BIO 312 exam 4

ascending tracts

• how sensory info reaches the cortex

• signals from most of body first enter SC through spinal nerves

• signals from face, mouth, and head use cranial nerves

somatosensory system

• touch

• temperature

• pain

• proprioception

• vibration

chain of neurons in ascending pathways

first order neurons:

• cell bodies in ganglion

• conduct to spinal cord or brain stem

second order neurons:

• cell bodies in dorsal horn

• conduct to thalamus or cerebellum

• axons cross to contralateral side

third order neurons:

• cell bodies in thalamus

• conduct info to somatosensory cortex

spinothalamic pathway

• somatic pain, temp, light touch, itch pressure

1. first order neuron comes in through dorsal horn of spinal cord, synapse with 2nd order neuron

2. 2nd order neuron crosses to contralateral side in the lumbar vertebrae and ascends to thalamus, synapses with 3rd order neuron

3. 3rd order neuron ascends to primary somatosensory cortex

• damage causes loss of pain and temp on the contralateral side of the body, from that vertebrae down

spinocerebellar pathway

• proprioception

1. first order neuron comes in through dorsal horn of spinal cord and synapses on the second order neuron

2. second order neuron ascends on the same side to cerebellum

• loss of same side proprioception (balance, coordination) on same side of body (ipsilateral)

dorsal column medial lemniscal pathway

• deep touch, visceral pain, vibration

1. first order neuron comes in through dorsal horn of spinal cord and ascends on the same side to the medulla, synapses with 2nd order neuron, then crosses over

2. second order neuron ascends on contralateral side to thalamus, synapses with 3rd order neuron

3. third order neuron ascends from thalamus to primary somatosensory cortex

• damage would cause loss of vibration, deep touch, and pain on same side of body because pathway does not cross until the brain

motor areas of the brain

primary motor cortex and brain stem house upper motor neurons, which go through spinal cord and synapse into lower motor neurons which innervate muscles

ventral corticospinal tract

2 neurons

• upper motor neuron cell bodies in cerebral cortex (precentral gyrus in the primary motor cortex)

• descend on same side until exit (lumbar) and then cross

• synapse on lower motor neuron

• skeletal muscle (voluntary)

damage causes reduced voluntary skeletal muscle control (especially posture), not total loss: often on both sides, will be slightly higher on the side damaged

lateral corticospinal tract

3 neurons

• upper motor neuron cell bodies in cerebral cortex (precentral gyrus in primary motor cortex)

• descend on same side until medulla, decussates to other side

• spinal cord axons from contralateral cortex

• synapse onto interneuron in lumbar

• interneuron synapses onto lower motor neuron

• skeletal muscle (voluntary)

damage causes loss of voluntary movement on the same side at or below where the damage is located

rubrospinal tract

3 neurons

• upper motor neuron cell bodies in midbrain

• decussates at the pons to contralateral side

• descends and synapses onto interneuron in ventral horn

• synapses onto lower motor neuron

• skeletal muscle (involuntary)

damage leads to loss of involuntary skeletal muscle movement (coordination and smooth control), on contralateral side

indirect pathways

subcortical region: UMN cell bodies in brainstem nuclei, can be influenced by cortical pathways

regulates: balance/posture, coordination of tracking objects

extrapyrimidal pathways versus pyrimidal pathways

pyramidal: lateral and ventral corticospinal pathways

• fask, skilled, voluntary movement

• decussate in medulla lateral or SC ventral

• originate in primary motor cortex

extrapyramidal: rubrospinal pathway

• originate in brainstem

• involuntary control, balance/posture

• some decussate some don’t

damage to dorsal and ventral regions

dorsal: loss of sensation or abnormal sensations, incomplete info about the enviroment around us

ventral: leads to paralysis

flaccid paralysis versus spastic paralysis

flaccid:

• lower motor neurons

• loss of tone

• nerve impulses can’t reach muscles, no communication between muscles and SC but there is between SC and brain, leads to atrophy

spastic: uncontrolled contractions

• upper motor neurons

• no voluntary movement but reflex activity still stimulates muscle

• communication between muscles and SC but not SC and brain

reflexes and reflex arc:

reflexes- fast, involuntary, stereotypes reactions of muscles to stimulation

basic reflex arc: sensation integration, motor output

• in many spinal reflexes the brain is not directly involved

• inputs to brain make you aware of stimulus and movement

components of a reflex arc

monosynaptic: 1. receptor 2. sensory neuron 3. motor neuron 4. effector

• only one synapse between sensory and motor neurons

polysynaptic: 1. receptor 3. sensory neuron 3. interneuron 4. motor neuron 5. effector

muscle spindle

• for proprioception

• information is constantly sent to brain (muscle status) via sensory neurons for tone, coordinated movement, balance

components of muscle spindle

extrafusal fibers: contract; innervated by alpha motor neurons

intrafusal fibers: no sarcomeres in the middle, only on ends so don’t usually contract; are innervated by gama motor neurons

primary afferent: monitor degree, length, and rate of stretch

secondary afferent: monitor degree of stretch

stretch reflex monosynaptic and reciprocal inhibition (polysynaptic)

monosynaptic:

1. receptor: intrafusal muscle fibers

2. primary afferent: sends signal to spinal cord and synapses with lower motor neuron

3. stimulate alpha motor neuron

4. alpha motor neuron synapses with extrafusal fibers and stimulates quadriceps to contract

reciprocal inhibition (polysynaptic):

5. primary afferent also stimulates interneurons

6. interneuron neuron synapses with alpha motor neuron on antagonistic side (hamstrings)

7. inhibit alpha motor neuron

8. alpha motor neuron inhibited=hamstring stays relaxed

muscle spindle length

• when stretched more AP’s in muscle spindle

coactivation: in voluntary muscle contraction both alpha and gamma fibers are activated together

• if only alpha MN’s were activated the , muscle spindle would become more slack not firing AP’s

• ensures muscle spindle can still detect length changes

• prevents muscle spindle from being useless as a receptor

muscle status

• state of stretch or contraction

• conveyed via AP frequency

• AP frequency decreases when muscle shortens

tendon organs

• proprioceptor for tendon reflex

• protective and prevents damage to muscle, respond to excessive tension

1. neuron from golgi tendon organ fires

2. motor neuron is inhibited

3. muscle relaxes

4. load is dropped

tendon reflex is opposite of stretch

1. quadriceps contract, tendon organs activated

2. afferent fibers synapse with interneurons in the spinal cord

3. efferent impulses to muscle are dampened due to stretched tendon, muscle relaxes decreasing tension

4. efferent impulses to antagonist muscle cause it to contract

muscle spindle sensitivity

• can be modified

brain increases gamma motor output: more sensitive, for balance reflexes

ex: bean routine

brain suppresses gamma motor output: less sensitive, for greater range of motion

ex: pitching a ball

flexor and crossed extensor reflexes

flexor(ipsilateral):

1. stimulus at receptor

2. sensory afferent, synapses at interneuron

3. a- interneuron to LMN flexors

b- interneuron IPSP to LMN extensors

4. a- motor neuron to flexors is stimulated

b- motor neuron to extensors is inhibited

5. a- alpha motor neuron synapses with extrafusal fibers of hamstring

b- alpha motor neuron synapses with extensors, signal is not sent because there is an IPSP, quads stay relaxed

extensor (contralateral):

2. same sensory afferent signal to contralateral side through interneuron

3. a-interneuron IPSP to LMN flexors

b- interneuron EPSP to LMN extensors

4. a- motor neuron to flexors is inhibited

b- motor neuron to extensors is stimulated

5. a- alpha motor neuron synapses with flexors, signal not sent bc of IPSP, hamstrings stay relaxed

b- alpha motor neuron synapses with extrafusal fibers of quads to contract

comparison of autonomic versus somatic NS

effectors:

• somatic: skeletal muscles

• ANS: Cardiac, smooth muscles and glands

efferent pathways and ganglia:

• single lower motor neuron to effector

• two-neuron chain pre ganglionic vs post ganglionic

• somatic=no ganglia (cell bodies in ventral horn)

• ANS=ganglia

neurotransmitters:

• somatic=Ach (always EPSP)

• ANS= Ach (sympathetic and parasympathetic) and norepinephrine (sympathetic)

NT release somatic versus ANS

ANS: broad release of NT to bloodstream (organs have receptors)

somatic: targeted release of NT to skeletal muscle

somatic nervous system

EPSP

cell body: ventral horn of SN

heavily myelinated

LMN- axon, single neuron from CNS to effector

NT(ACh): always excitatory, nicotinic receptor

target organ: skeletal muscle

sympathetic NS

EPSP or IPSP depending on receptor

cell bodies: lateral horns of SC or thorocolumbar

pre ganglionic neuron is short, post ganglionic neuron is long to reach target organ

light myselination in preganglionic neuron, no myelin in postganglionic neuron

ganglion is between pre and post ganglion

NT (norepinephrine)

effector: smooth muscle, cardiac muscle, glands

for neuron to blood vessel:

pre ganglionic neuron has light muselination, goes to adrenal gland, then blood vessel

NT: norepinephrine and epinephrine which go to alpha 1,2 and beta 1,2,3 receptors

• if they are released into bloodstream, slower, longer response

parasympathetic NS

• cell bodies; CNS craniosacral

• preganglionic neuron: light myselination, long axon

• post ganglionic neuron: short axon, no myelination

• ganglion in middle with Ach released which is excitatory and goes to nicotinic receptor(+) which excited muscle fibers or postganglionic cells, has Na+ channels or ionotropic receptors

• Ach released from post ganglionic neuron and goes to muscarinic receptors(+/-) goes to neuromuscular or neuroglandular junctions and metabotropic receptors like G proteins using secondary messangers

  • stimulatory: M1 M3 M5

  • inhibitory M2 M4

parasympathetic functions

• relaxation

• energy absorption

• food processing (digestion/absorption)

parasympathetic (craniosacral) division

• preganglionic fibers from cranial nerves and sacral regions of SC

• long preganglionic fibers

• all fibers release Ach

preganglionic cell bodies: brainstem, sacral segment of SC

postganglionic cell bodies: walls of organs

parasympathetic activation

• constriction of pupils

• secretion of digestive enzymes

• promotion of absorption

• changes in blood flow associated with sexual arousal

• increase in motility of GI tract

• contraction of urinary bladder during urination

• constriction of respiratory passageways

• reduction in HR

sacral versus cranial outflow of parasympathetic NS

sacral:

• s2-s4: half of large intestine, bladder, reproductive organs

Cranial:

• CN3: oculomotor: constrict pupils, lens rounding

• C7: facial nerve: salivary glands, tear glands

• c9: glossopharyngeal: salivary glands

• c10: vagus: slow HR, increase blood supply to viscera, bronchoconstriction

sympathetic NS functions

• heightened mental alertness

• increased metabolic rate

• reduced digestive and urinary function

• activation of energy reserves

• uncreased respiratory rate and dilation of respiratory pathways

• increased HR and BP

• activation of sweat glands

types of ganglia in sympathetic NS

sympathetic chain ganglia:

• lateral sides of vertebral column

• neurons originating here innervate body wall

collateral ganglia:

• anterior to vertebral column

• innervate abdominopelvic contents

• preganglionic fibers pass through sympathetic chain

• form splanchnic nerves that pass to collateral ganglia

adrenal medullae:

• center of adrenal glands

• cause release of NT into circulation

sympathetic chain pre vertebral (2 pathways)

first option:

1. pre ganglionic neuron axons go to sympathetic chain lateral horn, then ventral root, then spinal nerve, then ventral ramus, then gray ramus communicans

2. postganglionic neuron synapses at same level of sympathetic chain→ gray ramus communicans

3. synapses at effector→ releases NT

second option:

1. pregangionic neuron ascends or descends then synapses at lateral horn, goes to ventral root, then spinal nerve, then ventral ramus, then gray ramus communicans, then sympathetic chain where it ascends or descends

2. postsympathetic neuron is at different level of sympathetic chain→ gray ramus communicans

3. synapse at effector→ NT release

collateral ganglia (postvertebral)

1. preganglionic neuron axons go through sympathetic chain lateral horn, go to ventral root, then spinal cord, then ventral ramus, then gray ramus communicans, then passes through sympathetic chain

2. post ganglionic neuron synapses at same level of sympathetic chain→ collateral ganglion

3. synapses at effector→ NT release

adrenal medulla

some preganglionic fibers directly synapse on it

• secretes NE and E hormones into blood

• lengthens the effects

sympathetic vs parasympathetic

location of CNS visceral motor neurons

S: thoracolumbar

PS: CNS craniosacra;

location of PNS ganglion:

S: sympathetic chain or collateral ganglion

PS: at effector