BIOL20 - Autonomic + Somatic Nervous System

Exam 2

Somatic nervous system

sensory + motor neurons

sensory - touch, pain, temp, proprioception, sight, hearing, taste, smell, equilibrium

motor- SKM

Autonomic NS

sensory received from organs, blood vessels, muscles, NS

Somatic v. Autonomic Motor division

Main differences between Somatic and Autonomic

ANS motor neurons DO NOT directly innervate target

• have preganglionic and postganglionic neurons

SNS - axon of a single, myelinated somatic motor neuron extends from CNS to SKM fiber it innervates

Preganglionic neuron

• initial efferent neuron

• cell body in CNS

• all axons release ACh

Postganglionic neuron

• cell body in autonomic ganglion in PNS

• axons travel → target cells

• release either ACh or norepinephrine

• trigger specific changes → inhibit/excite responses

Anatomy of Autonomic Motor Pathways

2 neurons in series

1. preganglionic neuron:

   • cell body → CNS

   • axon → autonomic ganglion

   • myelinated

2. postganglionic neuron:

   • unmyelinated

   • axon extends from ganglion → effector

Autonomic NS includes which 2 divisions

Sympathetic + Parasympathetic

Sympathetic NS axon lengths

Preganglionic sympathetic axon: short

Postganglionic sympathetic axon: longggg

• thoracolumbar

Parasympathetic NS axon lengths

Preganglionic sympathetic axon: longggg

Postganglionic sympathetic axon: short

• craniosacral

Sympathetic preganglionic neuron

cell bodies are part of

→ lateral horns of grey matter

→ 1 2 thoracic + first 2/3 lumbar segments

Sympathetic trunk ganglia

anterior + lateral to vertebral column

How preganglionic axons connect with postganglionic axons neurons in the sympathetic trunk ganglia

1. axon synapses w/ postganglionic neuron

2. axon goes up or down a trunk

3. axon continues w/out synapsing (directly to target)

4. axon continues w/out synapsing through → sympathetic trunk gang → adrenal medullae

Pathways from sympathetic trunk

1. spinal nerves: postganglionic axons leave trunk via gray ramus communicans

   • innervates top part of body

2. cephalic periarterial nerve: superior part of neck, innervate blood vessels in/out of our head

3. sympathetic nerves: innervate heart + lungs

4. splanchnic nerves: pass through trunk forming a splanchnic nerve → prevertebral ganglion

Parasympathetic division

in nuclei of 4 cranial nerves ( III, VII, IX, X), in brain stem, and in lateral grey matter of sacral segments of spinal cord

Where are cell bodies of the Parasympathetic outflow located?

nuclei of brain stem + lateral grey matter of sacral spinal cord segments

Cranial parasympathetic outflow

extends from the brain stem in 4 cranial nerves

Sacral parasympathetic outflow

extends from 2nd through 4th sacral spinal nerves

2 major types of sympathetic ganglia

Sympathetic trunk ganglia: lie in a vertical row on either side of the vertebral column

Prevertebral ganglia: lie anterior to the vertebral column and close to the large abdominal arteries

Abdominal + Pelvic Plexuses

named after the artery

• celiac (solar) plexus

• superior mesenteric plexus

• inferior mesenteric plexus

• renal plexus

• hypogastric plexus

What are ANS neurons characterized by?

the neurotransmitters they produce and release

Cholinergic neurons

release ACh

Adrenergic neurons

release Norepinephrine (noradrenalin)

Sympathetic receptor classes - postganglionic

Adrenergic receptors

Cholinergic receptors

Adrenergic receptors

bind to epinephrine + norepinephrine

2 types:

1. alpha adrenergic

   1. alpha-1: walls of blood vessels, pupil dilation, ejaculation

   2. alpha-2: presynaptic neurons, release ACh

2. beta adrenergic

   1. beta-1: in p.mb of cardiac muscle cells, kidney cells, adipose cells

   2. beta-2: in p.mb of SM

   3. beta-3: in SM cells of digestive tracts walls

Cholinergic receptors

bind to ACh

2 types

1. Muscarinic receptors: on sweat glands in skin

2. Nicotinic receptors: in p.mb of all postganglionic neurons, w/in sym ganglia + adrenal medullae

Sensation

conscious + subconscious awareness of changes in the external or internal environment

Components of sensation

stimulation of the sensory receptor

→ transduction of stimulus

→ generation of nerve impulses (transmission)

→ integration of sensory input

Classification of Sensory Receptors

General senses - somatic and visceral

Special senses

Somatic sensory

tactile, thermal, pain, proprioceptive sensations

Visceral

provide info abt conditions within internal organs

Special senses

smell, taste, vision, hearing, equilibrium, balance

Types of sensory receptors

free nerve endings

encapsulated nerve endings

separate cells

Free nerve endings

pain + thermoreceptors

Encapsulated nerve endings

pacinian corpuscles, Meissner’s corpuscles

Separate cells

hair cells, photoreceptors, and gustatory receptor cells

Classification of sensory receptors based on location

exteroceptors: at/near body surface

Interoceptros: inside body (blood vessels, viscera, NS)

proprioceptors: muscles, tendons, joints, inner ear

Photoreceptors

specific to the retina

Chemoreceptors

smell, taste, O2, CO2 (pH)

Osmoreceptors

osmotic pressure (type of chemoreceptor)

Baroreceptor

pressure (type of mechanoreceptor)

Proprioceptors

type of mechanoreceptor

Sensory receptor adaptation

decrease in potentials during a maintained, constant stimulus

*desensitizes body

Rapidly adapting receptors

receptors that detect pressure, touch, and smell

Slowly adapting receptors

receptors that detect pain, body position, and chemical composition of blood

Somatic Sensations

sensory receptors in skin, muscles, tendons, and joints

• uneven distribution of receptors

4 modalities: tactile, thermal, pain, and proprioceptive

Tactile sensation

touch, pressure, vibration, itch, tickle

• in skin → Meissner corpuscles, hair root plexuses, Merkel discs, Ruffini corpuscles, Pacinian corpuscles, free nerve endings

Proprioceptive sensations

• slow adaptation

• weight discrimination

• 3 types: muscle spindles, tendon organs, and joint kinesthetic receptors

Somatic sensory pathways

carry info from somatic sensory receptors to the primary somatosensory area in the cerebral cortex to the cerebellum

• first order neurons

• second order neurons

• third order neurons

First-order neurons

impulses from somatic receptors → brain stem / spinal cord

Second-order neurons

impulses from brain stem + spinal cord → thalamus

Third-order neurons

impulses from the thalamus → primary somatosensory area of cortex on same side

Somatic sensory impulses ascend to the cerebral cortex along three general pathways

• posterior column-medial lemniscus pathway

• anterolateral (spinothalamic) pathway

• trigeminothalamic pathway

What is the primary somatosensory area?

Postcentral gyrus (both parietal lobes)

• each region receives info from different part of body on the opposite side

• each point of body maps to a specific region in the primary motor area

Lower motor neurons (LMNs)

nerves that extend out of the brain stem + spinal cord

• innervate SKM of face + head via cranial nerves

• innervate SKM of limb + trunk via spinal

Somatic motor pathways provide input into LMNs → Four distinct circuits of Somatic Motor Pathways

1. local circuit neurons

2. upper motor neurons

3. basal nuclei neurons

4. cerebellar neurons

Local circuit neurons

located close to LMNs in the brainstem + spinal cord

Upper motor neurons (UMNs)

input to both lower circuit neurons + LMNs

Basal Nuclei neurons

assist mvmt by indirectly providing input to UMNs

Cerebellar neurons

assist mvmt via control of activity of UMNs

UMNs extend to LMNs via:

1. Direct motor pathways: delivers signals to LMNs from cerebral cortex

2. Indirect motor pathways: deliver signals to LMNs from motor centers in basal nuclei, cerebellum, cerebral cortex

Primary Motor Area

Precentral gyrus