4/9 Quiz

Schwann cells

Glial cells of the PNS that support nerve regeneration by creating guiding channels and producing growth factors, enabling peripheral nerves to regrow after injury.

Wallerian degeneration

The breakdown of the distal portion of an injured nerve. In the PNS, followed by effective debris clearance enabling regeneration. In the CNS, debris clearance is far less effective.

Why can't the CNS regenerate like the PNS?

CNS lacks Schwann cells, has poor debris clearance, and upper motor neuron axons face an inhibitory environment with no effective guiding channels or sufficient growth factors.

4 strategies to repair spinal cord injuries

1. Stem cell therapies 2. Transplanted glial cells 3. Neurotrophic factors / cell adhesion molecules 4. Transplanting peripheral nerves as a bridge bypass

Corticospinal (pyramidal) system

The pathway from motor cortex to spinal cord. Called pyramidal because axons pass through the pyramids of the medulla. Decussates (crosses) at the lower medulla to control the opposite side of the body.

Decussation of the pyramidal tract

The crossing of motor axons from one side of the brain to control the opposite side of the body. Occurs at the lower medulla.

Motor homunculus

A distorted map of the body on primary motor cortex. Body parts requiring more precise/complex movement (hands, face, lips) have larger cortical representation than simpler parts (trunk, legs).

What does M1 represent?

Both muscles AND directions of movement. ~1/3 of M1 neurons correspond to specific muscles; most M1 neurons respond to movement in a particular direction regardless of which muscles are used.

Kakei et al. (1999) finding

Most M1 neurons fired for a particular movement direction regardless of which muscles accomplished it — showing M1 encodes movement direction, not just muscle activation.

Georgopoulos et al. (1982) finding

Individual M1 neurons fire preferentially before movements in a particular direction (e.g., vigorously before leftward movements, silent before rightward ones).

Basal ganglia function

Modulates movement by selecting correct motor programs and inhibiting incorrect ones. Receives input from wide cortical areas and outputs back to cortex in a loop. Initiates movement and maintains balance between excitation and inhibition.

Structures of the basal ganglia

Striatum (caudate nucleus + putamen), Globus pallidus external (GPe) and internal (GPi), Subthalamic nucleus (STN), Substantia nigra pars compacta (SNc)

Direct ("go") pathway

Cortex → Striatum → GPi inhibited → Thalamus disinhibited → Cortex excited. Thalamus is released from inhibition and sends excitatory signals to cortex, facilitating movement.

Indirect ("no-go") pathway

Cortex → Striatum → GPe inhibited → STN disinhibited → GPi excited → Thalamus inhibited → Cortex suppressed. Thalamic activation is reduced, suppressing movement.

Role of dopamine from SNc

Dopamine from the substantia nigra pars compacta modulates the striatum — facilitates the direct pathway (promoting movement) and inhibits the indirect pathway.

Extrapyramidal systems

Motor systems whose tracts are located outside the pyramids of the medulla. Two important components: the cerebellum and the basal ganglia.

Huntington's Disease

An inherited (genetic) brain disorder causing progressive degeneration of neurons, particularly in the basal ganglia (especially the striatum). Primarily damages the indirect pathway → excessive involuntary movement (chorea).

Chorea

Involuntary, excessive, irregular movements caused by loss of the indirect pathway's inhibitory function. Hallmark motor symptom of Huntington's Disease.

Parkinson's Disease

A progressive brain disorder caused by loss of dopamine-producing neurons in the substantia nigra. Primarily damages the direct pathway → bradykinesia, difficulty initiating movement, and tremors.

Bradykinesia

Slowness or reduction of movement. Hallmark symptom of Parkinson's Disease caused by disruption of the direct pathway due to dopamine loss from SNc.

What causes tremors in Parkinson's Disease?

A compensatory mechanism involving the cerebellum and thalamus — not directly from basal ganglia dysfunction itself.

Huntington's vs. Parkinson's Disease

Huntington's: indirect pathway → too much movement (chorea) → inherited. Parkinson's: direct pathway → too little movement (bradykinesia) → dopamine loss in SNc → tremors from cerebellum/thalamus compensation.

"When to move" experiment finding

Decision-making regions activate up to 10 seconds before a button press. Motor cortex activates ~5 seconds before. Conscious experience of "deciding" occurs only ~1 second before — brain predicts action before conscious awareness.

Brain-Computer Interface (BCI)

Technology that reads neural signals from motor cortex and translates them into commands to control external devices (computers, prosthetics) for people with motor impairments.

Closed-loop control

A motor control system that uses sensory feedback to adjust ongoing movement in real time (e.g., touch feedback for robotic grip). Contrasts with open-loop control, which uses no real-time feedback.

Upper motor neurons

Neurons whose axons travel within the CNS (corticospinal tract). Their axons are very difficult to repair after injury due to lack of regenerative support in the CNS.

Why can you reattach a finger but not repair a spine?

Finger nerves are peripheral (PNS) — Schwann cells guide regeneration and Wallerian degeneration clears debris. Spinal cord nerves are CNS (upper motor neuron axons) — no Schwann cells, poor debris clearance, inhibitory environment.

Somatotopic organization of motor cortex

Primary motor cortex is organized as a body map where each region controls a specific body part. More cortex is devoted to body parts requiring fine, complex movements (hands, lips) than to those requiring gross movement (trunk).

Which basal ganglia pathway initiates movement?

The direct ("go") pathway — disinhibits the thalamus, allowing it to send excitatory signals back to the cortex to initiate movement.

Which basal ganglia pathway suppresses movement?

The indirect ("no-go") pathway — ultimately inhibits the thalamus, suppressing its excitatory signals to cortex and preventing movement.

Substantia nigra pars compacta (SNc)

Dopamine-producing region of the basal ganglia. Its neurons are lost in Parkinson's Disease, disrupting the direct pathway and reducing movement initiation.

Circadian rhythm

A pattern of behavioral, biochemical, or physiological fluctuation that has a 24-hour period.

Diurnal

Active during the light periods of the daily cycle.

Nocturnal

Active during the dark periods of the daily cycle.

Suprachiasmatic nucleus (SCN)

A portion of the hypothalamus located just above the optic chiasm. It is the brain's biological clock, responsible for regulating the endogenous sleep/wake cycle.

Why is the endogenous sleep/wake cycle slightly longer than 24 hours?

Because the SCN's internal clock runs slightly longer than 24 hours. Without external light cues to entrain it, the active period drifts later each day (seen in both hamsters and humans).

What happens to circadian rhythms after an SCN lesion?

The animal can still use external light/dark cues to coordinate activity, but loses its endogenous rhythm. In constant dim light, activity becomes completely random — the internal clock is gone.

What does the SCN lesion experiment prove?

It shows the SCN is necessary for the endogenous circadian rhythm. Without it, the animal relies entirely on external light cues and has no internal clock.

Tau hamster

A genetic line of hamsters with an endogenous sleep/wake cycle of ~20 hours instead of the typical ~24 hours. Used to demonstrate the causal role of the SCN.

SCN transplant experiment

When the SCN from a tau hamster (20-hr rhythm) is transplanted into a normal hamster with no SCN, the normal hamster adopts the 20-hour rhythm of the donor. This proves the SCN causally produces the circadian rhythm.

What does the SCN transplant experiment establish?

Causality — the SCN is not just correlated with circadian rhythms, it directly produces them. The recipient animal's rhythm matches the donor SCN's rhythm.

Retinohypothalamic pathway

The neural pathway that carries light/dark information from the retina to the SCN, allowing external light to entrain (synchronize) the internal clock to the environment.

Melanopsin

A photopigment found in specialized retinal ganglion cells. These cells bypass rods and cones and project directly to the SCN via the retinohypothalamic tract to signal light levels.

How does light reach the SCN?

Specialized retinal ganglion cells containing melanopsin detect light and send signals directly to the SCN via the retinohypothalamic tract — bypassing the regular rods and cones used for vision.

Entrainment

The process by which external cues (especially light) synchronize the internal biological clock to the 24-hour environmental cycle.

Evidence that light entrains human behavior

People on the western side of a time zone go to bed later than those on the eastern side, even though their clocks show the same time. This is because the sun sets later in the west — showing humans are still entrained by actual sunlight, not clock time.

Endogenous human sleep/wake cycle

Slightly longer than 24 hours. When humans are isolated from all external light cues, their sleep period gradually drifts later each day, revealing the internal clock runs long.

Human isolation experiment finding

When isolated from external time cues for 77 days, a participant's sleep period shifted progressively later each day, confirming humans have an endogenous clock longer than 24 hours that must be entrained by light.

Exogenous sleep/wake cycle

A sleep/wake cycle driven by external cues (e.g., light, alarm clocks, social schedules) rather than the internal biological clock.

Endogenous sleep/wake cycle

A sleep/wake cycle driven by the brain's internal biological clock (the SCN), independent of external environmental cues.

How are daily sleep/wake cycles maintained?

The SCN acts as the internal biological clock, generating a ~24-hour rhythm. Light information reaches the SCN via the retinohypothalamic pathway (melanopsin cells), entraining the clock to match the external light-dark cycle.

What evidence shows the SCN causally sets the internal clock?

1. SCN lesions eliminate endogenous rhythms (only external cues remain). 2. Transplanting the SCN from a tau hamster (20-hr rhythm) into a normal SCN-lesioned hamster causes it to adopt the donor's 20-hr rhythm — proving the SCN generates the rhythm.

Phase shift

A change in the timing of the circadian activity cycle in response to a shift in the light-dark schedule (e.g., lights going on/off at a different time).

Free-running rhythm

The circadian rhythm expressed in the absence of external time cues. It reveals the true period of the internal clock, which is slightly longer than 24 hours in both hamsters and humans.