Neuro Unit 3

Somatomotor system

controls striated skeletal muscles --> this is how you move

Autonomic motor system

sympathetic and parasympathetic control of heart, blood vessels, digestive tract

Upper motor neurons

start in cerebral cortex, target lower motor neurons

Lower motor neurons

start in brainstem (cranial nerves) or spinal cord (spinal nerves, ventral horn), directly target muscles-- exit the ventral root and join w/ spinal nerves AFTER the dorsal root ganglion, traveling all the way to muscle fiber

neuromuscular junction

motor neuron-muscle connection

acetylcholine

motor neurons signal muscle contractions via the ___

muscle fascicles

muscles are made up of

muscle fibers

fascicles contain bundles of

each adult muscle fiber is targeted by how many neuromuscular junctions?

one

ligand-gated ion channels

acetylcholine receptors are___. Activation of the receptor causes depolarization-- sodium enters the muscle FIBER, potassium exits. This spreads across the fiber like an action potential --> calcium is released inside the fiber

actin and myosin

calcium released in response to acetylcholine binding to the muscle fiber activates ____ complex to cause muscle contraction

myosin; actin

___ molecules slide across ___ molecules during muscle contraction

Type I fibers

More mitochondria than type II, thin, slow speed, low force, slow fatigability, increased proportion by slow, sustained exercise.

Type IIa fibers

Thick, intermediate speed, intermediate force, intermediate fatigability, increased proportion due to short, high intensity exercise -- producing short bulky muscles that fatigue quickly.

Type IIb fibers

Thick, fast, high force, rapid fatigability, increased proportion due to short, high intensity exercise -- producing short bulky muscles that fatigue quickly.

motor units

a motor neuron and the muscle fibers it innervates. fine dexterity groups have small ___, course control muscles have large ___

LMN's are controlled by

local circuits (sensory neurons/reflexes & interneurons in spinal cord), upper motor neurons (cortical/pyramidal tracts for voluntary motor control, noncortical/nonvoluntary motor control)

Reflex circuits

Local and fast, no needed signal from brain (sensory input --> dorsal root, synapse in spinal cord on ventral horn, rotation back down through LMN's in ventral root)

corticobulbar tract

controls head and neck muscles to bilateral brainstem cranial nerve motor nuclei, descends thru pyramids of medulla

corticospinal tract

controls body/limb muscles (lateral and ventral/anterior), descends thru pyramids of medulla

lateral corticospinal tract

to entire contralateral spinal cord (mostly limbs), descends thru pyramids of medulla, crosses midline at lower medulla (decussation), descends spinal cord & synapses on LMNs in ventral horn

ventral/anterior corticospinal tract

to bilateral cervical and upper thoracic spinal cord (mostly central body), descends thru pyramids of medulla, descends on the same side of the body, then projects bilaterally to LMNs for control of trunk muscles

rubrospinal

nonpyramidal motor pathway; red nucleus to spinal cord, for coordinating fine limb movements

vestibulospinal

nonpyramidal motor pathways; vestibular nucleus to spinal cord for balance and head position adjustments

reticulospinal

nonpyramidal motor pathway; reticular formation to spinal cord for postural control and learned limb movement

tectospinal

nonpyramidal motor pathway; superior colliculus to cervical spinal cord for coordinating head and eye movements

ALS (amyotrophic lateral sclerosis)

progressive loss of motor control due to degeneration of UMNs or LMNs

Primary lateral sclerosis

Affects only UMNs

Progressive muscle atrophy

Affects only LMNs

Paralytic polio

Motor neurons are directly targeted and killed by the virus, causing muscle atrophy and paralysis

Basal ganglia

Subcortical nuclei critical for information processing related to decision making and movement selection. Main input --> cortex, main output --> thalamus

Striatum

Caudate nucleus, putamen, nucleus accumbens. Receives dense inputs from the cortex and dopamine neurons in the midbrain, divided by internal capsule (nucleus accumbens is only present in the anterior/rostral striatum)

Globus pallidus

External GPe and Internal GPi --> internal GPi is part of output of the basal ganglia. GP lies medial to putamen.

Subthalamic nucleus

Part of diencephalon, "sub" thalamus

Substantia nigra

Pars compacta (SNc) is major dopamine center, Pars reticulata (SNr) is part of the output of the basal ganglia. Part of the midbrain/mesencephalon, positioned between subthalamic nucleus and CEREBRAL peduncle. Dopamine neurons in SNc project to the dorsal striatum (caudate/putamen)

glutamate

Corticospinal neurons signal with ___ to excite MSNs in the striatum

GABA

MSNs signal with ___ to inhibit target neurons

Dopamine input from SNc can modify MSN activity

Excitatory via D1Rs; Inhibitory via D2Rs

GPi and SNr

Main output centers for BG, signal ventral anterior and ventral lateral nuclei of the thalamus with GABA

Hypokinesia

Reduced movement due to basal ganglia degeneration

Hyperkinesia

increased movement due to degeneration of basal ganglia

Akinesia

Impaired initiation

Bradykinesia

slowed/reduced movement

Rigidity

stiffness/resistance

Chorea

rapid/jerky movements

Athetosis

Slow writhing movements

Ballism

flailing movements

Nigrostriatal pathway

Nigrostriatal dopamine neurons are concentrated in the SNc, where they project to the dorsal striatum (caudate/putamen)

Parkinson's Disease

arising from degeneration of dopamine neurons in the SNc

L-DOPA

dopamine precursor molecule that crosses the BBB and is converted to dopamine

Deep brain stimulation

Surgical treatment for Parkinson's, OCD, etc. Stimulating electrodes are implanted into GPi or subthalamic nucleus.

Huntington's disease

degeneration of MSNs in the striatum, inheritance of dominant mutation producing a mutant protein that kills MSNs, produces hyperkinesia

Tourettes

Dysfunction in striatal and frontal cortical circuits, uncontrollable movements and vocalizations

Dystonia

Uncontrollable sustained muscle rigidity and contortions

Cerebellar lobes

anterior, posterior, flocculondular

vermis

midline cerebellum

hemispheres

lateral cerebellum

cerebellar cortex

external surface of cerebellum

folia

organization of cerebellar cortex -- similar to gyri of cerebral cortex

arbor vitae

internal white matter tracts of cerebellum

Cerebellum connects to the brainstem via three peduncles:

superior (midbrain), middle (pons), inferior (medulla)

Deep cerebellar nuclei

fastigial (medial), interposed, dentate (lateral) --> FID. Main output of the cerebellum.

Layers of cerebellar cortex

Molecular (outer, made up of axons, dendrites, smaller neurons), Purkinje (middle), Granular (inner)

Granule cells

most numerous cell type in the brain, send axons up into molecular layer. Axons sent up split and run in both directions parallel to the cerebellar folia.

Purkinje cells

Extensive, fan shaped dendritic tree reaches into molecular layer, the only way to leave the cerebellar cortex. Fan out into single plane, perpendicular to direction of the folia.

Mossy fibers

contact granule cells (come from many places, cerebral cortex, vestibular nucleus, spinal cord)

Climbing fibers

contact purkinje cells (come from olivary tract in rostral medulla)

Cerebellar interaction w/ rubrospinal tract

Inf. olivary nucleus sends climbing fibers to purkinje cells (only outputs of cortex) --> P. cells send axons to deep cerebellar nuclei (FID) --> FID send axons out of cerebellum

Cerebellar functional domains

Spinocerebellum (medial/vermis), cerebrocerebellum (lateral/hemispheres), vestibulocerebellum (flocculonodular lobe)

Spinocerebellum input

spinocerebellar tracts and cranial nerves carrying proprioceptive information in the body

Spinocerebellum output

Processed in vermis and intermediate hemispheres of cerebellar cortex, output is to interposed nuclei, which outputs to red nucleus, which projects to spinal cord and cranial nerve nuclei, coordinating movements for posture control/movement corrections

cerebrocerebellum input

from cerebral cortex via pontine nuclei (pons), sending axons to contralateral side of cerebellum via middle cerebellar peduncle, processed in lateral hemispheres of cerebellum

cerebrocerebellum output

processed in lateral hemispheres of cerebellum, output to dentate nucleus, then ventrolateral nucleus of thalamus, projecting to cerebral cortex, involved in coordinating movement planning, learned movements, cognition

vestibulocerebellum input

from vestibular nuclei and vestibular nerve, processed in flocculonodular lobe of cerebellum

vestibulocerebellum output

to fastigial nuclei and vestibular nucleus, project to spinal cord/cranial nerves to maintain balance, coordinate head/eye movements

Ataxia

Inaccuracy in speed and force movements, inaccurate estimate of distances

tremor

involuntary movements, shakes/jiggles

nystagmus

involuntary movements of the eyes

Central pattern generators (CPGs)

A network of neurons, typically in spinal cord, that are capable of generating rhythmic pattern of activity, importantly: without requiring sensory feedback

flexor (bending the limb); extensor (straightening the limb); mutual inhibition

locomotion requires rhythmic coordination of ____ and _____ through _____

Mutual inhibition

utilization of excitatory and inhibitory interneurons. coordination of ipsilateral flexor/extensor opposition as well as contralateral opposition.

EphA4

gene that encodes a receptor for the protein family ephrin, activation repels axon growth, preventing excitatory interneurons from crossing. No EphA4 gene --> growth of excitatory interneuron across spinal cord --> contralateral excitation (both sides moving at once).

Walking is _____ and can be stereotyped, but isn't rigid

automatic; ascending sensory info, without cortical involvement, is sufficient to control step speed (proprioceptive information, 1a afferents from muscle spindles/muscle stretch reflex, 1b afferents from GTO/muscle contraction).

Proprioceptive information Process

stretching hip flexor muscle --> activates muscle spindle in flexor muscle --> excite motor neuron to contract flexor muscle --> inhibit motor neuron to relax extensory muscle

Mesencephalic Locomotor Region (MLR)

Can initiate walking, even in decerebrated prep. MLR --> reticular formation (medulla) (axons in ventrolateral funiculus) --> CPG. Stimulation of MLR can modulate speed of walking.

glutamatergic

Descending pathways require _____ activation to initiate walking

Norepinephrine; Serotonin

_______ and _______ can be sufficient to initiate stepping, not necessary

Dorsal spinocerebellar tract

Proprioception

ventral spinocerebellar tract

Afferents from CPGs

Gaze fixation

Active process that holds the retinal image steady so photoreceptors can work

Foveal vision

requires the brain to decide where to direct the fovea, what to look at and how to allocate the brain's visual processing resources. the fovea corresponds to the central few degrees of the visual field, and a large proportion of the primary visual cortex represents the fovea.

Saccades

Rapidly shift the line of sight (fovea) to new objects in the environment -- are conjugate eye movements, fast, ballistic, do not use visual feedback and do not require a visual target.

Smooth Pursuit

Slowly moves the eyes to track moving visual targets, are conjugate eye movements, use visual feedback, and require a moving visual target

vergence

rotates the eyes inward/outward to focus on near/far objects, are disconjugate, and driven by visual processing in the cortex (that an object is in front or behind the current plane of fixation), v. cortex then projects to brainstem centers that control vergence eye movements

Reflexive eye movements

Several types (including VOR, which is driven by vestibular system and keeps line of sight fixed by rotating eyes opposite of head rotation, and nystagmus, which compensates for prolonged head rotation or visual motion-- quick phase is a saccade driving eyes to a new point of fixation, slow phase is VOR tracking the fixated object during prolonged head rotation or visual motion).

lateral rectus

abducens nerve, abduction, pulls the eye outward

medial rectus

adduction, pulls the eye inward, oculomotor nucleus

superior rectus

elevation, pulls the eye up, oculomotor nucleus

inferior rectus

depression, pulls the eye down, oculomotor nucleus

superior oblique

intorsion, top of the eye rotates inward, trochlear nucleus