PNB 2774 Module 5 CNS

central nervous system (CNS)

• consists of the brain and spinal cord

• separated from the peripheral nervous system by the blood-brain barrier

• made up of neurons and supportive glial cells

afferent connections

“inward”, carrying information from the outside world to the CNS

efferent connections

“outward”, carrying processed information from the CNS to other structures (e.g. muscles and organs)

white matter

made up of neuronal axons that are myelinated

gray matter

made up of unmyelinated axons

functions of neuroglia

• myelinate

• keep neurons in place

• support neurons with nutrients and O₂

• remove pathogens and dead cells

• assist in development

• facilitate synaptic transmission

astrocytes

• most abundant in neuroglia

• regulate the environment surrounding neurons

microglia

remove debris and damaged cells

multipolar neurons

common in the CNS, distinguished by a single axon and short dendrites

pyramidal neurons

the longest CNS neurons with longer axons and dendrites

interneuron

a neuron entirely within the CNS

meninges

• three layers of membrane that separate the skull bone from CNS tissue

• dura mater

• arachnoid mater

• pia mater

dura mater

• thickest later of the meninges

• responsible for draining blood and cerebrospinal fluid (CSF) from the brain into the bloodstream

• two layers with blood vessels, ECF, and lymphatic vessels in between

arachnoid mater

• middle layer of the meninges

• creates the sub-arachnoid space between the dura and pia mater

• facilitates the movement of fluid out of the sub-arachnoid space

pia mater

• layer of the meninges directly on the surface of the brain

• protects the CNS tissues as well as the arteries that supply blood to the brain

• contains CSF

cerebrospinal fluid

• produced by ependymal cells that line the ventricles

• provides another layer of “padding”for the brain from physical injury

• closely regulates the extracellular environments of neurons

• facilitates waste removal

• provides solutes and water to interstitial fluid (ISF)

• flows from the choroid plexus into the ventricles

ventricles

fluid-filled spaces in the brain

blood-brain barrier 

• consists primarily of endothelial cells that form tight junctions with each other, preventing solute movement

• capillary endothelium responsible for moving nutrients from blood into the brain

  • water-soluble molecules not specifically transported can’t cross

• hydrophobic compounds (O₂, CO₂, free fatty acids) are diffusion limited

• things like ethanol, THC, nicotine, propofol can diffuse within 10 seconds

carrier-mediated transport (BBB)

amino acids, glucose, nucleotides, small peptides

receptor-mediated transport (BBB)

hormones, growth factors, enzymes, small proteins

important differences between ISF and blood plasma

• lower pH due to elevated CO₂

• less protein

• less glucose

• lower potassium

subdural hematoma

physical damage causes bleeding onto the surface of the brain, causing pressure and damage to neurons

simple circuits

everything is feed forward

complex circuit

• has feedback

• the activity of the second neuron projects back and affects the activity of the first

• promotes additional information processing, but the activity can be very difficult to predict and understand

divergent circuit

• one neuron projecting onto many others

• the singular neuron’s activity influences and coordinates the activity of many others

convergent circuit

• multiple neurons project onto a single neuron, which integrates all of this input into a singular output

• gives you more sensitivity

rhythm circuit

• some can be as generated as little as two cells that communicate with each other 

• rhythms in larger populations of neurons are associated with important brain functions

chemotaxis

process where axons are “guided” to the right targets

electroencephalogram

measures brain activity with scalp electrodes; used for diagnosing epilepsy, sleep disorders, behavioral disorders

functional MRI (fMRI)

a more modern method that still detects functional activity in particular brain regions

structural MRI (sMRI)

same foundational technology as fMRI, but detects the brain’s structure

diffusional tensor imaging (DTI)

• detects how water travels along the axonal projections of the brain

• most useful for macro-scale connectivity

  • typically can’t tell you about the micro-scale connections defining a neuronal circuit

longitudinal fissure

divides the brain into left and right lobes

lateral fissure

divides into top and bottom sections

cerebrum

largest part of the brain, the top/outermost region

cerebral cortex

• outer layer of gray matter

• split into multiple lobes

• responsible for higher level processing and cognition

cerebral control

right side of the cerebral cortex controls left side of the body and vice versa

cerebral dominance

different sides of the brain are associated with different types of tasks

left side of the brain

language, mathematical reasoning, logical reasoning

right side of the brain

spatial awareness facial recognition, visual imagery, music

frontal lobe

• motor, speech, memory formation, personality, emotion

• divided into prefrontal cortex, motor cortex, and broca’s area

broca’s area

plays a key role in the generation of articulate speech

primary motor cortex

• functionally and anatomically stratified

• different regions are directly associated with controlling muscles in different regions of the body

• the larger the volume of the brain, the stronger the level of control of that area of the body

parietal lobe

• somatosensory cortex (touch, proprioception)

• integration of sensory information

• collects information from multiple sources

• manages taste (gustation), sight, touch, and smell

proprioception

awareness of your body in 3D space relative to other structures and itself

somatosensory cortex

• processes sensory input from contralateral cord and thalamus

• temp, touch (mechanoreception), proprioception, pain (nociception)

• integrates these inputs in order to perform skilled movements (sensory and motor signals)

temporal lobe

• hearing, speech, language, smell

• organization and language comprehension

• memory formation and retrieval

• wenicke’s area

• olfactory cortex

• auditory cortex

wernicke’s area

speech comprehension

aphasia

inability to understand words and communicate

olfactory cortex

processes smell

auditory cortex

processes hearing

occipital lobe

• visual processing, visual stimuli

• processing and integrating visual stimuli

  • initially processed in the visual cortex, then interpreted in the visual association area

projections

connections between subcortical regions

commissural connections

connections between lobes

association connections

connections within lobes

brainstem

• home of axons connecting the cerebrum and spinal cord

• contains nuclei responsible for autonomic responses and reflexes

  • including those associated with the cranial nerves

• gateway to the brain for 90% of the body via bidirectional pathway

• begins at the foramen magnum at the opening in the occipital bone (back and lower skull)

• key components include medulla oblongata, pons, midbrain, and reticular formation

medulla

• relay center for sensory, proprioceptive, and movement information

• between pons and spinal cord

• different portions have different functional roles

key functions of the medulla

• cardiovascular and respiration control maintain blood pressure, breathing rate

• vomiting (the emetic reflex)

• facial heat and pain

• proprioception, somatosensation, muscle tension, movement

• motor tracts

• nuclei associated with the cranial nerves

• different nuclei responsible for each of these functions

pons

• relay station for information transfer between cerebellum and cerebrum

• vital for breathing control

reticular formation

• nuclei associated with the brainstem

• control of respiration, sleep, alertness, eye movements, sexual activity, pain modulation

• damage leads to irreversible coma

cranial nerves

• 12 pairs of nerves arising from the brainstem

• carry sensory, autonomic and motor fibers

I

olfaction

II

optical

III

oculomotor

• control of eye and associated muscles (4 of 6 muscles)

IV

trochlear

• control of 1 of 6 eye muscles

V

trigeminal

• tri = three parts: ophthalamic, maxillary, mandibular

• facial sensations and control of jaw

VI

abducens

• controls the last eye muscle

VII

facial

• facial expression, taste, salivary glands, tears

VIII

vestibulchoclear

• hearing and balance

IX

glossopharyngeal 

• tongue and pharynx

• taste sensation, blood pH, swallowing, salivary glands

X

vagal/vagus nerve

• swallowing, speaking, cardiovascular, GI, various motor control

• vagus = wanderer, reflective of how many varied roles this nerve serves, all throughout the body (and conversely, projections throughout the brain)

• vagus nerve stimulation is a widely used medical treatment with an unclear mechanism

XI

accessory

• trapezius (upper back) and sternocleidomastoid (neck)

XII

hypoglossal

• tongue control

midbrain

• above brainstem

• connects diencephalon and cerebrum to the pons

• key elements: tectum and tegmentum

tectum

controls reflexes in response to visual/auditory stimuli (including eye movement)

tegmentum

movement control, contains cerebral peduncles and substantia nigra

basal ganglia

• fore/midbrain structures linking motivation to motor function

• key components:

  • caudate nucleus 

  • putamen

  • globus pallidus

  • claustrum

  • amygdaloid body

• associated with subconscious processing

• gets input from the cortex and thalamus, then sends output back to the cortex via the thalamus

• modulates motor output

basal ganglia circuitry

• the circuitry of the basal ganglia is very well studied, in part due to its complicity in parkinson’s disease

• the pathways between the striatum and thalamus are important and there are two: direct and indirect

direct pathway

• from striatum to the GPi (globus paladus internus)

• inhibitory

indirect pathway

inhibits the GPe (globus paladus externus) which itself inhibits the STN (subthalamic nucleus) which excites the GPi

diencephalon

• between the brainstem and cerebral cortex

• contains the thalamus, subthalamus, epithalamus, and hypothalamus

thalamus

• bidirectional relay station to and from cerebral cortex

• regulates motor function

• all conscious senses except olfaction converge onto the thalamus

• filters the “signal” from the noise

epithalamus

• includes the pineal gland which secretes melatonin

• vital for the circadian rhythm, your body’s 24 hour cycle

hypothalamus

• controls endocrine, autonomic, somatic systems

• 11 distinct nuclei that control:

  • hormone release, body temp, panting, sweating, circadian rhythms, blood pressure, heart rate, GI stimulation, satiety, eating, memory, pupil dilation, shivering, arousal, energy balance, learning, memory, sleep

• pituitary gland links CNS to endocrine system

• no blood brain barrier

  • needs access to signals from the blood

limbic system

• a ring around the diencephalon

• comprised of:

  • cingulate gyrus

  • hippocampus

  • parahippocampal gyrus

  • amygdala

  • olfactory bulbs

  • fornix

  • diencephalon nuclei

hippocampus

• a small but very important brain region

• vital for memory, learning, and emotions

• converts short-term memories into long-term memories

cerebellum

• master motor control

  • detects error

  • motor learning and adaptations

  • automating/optimizing motor behavior

• regions:

  • spinocerebellum

  • vestibulocerebellum

  • cerebrocerebellum

spinocerebellum

motor movement of trunk and limbs

vestibulocerebellum

input from brainstem, controls eye/head movements and balance

cerebrocerebellum

motor planning, sensory/cognitive/affective processing

spinal cord (background)

• controls flow of information into and out of the brain

• reflexes arise from the spinal cord without always rising to the brain itself

• spinal nerves originate here

  • control a variety of physiological processes involved in movement, sensation, etc

spinal cord (terminology)

• only 15-20% of cells in the spinal cord are neurons

  • about 75% are glial cells

• key terminology

  • alpha-motor neurons

  • gama-motor neurons

  • interneurons

  • axonal tracts

  • ipsilateral fibers

  • contralateral fibers

alpha-motor neurons

directly stimulate contraction of striated muscle

gamma-motor neurons

neurons that are thinner, less myelinated, and conduct slower

interneurons

neurons that stay entirely within the spinal cord

axonal tracts

fibers running up and down the cord

ipsilateral fibers

fibers that run along the same side

contralateral fibers

fibers that cross to the other side

spinal cord (meninges)

• a continuation of the structures that surround the brain

• provide the same type of protection

• CSF, ISF, found in these regions just as in the brain

spinal cord (anatomy)

• dorsal root ganglionic cell

  • synapse onto interneurons within the gray matter of the dorsal horn

• the inverse occurs in the ventral horn

  • connections are made with ventral roots that carry information from the CNS to muscles and glands

  • the ventral horn contains the cell bodies of motor neurons

• dorsal root cell body is outside the CNS; is sensory

• ventral root cell body originates within the spinal cord; is motor or autonomic

• divided into left and right halves by the dorsal medial sulcus (back) and ventral medial fissure (front)