Veterinary Physiology NEURO

How does information go from stimuli to effector?

receptors->sensory neuron->interneuron->motor neuron->effector

kinds of sensory receptors

- pacinian corpuscle

- golgi tendon organ

- meissners corpuscles

- photoreceptors

- taste and smell receptors

kinds of effectors

- other neurons

- muscle

- gland

kinds of glial cells

- astrocytes

- oligodendrocytes

- microglia

- Schwann cells

oligodendrocyte

Type of glial cell in the CNS that wrap multiple axons in a myelin sheath.

Schwann cells

Type of glia in the PNS. Wraps one axon in myelin. Can also surround axons without wrapping them

astrocyte

type of glia in CNS that forms a scaffold, regulates K+, and reabsorbs GABA, glutamate

Glutamate-Glutamine cycle

astrocytes take up GABA and glutamate and convert it into glutamine

Blood Brain Barrier (BBB)

maintained by astrocytes and endothelial cells

Microglia

immune cells of CNS from bone marrow

simple diffusion

movement of a solute from an area of high concentration to an area of low concentration

- slow

- lipid soluble, small

passive transport

kind of simple diffusion where the assistance of channels is needed

ion channels

A transmembrane protein channel that allows a specific ion to diffuse across the membrane through passive transport

- can be gated

kinds of gated channels

- voltage gated

- mechanically gated

- ligand gated

voltage gated channels

open and close in response to changes in membrane potential

have 2 kinds of gates: activation and inactivation

mechanically gated channels

open and close in response to physical deformation of receptors

ligand-gated channels

channel that opens when a neurotransmitter attaches

primary active transport

Active transport in which ATP is hydrolyzed, yielding the energy required to transport an ion or molecule against its concentration gradient.

Na/K ATPase

leads to net negative charge in cell

secondary active transport

Form of active transport which does not use ATP as an energy source; rather, transport is coupled to ion diffusion down a concentration gradient established by primary active transport.

symporter

secondary transporter that carries two different ions or small molecules, both in the same direction

antiporter

A carrier protein that secondary transports two molecules across the plasma membrane in opposite directions.

At rest, membrane is only permeable to...

K+

K+ leaky channels are only thing open

What channels defines MP during AP?

Na+ voltage channels

What channels defines MP at rest?

K+ leaky channels

axon hillock

graded potentials (EPSPs and IPSPs) summate here. Must depolarize to -55mV for AP to occur

EPSP (excitatory postsynaptic potential)

excitatory synapses that lead to post-synaptic opening of ligand gated Na+/Ca+ channels (depolarizing graded potential)

IPSP (inhibitory postsynaptic potential)

inhibitory synapse leads to opening of K+/Cl- channels causing hyperpolarization

Action potential steps

Voltage gated Na+ channels steps

Kinds of NTs

1. Amino acids: GABA, glycine, glutamate

2. acetyl choline

3. amines: seratonin, dopamine, norepinephrine, epinephrine

4. neuropeptides: oxytocin, vasopressin, neuropeptide gamma)

steps of AP at synapse

1. AP reaches terminus

2. voltage gated Ca+ channels open and let Ca+ in cell

3. NTs release into cleft

4. NTs bind to receptors on postsynaptic neuron

5. (not seen) NT removed from cleft

methods of removing NTs after transmission

1. ACh broken down by acetylcholinesterase. Choline taken up by presynaptic neuron and reused.

2. reuptake by presynaptic neuron (amines)

3. diffusion to be metabolized by astrocytes

Synaptic Toxins

- block release of NTs (botulism, tetanus)

- block reuptake of NTs (SSRI, SBARI)

- block receptor (atropine)

- block metabolism (tensilon, sarin)

NT receptor types

- ligand gated (nicotinic, ionotropic)

- G-protein coupled (muscarinic, metabotropic)

electrical synapse

a type of synapse in which the cells are connected by gap junctions, allowing ions (and therefore the action potential) to spread easily from cell to cell

intervertebral disc disease

rupture or protrusion of the cushioning disc found between the vertebrae that results in pressure on the spinal cord or spinal nerve roots

fibrocartilaginous embolic myelopathy

-spinal cord infarction

-fibrocartigalinous embolus obstructs ventral spinal artery (no blood flow to spinal segment)

-acute, non-progressive, nonpainful paralysis

- Asymmetrical signs

dura mater

thick, outermost layer of the meninges surrounding and protecting the brain and spinal cord

subdural space

space between dura mater and arachnoid mater filled with serous fluid

epidural space

space external to the dura mater. anesthetic site

arachnoid mater

weblike middle layer of the three meninges, CSF reabsorption

subarachnoid space

a space in the meninges beneath the arachnoid membrane and above the pia mater that contains the cerebrospinal fluid

pia mater

Innermost layer of the meninges, contains blood vessels

all spinal nerves carry...

sensory and motor fibers

how many spinal cord segments are associated with the 7 cervical vertebrae?

8; spinal nerve exits occur in front of C vertebrae except spinal nerve C8 which exits behind C7 vertebra

After spinal nerve C7, all spinal nerves exit...

behind their respective vertebrae

dermatome

Area of skin supplied by a single spinal segment

Myotome

muscle or group of muscles supplied by a specific spinal segment

cauda equina

terminal spinal segments that cluster around L4-L6 and stretch down canal caudal

cervicothoracic enlargement

C6-T2, supplies forelimb reflexes and LMNs

lumbosacral enlargement

L4-S2, supplies hindlimb reflexes and LMNs

dorsal column

a white matter tract on the dorsal side of the spinal cord, carrying touch and proprioceptive axons to the brain stem

dorsal horn

Crescent shaped projection of gray matter within the spinal cord where sensory neurons enter the spinal cord. Home of secondary sensory neurons

Secondary sensory neurons

cross the midline so that one side of the brain processes information from the opposite side of the body

lateral horn

(only in thoracic and a few lumbar and sacral regions)

- autonomic neurons

ventral horn

somatic motor neurons whose axons exit the cord via ventral roots

lateral column

white matter of the spinal cord between the posterior horn on one side and the axons from the anterior horn on the same side; composed of many different groups of sensory and motor tracts

ventral column

the ventral section of white matter in the spinal cord between the ventral horns. carries sensory and motor tracts

ventral spinal artery

Comes off the vertebral artery, supplies the ventral 2/3 of spinal cord (1).

Dorsolateral Spinal Arteries

Formed from fusion of dorsal radicular arteries; supplies dorsal spinal cord

somatic reflexes start and end in...

PNS; synapse in CNS but don't join tracts

components of a reflex arc

1. receptor

2. sensory neuron

3. interneuron (not always present)

4. motor neuron

5. effector

how to classify reflexes

1. Effector

- somatic

- autonomic

2. Integration site

- spinal cord = spinal reflex

- brain = cranial nerve reflex

3. # of synapses

- monosynaptic

- polysynaptic

Myostatic stretch reflex

flexor reflex (withdrawal reflex)

crossed extensor reflex

crosses over

muscle spindle steps

1. primary sensory neuron detects stretch. synapses with alpha motor neuron

2. alpha motor neuron synapses of muscle leading to contraction

3. secondary sensory neuron senses change in tension

4. gamma motor neuron tenses intrafusal fiber to just below threshold tension (resets)

myostenia gravis

generalized, degeneration of neuromuscular junctions. sensory works, reflexes weakened

spinal tracts

- ascending: carry sensory

- descending: carry motor

- sometimes decussate

- paired

Kinds of ascending tracts

- conscious

-> dorsal column

-> spinothalamic tract

-> non-specific multisynaptic pathways

- unconscious

-> spinocerebellar tract

ascending tracts

primary sensory neuron enters via dorsal root (cell body always in dorsal root ganglion). Usually decussates (exception proprioception fibers in unconscious tracts)

spinothalamic tract

- carries pain and temperature information

- located in ventrolateral areas of spinal cord white matter.

- Moderate myelination

- resistant to injury

dorsal column tract

- carries conscious proprioception + touch

- dorsal aspect of spinal cord

- includes medial fasciculus gracilis (caudal - T6, hindlimb) and more lateral fasciculus cuneatus (cranial - T6, forelimb)

- large myelinated fibers to cortex

- high injury risk

nonspecific multisynaptic pathways

- poorly myelinated

- ascend in white matter and terminate in reticular activating system (RAS)

- synapse multiple times as they ascend

- resistant to injury

spinocerebellar tracts

- unconscious proprioception

- adjusts activity of motor. neurons

- does not decussate

- enter cerebellum via caudal cerebellar peduncle

- heavily myelinated

- high injury risk

ataxia

inability to perform coordinated movements

types of ataxia

- spinal

- vestibular

- cerebellar

kinds of descending tracts

- major

-> rubrospinal

-> reticulospinal

-> vestibulospinal

- minor

-> corticospinal

-> tectospinal

descending tracts

- have 2-3 neurons in series that start at specific brain nuclei and end at spinal cord level where they synapse with LMNs

state the order of spinal pathway injury from least to most susceptible:

pain->motor->proprioceptive

rubrospinal tract

- red nucleus of thalamus -> spinal cord

- voluntary movement

- decussates in midbrain

reticulospinal tract

- reticular formation in medulla -> spinal cord

- balance, posture

- mostly ipsilateral

vestibulospinal tract

- vestibular nuclei -> spinal cord

- synergy of movements, equilibrium, balance

- ipsilateral

corticospinal tract

- cortex -> spinal cord

- fine muscle movement

tectospinal tract

- cortex -> cervical spinal segments

- head movement response to visual/auditory stimuli

Upper Motor Neurons (UMN)

motor neurons in the central nervous system that control the lower motor neurons in the peripheral nervous system

Lower Motor Neurons (LMN)

Motor neurons that leave spinal cord via ventral root. Cell bodies of LMN originate in the spinal cord. Multiple UMNs from different motor tracts can synapse at one LMN

UMN vs LMN damage

1. Evaluate tone

- LMN damage -> decreased tone (flaccid paresis/paralysis) + neurogenic atrophy

- UMN damage -> increased tone (spastic paresis/paralysis) + disuse atrophy over time. Increased tone results from reduced inhibitory signals summating on LMN

2. Evaluate reflexes

- LMN damage -> reduced reflex

- UMN damage -> increased reflex

Localizing a lesion steps

1. does animal have lesion?

- neurologic exam

2. what limbs affected?

- 1 limb -> peripheral nerve

- 2-4 limbs -> spinal cord/CNS

3. ataxia?

- spinal cord lesion would have proprioceptive tracts affected

4. what reflexes are decreased?

- forelimb decreased -> cervicothoracic enlargement lesion

- hindlimb decreased-> lumbosacral enlargement lesion

- no reflexes affected if lesion in C1-C5 or T3-L3

5. consider affected limbs

- hindlimb only -> lesion caudal to T2

- forelimb + hindlimb -> lesion at or above T2

6. Consider 4. and 5.

- hindlimb + hindlimb reflex decreased -> lesion in L4-S2

- hindlimb affected -> lesion in T3-L3

- forelimb + hindlimb + forelimb reflexes decreased -> lesion in C6-T2

- forelimb + hindlimb affected -> lesion in C1-C5

brachial plexus

pattern of organization of peripheral nerves that originate from C6 to T2

lumboscaral plexus

pattern of organization of peripheral nerves that originate from L4 to S3

what spinal segments make up the musculocutaneous nerve?

C6-8

what spinal segments make up the radial nerve?

C7-T2

what spinal segments make up the median nerve?

C8-T2

what spinal segments make up the femoral nerve?

L4-6

what spinal segments make up the obturator nerve?

L4-6

what spinal segments make up the sciatic nerve?

L6-S2

what spinal segments make up the pudenal nerve?

S1-3

neuromuscular transmission

1. sodium influx

2. depolarization

3. release of Ca2+ from SR

4. Actin/myosin interaction

5. shortening of sarcomere

6. Na+ channels close and Na+ pumped out

7. repolarizattion

8. Ca2+ channels close and Ca2+ pumped into SR

9. sarcomere relaxes