Chapter 13: The Spinal Cord, Spinal Nerves, and Somatic Reflexes

Conduction?

Send information up and down the cord

Connects parts of the body with each other and with the brain

Sensory information goes in and motor information goes out

Neural Integration?

Spinal neurons receive multiple inputs from multiple sources and integrates that information

Generate the appropriate → output

Locomotion?

Our motions require coordination with multiple muscle groups and neurons in the brain

Neurons in the spine coordinate these circuits with → outputs

Reflexes?

Reflexes from the spine are key in:

Posture

Motor coordination

Protective responses

Spinal cord?

Cylinder of nervous tissue that arises from the brainstem at the foramen magnum of the skull

Goes as far as L1/L2 vertebrae

Spinal cord is divided into cervical, thoracic, lumbar, and sacral regions

The cord ends before the sacral region, but spinal nerves emerge from this region

Cervical enlargement?

From the C4 to T1

Upper limbs nerves emerge

Lumbosacral enlargement?

T9 to T12

Pelvic region and lower limbs nerves emerge

Cauda equina?

Roots of the lower spinal nerves that angle inferiorly (like wisps of hair)

Innervate with pelvic organs and lower limbs

Just inferior to the lumbar enlargement is the conus medullaris (medullary cone)?

Where the spinal cord terminates around L1 and L2

Coming off of the conus medullaris is the filum terminale (terminal filum)?

Anchors the spinal cord to the → coccyx

Protecting the spinal cord is essential because?

if damaged, the cord cannot be regenerated

The vertebral column and the meninges are the protective structures for the spinal cord

The spinal cord is located within the?

Located within the vertebral canal of the column

within the → vertebral foreman

Meninges?

Three protective layers of connective tissue that encircle the → spinal cord and brain

Spinal meninges?

Surround the spinal cord and are continuous with cranial meninges

Dura mater?

Most superficial (outer meninge), thick layer composed of dense irregular connective tissue

Epidural space?

Cushion of fat and connective tissue between the dura mater and vertebral → wall

Arachnoid mater?

Middle of the meninges, thin, avascular covering made up of collagen and elastic fibers

Arranged like → spider web

Subdural space?

Filled with interstitial fluid, area between the dura mater and arachnoid mater

Subarachnoid space?

Space between the arachnoid mater and pia mater

Filled with → cerebrospinal fluid (CSF)

Pia mater?

Innermost layer, made up of thin transparent connective tissue

Adheres to the surface of the → spinal cord

Vascular, contains blood vessels that supply nutrients and oxygen to the spinal cord

White matter?

Of the spinal cord contains myelinated axon bundles

Carry signals from one part of the CNS to → another

Gray matter?

Of the spinal cord is shaped like a → H or a butterfly

Consists of dendrites, cell bodies, neurons, unmyelinated axons and neuroglia

Site of information processing, synaptic → integration

Spinal Tracts?

Fibers in each tract have similar origin, destination and function

Ascending tracts?

Carry sensory information → up

Descending tracts?

Carry motor information → down

Somatic sensory pathways?

Relay information from sensory receptors to the cerebral cortex and cerebellum

Pathways to the cortex follow sets of neurons called first, second, and third-order neurons

First-order neurons?

Detects stimulus and transmits signal to the → spinal cord or brainstem

Second-order neurons?

Continues to the thalamus at the upper end of the brainstem

Third-order neurons?

Carries the signal the rest of the way to the sensory region of the → cerebral cortex

The spinothalamic tract is part of what?

Is part of the anterolateral system

Carries signals for pain, pressure, temperature, light touch, tickle, and itch

Tract is composed of axons of second-order neurons

Upper motor neurons (UMN)?

Originates in cerebral cortex or brainstem and terminates → lower motor neurons

Lower motor neurons (LMN)?

Leave the brain stem or spinal cord to communicate with → skeletal muscles

Cell bodies are in the brain stem and spinal cord

Lateral and anterior corticospinal tracts carry what?

Carry signals from cerebral cortex for precise, finely coordinated → movements

Spinal nerves?

Act as paths of communication between the spinal cord and the body

How many pairs of the cervical nerves?

8 pairs

How many pairs of the thoracic nerves?

12 pairs

How many pairs of the lumbar nerves?

5 pairs

How many pairs of the sacral nerves?

5 pairs

How many pairs of the coccygeal nerves?

1 pair

How many total nerves?

62 total pairs

Nerve pairs are connected to spinal nerve segments by two bundles of axons called?

Roots

Spinal nerves are associated with the spinal cord, but they are part of the?

PNS

Parallel bundles of axons along with connective tissue layers and neuroglia cells

Dorsal (posterior) root?

Conduct nerve impulses from sensory receptors (in skin, muscles, and internal organs) to the → CNS

Each has a swelling called the dorsal (posterior) root ganglion – contains cell bodies of sensory neurons

Ventral (anterior) root?

Contains axons of motor neurons

Send nerve impulses from the CNS through motor neurons to effectors (muscles or glands)

Reflexes?

Quick, involuntary, stereotyped reactions of glands or muscle to stimulation

Reflexes require?

Stimulation

Not spontaneous actions, but responses to sensory input

Reflexes are?

Quick

Involve few, if any, interneurons and minimum synaptic delay

Secondly, Reflexes are?

Involuntary

Occur without intent and are difficult to suppress

Thirdly, Reflexes are?

Stereotyped

Occur essentially the same way every time

Reflexes include what?

Glandular secretion and contraction of all three types of muscle

Somatic reflexes?

Reflexes involving the somatic nervous system, innervating skeletal muscle

Visceral reflexes?

Reflexes involving organs such as heart (cardiac muscle) and intestines (smooth muscle)

Reflex arc?

Nerve impulse traveling into, through, and out of the → CNS

(1) Sensory (somatic) receptor?

Dendrites of a sensory (afferent) neuron or an associated sensory structure

Responds to a stimulus – change in the internal or external environment

Produces a local potential

Reaches threshold and triggers a → action potential

(2) Sensory (afferent) neuron?

Axon receives the nerve impulse from the sensory receptor

Travels to the gray matter of the spinal cord through the dorsal (posterior) root

Relay the reflex nerve impulses to the parts of the brain responsible for → conscious awareness

(3) Integrating center?

Regions of the gray matter within the → CNS

Integration will occur in multiple regions for a complex reflex

(4) Motor (efferent) neuron?

Picks up the impulses triggered by the integrating center and sends it out of the CNS to an effector through the ventral (anterior) root

(5) Effector?

Part of the body that responds to a motor nerve impulse (muscle or gland)

The response is called a → reflex

Stretch (myotatic) reflex?

When a muscle is stretched, it “fights back” and → contracts

Helps maintain equilibrium and posture

Head starts to tip forward as you fall asleep and muscles contract to raise the head

Stretch reflex stabilizes what?

Stabilize joints by balancing tension in extensors and flexors, smoothing muscle actions

Stretch reflex is mediated primarily by?

The brain, but its spinal component can be greater if muscle is suddenly stretched by a tendon tap (knee jerk)

Knee-jerk (patellar) reflex?

Is a monosynaptic reflex arc

One synapse between the afferent and efferent neurons

Testing somatic reflexes helps diagnose many → diseases

Reciprocal inhibition?

Reflex phenomenon that prevents muscles from working against each other by inhibiting antagonist when agonist is excited

Stretching of muscle stimulates?

Sensory receptors called muscle spindles (monitor changes in muscle length)

Muscle spindles generate?

Nerve impulses that travel up somatic sensory neurons to the → CNS

Spinal cord contains the?

Integrating center, which actives a motor neuron in the gray matter

Motor neuron sends a?

Signal along its axon out to the PNS nerves to → stimulate muscle

Action potential is?

Triggered in the stretched muscle and it → contracts

At the same time, an inhibitor signal is?

Being sent out another motor (efferent) neuron

Inhibits contraction of?

The knee flexor preventing overstretching of the knee extender

Tendon reflex?

In response to excessive tension on the tendon

Inhibits muscle from contracting strongly

Moderates muscle contraction before it tears a tendon or pulls it loose from the muscle or bone

Tension is applied to a?

Tendon, tendon organ (sensory receptor) is stimulated and depolarizes

Nerve impulse travels to the CNS along a → sensory neuron

Spinal cord acts as the?

Integrating center, activates an inhibitory interneuron, which sends a signal to motor neurons

Hyperpolarization (inhibition) occurs in the motor neuron

Muscle relaxes and → eases tension

Flexor?

The quick contraction of flexor muscles resulting in the withdrawal of a limb from a painful stimulus

Triggers contraction of the flexors and relaxation of the extensors in that limb

Polysynaptic reflex arc?

Pathway in which signals travel over many synapses on their way to the muscle

If you step on something sharp dendrites of pain sensory receptors are?

Stimulated

Generates a nerve impulse to the → CNS

In the integrating center (spinal cord) activates interneurons extend to several spinal cord segments

Through ascending and descending → tracts

Interneurons activate?

Motor neurons, in several spinal cord segments, generate nerve impulses that travel to motor→ axon terminals

Motor neuron signals cause flexor muscles in the thigh to contract, making you withdraw your leg (protective)

Cross extensor reflex?

Contraction of extensor muscles in limb opposite of the one that is withdrawn

Maintains balance by extending other leg

Flexor reflex uses an?

Ipsilateral reflex arc, (stimulus and response on same side)

Crossed extension reflex uses a?

Contralateral reflex arc (input and output are on opposite sides)

Intersegmental reflex?

One in which the input and output occur at different levels (segments) of the spinal cord

Pain in foot causes contraction of abdominal muscles

If you step on a tack what happens?

Pain receptors in that foot are stimulated

Generates a nerve impulse, which goes to the→ spinal cord

Integrating center (spinal cord) activates?

Interneurons of the uninjured foot through several spinal cord segments

Pain signals move to?

The opposite side though interneurons and go several segment levels above and below the point of entry into the cord

Motor neurons are?

Excited in several spinal cord segments that connect to extensor muscles and send nerve impulses toward axon terminals

Motor neurons cause extensor muscles in the thigh (effectors) of the uninjured leg to?

Contract

Helps you to put weight on that foot to→ support the body