Chapter 12 lecture: Spinal Cord, nerves, reflexes

both brain and spinal cord

• Receive sensory input from receptors

• Contain reflex centers

• Send motor output to effectors

Reflex

• Rapid, automatic response triggered by stimuli

spinal reflexes

• Controlled in the spinal cord-No brain input

• Control some of the most rapid changes to environmental changes

Conduction

Carry information up and down spinal cord

locomotion

• repetitive, coordinated actions of several muscle groups

• Neurons controlling flexor and extensor muscles

spinal cord structure

• Adult length: about 45 cm (18 in)

• ends at L1-L2 vertebrae

Segments of the spinal cord

• 31 segments:

• 8 cervical

• 12 thoracic

• 5 lumbar

• 5 sacral

• 1 coccygeal

conus medullaris

Cone shaped end of spinal cord at L1-L2

cauda equina

• Nerve roots from spinal cord to lower intervertebral foramen to exit

• L2 to S5 and filum terminale

filum terminale

• anchors the spinal cord to the coccyx and sacrum

• Pia mater

• terminal thread

white matter of spinal cord

• Primarily myelinated axons, but also has unmyelinated axons

• Contains major sensory and motor tracts to and from the brain

gray matter of spinal cord

• Mostly neuron cell bodies, neuroglia, unmyelinated axons

• Has central canal in the middle - contains cerebrospinal fluid

• Site for integration of postsynaptic potentials

posterior gray horn

A

Location of sensory nuclei

lateral gray horn

B

Contains Motor nuclei (visceral)

only in thoracic and lumbar vertebrae

anterior gray horn

C

Contains Motor nuclei (Somatic)

posterior white column

Sends information up to the brain

• sensory info

lateral white column

Information descends from the brain

anterior white column

Information descends from the brain (Brings info back to the spinal cord)

• motor commands

central canal of spinal cord

contains cerebrospinal fluid

posterior median sulcus

anterior median fissure

Posterior Root

Carries sensory neurons

posterior root ganglion

Contains sensory nerve cell bodies

Anterior root

carries motor information

Spinal Nerve

Collection of axons

posterior ramus

• Letter A in image

• relates to muscles, joints, and skin of the back

• Sensory info comes in and motor response going out

Anterior ramus

• Letter B in image

• Impacts Anterior and lateral trunk, as well as limbs

• Sensory info coming in and motor going out

rami communicantes

• Spinal nerves of autonomic nervous system

• See chapter 12 supplemental video for image

Sympathetic nerve

Brings in sensory information from visceral organs

Carries out motor information to visceral organs

See chapter 12 supplemental video for image

spinal meninges

• 3 specialized membranes surrounding spinal cord and brain

• Stability, shock absorption, carry blood supply (oxygen and nutrients)

3 spinal meninges (Superficial to deep)

dura mater, arachnoid mater, pia mater

Dura mater

tough outer layer of the meninges

arachnoid mater

Made of simple squamous epithelium

Pia mater

thin, delicate inner membrane of the meninges

subarachnoid space

• between arachnoid mater and pia mater

• filled with cerebrospinal fluid

  • shock absorber; diffusion of gases, nutrients, etc.

• blood vessels for spinal cord

epidural space

• space between dura mater and vertebrae

denticulate ligaments

• pia mater, through the arachnoid mater, to dura mater

• Prevents lateral movement

Lumbar puncture

• Withdrawal of cerebrospinal fluid

• needle inserted into subarachnoid space between two lumbar vertebrae.

• Also known as spinal tap

3 connective tissue layers of a spinal nerve

epineurium,

perineurium, endoneurium

Epineurium

surrounds the entire nerve

Perineurium

surrounds group of axons (fascicles)

Endoneurium

Surrounds individual axons

dermatome

• Area of skin innervated by spinal nerves

• sensory

• Not face (CN V) or scalp

• Some boundaries overlap

Shingles

• varicella zoster virus

• Causes chicken pox

• Can later attack neurons in posterior roots and sensory ganglia

nerve plexus

• anterior rami of adjacent spinal nerves

• 4 major nerve plexuses:

• Cervical plexus - neck + diaphragm

• Brachial Plexus - pectoral girdle, upper limb

• Lumbar Plexus - pelvic girdle, lower limb

• Sacral Plexus - pelvic girdle, lower limb

Cervical Plexus

• Anterior Rami of C1-C5

• Branches innervate skin/muscles of the neck

• Contains the phrenic nerve

phrenic nerve

C3-C5 to the diaphragm, allows breathing.

brachial plexus

• Anterior Rami of C5-T1

• Innervates skin and muscles of Pectoral girdle, upper back, and upper limbs

lumbar plexus

• Anterior rami of T12-L4

• Innervates skin and muscles of pelvic girdle and lower limbs

sacral plexus

• Anterior rami of L4-S4

• Innervates skin and muscles of pelvic girdle and lower limbs

Subunits of nerves (medial to lateral)

• Anterior rami unites into Trunks. Trunks branch out into divisions, which come together again to form cords

• Lastly, the cords divide into individual nerves

sciatic nerve

• lots of leg movement is associated with this nerve

Step 1 of a simple reflex arc

• Activation of a receptor by stimulus

• Ex: Lean on tack, stimulates pain receptors

Step 2 of a simple reflex arc

• Activation of a sensory neuron

• Graded potential turns into an action potential in a sensory neuron

Step 3 of a simple reflex arc

• Information processing in the CNS

• Sensory neuron

• → excitatory postsynaptic potentials (EPSP) at interneuron

Step 4 of a simple reflex arc

• Activation of motor neuron

• Interneuron stimulates motor neurons

• Collaterals from interneuron may send pain sensations to other areas in CNS

Step 5 of a simple reflex arc

• Response of a peripheral effector

• Ex: Skeletal muscle contraction to pull away from the tack

Classification of reflexes

• development, nature of response, complexity of circuit, processing site

development of reflexes (2 types)

• innate and acquired

innate reflexes

• basic neural reflexes formed before birth (sucking, baby grabbing your finger if you put it in their hand)

acquired reflexes

• rapid, automatic, learned motor patterns

• (slamming foot on break while driving)

nature of response (2 types)

• somatic reflexes and visceral reflexes

somatic reflexes

• involuntary control of skeletal muscles (withdrawal reflex)

Visceral reflexes (autonomic reflexes)

• Controls involuntary effectors: smooth muscles, cardiac muscle, glands, adipose tissue

Complexity of circuit (2 types)

• monosynaptic and polysynaptic reflexes

monosynaptic reflex

Single synapse—simplest reflex arc

polysynaptic reflex

1+ interneuron, multiple synapses

processing site reflexes (2 types)

• Spinal and cranial reflexes

spinal reflexes

• processing occurs in the spinal cord, 1+ spinal cord reflexes

Cranial reflexes

• Processing occurs in the brain

ascending tracts

sensory information in posterior columns of white matter

descending tracts

motor commands in anterior columns of white matter

reflexes

• rapid automatic responses to specific stimuli

• preserve homeostasis through rapid adjustments in organ/organ system function

• little variability in response

Gray commissures

where axons cross, anterior and posterior sides