Unit 10 - Nervous System, Spinal Cord, Spinal Nerves

• Nervous System

1. Nervous Tissue (Ch. 12)

• Overview of Nervous System (12.1)
• Properties of Neurons (12.2)
• Supportive Cells (12.3)

1. 2. The Spinal Cord, Spinal Nerves, (Ch. 13)

• The Spinal Cord (13.1)
• Spinal Nerves (13.2)
• Internal Communication
• Endocrine and nervous system maintain internal coordination

1.Endocrine system

• uses hormones secreted into blood
• “long-distance” communication
• Slow

2.Nervous system

• Uses electrical and chemical means
• sends message from cell to cell
• Very quick
• Nervous system components

1. Central nervous system (brain and spinal cord)
   • Enclosed by cranium and vertebral column (very protected).
2. Peripheral nervous system (everything else)

• Nervous System Function

1. Sense organs receive information and transmit coded messages to the CNS (brain, spinal cord)(sensory/afferent).
2. CNS processes information, determines appropriate response.
3. CNS issues commands to muscles and gland cells to carry out response (motor/efferent).

Visceral: heart, stomach, bladder

• Peripheral Nervous System

1. Sensory (afferent) division – carries signals from receptors to

CNS [Receptors]

• Somatic sensory division
  • Signals from skin, muscles, bones, and joints
• Visceral sensory division
  • Signals from the viscera (heart, lungs, stomach, and urinary bladder)

2. Motor (efferent) division – carries signals from CNS to

effectors (glands/muscles) [Effectors]

• Somatic motor division
  • Carries signals to skeletal muscles
• Visceral motor division(autonomic nervous system)
  • Signals to glands, cardiac, and smooth muscle
    • Involuntary responses are visceral reflexes.
• Visceral Motor Division (a.k.a Autonomic Nervous System)
• Subdivisions:

1. Sympathetic Division

• Arouses body for action (“fight or flight”)
  • Increases heart rate and respiration
  • Inhibits digestive and urinary systems

1. Parasympathetic Division

• Calming effect (“rest and digest”)
  • Slows heart rate and breathing
  • Stimulates digestive and urinary systems
• Properties of Neurons
• Neurons (nerve cells) all have 3 fundamental properties:

1. Excitability

• Will respond to stimuli (environmental changes)

1. Conductivity

• Produce electrical signals that are quickly conducted to other cells at distant locations

1. 3.Secretion
2. When signal reaches the end of a nerve fiber, a neurotransmitter will be released that crosses the gap and influences the next cell.

• Functional Classes Neurons

1. Afferent (sensory) neurons

• Detect stimuli and transmit the information toward the CNS

1. Interneurons

• ~90% of all neurons connect neurons together
• Receive signals from many neurons and carry out integrative functions (make decisions)

1. Efferent (motor) neurons

• Sends signals to muscles and/or gland cells.
• Neuron Structure
• Soma
  • Control center of neuron
  • a.k.a. Neurosoma or cell body
• Dendrites
  • Branches off of the soma.
  • Receiving signals from other neurons.
• Axon
  • Originates from a mound on the soma called the axon hillock
  • For rapid conduction of signals to distant points
  • Axoplasm - cytoplasm of axon
  • Axolemma - plasma membrane of axon
  • Myelin sheath may enclose it.
  • Distal end has terminal arborization.
  • Synaptic knob (terminal button) is swelling that forms junction (synapse) with next cell.
  • Contains synaptic vesicles full of neurotransmitter.

• Neuron Types

1. Multipolar neuron

• One axon, multiple dendrites
• Most common neuron, most neurons in CNS are this kind

1. Bipolar neuron

• One dendrite and one axon
• Olfactory cells, retina, inner ear

1. Unipolar neuron

• Single process leading away from soma
• Sensory cells from skin and organs to spinal cord

1. Anaxonic neuron

• Many dendrites, no axon
• Retina, brain, and adrenal gland
• Neuroglia (Supportive Cells)
• Outnumber neurons 10:1
• Protect neurons and help them function
• Bind neurons together and form framework for nervous tissue
• 4 Types in CNS

1. Oligodendrocytes

• Form myelin sheaths that speed conduction.

1. Ependymal cells

• Line internal cavities of brain, secrete and circulate cerebrospinal fluid (CSF)
• Cuboidal epithelium

1. Microglia

• Wander through CNS looking for damage/debris
• For debris and foreign matter
• Found in places fighting infection, injury

1. Astrocytes

• Most abundant in CNS, clever entire brain surface and most non-synaptic regions of neurons.

• Neuroglia Types - CNS
• Astrocytes
• Functions:

1. Form supportive framework.
2. Perivascular feet (extensions) contact blood capillaries to form seal (blood-brain barrier).
3. Convert glucose to lactate for neurons.
4. Secrete nerve growth factors
5. Communicate electrically with neurons.
6. Absorb excess neurotransmitters and ions.

Neuroglia in PNS:

1. Schwann cells

• Envelop nerve fibers and produce a myelin sheath in PNS
  Assist in regeneration of damaged fibres

1. Satellite cells

• Surround neurosomas in ganglia of PNS and provide electrical insulation around soma
• Regulate chemical environment of neurons

Blood Brain Barrier, Clinical Insight

• The blood-brain barrier is a tightly packed layer of cells that line the blood vessels in the brain and spinal cord. Separates the capillary blood from the surrounding interstitial fluid.
• Prevents large molecules (most drugs), immune cells, and disease-causing organisms such as bacteria and viruses form passing from the bloodstream, into the central nervous system (CNS)
• Protective, semi-permeable and highly selective. This also plays a role in homeostasis

Brain Tumors- Gliomas

• A tumor is a mass of rapidly dividing cells. Most adult brain tumors are composed of glial cells
• Glial cells usually grow rapidly and are highly malignant
• Because of the blood-brain barrier, they usually do not yield to chemotherapy and must be treated with radiation or surgery

Myelin

• Myelin sheath is insulation around nerve fibre
• Formed by:

1. Oligodendrocytes in CNS
2. Schwann cells in PNS

• Consists of the plasma membrane of glial cells
• Begins in fetal development but proceeds rapidly in infancy.

Myelination in the PNS

• Schwann cell spirals repeatedly around single nerve fibre
  • as many as 100 layers of membrane
  • no cytoplasm between membranes

Myelination in the CNS

• Oligodendrocyte
  • Myelinates several nerve fibres in immediate vicinity
  • Since it is anchored to multiple nerve fibers, it cannot migrate around any of them
  • Must push newer layers of myelin under the older ones
    • myelination spirals inward toward nerve fibre

Myelin

• Many schwann cells or oligodendrocytes needed to cover one nerve to fibre
• Sheath is segmented:
  • Nodes of ranvier (myelin sheath gap)
    • Gaps between segments
  • Internodes (intermodal segments)
    • Myelin covered segments from one gap to the next
  • Initial segment (in trigger zone)
    • Short section of nerve fibre between one axon hillock and the first glial cell

Multiple Sclerosis

• Degenerative disorder of the myelin sheath
• The oligodendrocytes and myelin sheaths of the CNS deteriorate and are replaced by hardened scar tissue, especially between the ages of 20 and 40. Fatal from 25-30 years after diagnosis
• Nerve conduction are disrupted, with effects that depend on what part of the CNS is involved- double vision, blindness, speech defects, neurosis, tremors, or numbness, for example.
• The cause of MS remains uncertain; autoimmune triggered by virus?

Concept Check: Fill-in-the-blank

1. Neurons that convey information to the CNS are called sensory, or afferent neurons.
2. To perform their role, neurons must have the properties of excitability, secretion, and conductivity
3. In the CNS, myelin is produced by glial cells call oligodendrocytes.

Nervous System

2. The Spinal cord, Spinal nerves (CH.13)

• The spinal cord (13.1)
• Spinal nerves (13.2)

Spinal Cord

• Information highway that connects brain with lower body
• We study the spinal cord, as well as the spinal nevers that arise from each vertebral level
• The central and peripheral nervous systems are linked structurally and functionally

Functions:

1. Conduction

• Nerve fibers conduct sensory and motor information up and down spinal cord (2 directions!)

1. Neural integration

• Spinal neurons receive input from multiple sources, integrate it, and execute appropriate output (e.g., bladder control)

1. Locomotion

• Spinal cord contains central pattern generators: groups of neurons that coordinate repetitive sequences of contractions for walking

1. Reflexes

• Involuntary responses to stimuli vital to posture, coordination and protection

Spinal Cord Surface Anatomy

• Cylinder of nervous tissue that arises from brainstem at foramen magnum of the skull
  • Occupies upper ⅔ of vertebral canal
  • Inferior margin ends at L1 or slightly beyond in adults
  • Gives rise to 31 pairs of spinal nerves
  • 45 cm long

The Vertebral Column

Spinal Cord Surface Anatomy

Spinal cord divided into 4 major regions:

1. Cervical (7 vertebrae, 8 spinal nerve)
2. Thoracic (12)
3. Lumbar (5)
4. Sacral (5)

Two enlargements where the cord is thicker:

1. Cervical enlargement - nerves to upper limbs
2. Lumbar enlargement - nerves to pelvic region and lower limbs

Medullary cone

• Cord tapers to a point inferior to lumbar enlargement

Cauda equina

• Bundle of nerve roots L2-S5
• Cauda = tail, Equine = horse
• Innervates the pelvic organs and lower limbs

Meninges

• 3 fibrous membranes enclose braina nd spinal cord
• Seprate soft tissue of CNS fro, cranium and verteral canal
• Consist of (superficial to deep)

1. Dura Mater - dura = tough outermost layer (dura- durable)
2. Arachnoid mater looks like spider web (arachnoid=spider) 5-6 layers.
3. Pia mater thinnest, 1-2 layers of squamous cubodial, innermost layer

• Dural sheath surrounds spinal cord and is seperated from vertebrae by epidural space (fat, blood vessels, connective tissue): anesthetics can be introduced here and block pain signals during childbirth or surgery.

Arachnoid Mater

• Adheres to dura
• Separated from pia mater by subarachnoid space
  • Filled with cerebrospinal fluid (CSF)
  • Region inferior to medullary cone occupied by cauda equina and CSF
    • Lumbar puncture (spinal tap) samples CSF

Pia Mater

• Membrane that follows contours of spinal cord

When a sample of CSF is needed for clinical purposes, it is taken from the lumbar cistern by a procedure called lumbar puncture (or colloquially, spinal tap). A spinal needle is insrted between two vertbrae at level L3/L4 or L4/L5, where there is no risk of accidntial injuryt to the spinal cord (which ends at L1 to L2). CSF drips fromt he spinal needle into a collection tube; usually 3 to 4 mL of CSF is collected.

Spinal Tap (Lumbar Puncture)

Spina Bifida

• Cogenital defect
• One or mor evertebrae fail to form complete vertebral arch for enclosure of the spinal cord
  • 1 baby in 1,000
• Common in lumbosacral region
• Sac protrudes from spine and may contain meninges, CSF, parts of spinal cord and nerve roots
• Folic acid reduces incidence
  • Start 3 months before conception as defcet in the first 4 weeks

Concept Check: Question

Which of these is not a region of the spinal cord?

1. Cervical
2. Thoracic
3. Pelvic
4. Lumbar
5. Sacral

Cross-sectional Anatomy of the Spinal Cord

• Gray matter
  • Central area
  • Neuron cell bodies with little myelin (site of information processing, synaptic integration)
• White matter
  • Surrounds gray matter
  • Abundantly myelinated axons (carry signals in CNS)
    • Lighter in colour because of this

Gray Matter

• Pair of posteriori (dorsal) horns
  • Posterior (dorsal) root of spinal nerve carries sensory nerve fibers
  • Pair of thicker anteiroir (ventral) horns
    • Anterior (ventral) root of spinal nerve carries only motor nerve fibers
  • Lateral horn
    • Visible from T2 through L1
    • Contains neurons of sympathetic nervous system
  • Gray commissure connects right and left sides
    • Central canal lined with ependymal cells and filled with CSF

• Near its attachment to the spinal cord, a spinal nerve branches into a posterior (dorsal) root and anterior (ventral) root
• The posterior root carries sensory nerve fibers
• The anterior horns contain the large somas of the somatic motor neurons, which exit by way of the anteriori root of the spinal nerve and lead to the skeletal muscles
• A lateral horn is visible on each side of the gray matter from segments T2 through L1 of the cord. It contains neurons of the sympathetic nervous system

White Matter

• Surrounds gray matter
• Bundles of axons up and down cord providing communication between different levels of CNS
• Columns (funiculi):
  • Three pairs of columns on each side

1. Posterior (dorsal)
2. Lateral
3. Anterior (ventral)

• Each column consists of subdivisions = tracts or fasciculi

3

For your Practice- Label

Peripheral Nervous System

• Two divisions (sensory and motor), each with somatic and visceral subdivisions
• Composed of:
  • Nerves
    • Bundles of nerve fibers (axons) wrapped in fibrous conencitve tissue
  • Ganglion
    • Knot-like swelling in a nerve where neuron cell bodies are concentrated
  • Plexus
    • Interconnecting ganglia (network of interwoven nerves)

The Spinal Nerves

Nerve:

• Cord-like organ composed of numerous nerve fibres bound together by conenctibe tissue
• Mixed nerves
  • Contain both afferent and efferent fibres
• Nerve fibres of PNS have a neurolemma (outmost sheath of Schwann cells) and myelin sheath from Schwann cells
• Endoneurium
  • Loose connective tissue external to neurolemma
• Perineurium
  • Layers of overlapping squamous cells that wrap fascicles (bundles of nerve fibres)
• Epineurium
  • Dense irregular connective tissue that wraps entire nerve
• Blood vessels penetrate connectibve tissue coverings

Classification of Nerve Fibers

• Both afferent and efferent nerves can be:

1. Somatic

• innervate skin, skeletal muscles, bones, and joints

1. Visceral

• Innervates blood vesels, glands and viscera

Also:

1. General

• innervate widespread organs such as msucles, skin, glands, viscera, and blood vessels

1. Special

• Innervate more localized organs in the head (eyes, ears, olfactory and taste receptors, and muscles of chewing, swallowing, and facial expression)

Question

The outermost connectibve tissue wrapping of a nerve is called the:

1. Epineurium
2. Perineurium
3. Endoneurium
4. Arachnoid mater
5. Dura mater

Ganglion

• Cluster of neurosomas outside the CNS
  • Enveloped with epineurium continuous with that if nerve
• Among neurosomas are bundles of nerve fibres leading into and out of the ganglion
• Rootlets:
  • Emerge from anterior surface → coverage to from anterior (ventral) root
  • Emerge from posteriori surface → coverage to form posterior (dorsal) root
    • Swells into Dorsal Root Ganglion

Proximal Branches of a Spinal Nerve

Distal Branches of a Spinal Nerve

Rami of the Spinal nerves

Anterior ramus: in thoracic region, it gives rise to intercostal nerves in other regions, anterior rami form plexuses

Posterioir ramus: innervates the muscles and joints in that region of the spine and the skin of the back:

Meningeal branch: reenters the vbertbral canal and innervates the meninges, vertebrae, and spinal ligaments

• Spinal nerve plexuses:
  • Except the thoracic region, nerves branch and merge repeatedly to form five webs = nerve plexuses
  • 31 pairs of spinal nerves (mixed)
    • 8 cervical
      • 1st cervical exits between skull and atlas, all others at intervertebral foramina
    • 12 thoracic
    • 5 lumbar
    • 5 sacral
    • 1 coccygeal
• Cervical plexus (C1-C5):
  • Receives fibers from the anterior rami of nerves C1-C5 and gives rise to these nerves.
  • Most important: phrenic nerves to diaphragm. C3,4,5 are important for breathing

• Cervical plexus (C1-C5):
  • Hypoglossal nerve (XII)
    • Motor- innervates muscles of tongue
  • Lesser occipital nerve
    • Somatosensory- skin on parts of ear and neck
  • Great auricular nerve
    • Somatosensory- most of external ear, salivary gland
  • Transverse cervical nerve
    • Somatosensory- anterior and lateral neck, underside of chin
  • Ansa cervicalis
    • Motor- omohyoid, sternohyoid, and sternothyroid neck muscles
  • Supraclavicular nerves
    • Somatosensory- neck, shoulder, anterior chest
• Brachial plexus (C5-T1):
  • Innervates upper limb and some muscles of neck and shoulder
  • Median nerve → carpal tunnel
• 
• Brachial plexus (C5-T1):
  • Musculocutaneous: sensory: skin of anterolateral forearm; elbow joint; motor: brachialis, biceps brachii, coracobrachialis muscles
  • Axillary: sensory: skin of lateral shoulder and arm; shoulder joint; motor: deltoid and teres minor muscles (rotator cuff of shoulder)
  • Radial: sensory: skin of posterior arm; posterior and lateral forearm and wrist; joints of elbow, wrist, and hand; motor: extensor muscles of posterior arm and forearm
  • Median: sensory: skin of lateral ⅔ of hand; tips of digits I-IV; joints of hand; motor: forearm flexors, thenar group and lumbricals I-II of hand
  • Ulnar: sensory: skin on palm and med hand and digits II-V; joints of elbow and hand
• Lumbar plexus (L1-L4):
  • Innervates abdominal wall, anterior thigh, genitalia
  • 5 roots and 2 divisions

• Lumbar plexus (L1-L4):
  • Iliohypogastric- sensory: skin of lower abdomen, gluteal region; motor: abdominal muscles
  • Ilioinguinal- sensory: skin of upper thigh; scrotum and penis, labia; motor: abdominal muscles
  • Genitofemoral- sensory: skin of thigh; scrotum, labia; motor: male cremaster muscle
  • Lateral femoral cutaneous- sensory: skin of thigh;
  • Femoral- sensory: skin of thigh and knee; skin of leg and foot; hip and knee joint; motor: muscles of hip, quadriceps
  • Obturator- skin of thigh; thigh and knee joints; motor: obturator externus (pelvis); thigh muscles
• Sacral and Coccygeal Plexuses:
  • Sacral (L4-S4):
    • Innervates remainder of lower trunk and lower limb
    • Sciatic nerve; injury= 90% of cases result from herniated intervertebral disc or osteoarthritis of lower spine
    • Coccygeal (S4-Co1):

• Sacral and Coccygeal Plexuses:
  • Superior gluteal: motor: gluteus, hip muscle
  • Inferior gluteal: motor: gluteus
  • Posterior cutaneous: sensory: skin of gluteal region, perineum, posterior thigh and leg popliteal fossa
  • Tibial: sensory: skin of posterior leg, plantar skin, knee and foot joints. Motor: hamstring muscles, posterior leg, foot
  • Fibular: sensory: skin of anterior distal third of leg, dorsum of foot and toes, knee joint. Motor: biceps femoris muscle, anterior muscles of leg, extensor muscles of foot
  • Pudendal: sensory: skin of penis and scrotum; clitoris, labia, lower vagina. Motor: muscle of perineum

• Dermatome Map:
  • Dermatome: 4 of them: cervical nerves, thoracic nerves, lumbar nerves, and sacral nerves.
    • Specific area of skin that sends sensory input to a specific spinal nerve
    • Oversimplified -50% overlap edges
    • Necessary to sever or anesthetize three successive spinal nerves to produce total loss of sensation from one dermatome
  • E.g. spinal nerve damage can be assessed by testing the dermatomes with pressure/pain and noting areas the pressure has no sensation

• Concept check: Fill-in-the-blank:
  • 1. Outside the CNS, the somas of neurons are clustered in swellings called glanglioll?
  • The phrenic nerves arise from the cervical plexus and innervates the diaphragm.
  • 3. The sciatic nerve is a composite of two nerves, the fibular and tibia.