Divisions of the nervous system

CNS

PNS

EFFERENT AFFERENT

Somatic SomaticANS Visceral

Skeletal muscle-voluntary

Cardiac & smooth muscles Glands-involuntary

Skeletal muscle, tendonsjoints, skin

Cardiac & smooth muscles Glands

PNS Terminology• Ganglia – neuron

cell bodies• Peripheral nerves –

neuronal axons• PNS neuroglia

– Satellite cells• Enclose neuron cell

bodies in ganglia

– Schwann cells• Cover peripheral

axons

Efferent Division of the PNS

• the somatic nervous system and part of the autonomic nervous system

• the somatic – control of skeletal muscle

• the ANS – involuntary control over cardiac and smooth muscle + gland secretion

I - OlfactoryII - OpticIII - OculomotorIV-TrochlearV - TrigeminalVI - AbducensVII - FacialVIII - AcousticIX - GlossopharyngealX - VagusXI - AccessoryXII - Hypoglossal

-cranial nerves – 12 pairs-considered part of the peripheral nervous system (PNS)-olfactory & optic contain only sensory axons = sensory nerves-remaining are motor or mixed nerves (both motor and sensory axons)

Spinal Nerve•after passing through intervertebral foramina the spinal nerve branches = ramus/rami•Dorsal ramus

Sensory/motor innervation to skin and muscles of back

•Ventral ramus-Ventrolateral body surface, body wall structures, muscles of the upper and lower limbs

•in addition to these rami - the spinal nerves also give off a meningeal branch - reenters the vertebral canal and supplies the vertebrae, vertebral ligaments and meninges

• rami communicantes = branches from the spinal nerve

-defined as a connection between a spinal nerve and the sympathetic trunk of the ANS

sympathetic trunk

Nerve Plexuses• Four major plexuses

– Cervical plexus– Brachial plexus– Lumbar plexus– Sacral plexus

• Joining of ventral rami of spinal nerves to form nerve networks or plexuses

• Found in neck, arm, low back & sacral regions

• No plexus in thoracic region

Somatic Nervous System

• considered the voluntary aspect of the PNS– but the muscles of posture and balance are controlled involuntarily by

the lower brain centers (brain stem, cerebellum)

• cell bodies located in the ventral gray horn of the spinal cord• the axon of a somatic motor neuron extends from the CNS

continuously to its skeletal muscle target• terminals release acetylcholine – contraction• can only stimulate its target

Somatic Nervous System

- somatic/motor axons emerge from the ventral gray horn and travel into the spinal nerve

- they then travel through either the:– dorsal ramus to end up at

the muscles of the back– OR the ventral ramus to

end up at the muscles of the limbs and body wall (chest/abs/pelvis)

Somatic Nervous System• somatic motor neurons that originate in

the ventral gray horn (or the brain stem) receive incoming information from many converging presynaptic neurons– both excitatory and inhibitory on these

motor neurons– the neurons that synapse with these motor

neurons are:– 1. reflex neurons originating in the spinal

cord– 2. neurons from motor areas of the brain –

form the descending white matter tracts– these neurons synapse with the somatic

motor neurons and regulate their activity• activation – impulse sent to muscles• inhibition – no impulse, no contraction

Somatic Nervous System

• no matter what motor pathway you learn – they eventually control the somatic motor neuron in the ventral gray horn or the brain stem

• therefore the somatic motor neuron is considered the final common pathway– considered the only way any other part of the nervous system can

influence muscle activity

Somatic Motor pathways

• all excitatory and inhibitory signals that control skeletal muscle movement converge on the somatic motor neurons

• these somatic motor neurons originate in one of two places– 1. Brain Stem– 2. Ventral Gray Horn of Spinal Cord

• these motor neurons extend from the brain stem and SC to innervate the skeletal muscles– also called lower motor neurons (LMNs)– their axons extend through the cranial and spinal nerves to skeletal

muscle– only LMNs provide output from the CNS to skeletal muscle fibers

Somatic Motor pathways

• neurons in four distinct circuits control movement by providing input to these LMNs– 1. local circuit neurons– 2. upper motor neurons (UMNs)– 3. basal ganglial neurons– 4. cerebellar neurons

Somatic Motor pathways

• 1. local circuit– input arrives at LMNs from nearby interneurons called local circuit

neurons– receive input from somatic sensory receptors and higher centers of

the brain– help coordinate rhythmic activities in muscle groups

• 2. UMNs: Upper Motor Neurons– provide input to the local circuit and LMNs– essential for planning, initiating and directing sequences of

voluntary movements– extend from the brain to the LMNs via two types of

somatic motor pathways• 1. direct motor pathways: nerve impulses for

voluntary movement– lateral corticospinal, anterior corticospinal and

corticobulbar (brain stem)– UMNs originate in the motor cortex and travel

down the spinal cord as the corticospinal tracts to synapse with the LMN

– OR – UMNs exit the brain stem– the LMN emerges as spinal nerves or through

the brain stem and out as cranial nerves

• 2. indirect motor pathways: or extrapyramidal pathways

– nerve impulses follow complicated circuits that involve the cortex, basal ganglia, thalamus and brain stem

– descending axons/tracts pass outside the pyramids of the medulla

– 1. rubrospinal – 2. reticulospinal– 3. vestibulospinal

• 3. Basal ganglia pathways– assist movement by providing input to

the UMNs– “okays” the motor pathways that

emerge from the motor cortex– also suppresses unwanted

movements and initiates and terminates movement

– the production of dopamine by the substantia nigra also effects muscle tone by modifying this path

– caudate nucleus and putamen receive sensory input from several areas of the brain – to know what muscles are doing

Motor Cortex

Basal Ganglia

Thalamus

Somatic Motor pathways

• 4. Cerebellar– function involves four activities

• 1. monitoring intentions for movement

• 2. monitoring actual movement• 3. comparing the command

(intention and movement) with sensory information

• 4. correction – to UMNs– travels via the thalamus to the

UMNs in the cerebral cortex – or can go directly to the UMNs

in the brain stem

Cerebellum

Thalamus

Motor Cortex

Midbrain

The Neuromuscular Junction• end of the lower motor neuron (synaptic terminal or axon bulb) in very close association with a muscle fiber/cell• distance between the bulb and the folded sarcolemma = synaptic cleft• nerve impulse leads to release of a specific neurotransmitter (acetylcholine)•this release will result in activation of the muscle cell and contraction•therefore the NMJ is ALWAYS excitatory•the only way inhibition can take place is through the inhibition of the neuron “connecting” with the muscle –i.e. upper motor neurons

http://www.blackwellpublishing.com/matthews/neurotrans.html

ANS• two divisions that innervate the same organs • efferent branch regulates “visceral” activities

(motor commands, involuntary, organs)• also has an afferent branch that receives

sensory information from these areas• generally divided into the:

– A. Parasympathetic– B. Sympathetic

ANS• involuntary motor commands and sensory information • supplies cardiac and smooth muscle, glands (i.e. viscera)• comprised on two neurons

– preganglionic and postganglionic– preganglionic synapses with the cell body of the postganglionic within

the ganglion– the pregang and postgang neurotransmitters can differ– the postganglionic neuron is unmyelinated– glands are innervated by the preganglionic neuron – e.g adrenal gland

which then releases epinephrine or norepinephrine in response

Parasympathetic Division• cell bodies of the preG neurons

are located in the brain stem– axons form the four cranial nerves III,

VII, IX and X – known as cranial parasympathetic

outflow• also found in the lateral gray

horns of the sacral spinal nerves 2 through 4

– emerge as part of the cranial or spinal nerve

– known as sacral parasympathetic outflow

Parasympathetic Division• parasympathetic ganglia are located

near or in the target• called terminal ganglia

– the preG fibers are very long because they must extend from the CNS to an organ

– synapse with postG within the terminal ganglia

– four major terminal ganglia are located close to the organ they innervate

– 1. otic (parotid gland)– 2. submandibular (submandibular and

sublingual glands)– 3. pterygopalatine (lacrimal gland)– 4. ciliary (pupils)

Sympathetic Division• for visceral motor commands• cell bodies of the preG neurons are located in

the lateral gray horns of T1 to L2• sympathetic ganglia:

– site of the synapse between the preG and postG neurons

– short preG lead into these ganglia– long postG axons lead out to target

– two groups:1. sympathetic trunk ganglia: or paravertebral chain ganglia

-vertical row lateral to the vertebral column

-3 cervical, 11 or 12 thoracic, 4 or 5 lumbar and 4 or 5 sacral2. prevertebral ganglia: or the collateral ganglia-postG neurons innervate the abdominal organs -anterior to the vertebral column and are near the large abdominal arteries-three major prevertebral ganglia: celiac, superior mesenteric and inferior mesenteric

celiacganglion

Sympathetic Division

• axons exit the lateral gray horn through the ventral root of the spinal cord

• axons form part of the spinal nerves T1 to L2

• form part of the spinal nerve along with somatic motor nerve axons and parasympathetic preG axons

• BUT the axons then enter the rami communicantes and pass to the nearest sympathetic trunk ganglion – synapse with the postG neuron

celiacganglion

*** whether it is sympathetic or parasympathetic – the preG neurons release AcH

Sympathetic Dominance

• fight or flight• protective response• elevated heart rate, blood pressure,

respiration rate• increase blood flow to skeletal muscles, lungs,

heart, brain• decrease blood flow to digestive, reproductive

and urinary organs

Parasympathetic Dominance

• “rest and digest” response• dominates in quiet, stress-free situations• resets the system after sympathetic

stimulation– e.g. slow the heart rate and lower blood pressure

ANS Neurotransmitters

• specific neurons release specific NTs – have distinct names• cholinergic neurons –release of AcH

– all preG neurons from sympathetic and parasympathetic neurons– all parasympathetic postG neurons– two types of receptors

• 1. nicotinic• 2. muscarinic

• adrenergic neurons – release of NE– most sympathetic postG are adrenergic– two types of receptors

• 1. alpha – 1 and 2• 2. beta – 1 and 2 and 3

ANS receptors

• the NTs released by the ANS can either stimulate or inhibit its target – depends on the receptors located in the target1. Cholinergic receptors – respond to AcH

• a. nicotinic – named because they are also activated by nicotine– found on the cell bodies of the postG neurons within the ganglia of the symp. and

parasymp. division – i.e. respond to AcH release from symp and parasympathetic preG fibers– binding of AcH opens channels for the movement of multiple ions including Na and K– if more positive ions (e.g. Na) enter the postG neuron within the ganglion –

depolarization and initiation of an AP by the postG neurons

nicotinic R muscarinic R

adrenergic R

• b. muscarinic receptors• can bind either Ach or muscarene (Amanita muscaria mushroom)• called metabotropic receptors - specific for one kind of ion

– e.g. ligand-gated Na channel• expressed on tissues “downstream” of post-ganglionic neurons – at the

target tissue– e.g. neuromuscular junction (ligand-gated sodium channel)

nicotinic R muscarinic R

adrenergic R

2. Adrenergic receptors – respond to NE/Epi• alpha and beta classes – , 2, 1, 2, 3• distributed in a specific pattern in target tissues and respond to either NE

or Epi or both– epinephrine made by the adrenal glands and NOT by neurons

• respond to activation by activating G proteins -> second messengers (cAMP or Ca)

• therefore they are called G protein coupled receptors

nicotinic R muscarinic R

adrenergic R

Reflex arc •Neural “wiring” of reflex•Requires 5 functional components: 1. sensory receptor, 2. sensory neuron, 3. integrating center (SC or BS), 4. motor neuron & 5. effector

Reflexes

Classification of Reflexes

• By development – Innate, acquired

• Where information is processed– Spinal, cranial

• Motor response– Somatic, visceral

• Complexity of neural circuit– Monosynaptic

Spinal Reflexes

• Stretch reflex: causes contraction in response to stretch

• regulates skeletal muscle length and tone

• monosynaptic - only one synapse in the CNS - between and single sensory and motor neuron

• all monosynaptic reflexes are ipsilateral reflexes - input and output on same side

• sensory receptors are found in muscle spindles – activated when stretched

– e.g. Patellar reflex – hit with a mallet stretches the quadriceps and its tendon - results in contraction

– muscle spindles in the quadriceps muscles are activated

– reflex results

Spinal Reflexes

•Tendon reflexes: controls muscle tension by causing muscle relaxation before muscle contraction rips tendons

•generally polysynaptic - more than one CNS synapse involved between more than two different neurons•sensory receptor synapses with 2 interneurons

• 1. an inhibitory IN - synapses with motor neurons and causes inhibition and relaxation of one set of muscles

• 2. a stimulatory IN synapses with motor neurons and causes contraction of the antagonistic muscle

withdrawlcrossedextensor

Postural Reflex