Peripheral Nervous System

• Consists of sensory / afferent nerves that conduct information to the central nervous system

  • Along with motor / efferent nerves that conduct information from the central nervous system

• Two Divisions :

  1. Somatic nervous system

  2. Autonomic nervous system

     • Sympathetic

       • Lumbar and Thoracic

     • Parasympathetic

       • Brain Stem and Sacral Region

     • Enteric

Sensory

• PNS receptors receive stimuli and send that information to the CNS to be interpreted

  • Information sent to the CNS via sensory axons

• Lidocaine = Voltage-gated $N{a}^{+}$$Na^+$ channels antagonist

  • Doesn't allow opening

• Receptor classes are based on location and what stimulates them

Somatic Receptors

• Located in the skin , skeletal muscles and joints

• Exteroceptors

  • Receive stimuli from the external environment at the body surface
  • eg , detect pressure , pain , temperature , itch , tickle
  • eg , Merkel's disk , Pacinian corpuscle , free nerve endings

• Proprioceptors

  • Receive stimuli from within skeletal muscles and tendons
  • eg , detect muscle stretch , joint angles
  • eg , gogli tendon organ , muscle spindles

Autonomic / Visceral Receptors

• Located in the viscera ( organs )

• Visceroceptors

  • Receive stimuli from the internal environment
  • eg , detect blood pressure , pH , carbon dioxide , osmolarity
  • eg , baroreceptor , chemoreceptor , osmoreceptor

How Do Receptors Perform Their Function?

• Receive a stimulus

  • Causes opening or closing of certain channels

    • eg , opening of a $N{a}^{+}$$Na^+$ channel or a closing of a $K^{+}$$K^+$ channel

    • Leads to depolarization

      • Proportional to the size of the stimulus

      • Results in an action potential if threshold is reached

        • Frequency proportional to size of the stimulus
        • Conduct to the CNS to be interpreted

Seonsory Adaptation

• Adaptation to a constant stimulus by a receptor

• Phasic Receptors

  • Receptors that adapt
  • Action potential frequency drastically slows or ceases over time
  • eg , many exteroreceptors

• Tonic Receptors

  • Receptors that do NOT adapt ( or adapt very little )

    • They don't adapt because its dangerous if they did

  • Action potential frequency is unchanged with a constant stimulus

  • eg , pain receptors and most visceroceptors , free nerve endings

• Example

  • Over time less and less sodium channels are open in response to stimulus

    • until action potential no longer fire even in the presence of a stimulation

• Rebound Effect = the reopening of sodium channels , and restarting the generation of action potentials

Cholinergic

• Associated with acetylcholine

• Neurons that release acetylcholine are cholinergic neurons

• Receptors that bind acetylcholine are cholinergic

• Type types of acetylcholine receptors

  • Nicotinic
  • Muscarinic

Adrenergic

• associated with epinephrine and norepinephrine

• Neurons that release norepinephrine are adrenergic

• Receptors that bind norepinephrine are adrenergic

• Two types of adrenergic receptors

  • Alpha
  • Beta

Somatic Motor Output

• Controls movement

  • Via motor nerves of the brainstem and all regions of the spinal cord
  • Voluntary

• Effector of motor output = skeletal muscle

• One efferent motor neuron between the CNS and the effector

  • Myelinated

  • Efferent motor neuron releases ACh

    • ACh binds to nicotinic receptors on skeletal muscle cells

Autonomic Motor Output

• Controlled ONLY at brain stem and thoracic level

  • NO cervical output

• Controls visceral cells

  • Via motor nerves of the brainstem and certain regions of the spinal cord
  • Helps maintain homeostasis via autonomic regulation
  • Involuntary

• Effectors of motor output = cardiac muscle , smooth muscle , cells of viscera

• Two efferent motor neurons between CNS and effector

  • Preganglionic Neurons

    • Arise from the CNS and then synapse with postganglionic neurons
    • Myelinated

  • Postganglionic Neurons

    • Synapse with the preganglionic neurons
    • Synapse with and transmit motor information to effectors
    • Unmyelinated
    • Cell bodies of postganglionic neurons make up ganglia of autonomics

Sympathetic Nervous System

• Most active during stressful situations ( "fight or flight" ) and physical activity

• Thoracic spinal nerves and lumbar spinal nerves ( from T1 to L2 )

  • Sometimes referred to as the thorocolumbar division

• Preganglionic Neuron = soma originates in the lateral horn and axon is short

• Postganglionic Neuron = axon is long

• Paravertebral sympathetic ganglion chain

  • Ganglia located bilaterally along the length of the vertebral column
  • Location of most postganglionic neuronal cell bodies

• Collateral ganglia

  • Ganglia located in the abdomen
  • Location of some postganglionic neuronal cell bodies

• Preganglionic neuron that innervates the adrenal gland does so directly

  • Releases ACh onto nicotinic receptors of the adrenal gland

• Neurotransmitters

  • Preganglionic neuron releases ACh

    • Binds to nicotinic receptors on postganglionic neuron

  • Postganglionic neuron releases norepinephrine ( NE )

    • Exception = Seat Gland

      • Postganglionic Neuron to sweat gland releases ACh

      • Sweat gland contains cholinergic receptors

        • Muscarinic ACh receptors

  • Alpha Receptors

    • $\alpha -1$$\alpha-1$ = Excitatory

      • eg , excites smooth muscle of blood vessels of the skin and visceral cells

        • Causes blood vessels to constrict

    • $\alpha -2$$\alpha-2$ = Inhibitory

      • eg inhibits smooth muscle of the digestive tract

        • Causes digestive tract to relax

  • Beta Receptors

    • $\beta -1$$\beta-1$ = excitatory

      • eg , excites cardiac muscle and conduction system

        • Increases heart contractility and heart rate

    • $\beta -2$$\beta-2$ = inhibitory

      • eg , inhibits smooth muscle of airways

        • Causes airways to relax

    • $\beta -3$$\beta-3$ = excitatory

      • eg , found in adipose tissue

        • Causes lipolysis

Parasympathetic Nervous System

• Active during normal relaxed states ( "rest and digest " )

  • Active most of the time

• Cranial nerves ( from brainstem ) and sacral spinal nerves

  • Sometimes referred to as the craniosacral division

  • 75% of all parasympathetic output is from the vagus nerve ( CN 10 )

  • Pelvic nerves from sacral spinal region

    • Innervates lower portion of the large intestine , urinary system and reproductive system

  • Preganglionic neuron = soma originates in brainstem and sacral region , axon is long

  • Postganglionic neuron = axon is short

  • Terminal ganglia = ganglia located very near the effectors

• Neurotransmitters

  • Preganglionic neuron releases ACh

    • ACh binds to nicotinic receptors

  • Postganglionic neuron releases ACh

    • ACh binds to muscarinic receptors of effectors

      • Muscarinic receptors are excitatory or inhibitory

        • Excite smooth muscle of the digestive tract

          • Causes digestive tract to constrict

        • Excite smooth muscle of the airways

          • Causes airways to constrict

        • Excite salivary glands

          • Causes salivary glands to produce saliva

        • Inhibit conduction system of the heart

          • Decreases hear rate
          • No direct effect on contractibility

Neuropathy

• Damage to nerves

• Somatic neuropathy ( peripheral neuropathy )

  • Sensory

    • Symptoms = pins , needles , burning , numbness , pain

  • Motor

    • Symptoms and Signs = muscle weakness , muscle atrophy , diminished reflexes

• Automonic neuropathy

  • Would lead to lack of autonomic regulation

    • eg , loss of blood pressure regulation
    • eg , loss of digestive system regulation
    • eg , loss of heart rate regulation

Regulation of Autonomic Nervous System

• Central Nervous System

  • Hypothalamus = emotions affecting autonomic output

  • Brainstem = reflex centers ( bundle of axons )

    • eg , heart rate , blood pressure , breathing , coughing

  • Spinal Cord = reflex centers

    • eg , defecation , micturition , erection , ejaculation

• Autonomic Reflexes

  • Visceroceptors receive stimuli and send information to the CNS

    • CNS interprets information and responds accordingly

      • Response sent to effectors

  • eg , Baroreceptor reflexes

    • Increase in blood pressure

      • Stimulates baroreceptors

        • Signals reflex center in the brainstem

          • Stimulates parasympathetics = decreases blood pressure
          • Inhibits sympathetics = decreases blood pressure

    • Decrease in blood pressure

      • Inhibits baroreceptors

        • Signals reflex center in the brainstem

          • Inhibits parasympathetics = increases blood pressure
          • Stimulates sympathetics = increases blood pressure

  • eg , Micturition Reflex

    • Bladder fills with urine ( stretches the bladder wall )

      • Stimulates mechanoreceptors of the bladder wall

        • Signals reflex center in the sacral spinal cord

          • Stimulates parasympathetics

            • Contraction of bladder
            • Relaxation of internal sphincter

Sympathetics versus Parasympathetics

• Most visceral tissue is regulated by both the sympathetics. and parasympathetics

  • Sympathetics and parasympathetics have opposite effects

    • If one stimulates the other inhibits
    • Exception = salivary glands are stimulated by both

  • Exceptions exist

    • Solely regulated by the sympathetics

      • Smooth muscle of most blood vessels
      • Adrenal glands
      • Sweat glands
      • Arrector pili muscles
      • Adipose tissue

• Changing autonomic tone

  • Increased tone = increased activity

    • eg , increased sympathetic tone = increased sympathetic activity

  • Decreased tone = decreased activity

    • eg , decreased sympathetic tone = decreased sympathetic activity