The nervous system is broadly divided into the central nervous system(CNS) and the peripheral nervous system(PNS). A reflex arc is a pathway for a rapid response that does not involve conscious awareness.
Basic Knowledge
Central nervous system(CNS): cerebral cortex(sensation・movement・thought)・cerebellum(motor coordination・balance)・brainstem=midbrain・pons・medulla oblongata(respiration・heartbeat・centers for maintaining life)・diencephalon=thalamus(sensory relay)+hypothalamus(the highest center for autonomic nerves・hormones)・spinal cord(conduction pathways・reflex centers).
Peripheral nervous system(PNS): somatic nervous system=sensory nerves(afferent)+motor nerves(efferent). Autonomic nervous system=sympathetic nerves+parasympathetic nerves(which control internal organs antagonistically).
Reflex arc: receptor→sensory nerve→spinal cord(reflex center)→motor nerve→effector. It is fast because it does not pass through the cerebrum. Examples: patellar tendon reflex・pupillary reflex.
Explain the reflex arc of the patellar tendon reflex, including the nerves and centers through which it passes.
Answer: The muscle spindle(receptor) in the quadriceps muscle receives a stretching stimulus, and the sensory nerve sends a signal to the spinal cord. Through an interneuron in the spinal cord, the motor nerve sends a signal to the quadriceps muscle(effector), causing it to contract. Because it does not pass through the cerebrum, it occurs unconsciously as a monosynaptic reflex.
- CNS=cerebrum・cerebellum・brainstem・diencephalon・spinal cord
- Hypothalamus=the highest center for autonomic nerves・hormones
- Somatic nervous system=sensory(afferent)+motor(efferent)
- Autonomic nervous system=sympathetic・parasympathetic(antagonistic)
- Reflex arc=receptor→sensory nerve→spinal cord→motor nerve→effector
- Reflexes are fast because they do not pass through the cerebrum
- Medulla oblongata=life-support center for respiration・heartbeat
Points to Note
① A reflex is completed in the spinal cord and therefore does not pass through the cerebrum. ② The sympathetic and parasympathetic nerves work antagonistically. Heartbeat: sympathetic↑・parasympathetic↓. Digestive tract: sympathetic↓・parasympathetic↑. ③ The medulla oblongata is essential for maintaining life, and its destruction causes immediate death.
Practice
- Where is the highest center for the autonomic nerves・hormone secretion?
- State the five elements of a reflex arc in order.
- In which part of the brain are the life-support centers for respiration・heartbeat located?