Exam 3

Central Nervous System

• CNS has >100 billion neurons

• Receives and processes millions of signals from sensory nerves and organs to determine bodily responses

• Presynaptic neurons release neurotransmitters

• Neurons don’t have direct physical contact

• Communication occurs chemically across synaptic cleft via neurotransmitters

• Contains brain and spinal cord

Neurons

• Basic functional unit of CNS

• Found in brain motor cortex

• Input signal received by dendrites and cell body

  • For different types of neurons, there may be only a few hundred or as many as 200,000 synaptic connections from input fibers

• Output signal via single axon → branches to various parts of CNS or peripheral body to distribute the signal

• Neurons may have hundreds to 200,000 synaptic connections from input fibers

Motor neurons

• Carry nerve impulses from CNS to muscles or glands

• Use efferent pathways

• Are multipolar (many dendrites, one axon)

• Cause effector response:

  • Muscle fibers to contract

  • Glands to secrete

Sensory neurons

• Carry nerve impulses from sensory receptors to the CNS

• Use afferent pathways

• Are unipolar

• Sensory receptors may be:

  • Simple (naked nerve endings like pain receptors)

  • Complex (in the eye or ear)

Interneurons

• Association neurons

• Found entirely within CNS

• Are multipolar

• Convey impulses between various parts of CNS

• Lie between sensory and motor neurons

• Transmit messages from:

  • brain to spinal cord or vice versa

  • one side of spinal cord to the other

Peripheral Nervous system

• All neurons outside CNS

• 31 pairs of spinal nerves + 12 pairs of cranial nerves (III-XII)

  • Each spinal nerve is a mixed nerve (somatic afferent/efferent and visceral afferent/efferent)

• Somatic nervous system: somatic and visceral

  • Afferent

  • Efferent

• Autonomic nervous system:

  • Sympathetic

  • Parasympathetic/cholinergic

Afferent pathways

Peripheral/sensory receptors transmit signals to CNS

Efferent pathways

CNS transmits signal to effector/peripheral organs

3 major areas of the brain

• Cerebrum: telencephalon, diencephalon

• Cerebellum

• Brainstem: midbrain, pons, medulla oblongata

Cerebrum

Composed of:

• Telencephalon (cerebral cortex) → contains grey matter because nerve fibers lack white myelin coating

• Diencephalon (thalamus, hypothalamus)

Cerebral Cortex

Contains:

• Longitudinal fissures: separate cerebrum into left and right hemispheres

• Central sulcus: separates motor and sensory cortex

Lobes in the cerebral cortex

• Parietal (2)

• Frontal

• Temporal (2)

• Occipital

Functions of the cerebral cortex

• Intellectual processes: thought, intelligence

• Processes sensory information and integrates with past experience to produce motor response

Diencephalon

• Thalamus

• Hypothalamus

Thalamus

• Relays stimuli received from all sensory neurons to cerebral cortex for interpretation

• Relays signals from cerebral cortex to proper area for further processing

Hypothalamus

• Most important in controlling body function

• Helps maintain homeostasis: temperature, blood glucose levels, hormone levels

• Signals the pituitary gland via releasing factors

• Signals lower neural centers

Cerebellum

• Located behind the brainstem

• Helps monitor and regulate voluntary movement

• Integrates postural adjustments, maintenance of equilibrium, perception of speed, and other reflexes related to fine tuning of movement

Brainstem

• Composed of midbrain, pons, and medulla oblongata

• Supports basic life support centers → maintains vegetative functioning

  • respiratory control center

  • cardiovascular control center

• Mediates reflexes

Spinal cord

• Contains both grey and white matter

• Gray matter is H-shape in core of cord

Gray matter

• Regions of brain and spinal cord made up primarily of cell bodies and dendrites of nerve cells

• Gray matter → interneurons in spinal cord (small nerves that don’t leave the spinal cord) → axon terminals

White matter

• Contains tracts or pathways made up of bundles of myelinated nerves

• Carries ascending and descending signals

• Ascending pathway: transmits sensory impulses upwards through dorsal root

  • from sensory receptors to thalamus and cerebral cortex

• Descending pathways (motor): transmits motor impulses down from cortex through ventral root → axon terminal neuromuscular junctions (contact w/ muscle fiber)

  • Pyramidal tract: excites motoneurons and control muscles

    • Corticospinal tract

    • Rubrospinal tract

  • Extrapyramidal tracts: originate in brainstem and control posture

    • Medullary reticulospinal tract

Somatic Nervous System

• Somatic afferent (sensory): via dorsal root

  • Carry sensations from periphery to spinal cord

  • Includes exteroceptive (pain, temperature, touch) and proprioceptive input

• Somatic Efferent (Motor): via ventral root

  • Communicate from spinal cord to skeletal muscles

Sensory receptors

• Nervous system activity initiated by sensory experiences that excite sensory receptors

• Sensory experiences can either cause:

  • immediate reactions from the brain

  • or memories of the experiences can be stored in the brain

Somatic Sensory System

• Transmits sensory information from receptors across the entire body surface and some deep structures

• Sensory information enters CNS through peripheral nerves

• Sensory Pathway:

  1. spinal cord at all levels

  2. reticular substance of medulla, pons, and mesencephalon

  3. cerebellum

  4. thalamus

  5. somatosensory and motor areas of cerebral cortex

Sensory receptors in the skin

Located in dermis

• Free nerve endings: pain, cold, warmth

• Pacinian corpuscle: pressure (tactile)

• Meissner’s corpuscle: touch

• Kinesthetic receptors

Sensory receptors in joints

Kinesthetic receptors

Sensory receptors in muscles

• Golgi tendon apparatus

• Muscle spindle

Skeletal motor nerve axis

• Skeletal muscles can be controlled by multiple levels of the CNS:

  • spinal cord

  • reticular substance of medulla, pons, and mesencephalon

  • basal ganglia

  • cerebellum

  • motor cortex

• Lower regions: handle automatic, instantaneous muscle responses to sensory stimuli

• Higher regions: responsible for deliberate, complex muscle movements controlled by thought processes

Autonomic Nervous System

• Controls most visceral functions

• Sympathetic NS (adrenergic)

• Parasympathetic NS (cholinergic)

• Autonomic motor pathway: include preganglionic fiber (leaves CNS) and postganglionic fiber (innervates effector)

• All organs receive dual signals: parasympathetic & sympathetic

Sympathetic NS

• Responsible for increasing activity in most systems (except GI)

• Adrenergic fibers release epinephrine

Sympathetic Nerve Fibers

• First pass into sympathetic chain, then to tissues and organs

• Heart receives the most sympathetic fibers

Where do sympathetic fibers originate from?

In spinal cord with spinal nerves between T1 and L2

Sympathetic Nervous System Components

1. Paravertebral sympathetic chains of ganglia: on both sides of vertebral column (2 chains total) → interconnected with spinal nerves

2. Prevertebral ganglia

3. Nerves extending from ganglia to internal organs

Sympathetic vs Skeletal Motor Pathways

• Sympathetic pathways use two neurons:

  • Preganglionic neuron

  • Postganglionic neuron (goes to effector organ)

• Skeletal motor pathways use only a single neuron

Preganglionic neurons

• Cell body lies in intermediolateral horn of spinal cord

• Fibers pass through ventral root of the cord into the corresponding spinal nerve

Preganglionic Sympathetic Fibers Pathway

• After leaving the spinal canal, preganglionic sympathetic fibers leave the spinal nerve

• They pass through white ramus into one of the ganglia of the sympathetic chain

  1. Synapse with postganglionic sympathetic neurons in the ganglion they enter

  2. Pass upward or downward in the chain to synapse in another ganglion

  3. Pass through the chain and synapse in a peripheral sympathetic ganglion (in the effector organ) via sympathetic nerves radiating outward

Sympathetic nerve fibers in the skeletal nerves

• Some postganglionic fibers return from sympathetic chain into spinal nerves via gray rami at all levels of the cord

• These sympathetic fibers are small type C fibers → extend to all parts of the body via skeletal nerves

• Control blood vessels, sweat glands, and piloerector muscles (hair)

• 8% of fibers in skeletal nerves are sympathetic fibers

Parasympathetic NS

• Responsible for slowing activity in most systems (except GI)

• Cholinergic fibers release acetylcholine

• Controls GI excitatory activity

• Synapse in ganglia close to the effector organ

Where do parasympathetic fibers originate from?

Brainstem and sacral region of spinal cord → synapse in ganglia close to the effector organ

How do parasympathetic fibers exit the CNS?

• Leave through cranial nerves III, VII, IX, and X

• Additional fibers exit via 2nd and 3rd sacral spinal nerves, and occasionally 1st and 4th sacral nerves

Vagus Nerve (CN X)

• Carries about 75% of all parasympathetic nerve fibers

• Supplies entire thoracic and abdominal regions

Autonomic reflex

• Monosynaptic reflex arc

• Knee jerk response: Patella tendon reflex

Complex reflexes

• Involve multiple synapses

• Crossed extensor reflex

Motor unit

• A single motor neuron (dendrite, cell body, axon) and all of the muscle fibers it innervates

• Represents functional unit of movement

• Ratio of muscle fibers to nerve relates to muscle’s movement function

Sensory nerves

• Enter the spinal cord on dorsal side

• Cell bodies lie outside the spinal cord in dorsal root ganglia

Motor nerves

• Exit the spinal cord on ventral side

• Cell bodies lie within grey matter of spinal cord

• Somatic: innervates skeletal muscle

• Autonomic (visceral): innervates organs / smooth muscle

Axons

Carry impulses away from cell body

Myelin

• Schwann cells wrapped around axons of some neurons

• Insulator: increase speed of action potential conduction

• Appear as multiple lipid-protein layers

• Are a continuous cell

Nodes of Ranvier

• Gaps between Schwann Cells → not covered by myelin

• Saltatory conduction: impulse jumps from node to node (fast transmission)

Dendrite

Receives stimuli and carry it to cell body

Cell body

Site of cellular activity

Synapse

• Junction between dendrites of one neuron and axon of a second neuron

• Nerves communicate by releasing chemical messenger at synapse

  • Neurotransmitters released from axon and bind to receptors on dendrite of second neuron

Important neurotransmitters

• Monoamines

• Neuropeptides

• Nitric oxide

Alpha motor nerves

• Larger fibers

• Conduct impulses faster

• Innervate regular muscle fibers

Gamma motor nerves

• Smaller fibers

• Conduct impulses more slowly

• Innervate proprioceptors such as muscle spindles

Properties of nerves

• Irritability: able to respond to stimuli

• Conductivity: able to transmit electrical potential along the axon

Resting membrane potential

• Difference in charge between the inside and outside of the cell

• Na⁺: greater concentration outside the cell

• K⁺ and anions: greater concentration inside the cell

• Greater membrane permeability for potassium than for sodium

• Na⁺ / K⁺ Pump: moves Na⁺ out & K⁺ in

Generating Action Potentials in neurons

• Depolarization & Repolarization via voltage gated ion channels

  • Dep: Na+ channels open → Na+ rushes in

  • Na+ channels close → stops inward Na+ flow

  • Rep: K+ channels open → K+ rushes out

• Net Effect: depolarization followed by repolarization

• Electrical Flow: created by ionic flow, not by electron flow

Na+ / K+ Pump

• Membrane bound protein

• Utilizes ATP: to move ions against concentration gradient

  • 3 Na+ out (low inside → high outside)

  • 2 K+ in (low outside → high inside)

• Maintains resting membrane potential → establishes sodium & potassium concentration gradients

Neuromuscular junction

• Motor neuron cell body and dendrites in gray matter of spinal cord

• Axon leaves spinal cord → extend to muscle

• Axon’s terminal branches end in synaptic knob → has synaptic vesicles containing acetylcholine

Motor end plate

• Area beneath terminal branches of axons

• Contains acetylcholine receptor complexes → acetylcholine binding opens receptor complex

• Cholinesterase in the synaptic cleft, degrades acetylcholine into acetate and choline

Where are neurotransmitter receptors located?

On postsynaptic membrane of muscle mass

All or None Law

• When a neuron reaches threshold, it generates an action potential that is conducted along entire axon without any change in voltage

• When the nerve fires, all muscle fibers it innervates contract

Temporal summation

Additive effect of successive stimuli from a single axon

Spatial summation

Additive effect of stimuli from various axons

Gradation of force

Force of muscle varies from slight (twitches) to maximal (tetanus):

• Increase number of motor units recruited (summation)

• Increase frequency of motor unit discharge

Proprioceptors

• Muscle spindles

• Golgi tendon organs

• Pacinian corpuscles

• Ruffini endings

Muscle spindles

• Encapsulated fibers within muscle belly

• Monitor rate of changes in muscle length

• Respond by causing muscle contraction

Golgi Tendon Organs

• Encapsulated receptors

• Located at musculotendinous junction

• Monitor tension within tendons

• Respond by causing muscle relaxation

Pacinian Corpuscles & Ruffini Endings

• Encapsulated receptors

• Located near joints, in muscle, tendon, and bone

Alimentary tract

Provides body with continual supply of water, electrolytes, vitamins, and nutrients → requires

• Movement of food through the alimentary tract

• Secretion of digestive juices and digestion of food

• Absorption of water, electrolytes, vitamins, and digestive products

• Circulation of blood through GI organs to carry away absorbed substances

• Control of all functions by local, nervous, and hormonal systems

Organ within the alimentary tract that is shared by the digestive and respiratory tract?

Oropharynx

Mechanical Digestion

• Chewing (mouth)

• Churning (stomach)

• Segmentation (small intestine)

Propulsion

• Swallowing (oral pharynx)

• Peristalsis (esophagus → stomach → small and large intestines)

Chemical Digestion

• Begins with chemical digestion of carbohydrates in the mouth

• Breakdown of nutrients in the stomach → formation of chyme

Absorption in the GI tract

• Nutrients absorbed mainly in small intestine

• Water & electrolytes absorbed in large intestine

• Absorption into blood vessels and lymph vessels

Intramural plexus of the enteric nervous system

Meissner’s Plexus (Submucosal Plexus)	Auerbach’s Plexus (Myenteric Plexus)
Network of parasympathetic nerve fibers and cell bodies	Linear chain of interconnecting neurons running the entire length of GIT
Located in submucosal tissue of the intestinal wall	Between the circular and longitudinal muscle layers
Controls GI secretions and local blood flow	Controls GI movements

Layers of smooth muscles in the GI tract

Innermost → Outermost

1. Mucosa

2. Submucosa

3. Muscularis

   • Inner circular layer

   • Outer longitudinal layer

   • Responsible for peristalsis

4. Serosa (outermost layer)

   • Connective tissue layer

   • Covered by peritoneum in intraperitoneal organs

Electrical Activity of GI Smooth Muscle

• GI smooth muscle is excited by continuous, intrinsic electrical activity

  1. Slow waves: baseline rhythmic fluctuations in membrane potential

  2. Spikes: true action potentials → triggered when slow waves reach threshold

• Resting Membrane Potential (RMP) can change to different levels → influence motor activity in GIT

  • Mainly slow waves: stimulate intermittent spike potentials

Slow wave electrical activity in GI smooth muscles

• Stimulate muscle contractions

• Stimulated by excitatory activity:

  • stretch

  • acetylcholine

  • parasympathetics

Regulation of Digestive Activities

• Neural mechanisms

• Hormonal mechanisms

• Local mechanisms (gastrointestinal reflexes)

Neural Control of the Gut Wall (Enteric Nervous System)

Myenteric and Submucosal plexuses

• Extrinsic control via sympathetic and parasympathetic nervous systems

• Sensory (afferent) fibers: pass from luminal epithelium and gut wall → enteric plexuses → prevertebral ganglia → spinal cord → vagus nerves → brainstem

Parasympathetic stimulation of Enteric NS

• Postganglionic neurons located in myenteric and submucosal plexuses

• Stimulation causes:

  • increased activity of enteric nervous system

  • enhancement of most GI functions

Sympathetic stimulation of Enteric NS

• Inhibits GI tract activity

• Acts through norepinephrine in two ways:

  1. Direct inhibition of GI smooth muscle (except mucosal muscle, which it excites)

  2. Major inhibitory effect on neurons of enteric nervous system

• Strong stimulation can inhibit motor movements of the gut, blocking food movement through GIT

GI reflexes

• Local reflexes

• Short reflexes

• Long reflexes

Local reflexes

• Reflexes integrate within the gut wall ENS

• These reflexes control:

  • GIT secretion

  • Peristalsis

  • Mixing contractions

  • Local inhibitory effects

Short reflexes

• Transmit over long distances to other areas of the GIT

• Reflexes from gut → prevertebral sympathetic ganglia → back to GIT

  • Gastrocolic reflex

  • Enterogastric reflex

  • Colonoileal reflex

  • Ileogastric reflex

Short Reflex Pathway

1. Distension of stomach by food

2. Stimulation of mucous membrane of stomach

3. Afferent signals sent to internal plexus

4. Efferent signals from internal plexus activate:

   • Gastric glands → release of gastric juice

   • G-cells in pyloric glands → gastrin secretion

Gastrocolic (gastroileal) reflex - Short Reflex

• Trigger: stomach activity

• Effect:

  • Ileocecal valve relaxation

  • Increased mass movements in colon

• Mediated through:

  • Short and long nervous pathways (intrinsic and extrinsic)

  • Hormones: CCK, gastrin

• Most evident after first meal of day

  • After a meal, reflex intensifies peristalsis in ileum → emptying of ileal content into cecum

• Often followed by an urge to defecate

  • Newborns often defecate after feeding due to strong reflex

Enterogastric reflex - Short reflex

• Trigger: fat or protein chyme reaches duodenum

• Effect: duodenal receptors send sensory signals to enteric nerves of stomach → inhibits stomach motility/peristalsis → delays gastric emptying

Colonoileal reflex - Short reflex

Reflexes from colon → inhibits emptying of ileal contents into the colon

Ileogastric Reflex - Short reflex

• Trigger: distension of ileum → inhibits gastric motility

• Result: Prevents more chyme from entering the ileum

Long reflexes

• Reflexes from gut → prevertebral sympathetic ganglia → back to GIT

• Vago-vagal Reflexes

  • From stomach and duodenum → brainstem → back to stomach via vagus nerve

  • Controls gastric motor and secretory activity

  • Afferent and efferent signals via vagus nerve

• Pain Reflexes: cause general inhibition of GI activity

• Defecation Reflexes

  • Travel from colon and rectum → spinal cord → return to GI tract

  • Produce powerful colonic, rectal and abdominal contractions for defecation

Types of Neurotransmitters in the Enteric Nervous System

• Acetylcholine: excites GI activity

• Norepinephrine: inhibits GI activity

Gastrin

• Stimuli: Protein, stomach distention, nerve

  • (Inhibited by acid)

• Site of Secretion: G cells of antrum, duodenum, jejunum

• Actions:

  • Stimulates gastric acid secretion

  • Promotes mucosal growth

Cholecystokinin (CCK)

• Stimuli: Protein, fat, acid

• Site of Secretion: I cells of duodenum, jejunum, ileum

• Actions:

  • Stimulates pancreatic enzyme secretion

  • Stimulates pancreatic & biliary bicarbonate secretion

  • Gallbladder contraction

  • Growth of exocrine pancreas

  • Inhibits gastric emptying

Secretin

• Stimuli: Acid, fat

• Site of Secretion: S cells of duodenum, jejunum, ileum

• Actions:

  • Stimulates pepsin secretion

  • Stimulates pancreatic & biliary bicarbonate secretion

  • Promotes growth of exocrine pancreas

  • Inhibits gastric acid secretion

Gastric Inhibitory Peptide (GIP)

(Glucose-dependent insulinotropic peptide)

• Stimuli: Protein, fat, carbohydrate

• Site of Secretion: K cells of duodenum and jejunum

• Actions:

  • Stimulates insulin release

  • Inhibits gastric acid secretion

Types of movements in the GIT

• Propulsive Movements (Peristalsis)

  • Move food forward along the tract

  • Occur at a rate appropriate for digestion and absorption

• Mixing Movements: thoroughly mix intestinal contents

Propulsive Movements – Peristalsis

• Basic propulsive movement of the gastrointestinal tract.

• Contractile ring forms around gut wall → ring moves forward, pushing contents in front of it, forward

• Stimulus: distention of the gut (stretching due to food)

  • Stretch activates enteric nervous system

  • Causes contraction behind distended area