The Nervous System

1. Structure

1.1. Central Nervous System (CNS)

1.1.1. Brain and spinal chord

1.1.2. Processing centers

1.2. Peripheral Nervous System (PNS)

1.2.1. Cranial nerves and spinal nerves

1.2.2. Sensory input and motor response pathways

2. Neuronal Pools

2.1. - Interneurons organized into functional groups of interconnected neurons (can be scattered) - Each has limited input sources and output destinations - May stimulate or depress parts of brain or spinal cord

2.2. Sensory neurons

2.2.1. About 10 million; bring information into CNS (incoming)

2.3. Motor neurons

2.3.1. About 1/2 million; send commands from CNS out to peripheral effectors (outgoing)

2.4. Interneurons

2.4.1. - About 20 billion; located within CNS - Interpret, plan, and coordinate signals coming in and out

2.5. 5 patterns of neural circuits in neuronal pools

2.5.1. 1. Divergence

2.5.1.1. - Spreads information from one neuron or neuronal pool to many - Especially common in sensory pathways

2.5.2. 2. Convergence

2.5.2.1. - Several neurons synapse on a single neuron

2.5.2.1.1. Ex: subconscious and conscious control of diaphragm in breathing -- two neuronal pools synapse with the same motor neurons

2.5.3. 3. Serial processing

2.5.3.1. - Information moves along a single path, sequentially from one neuron or neuronal pool to the next

2.5.3.1.1. Ex: pain signals pass two neuronal pools to reach conscious pain

2.5.4. 4. Parallel processing

2.5.4.1. - Several neurons/neuronal pools process the same information at the same time

2.5.4.1.1. Ex: step on a nail; signal spreads and you shift weight, lift foot, and feel pain at the same time

2.5.5. 5. Reverberation

2.5.5.1. - Collateral branches of neurons extend back and continue stimulating presynaptic neurons - Forms positive feedback loop; continues until synaptic fatigue or inhibition occurs

2.5.5.1.1. - Ex: may maintain consciousness, breathing, muscle coordination

3. Spinal Cord

3.1. Functions:

3.1.1. Housed within protective membrane (meninges) and vertebral column

3.1.2. Carries sensory and motor information between brain and most other parts of the body

3.1.3. Gives rise to spinal nerves and allows communication of peripheral organs

3.2. Anatomy:

3.2.1. 18 in. (45 cm) long, 1/2 in. (14 mm) wide

3.2.2. From brain only to vertebrate L1 and L2 (stops lengthening around age 4, but the vertebral column still grows)

3.2.3. 4 regions: - Cervical - Thoracic - Lumbar - Sacral

3.2.4. Has bilateral symmetry (same on both sides)

3.2.5. 31 segments; give rise to spinal nerves

3.2.6. Grooves divide spinal chord into left and right - Posterior median sulcus (posterior side) - Anterior median fissure (deeper, anterior groove)

3.2.7. Central canal contains cerebral spinal fluid (CSF) - Acts as a shock absorber - Allows diffusion of gases, nutrients, chemical messengers

3.2.8. Distal

3.3. Enlargements of the spinal chord

3.3.1. Areas of the spinal chord that supply the limbs have more gray matter and are visibly wider

3.3.1.1. Cervical enlargement: supplies shoulder and upper limb

3.3.1.2. Lumbosacral enlargement: supplies pelvis and lower limb

3.4. Spinal Roots and Ganglia

3.4.1. Conus medullaris: tapered, conical end of chord below lumbar enlargement

3.4.2. Cauda equina: - Nerve roots extending below conus medullaris - Looks like a horse's tail

3.4.3. Filum terminale: - Thin thread of fibrous tissue at end of conus medullaris - Attaches to coccygeal ligament

3.4.4. 2 branches form spinal nerves

3.4.4.1. 1. Anterior root (ventral root): axons of motor neurons

3.4.4.2. 2. Posterior root (dorsal root): axons of sensory neurons - Spinal nerve roots divide into rootlets before entering or leaving the spinal chord

3.4.5. Spinal Ganglia (aka dorsal root ganglia) - Contain cell bodies of sensory neurons that form the posterior root - Located between pedicles of adjacent vertebrae

4. Reflexes

4.1. - Rapid, automatic responses to specific stimuli - Basic building blocks of neural function - A specific reflex produces the same motor response each time

4.2. 5 components of a reflex arc

4.2.1. 1. Sensory receptor

4.2.2. 2. Sensory neuron

4.2.3. 3. Information processing in CNS

4.2.4. 4. Motor neuron

4.2.5. 5. Effector

4.3. Events in a spinal reflex arc

4.3.1. Step 1: Stimulus activates a receptor Step 2: With enough stimulation, action potential is generated in sensory neuron. Axon enters spinal cord via posterior root Step 3: Information processing in spinal cord usually occurs at one or more interneurons Step 4: Interneurons stimulate action potentials in motor neuron; its axon leaves via anterior root Step 5: Motor neurons stimulates effector (muscle/gland)

4.4. 4 classifications of reflexes

4.4.1. 1. Development

4.4.1.1. Innate reflexes

4.4.1.1.1. - Basic neural reflexes formed before birth - Genetically programmed (inborn) - Ex: withdrawal, chewing, visual tracking, sucking

4.4.1.2. Acquired reflexes

4.4.1.2.1. - Rapid, automatic learned motor patterns - Repetition enhances them - Ex: braking a car in emergency

4.4.2. 2. Motor response

4.4.2.1. Somatic reflexes: Control muscle contractions

4.4.2.1.1. - Superficial reflexes: stimuli in skin/mucous membranes - Stretch or deep tendon reflexes (such as patellar reflex) - Immediate: important in emergencies (slipping, cutting finger)

4.4.2.2. Visceral reflexes: Control other effectors (autonomic)

4.4.2.2.1. - Smooth muscle, cardiac muscle, or glands

4.4.3. 3. Complexity of neural circuit

4.4.3.1. Monosynaptic reflex

4.4.3.1.1. - Single synapse--simplest reflex arc - Sensory neuron synapses directly with motor neuron - Fast response

4.4.3.2. Polysynaptic reflex

4.4.3.2.1. - At least one interneuron between sensory neuron and motor neuron; most common - Slower response; delay increase with number of synapses involved (longer path=longer delay) - Intersegmental reflex arcs--many cord segments interact; produce viable response

4.4.4. 4. Site of information processing

4.4.4.1. Spinal reflexes: processing occurs in spinal cord

4.4.4.2. Cranial reflexes: processing occurs in brain

5. Gray Matter & White Matter

5.1. Gray Matter - cell bodies of neurons, neuroglia, and unmyelinated axons

5.1.1. Masses of gray matter within CNS are called nuclei and are organized into regions called horns

5.1.1.1. Posterior horns - somatic and visceral sensory nuclei (incoming information from receptors)

5.1.1.2. Anterior horns - somatic motor nuclei (outgoing information to effectors)

5.1.1.3. Lateral horns - thoracic and lumbar segments: visceral motor nuclei

5.1.2. Gray commissures

5.1.2.1. - Narrow bands of gray matter around central canal - Axons cross here to the other side of spinal cord

5.1.2.2. Sensory or motor nucleus location within gray matter determines which body part it controls

5.1.2.2.1. Sensory: Coming in messages from peripherals

5.1.2.2.2. Motor: Issue motor command to peripherals (going out)

5.2. White Matter

5.2.1. 3 columns (regions)

5.2.1.1. Posterior white columns- between posterior horns and posterior median sulcus

5.2.1.2. Anterior white columns- between anterior horns and anterior median fissure - Anterior white commissure: where axons cross from one side of spinal cord to the other

5.2.1.3. Lateral white columns- on each side of spinal cord, between anterior and posterior columns

5.2.2. Tract = bundles of axons in CNS

5.2.2.1. - Relay same type of information in same direction - Ascending tracts: sensory information up toward the brain - Descending tracts: motor commands down to the spinal chord

6. Spinal Nerves

6.1. 3 Connective tissue layers surround spinal nerves

6.1.1. 1. Epineurium: outermost; network of collagen fibers

6.1.2. 2. Perineurium: middle layer; separates nerve into fascicles (axon bundles)

6.1.3. 3. Endoneurium: innermost; surrounds individual axons

6.2. Spinal nerves

6.2.1. - Pair of spinal nerves emerges laterally from each spinal cord segment - Form by junction of anterior and posterior roots - All spinal nerves are mixed nerves (somatic, visceral, sensory, motor)

6.3. Peripheral nerves

6.3.1. - Form from branching and re-sorting of spinal nerves - All are mixed nerves (sensory and motor) - Same connective tissue layers as spinal nerves (continuous with each other)

6.3.1.1. Injury: relieve compression = high chance of healing Severed nerve = lower chance of healing with no repair (Walleran regeneration, nerve repair)

6.3.2. Peripheral distribution of spinal nerves

6.3.2.1. Sensory nerves: Posterior, anterior, and white rami also carry sensory information

6.3.2.2. Dermatome: specific bilateral region of skin supplied by a single pair of spinal nerves

6.3.2.3. Peripheral neuropathies: regional losses of neural function that affect dermatomes, often from nerve trauma, compression, various illnesses.

6.3.2.4. Shingles: rash/symptoms occur along dermatomes

7. Spinal Reflexes

7.1. Steps in a stretch reflex

7.1.1. 1. Stimulus = muscle stretching 2. Distortion of receptor sends action potential through sensory neuron 3. Sensory neuron synapses with motor neurons in spinal cord 4. Motor neurons send signals to motor units; triggers reflexive contraction of stretched muscle

7.2. Muscle spindle structure: receptors in stretch reflexes

7.2.1. - Made of bundles of small, specialized, intrafusal muscle fibers - Innervated by sensory and motor neurons (Gamma motor neurons = their axons are called gamma efferents) - Muscle spindle is surrounded by extrafusal muscle fibers (maintain muscle tone and contract muscle) - Dendrites of sensory neurons wind around central region of intrafusal fibers - Sensory neuron axon enters CNS in posterior root - Sensory neurons synapse in spinal cord directly with motor neurons - Gamma efferents complete reflex arc by synapsing back at the intrafusal fibers - Muscle contracts back to its resting length

7.3. Postural reflexes

7.3.1. - Include both stretch reflexes (monosynaptic) and also complex polysynaptic reflexes - Maintain normal upright posture - Often involve multiple muscle groups (ex: back and abdomen) - Maintain firm muscle tone - Extremely sensitive receptors allow constant fine adjustments to be made as needed

7.4. Polysynaptic reflexes

7.4.1. - More complicated than monosynaptic - Interneurons can control multiple muscle groups - Produce either EPSPs or IPSPs, stimulating some muscles and inhibiting others - Ex: Tendon reflex, Withdrawal reflexes, Crossed-extensor reflexes

7.4.2. 5 general characteristics

7.4.2.1. 1. Involve pools of interneurons (EPSP & IPSP)

7.4.2.1.1. - May cause excitation or inhibition

7.4.2.2. 2. Involve more than one spinal segment

7.4.2.2.1. - Can activate muscles in multiple areas

7.4.2.3. 3. Involve reciprocal inhibition

7.4.2.3.1. - Coordinates contractions and reduces resistance

7.4.2.4. 4. Have reverberating circuits

7.4.2.4.1. - Prolongs reflexive motor response

7.4.2.5. 5. Several reflexes may cooperate

7.4.2.5.1. - To produce coordinated, controlled response

7.5. Tendon reflex

7.5.1. Prevents skeletal muscles from: - Developing too much tension - Tearing or breaking tendons

7.5.2. Sensory receptors are Golgi tendon organs: - Stimulated when collagen fibers are overstretched - Stimulate inhibitory interneurons in spinal cord - More muscle tension leads to more muscle inhibition

7.6. Withdrawal reflexes

7.6.1. - Move body part away from stimulus (pain or pressure) Ex: flexor reflex in limbs; pulls hand from hot pan - Strengthen and extent of response depends on intensity and location of stimulus

7.7. Reciprocal inhibition

7.7.1. For flexor reflex to work, stretch reflex of antagonistic (extensor) muscles must be inhibited (reciprocal inhibition) by interneurons in spinal cord. - When flexors contract, extensors relax - When extensors contract, flexors relax

7.8. Reflex arcs

7.8.1. Ipsilateral reflex arcs

7.8.1.1. - Occur on same side of body as stimulus - Stretch, tendon, and withdrawal reflexes

7.8.2. Crossed extensor reflexes

7.8.2.1. - Occur on side of opposite stimulus - Coordinated with flexor reflex - Step on something sharp (cross extensor straightens other foot) - Maintained by reverberating circuits

8. Spinal Cord (cont)

8.1. Spinal nerves

8.1.1. - Formed by union of posterior and anterior roots - Pairs: one from each side at each vertebral level - Each has a white ramus communicans and a gray ramus communicans that innervate glands and smooth muscle - Mixed nerves: contain both afferent (sensory) and efferent (motor) fibers - Each spinal nerve quickly divides into rami

8.1.1.1. - Posterior ramus supplies skin/muscles of back - Anterior ramus supplies most of body wall, skin, limbs

8.2. Spinal Meninges

8.2.1. 3 specialized membranes surrounding spinal chord

8.2.1.1. Dura mater: outermost layer

8.2.1.1.1. - Tough with dense collagen fibers - Continuous with cranial dura mater and fuses with periosteum of occipital bone - Distal end tapers to dense cord of collagen fibers; joins filium terminale in coccygeal ligament

8.2.1.1.2. Epidural space: - Between vertebrae and dura mater (superficial to dura mater) - Contains loose connective and adipose tissue

8.2.1.1.3. Subdural space: - potential space deep to dura mater (usually found in autopsies, not healthy individuals)

8.2.1.2. Arachnoid mater: middle layer

8.2.1.2.1. Arachnoid membrane: weblike layer of simple squamous epithelia

8.2.1.2.2. Arachnoid trabeculae: network of collagen/elastic fibers between arachnoid membrane

8.2.1.2.3. Subarachnoid space: - Space with arachnoid trabeculae, between arachnoid mater and pia mater - Filled with cerebrospinal fluid (CSF) that carries dissolved gases, nutrients, wastes - Lumbar puncture or spinal tap withdraws CSF from subarachnoid space

8.2.1.3. Pia mater: innermost layer

8.2.1.3.1. - A mesh of collagen and elastic fibers - Firmly attached to underlying neural structures it surrounds - Blood vessels for spinal cord are on surface of the pia mater, within subarachnoid space - Paired denticulate ligaments (one on each side) * Anchor pia mater to dura mater * Prevent lateral movement of spinal cord

8.2.2. Functions: - Protect spinal chord - Carry blood supply - Provide shock absorption - Bring nutrients to spinal chord along with oxygen

8.2.3. Continuous with cranial meninges

8.2.3.1. Meningitis: viral or bacterial infection of meninges

9. Brain Can Alter Spinal Reflexes

9.1. Voluntary movements and reflex motor patterns

9.1.1. - Spinal reflexes produce characteristic response for a given stimulus - Brain can also activate these same motor patterns through descending pathways (from brain to peripherals)

9.1.1.1. - Can facilitate, inhibit, or "fine-tune" the established motor response - Ex: walking, jumping, running

9.2. Reinforcement of spinal reflexes

9.2.1. - Higher centers can adjust sensitivity reflexes by stimulating excitatory or inhibitory interneurons in brainstem or spinal cord - When excitatory synapses are chronically stimulated, postsynaptic neurons can be in general facilitation (action) - This reinforcement enhances spinal reflexes

9.3. Inhibition of spinal reflexes

9.3.1. - Higher centers inhibit spinal reflexes by:

9.3.1.1. - Stimulating inhibitory neurons - Creating IPSPs at reflex motor neurons - Suppressing postsynaptic neurons, thus inhibiting the reflex

9.4. Plantar reflex

9.4.1. - Normal in adults - Stroke lateral sole, causes reflex toe-curling

9.5. Babinski reflex

9.5.1. - Normal in infants (toes spread like webs) - May indicate CNS damage in adults

10. Nerve plexuses (grouping)

10.1. - Complex, interwoven networks of nerve fibers - Formed from blended fibers of anterior rami of adjacent spinal nerves - Allows multiple spinal nerves to supply the same structures

10.2. 4 major nerve plexuses

10.2.1. 1. Cervical plexus

10.2.1.1. - Includes anterior rami of spinal nerves C1-C5 - Innervates scalp behind ear, neck, and diaphram

10.2.1.2. Major cervical plexus nerves

10.2.1.2.1. - Phrenic nerve: from C3-C5; controls diaphram - Lesser occipital nerve - Great auricular nerve - Transverse cervical nerve - Supraclavicular nerves

10.2.2. 2. Brachial plexus

10.2.2.1. - Innervates pectoral girdle, upper back, upper limb - Anterior rami of C5-T1 - These rami first form three large trunks (superior, middle, and inferior trunks) - Trunks re-sort their axons to form three cords (lateral, posterior, and medial cords) - Cords are named by their positions relative to axillary artery - Most nerves of brachial plexus come off the cords; a few originate at the trunks

10.2.2.2. Major brachial plexus nerves

10.2.2.2.1. - Musculocutaneous nerve (lateral cord) - Median nerve (lateral and medial cords) - Ulnar nerve (medial cord) - Axillary nerve (posterior cord) - Radial nerve (posterior cord)

10.2.3. 3. Lumbar plexus

10.2.3.1. - Includes anterior rami of spinal nerves T12-L4

10.2.3.1.1. Major nerves

10.2.4. 4. Sacral plexus

10.2.4.1. - Includes anterior rami of spinal nerves L4-S4

10.2.4.1.1. Major nerves

10.3. Sensory innervation of foot

10.3.1. - Saphenous nerve - Sural nerve - Fibular nerve - Tibial nerve