lecture7 pns efferent division

somatic nervous system (voluntary movements)

consists of a motor neuron between CNS and skeletal muscle cells

• cell bodies of these neurons located in brain stem or ventral horn of spinal cord

• neurotransmitter released is Ach (acetylcholine)

• leads to contraction of skeletal muscle cells

autonomic nervous system (involuntary movements)

• 2 neuron chain (connected by a synapse) between CNA and effector organ (in tissues other than skeletal muscle)

skeletal muscle activity occurs

in the neuromuscular junction (NMJ)  

• a special intercellular connection (chemically linked) between CNS and skeletal muscle fibers 

a motor unit 

consists of one motor neuron and all skeletal muscle fibers it controls 

• when the neuron fires, all its muscle fibers contract together 

motor end plate

a shallow depression in the sarcolemma (muscle cell membrane) at the neuromuscular junction (NMJ)

• lies directly under the axon terminal of the motor neuron

• contains ACh receptors that bind to ACh released from the neuron’s axon terminal

• leads to muscle contraction

1. release of ACh at NMJ

an action potential travels down the motor neuron to the axon terminal

• this causes voltage-gated Ca2+ channels to open, allowing Ca2+ to enter terminal

• calcium triggers exocytosis- vesicles release acetylcholine into the synaptic cleft

2. formation of end-plate potential (EPP) 

ACh binds to nicotinic ACh receptors on the motor end plate

• these are chemically-gated ion channels, which open and let Na+ enter the muscle fiber 

• the influx on Na+ causes local depolarization (graded potential) called the end-plate potential (EPP)

3. Initiation of an action potential

The EPP triggers an AP

• the AP spreads a long the sarcolemma, leading to Ca2+ releases inside the muscle contraction

acetylcholinesterase ends ACh activity at NMJ 

acetylcholinesterase (AChe), an enzyme on the motor end plate, breaks down ACh in the cleft 

• this stops ACh’s action, ending the electrical signal 

• without stimulation, the muscle fiber relaxes

black widow spider venom

causes explosive release of acetylcholine (ACh) at the neuromuscular junction

• as a result the muscle experiences spams, cramps. It can’t relax.

botulinum toxin (botox)

prevents the release of acetylcholine from the axon terminal at the neuromuscular junction

• as a result, the muscle cannot contract and becomes paralyzed

• flaccid paralysis- muscles stay relaxed and weak

autonomic ganglia 

groups of nerve cell bodies outside the CNS

• they hold the postganglionic neurons that send signals to organs and glands (visceral effectors)

sympathetic fibers

leave from thoracic (chest) and lumbar regions of spinal cord

• these fibers prepare the body for “fight or flight”- increasing heart rate, dilating pupils, and slowing digestion 

parasympathetic fibers

leave from brainstem and sacrum

• these fibers promote “rest and digest” - slowing heart rate, aiding digestion, and conserving energy 

sympathetic division

it has short preganglionic and long postganglionic neurons that prepare the body for action (fight or flight) short 

short pre-ganglionic fibers (sympathetic division)

located between segments T1 and L2, cell bodies in lateral horns

major sympathetic division neurotransmitters 

• acetylcholine (ACh) is released at the preganglionic synape (acticated postganglionic neuron)

• norepinephrine (NE) and sometimes epinephirne (Epi) are released at the postganglionic synapse (releases neurotransmitters onto target organ)

respomses to increased sympathetic activity

• heightened mental alertness, increases metabolic rate

• reduce digestive and urinary functions

• activation of energy reserves: glycogenolysis and liposis

• incr respiratory rate and dilation of respiratory passageways

• pupil dilation and adjusts eye for far vision

• increased heart rate and blood pressure (blood vessels constrict)

• activation of sweat glands

parasympathetic division

long pre-ganglionic in brainstem and sacral segment of spinal cord

• autonomic nuclei are in all parts of brainstem and lateral horns of S2-S4

• short post-ganglionic synapes in or near target organs

• most neurotransmitter is ACh at pre- and post-ganglionic synapses

• “rest and digest” system

parasympathetic effects 

• decreased metabolic rate 

• increased smooth muscle activity (motility) and blood flow to digestive tract 

• constriction of respiratory passageways

• reduction on heart rate and force of contraction

nicotinic receptors 

cholinergnic receptor - binds with ACh (acetylcholine) 

• located: on postganglionic cell bodies in all autonomic ganglia 

• ACh is released from sympathetic and parasympathetic preganglionic fibers 

preganglionic