Motor (efferent) system

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Motor (efferent) system

Divided into:

Somatic motor division that controls skeletal muscle

Visceral/autonomic – controls smooth and cardiac muscles and the glands (exocrine and endocrine)

These 2 system differ not only in the effector but also in their descending pathway and the response


ANS Versus Somatic Nervous System (SNS)

The ANS differs from the SNS in the following three areas

1. Effectors

The effectors of the SNS are skeletal muscles

The effectors of the ANS are cardiac muscle, smooth muscle, and glands



2. Efferent pathways

In the SNS heavily myelinated axons of the somatic motor neurons extend from the CNS to the effector (one neuron)

Efferent pathways in the ANS are a two-neuron chain

The preganglionic (first) neuron has a lightly myelinated axon

The post-ganglionic (second) neuron extends to an effector organ

Pre-ganglionic

Ganglion

Post-ganglionic


3. Target organ responses

All somatic motor neurons release Acetylcholine (ACh), which has an excitatory effect

In the ANS:

Preganglionic fibers release ACh

Postganglionic fibers release norepinephrine or ACh and the effect is either stimulatory or inhibitory

ANS effect on the target organ is dependent upon the neurotransmitter released and the

receptor type of the effector



Characteristic Somatic Nervous System Autonomic Nervous System

Effectors Voluntary (skeletal) muscleSmooth muscle, Cardiac

Muscle, Glands

General functionAdjustment to external

environment.

Adjustment within the internal environment

(homeostasis)

Number of neurons from CNS to effectors

1 2

Ganglia outside the CNS

0Chain ganglia, collateral

ganglia or terminal ganglia

Neurotransmitter AcetylcholineAcetylcholine, adrenaline,

noradrenalin

On NMJ - nicotinic receptors – ACh always excitatory


Receptors for neurotransmitters in the motor division

Receptors are divided into 2 groups:

Cholinergic – receive and respond to acetylcholine (ACh):

Two subgroups that are names of chemicals that mimic some of the

actions of Ach:

Muscarinic (chemical found in the mushroom Amanita

muscarina)

Nicotinic (chemical found in the tobacco plant – nicotina

tabacum)

Adrenergic – receive and respond to norepinephrin (NE) /

epinephrine (E)

Divided into alpha and beta


Cholinergic Nicotinic Receptors

Found on

Motor end plates of skeletal muscle cells

All ganglionic neurons (sympathetic and

parasympathetic)

Hormone-producing cells of the adrenal medulla

Effect of ACh at nicotinic receptors is always direct and

stimulatory


Direct effect – receptors are part of the ion channel

Nicotinic receptors


Cholinergic Muscarinic Receptors

Found on

All effector cells stimulated by postganglionic

cholinergic fibers

The effect of ACh at muscarinic receptors

Can be either inhibitory or excitatory

Depends on the receptor type of the target organ


Indirect effect – through G-protein and 2nd messenger

Muscarinic (cholinergic) and all adrenergic receptors


ACh (cholinergic) receptors

Nicotinic receptors (nACh)Pre-ganglionic

Muscarinic receptors (mACh)Post-ganglionic

Direct mechanism – open Na+ channels (depolarization)

Fast excitatory effect

Indirect mechanism – use of G-protein and 2nd messenger system

Slow excitatoryM1, M3, M5

Slow inhibitoryM2, M4

Inhibition of adenylate cyclase

Increase intracellular Ca2+ release


Adrenergic Receptors

Two types

Alpha () (subtypes 1, 2) – excited

Beta () (subtypes 1, 2 , 3)

Effects of NT depend on which subclass of receptor

predominates on the target organ

Norepinephrine excites mainly Alpha () receptors

Epinephrine excites both alpha and beta equaly


NE (adrenergic) receptors - all indirect through G-protein

1 1

Slow excitation

2nd messenger – cAMPInhibition or activation of

adenylate cyclase

slow excitation

Slow inhibition

2 2 3

Inhibition of adenylate cyclase

Lypolysis(excitation)

Increase intracellular Ca2+ release

Slow inhibition

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Autonomic Nervous System (ANS) function

The ANS major function is to maintain homeostasis

The ANS :

functions via visceral (subconscious) reflexes

The autonomic system is coordinated primarily by the hypothalamus and the medulla (higher centers)

The brain stem and the spinal cord seem to have direct influence over autonomic functions


Autonomic reflexes Autonomic reflexes are mediated by neural circuits in the

spinal cord and brain stem

The afferent pathways include both visceral and somatic fibers

The reflex includes interneurons that receive convergent input from visceral and somatic fibers

Efferent pathway can be sympathetic or parasympathetic

Main difference: visceral reflex arc has two neurons in the motor pathway


Hypothalamic Function Function as the main visceral control in the body

Regulates blood pressure, rate and force of heartbeat, digestive tract motility, rate and depth of breathing, eye pupil size etc.

Perception of pleasure, fear, and rage

Maintains normal body temperature (contains body’s “thermostat”) by initiating cooling or heat-generating processes

Regulates food intake (hunger)

Regulation of water intake and thirst – produce ADH, contain thirst center

Regulates sleep and the sleep cycle


The Hypothalamus other functions

Controls the the pituitary gland via the infundibulum (will be discussed in APII)

Contains the Mammillary bodies

Relay station for olfactory pathways

The supraoptic and paraventricular nuclei produce ADH and oxytocin


Autonomic control – brain stem and spinal cord

The brain stem and the spinal cord have direct influence over autonomic functions


Brain Stem

Consists of three regions – midbrain,

Pons

medulla oblongata

Similar to spinal cord – deep gray matter surrounded by white tracts but contains embedded nuclei in the white matter

Controls automatic behaviors necessary for survival (breathing, digestion, heart rate, blood pressure)

Provides the pathway for tracts between higher and lower brain centers

Associated with 10 of the 12 pairs of cranial nerves


Midbrain

Located between the diencephalon and the pons

Midbrain structures include:

Cerebral peduncles of the midbrain – two structures that contain descending pyramidal motor tracts

Cerebral aqueduct – hollow tube that connects the third and fourth ventricles

Various nuclei (what are nuclei)


Midbrain Nuclei

Nuclei that control cranial nerves III (oculomotor) and IV (trochlear)

Corpora quadrigemina

2 Superior colliculi – visual reflex centers; coordinate head and eye movement when we follow a moving object (does not have to be a conscious decision)

2 Inferior colliculi – auditory relay centers between hearing receptors to sensory cortex.

Reflexive responses to sound – turn head toward sound


Midbrain Nuclei

Substantia nigra – functionally linked to basal nuclei, contains melanin pigment (precursor of dopamine - NT)

Red nucleus – largest nucleus (rich blood supply) of the reticular formation; relay nuclei for some descending motor pathways


Pons

between the midbrain and the medulla oblongata

Forms part of the anterior wall of the fourth ventricle

Fibers of the pons:

Connect higher brain centers and the spinal cord

Relay impulses between the motor cortex and the cerebellum

Origin of cranial nerves V (trigeminal), VI (abducens), and VII (facial)

Contains nuclei of the reticular formation


Medulla Nuclei related to autonomic system

The medulla has an important role in the autonomic reflex center that maintain homeostasis:

Cardiovascular control center – adjusts force and rate of heart contraction

Respiratory centers – control rate and depth of breathing

Additional centers – regulate vomiting, hiccuping, swallowing, coughing, and sneezing

There is an overlap between medulla and hypothalamus that uses medullary centers to carry out instructions


Other Medulla nuclei and functions Pyramids – two longitudinal ridges formed by corticospinal tracts

descending from the motor cortex (will be discussed with the somatomotor pathways)

Inferior olivary nuclei – gray matter that relays sensory information regarding the stretch of muscle and joints

Cranial nerves IX (glosopharyngeal, X (vagus), XI (accessory; neck muscle), and XII (hypoglossal; tongue) are associated with the medulla

Vestibular nuclear complex – synapses that mediate and maintain equilibrium

Ascending sensory tract nuclei, including nucleus cuneatus and nucleus gracilis

Serve as relay station for general somatic sensation

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Motor (efferent) system