innate vs. acquired– somatic vs. visceral– cranial vs. spinal– monosynaptic, polysynaptic, or

intersegmental

SUMMARY (6 of 7)

• Characteristics of monosynaptic reflexes:– stretch reflex, postural reflex, muscle

spindles

• Characteristics of polysynaptic reflexes:– tendon, withdrawal, flexor, and

crossed extensor reflexes

End

Chapter 13